computation

Review
Thermal Analysis and Cooling Strategies of High-Efficiency
Three-Phase Squirrel-Cage Induction Motors—A Review
Yashwanth Reddy Konda 1 , Vamsi Krishna Ponnaganti 2 , Peram Venkata Sivarami Reddy 3 , R. Raja Singh 4 ,
Paolo Mercorelli 5, * , Edison Gundabattini 6, * and Darius Gnanaraj Solomon 7

1 SQL Database Administration, Infosys Technologies Limited, Hyderabad 500032, India;

yashwanthreddy.k@infosys.com
2 Manufacturing Systems and Engineering Management, California State University,
Northridge, CA 93012, USA; vamsi-krishna.ponnaganti.066@my.csun.edu
3 Cognizant Technology Solutions, Hyderabad 500019, India
4 Department of Energy and Power Electronics, Vellore Institute of Technology, Vellore 632014, India;
rrajasingh@vit.ac.in
5 Institute for Production Technology and Systems—IPTS, Leuphana University of Luneburg,
21335 Lüneburg, Germany
6 Department of Thermal and Energy Engineering, School of Mechanical Engineering, Vellore Institute of
Technology, Vellore 632014, India
7 Department of Design and Automation, School of Mechanical Engineering, Vellore Institute of Technology,
Vellore 632014, India; dariusgnanaraj.s@vit.ac.in
* Correspondence: paolo.mercorelli@leuphana.de (P.M.); edison.g@vit.ac.in (E.G.)

Abstract: In recent times, there has been an increased demand for electric vehicles. In this context, the
energy management of the electric motor, which are an important constituent of electric vehicles, plays
a pivotal role. A lot of research has been conducted on the optimization of heat flow through electric
motors, thus reducing the wastage of energy via heat. Futuristic power sources may increasingly
rely on cutting-edge innovations like energy harvesting and self-powered induction motors. In this
context, effective thermal management techniques are discussed in this paper. Importance was given
Citation: Konda, Y.R.; to the potential energy losses, hotspots, the influence of overheating on the motor efficiency, different
Ponnaganti, V.K.; Reddy, P.V.S.; cooling strategies, certain experimental approaches, and power control techniques. Two types of
Singh, R.R.; Mercorelli, P.;
thermal analysis computation methods, namely the lumped-parameter circuit method (LPCM) and
Gundabattini, E.; Solomon, D.G.
the finite element method (FEM), are discussed. Also, this paper reviews different cooling strategies.
Thermal Analysis and Cooling
The experimental research showed that the efficiency was greater by 11% with the copper rotor
Strategies of High-Efficiency
compared to the aluminum rotor. Each rotor type was reviewed based on the temperature rise and
Three-Phase Squirrel-Cage Induction
Motors—A Review. Computation 2024, efficiency at higher temperatures. The water-cooling method reduced the working temperatures by
12, 6. https://doi.org/ 39.49% at the end windings, 41.67% at the side windings, and by a huge margin of 56.95% at the yoke
10.3390/computation12010006 of the induction motor compared to the air-cooling method; hence, the water-cooling method is better.
Lastly, modern cooling strategies are proposed to provide an effective thermal management solution
Academic Editor: Andry Sedelnikov
for squirrel-cage induction motors.
Received: 29 July 2023
Revised: 23 October 2023 Keywords: induction motor; hotspots; cooling strategy; thermal analysis; heat transfer coefficient;
Accepted: 29 November 2023 thermal management; power control
Published: 4 January 2024

1. Introduction—Induction Motors
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland. Electric motors are used in a variety of industries, ranging from home electronics
This article is an open access article to aviation and including automotive and aerospace. Highly efficient motors can bring
distributed under the terms and substantial advantages, such as minimizing energy expenditures and lessening greenhouse
conditions of the Creative Commons gas emissions. Thanks to its high starting torque, sufficient speed control, and fair overload
Attribution (CC BY) license (https:// power, a particular form of motor known as an induction motor has gained popularity in
creativecommons.org/licenses/by/ recent years [1]. The interactions between the magnetic field formed by the stator winding
4.0/). and the cage bar generate mechanical power [2]. Furthermore, since rare-earth metals are

Computation 2024, 12, 6. https://doi.org/10.3390/computation12010006 https://www.mdpi.com/journal/computation

Computation 2024, 12, x FOR PEER REVIEW 2 of 22

Computation 2024, 12, 6 2 of 21

winding and the cage bar generate mechanical power [2]. Furthermore, since rare-earth
metals are in short supply, induction motors, which are a form of non-magnetic motor,
are emerging
in short supply, as induction
a promising choice.
motors, However,
which are a forma major downside of motor,
of non-magnetic induction are motors
emerging is
their underlying high heat factor, which affects their lifetime and
as a promising choice. However, a major downside of induction motors is their underlying performance.
highOn heat the otherwhich
factor, hand,affects
it is crucial to understand
their lifetime the rotor temperature of squirrel-cage
and performance.
induction motors (SCIMs) to guarantee
On the other hand, it is crucial to understand thetheir steady androtor
stable operation. Rotor
temperature overheat-
of squirrel-cage
ing seriously
induction affects
motors the end
(SCIMs) torings and rotor
guarantee theirbars
steady[3].and
SCIMs areoperation.
stable mainly used in cases
Rotor where
overheating
the motor affects
seriously needs theto operate
end rings at aandconstant speed
rotor bars [3].and
SCIMsshouldare start
mainly by used
itself.inSquirrel-cage
cases where
induction
the motormotors needs to also have many
operate applications
at a constant speed in and
the industrial
should start sector. TheseSquirrel-cage
by itself. motors need
ainduction
very lowmotors level of maintenance, so most industries prefer the usage
also have many applications in the industrial sector. These motors of SCIMs inneed
thesea
types of situations. Three-phase induction motors (IMs) are used
very low level of maintenance, so most industries prefer the usage of SCIMs in these types in woodworking ma-
chines, compressors,
of situations. elevators,
Three-phase and conveyors
induction in the mining
motors (IMs) are usedindustry, the chemical
in woodworking indus-
machines,
try, the automotive
compressors, industry,
elevators, and in railway
and conveyors applications
in the mining [4,5].
industry, The
the widespread
chemical usethe
industry, of
squirrel-cage
automotive industry,induction andmotors
in railwayin various applications
applications [4,5]. The can be attributed
widespread use to their simple
of squirrel-cage
structure,
induction high motors reliability,
in various and affordability.
applications canAs beelectric
attributedmotors continue
to their simpletostructure,
expand their
high
reach in various industries, it becomes crucial to monitor their conditions
reliability, and affordability. As electric motors continue to expand their reach in various and diagnose
unforeseen
industries, itproblems
becomes and faults
crucial that may
to monitor occur
their during and
conditions operation,
diagnose such as overheating
unforeseen problemsof
the
androtor
faultsand thatstator
may or different
occur during types of faults
operation, in these
such components
as overheating [6–8].
of the rotor and stator or
Figure
different 1 shows
types of faultstheinstructure of an SCIM[6–8].
these components and its squirrel-cage rotor. Under normal
Figuremost
operation, 1 shows
of the theheat
structure of anby
generated SCIM and its squirrel-cage
the motor’s rotor. Under
losses is dissipated through normal
heat
operation,
transfer most
to the of the heat environment.
surrounding generated by However,
the motor’s lossessuch
factors is dissipated
as heavy through heat
overloading,
transfer to the surrounding environment. However, factors
prolonged starting, or impaired cooling conditions during operation (due to a broken such as heavy overloading,
prolonged
cooling fan starting,
or clogged or motor
impaired cooling
casing) conditions
can affect duringcooling
the motor’s operation (due to[9].
capability a broken
When
cooling
the coolingfan capacity
or clogged of motor
an SCIM casing) can affect thethe
is compromised, motor’s coolingof
temperature capability
the rotor[9].barsWhen
and
the cooling
end capacity
rings, stator of an SCIM
windings, andistheir
compromised,
insulation the cantemperature
surpass their of maximum
the rotor bars and end
thresholds
rings,
[10]. stator windings,
Therefore, and their
the real-time insulation
and accurate can surpass
thermal their of
monitoring maximum
the SCIM’s thresholds
temperature[10].
Therefore, the real-time and accurate thermal monitoring of the
is critical in order to minimize thermal damage and ensure reliable and safe operation [7]. SCIM’s temperature is
critical in order to minimize thermal damage and ensure reliable and safe operation [7].

Figure 1. Diagrammatic representation of a squirrel-cage induction motor.

Figure 1. Diagrammatic representation of a squirrel-cage induction motor.

Estimating
Estimating anan SCIM’s
SCIM’s rotor
rotor temperature
temperature is is carried
carried out
out by
by the
the thermal
thermal monitoring
monitoring
technique,
technique, which involves three methods. The first method estimatesthe
which involves three methods. The first method estimates therotor
rotortempera-
temper-
ture
atureusing
using SCIM
SCIMparameters
parameters such as as
such thethe
rotor resistance.
rotor TheThe
resistance. second method
second methodusesuses
estima-
esti-
tors thatthat
mators predict the rotor
predict temperature
the rotor according
temperature to thetoSCIM’s
according thermal
the SCIM’s model.
thermal The third
model. The
method combines
third method the first
combines and
the second
first methods.
and second However,
methods. since thesince
However, temperature estima-
the temperature
tion methods
estimation consider
methods large large
consider partsparts
of the SCIM,
of the SCIM,they cannot
they cannotidentify
identifythe
therotor’s
rotor’s local
local
hotspots.
hotspots. Therefore, methods of directly measuring the rotor temperature are superior
Therefore, methods of directly measuring the rotor temperature are superior toto
estimation
estimation methods
methods in
in this
this regard
regard [11].
In
In this paper,
paper, the
thelosses
lossesandandhotspots
hotspotsinin a squirrel-cage
a squirrel-cage induction
induction motor
motor are listed
are listed and
and briefed.
briefed. The temperatures
The high high temperatures caused
caused by theseby these
losses andlosses andand
hotspots hotspots and their
their influence on
the performance of the induction motor are briefed. A steady-state analysis of a three-phase
IM using a popular computation approach, the lumped-parameter thermal network (LPTN)
Computation 2024, 12, 6 3 of 21

modeling method, is explained as it is super-fast, mathematically super-simple, and easy to

implement, and this method is used to model the main heat transfer paths. The applicability
of the thermal investigation through another outstanding computation technique, the finite
element method (FEM), is also discussed, as induction machines with components with
large temperature gradients take time to process, unlike with thermal network modeling.
Various experimental works on the motor efficiency of copper and aluminum rotors are
discussed. Various cooling schemes such as water cooling and oil cooling employed for the
cooling of the induction motor are highlighted. Also, methods for improving the efficiency,
such as introducing nano-fluids into the coolant pipes and changing the material of the
casing, the stator, and the rotor, are presented.

1.1. Losses in a Squirrel-Cage Induction Motor

Every electrical machine has losses in the form of heat. These losses can be separated
into I2 R losses, iron core losses, eddy-current losses, hysteresis losses, surface losses, losses
because of flux pulsation, and mechanical losses [12–14]. Mechanical losses cannot be
estimated analytically, so these losses will not be considered in the following simulations.
I2 R losses in the stator and rotor windings can be represented with Equation (1).

∆P = m.( R1 + R2 ).I 2 (1)

where R1 and R2 represent the winding resistance, m represents the number of phases, and
I is the supply current.
Iron core losses can be calculated by Equation (2).
 β 
f 
∆PFEhj = ∆P1,0 . . k dj .B2j .mj (2)
50

The β coefficient depends on the lamination used, P1,0 represents the specific losses
of the iron used, f represents the frequency of the supply, kdj represents the coefficient of
heterogeneous distribution of the FD, Bj is the average FD, and mj represents the magnetic
circuit weight of division. The surface losses of the iron core in the air gap are described by:

Q1,2 .n 1,5
 
π
∆Ppo = D1 .α.l e .K0 . .(td1,2 .β 0x .β δ .k c .1000)2 (3)
2 10000

D1 represents the inner diameter of the stator, α is the coverage coefficient of poles,
le is the stator/rotor packet length, K0 is the surface loss factor, Q1,2 is the number of slots,
n is the RPM, td1 is the slot pitch, and βδ is the pulsation in air gap FD. β0x depends on the
ratio of slots opening and the air gap length in the stator teeth. The pulsating-magnetic-
flux-caused set teeth losses are described by:
 2
Q1,2 .n
∆Pp1,2 ≈ 0.11. .β .m j1,2 (4)
1000 p1,2

n is the RPM, mj1,2 is the stator/rotor teeth weight, Bp1,2 is the FD magnitude of the
saturating stator/rotor teeth, and Q1,2 is the number of slots.

1.2. Hotspots in a Squirrel-Cage Induction Motor

An induction motor is designed to work according to industrial standards; sometimes,
it produces heat more than what is predicted, necessitating experimental research on
the heat development in the motor, as shown in Figure 2. In general, heat is produced
everywhere in the motor, but some specific parts play a vital role in producing the energy,
and studying those parts (as different parts will have different temperature limits) could
result in a new analysis and scope of improvement in that area, which could increase the
efficiency of the electric motor as a whole. Bearings and stator windings are very important
 Computation 2024, 12, x FOR PEER REVIEW 4 of 22
Computation 2024, 12, 6 4 of 21

important
for a motor for a motor
to work to work[13,15].
efficiently efficiently
Since[13,15]. Since the
the winding is awinding is a major
major source source
of heat and itsof
heat and its insulation is thermally sensitive, lowering the winding temperature
insulation is thermally sensitive, lowering the winding temperature is critical to improving is critical
to improving
the the motor’s
motor’s stability. stability. An
An enhanced enhanced
power power
loading and aloading and a density
high torque high torque density
necessitate
necessitate a wide-end winding. The losses in the system are controlled by
a wide-end winding. The losses in the system are controlled by those from the winding those from the
winding owing to the reduced speed and application
owing to the reduced speed and application of high torque [13].of high torque [13].

Figure2.2.Overheating
Figure Overheatingof
ofaasquirrel-cage
squirrel-cageinduction
inductionmotor
motor[16].
[16].

Increasedtemperature
Increased temperatureisisthe thelargest
largestinfluencing
influencingfactor factorof ofthe
theefficiency
efficiencyof ofinduction
induction
motors,as
motors, asshown
shownin inFigure
Figure3.3.An Anelevated
elevatedtemperature
temperaturehas hasaaconsequence
consequenceon onnumerous
numerous
elements of
elements of motor function.
function.An Aninduction
induction motor’s
motor’s efficiency
efficiency declines as the
declines as winding
the winding tem-
perature increases.
temperature increases.AsAs thethe
winding
windingtemperature
temperatureisisraised,
raised,the the input
input energy consumption
consumption
increases.Raising
increases. Raisingthe thewinding
windingtemperature
temperatureof ofthe
themotor
motorinduces
inducesthe themotor
motortorque
torqueto tofall
fall
and
and the
the motor efficiency
efficiency to to deteriorate.
deteriorate.The Thestator
stator winding
winding temperature
temperature is critical
is critical to
to un-
understand,
derstand, as as it damages
it damages thethe insulation
insulation system
system sincesince
highhigh temperatures
temperatures affect affect thespan
the life life
span
of theofstator.
the stator. The temperature
The temperature affects affects the winding
the winding impedance
impedance and theandmagnetic
the magnetic flux flux
den-
density. Whenever
sity. Whenever the the temperature
temperature of theof motor
the motor rises,
rises, the winding
the winding impedance
impedance increases.
increases. This
This may harm the motor’s maximum torque potential
may harm the motor’s maximum torque potential at maximum speed as well as its effi-at maximum speed as well as
its efficiency. Whenever the temperature of the material rises,
ciency. Whenever the temperature of the material rises, atomic oscillations cause previ- atomic oscillations cause
previously oriented
ously oriented magneticmagnetic
dipolesdipoles to “dynamically
to “dynamically resize”,resize”, resulting
resulting in a in
in a drop dropthein the
inten-
intensity
sity of theofmagnetic
the magnetic flux. Hence,
flux. Hence, the fluxthe flux density
density of the magnetic
of the magnetic materialsmaterials
will diminish will
diminish as the temperature
as the temperature rises. The rises. The torque
torque falls asfalls
the as the magnetic
magnetic field intensity
field intensity decreases.
decreases. As a
As a result, a drop in the magnetic flux and an increase in the inductance
result, a drop in the magnetic flux and an increase in the inductance will have a substantial will have a
substantial influence on the motor’s efficiency. The operating
influence on the motor’s efficiency. The operating temperature of the motor has the high- temperature of the motor
has
est the highest
influence oninfluence
the element on the elementfor
durability durability
both the for both the
bearings andbearings
the statorand the stator
winding. An
winding. An excessive bearing temperature will degrade the lubrication
excessive bearing temperature will degrade the lubrication system, leading to the need for system, leading to
the need for frequent lubrication, as shown
frequent lubrication, as shown in Figure 3 [17,18]. in Figure 3 [17,18].
To remove heat from the motor, the forced convection method is used with fan-cooled
motors. Various factors like the speed of the motor, the air temperature around it, and
tribology factors impact the bearing temperature loss. However, the primary loss of heat
dissipation is produced at the stator of the motor. The temperatures at different parts of the
squirrel-cage induction motor examined in this study at different operation times are listed
in Table 1.
Computation 2024,
Computation 2024, 12,
12, 6x FOR PEER REVIEW 5 of 22
5 of 21

Figure 3.
Figure 3. Adverse effects
effects of increased
increased temperature
temperature on
on the
the induction
induction motor.
motor.

To remove heat from the motor, the forced convection method is used with fan-
Table 1. Temperatures at various parts of the SCIM.
cooled motors. Various factors like the speed of the motor, the air temperature around it,
Temperature, and
◦C tribology factors
Temperature, ◦C impact the bearing
Temperature, ◦C temperature
Temperature, ◦ C loss. However,
Temperature, ◦ Cthe Temperature,
primary loss ◦ Cof
t (s)
heat dissipation is produced at the stator of the motor. The temperatures at different parts
(Rotor Tooth) (Rotor Surface) (Stator Tooth) (Stator Surface) (End Cap) (Shaft Surface)
0 40 of the squirrel-cage
40 induction 40 motor examined 40 in this study at40different operation 40 times
4.5 40.4 are listed in Table
40.3 1. 40.1 40.2 40.3 40.1
9 41.3 40.4 40.2 40.5 40.4 40.1
Table 1. Temperatures at various parts of the SCIM.
13.5 41.9 40.6 40.3 40.6 40.6 40.2
18Temperature, °C
42.3 Temperature,40.8
°C Temperature, 40.4°C Temperature, 40.8 °C Temperature, 40.8 °C Temperature,40.3 °C
t (s)
22.5 (RotorTooth)42.5 (Rotor Surface) 41 (Stator Tooth)
40.5 (Stator Surface)
41.1 (End Cap)
41 (Shaft Surface)
40.3
0 27 40 42.9 40 41.2 40 40.8 40 41.3 4041.2 40
40.4
4.5 31.5 40.4 43.3 40.3 41.5 40.1 40.8 40.241.6 40.3
41.5 40.1
40.5
9 36 41.3 43.5 40.4 41.6 40.2 40.9 40.541.8 40.4
41.6 40.1
40.6
13.5 40.5 41.9 43.8 40.6 41.8 40.3 40.9 40.642.1 40.6
41.9 40.2
40.8
18 42.3 40.8 40.4 40.8 40.8 40.3
22.5 42.5 As depicted in Table 1, the highest temperature, i.e., 43.8 ◦ C, was recorded 40.3
41 40.5 41.1 41 at the rotor
27 42.9 tooth41.2 40.8 temperature,41.3
region, and the lowest i.e., 40.8 ◦ C, was41.2
recorded at the shaft40.4surface.
31.5 43.3 The 41.5
temperature at the40.8 41.6
majority of the components 41.5
was recorded 40.5of from
in the range
36 43.5 ◦
40.6 41.6 ◦ 40.9
C to 46 C. The recorded 41.8were quite low41.6
temperatures because they were 40.6
recorded
40.5 43.8 after41.8
a short time interval.
40.9Nevertheless, these
42.1 temperatures would 41.9 be bound to40.8 increase as
the time of the operation increases. Hence, the thermal management of an electric machine
is vital
Astodepicted
safeguarding the1,machine
in Table andtemperature,
the highest its efficiency.i.e.,
An43.8
analysis of the
°C, was temperature
recorded rise
at the rotor
under variousand
tooth region, cooling
the settings makes it simple
lowest temperature, i.e.,to40.8
select
°C,anwas
appropriate
recorded cooling strategy
at the shaft [18].
surface.
The temperature at the majority of the components was recorded in the range of from 40.6
°C to 46 °C. The recorded temperatures were quite low because they were recorded after
a short time interval. Nevertheless, these temperatures would be bound to increase as the
time of the operation increases. Hence, the thermal management of an electric machine is
 vital to safeguarding the machine and its efficiency. An analysis of the temperature rise
under various cooling settings makes it simple to select an appropriate cooling strategy
[18].
Computation 2024, 12, 6 6 of 21
2. Thermal Analysis of High-Efficiency Three-Phase Cage Induction Motors
2.1. LPTN—Lumped-Parameter Thermal Network Modeling Method, a Computational
2. Thermal Analysis of High-Efficiency Three-Phase Cage Induction Motors
Approach
2.1. LPTN—Lumped-Parameter
Thermal Thermalthe
network modeling divides Network
motorModeling Method,
into simple a Computational
thermal Approach
elements compris-
Thermal network
ing combinations of modes modeling divides such
of heat transfer, the motor into simple
as radiation, thermaland
convection, elements compris-
conduction
ing
[19]. The combinations
thermal network of modes
of an IM of isheat
not transfer,
much different such if aswe radiation,
think of theconvection,
electric re-and
conduction
sistance as thermal[19].resistance,
The thermal network
current sourcesofasan IM issources,
power not much different
electric if we
capacitors asthink
ther- of
the electric resistance as thermal resistance, current sources as power
mal capacitors, voltage as nodal temperatures, and the current flowing through resistance sources, electric capac-
itors as thermal capacitors, voltage as nodal temperatures, and
as the power flow. All these thermal resistances, thermal capacitance, and component ge-the current flowing through
resistance
ometries of a as the power
motor can be flow. All these
determined thermal resistances,
mathematically. Thesethermal
parameterscapacitance, and com-
and associated
ponent
losses geometries
are fitted of a motor
into thermal can betodetermined
networks mathematically.
obtain the temperature These parameters
distributions across theand
associated
motor at differentlosses are fittedconditions
operating into thermal networks to obtain the temperature distributions
[16,20].
across the motor at
A steady-state different
analysis of operating conditions
a three-phase IM can [16,20].
be carried out using thermal re-
A steady-state analysis of a three-phase
sistance and thermal sources amid the motor component IM can be carried
nodes;out using thermal
however, resistance
with transient
analysis, the thermal capacitance is used to observe the internal energy change in theanalysis,
and thermal sources amid the motor component nodes; however, with transient com-
the thermal
ponents capacitance
over time. To find is theused to observeof
temperatures the internal
various energy change
components, in the components
a network founded
on over time. To find the
a lumped-parameter temperatures
thermal networkofcould various components,
be established, anda network
the geometryfoundedof the on a
lumped-parameter
components and their thermal
thermalnetwork
propertiescould be established,
could and thethe
be used to express geometry
developed of the compo-
model
nents and their thermal properties could be used to express the
[21,22]. This would have the advantages of being super-fast, mathematically super-simple, developed model [21,22].
and easy to implement, but it would need some experience to model the main heat transferand
This would have the advantages of being super-fast, mathematically super-simple,
easy to implement, but it would need some experience to model the main heat transfer
paths accurately. This method assumes that the temperature gradient within a solid is
paths accurately. This method assumes that the temperature gradient within a solid is
negligible [23]. The model could be upgraded by increasing the number of nodes and ac-
negligible [23]. The model could be upgraded by increasing the number of nodes and
curately designing heat flow directions [24]. Motor-CAD and Open Modelica are some of
accurately designing heat flow directions [24]. Motor-CAD and Open Modelica are some
the software programs used to model thermal networks [19,25].
of the software programs used to model thermal networks [19,25].
Badran et.al. [26] used a Y 90S-2 three-phase squirrel-cage rotor induction motor and
Badran et.al. [26] used a Y 90S-2 three-phase squirrel-cage rotor induction motor and
produced a thermal network with 10 nodes and 14 thermal resistances. The stator of the
produced a thermal network with 10 nodes and 14 thermal resistances. The stator of the
motor had networks for the stator iron, stator windings, and end windings. The heat trans-
motor had networks for the stator iron, stator windings, and end windings. The heat
fer to the stator from the rotor windings through the air gap occurred immediately with
transfer to the stator from the rotor windings through the air gap occurred immediately
an insignificant effect on the stator teeth. Finally, the rotor, stator, and frame were con-
with an insignificant effect on the stator teeth. Finally, the rotor, stator, and frame were
nected through
connected thermalthermal
through resistances. The following
resistances. components
The following are those
components arethat were
those thatcon-
were
sidered for modeling the thermal networks in this study: the rotor
considered for modeling the thermal networks in this study: the rotor bars, slot statorbars, slot stator wind-
ings, stator core,
windings, statorright end
core, windings,
right right end-cap
end windings, air, left air,
right end-cap endleft
windings, left end-cap
end windings, air,
left end-cap
round frame,frame,
air, round right side
rightframe, and left
side frame, andside
left frame. The models
side frame. The models werewere
mademadebased on on
based thethe
main heat flow paths as represented in the motor shown
main heat flow paths as represented in the motor shown in Figures 4 and 5.in Figures 4 and 5.

Figure 4. Schematic
Figure depicting
4. Schematic the the
depicting direction of heat
direction flow
of heat [26].
flow [26].
 Computation
Computation 2024,
2024, 12, 12, 6PEER REVIEW
x FOR 7 of 227 of 21

FigureFigure
5. Graphic representation
5. Graphic of theof
representation thermal network
the thermal [26]. [26].
network

The heat flow flow

The heat can becanwritten as the
be written induced
as the inducedheatheat
transferred
transferred from thethe
from rotor
rotorbars
barstoto the
the stator
statorwindings
windingsthrough
throughthetheair
airgap.
gap.This
Thisheat
heatwould
would then
then flow
flow to the stator iron
iron followed
fol-
lowedbybythetheround
roundframe
frameviaviaconvection
convectionto to the
the atmosphere.
atmosphere. ThereThere isisheat
heattransfer
transferfrom
from the
the stator
statorend
endwindings
windingsand androtor
rotorbar
barsides
sidestotothe
theend-cap
end-capair airbybyconvection,
convection,and,
and,finally,
finally, the
the side frame
side frame passes it on
passes to the
it on atmosphere
to the atmosphere by convection.
by convection. The The
heatheat
sources are the
sources arelosses
the losses
generated
generated in theininduction
the induction
motormotor
[26]. [26]. The thermal
The thermal behavior
behavior of a node
of a node in anininduction
an induction
motormotor
can becan be represented
represented by thebyfollowing
the following equation:
equation:
𝑑𝑇
𝜌𝐶 dT
ρC𝑉p V
𝑑𝑡
= 𝑄= Q
−𝑄−Q
in
+𝑄 +Q
out gen
(5) (5)
dt
In other words, the rate of energy stored within the system is the difference between
In other
the heat inflow ratewords,
to the the rate and
system of energy stored
the heat within
outflow the
rate system
from is the difference
the system between
plus the rate
the heat inflow rate to the system and the heat outflow rate from the system plus the rate of
of heat generation within the system [26,27].
heat generation within the system [26,27].
(i) Conduction
(i) Conduction ∆𝑇
𝑄 = 𝐾𝐴 (6)
∆𝑥 ∆T
Qth = KA (6)
∆𝑇 ∆𝑥 ∆x
𝑅 = = (7)
𝑄 ∆T𝐾𝐴 ∆x
Rth = = (7)
(ii) Convection Qth KA
(ii) Convection
𝑄 = ℎ𝐴(𝑇 − 𝑇 ) (8)
 
∆𝑇 Ts41− T∞
Q = hA 4
(8)
𝑅 = = (9)
𝑄 𝐴ℎ
Heat transfer through the air gaps can ∆T 1 by Taylor’s number and can be
Rthbe
= described
= (9)
used to find the convection coefficient [28,29]. Q th Ah
(iii) Radiation
Computation 2024, 12, 6 8 of 21

Heat transfer through the air gaps can be described by Taylor’s number and can be
used to find the convection coefficient [28,29].
(iii) Radiation
 
Q =∈ σSB T14 − T24 (10)

2.2. Thermal Investigation through Finite Element Method (FEM), a Computational Approach
In finite element examination, the governing equations contain finite elements that
are joined to a large group of equations that represent the physics of the problem. This
method is usually used for machines with components with large temperature gradients
and takes time to process compared to thermal network modeling [19]. The capacity to
solve nonlinear, circular geometry, and time-dependent problems make FEA superior to
other numerical methods. ANSYS is one of the software programs used for finite element
analysis. The Biot–Fourier equation is used assuming the heat flow is transient.

∂T
ρ.c. − ∇.(λ.∇ T ) = Q (11)
∂t
T is the temperature, c is the specific heat capacity, t is the time, ρ is the density, and
Q is the internal heat generation.
Usually, there are two kinds of boundary conditions in thermal analysis, namely,
Dirichlet’s condition and Neuman’s condition. A constant temperature is set for a solid
body surface using Dirichlet’s condition. Neuman’s condition is presented at the cooling
surface through the heat transfer coefficient. The HTC can be calculated analytically from
Newton’s law in the following equation [12]:
 
q = −λ.∇ T = h. Ts − T f (12)

The HTC can be determined through CFD:

 
q = π.d.λ.Nu. Ts − T f (13)

where λ is the fluid thermal conductivity, d is the specific diameter of the solid at the
boundary, q is the heat flux, Ts is the solid body temperature, Tf is the fluid temperatures,
and Nu is the Nusselt number, which is represented as:
 c 1
3
Nu = 2 + 0.6Re0.5 µ (14)
λ
µ is the viscosity, and c is the specific heat of the fluid.
The thermal utilization of an induction machine can also be found using CFD. To
make sure that CFD simulations are trustworthy, the results obtained by these simulations
need to be validated properly [30].

2.3. Experimental Analysis to Compare the Motor Efficiency of Copper and Aluminum Rotors
Cheng et al. [31] performed certain calculations comparing the efficiency of copper
and aluminum rotors for induction motors. To validate their results, certain experiments
were carried out involving both of the induction motors (aluminum and copper) along with
other test equipment items like a MagtrolHD-825 Dynamometer, a YASKAWA Varispeed
G7 Inverter, and so on.
The color indicator for efficiency was mentioned in Figures 6 and 7. The brightness of
the red color indicates a higher efficiency of the rotor. So, it is quite evident that the area of
red color in the copper graph, Figure 7, is visibly greater than that of the aluminum graph.
For example, with the rotor speed set at 2500 rpm and at 10 Nm torque, the copper rotor
was 15% more efficient compared to the aluminum rotor. This ultimately suggests that
Computation 2024, 12, 6 9 of 21

Computation
Computation 2024,
2024, 12,
12, xx FOR
FOR PEER
PEER REVIEW
REVIEW 99 of
of 22
22
the efficiency of a motor with a copper rotor is higher in comparison to a motor with an
aluminum rotor [31].

Figure 6.
Figure6. Efficiency
6.Efficiency
Efficiencyof of an
ofan aluminum
analuminum rotor
aluminumrotor [31].
rotor[31].
[31].
Figure

Figure 7.
Figure7. Efficiency
7.Efficiency
Efficiencyof of
ofaaacopper
copper rotor
copperrotor [31].
rotor[31].
[31].
Figure
The
The peak
Thepeak efficiency
peakefficiency
efficiencyof of the
ofthe motor
themotor
motorwith with
withan an aluminum
analuminum
aluminumrotor rotor
rotorwaswas obtained
wasobtained
obtainedat at 2500
at2500 RPM
2500RPMRPM
and
and
and 1313 Nm
13 Nm torque,
Nm torque, whereas whereas
whereas the the peak
thepeak efficiency
peakefficiency of
efficiencyofofthe the motor
themotor
motor withwith
with a copper
a copper
a copper rotor was
rotor
rotor waswasob-
ob-
tained
obtained at
tained atat 2300
2300
2300 RPM
RPM RPM and
andand15 Nm
1515Nm torque.
Nmtorque. So,
torque.So,So, clearly,
clearly,
clearly, the
the speed
the speed
speed is directly
is is directly
directly proportional
proportional
proportional to
to
the
to
the efficiency
the efficiency
efficiency of
ofof the
the
the motor.
motor. The
motor.The The lowest
lowest
lowest efficiencies
efficiencies
efficiencies werewere
were achieved
achieved
achieved at both
at at
bothboththe
the theextremes
extremes
extremes of
of
the
of
the experiment,
the experiment,
experiment, i.e., at
at high
i.e.,i.e., torque
at high
high torque
torque and
andand low
lowlow speed. The
speed.
speed. TheThelowest
lowest efficiency
lowest efficiency
efficiency of
of the
theofmotor with
the motor
motor with
an
with aluminum
an aluminum rotor rotorwas obtained
was obtained at 500
at 500RPM
an aluminum rotor was obtained at 500 RPM and 13 Nm torque. In the RPM andand 1313 NmNm torque.
torque. InIn the
the same
same way,
way, the
way, the
the
lowest
lowest efficiency
efficiency of
of the
the motor
motor with
with aa copper
copper rotor
rotor was
was
lowest efficiency of the motor with a copper rotor was obtained at 550 RPM and 17 Nm obtained
obtained at
at 550
550 RPM
RPM and
and 17
17 Nm
Nm
torque.
torque.
torque.So,So, clearly,
So,clearly,
clearly,high high torque
torqueisis
hightorque indirectly
isindirectly proportional
proportionaltoto
indirectlyproportional the
tothe efficiency
efficiencyofof
theefficiency ofthethe motor.
themotor.
motor.
One
One more
Onemore
moremajormajor statistical
majorstatistical inference
statisticalinference
inferencefrom from Figures
fromFigures
Figures66and6 and 7 is
and77isisthatthat
thatthethe effectiveness
theeffectiveness
effectivenessof of
of
the
the
theIMIM with
IMwith a copper
witha acopper
copper rotor
rotor was
rotor was
was 87% 87%
87% at a higher
at aathigher
a higher RPM. RPM.
RPM. This
ThisThis percentage
percentage
percentage could
couldcould even
even even reach
reachreach
90%
90%
at some
90% at
at some
some discrete
discrete points points
discrete in
in the
in the graph
points graph
the[31],
graph [31],
[31], whereas
whereas for
for the
for the aluminum
whereas the aluminum rotor,
rotor, the
rotor, the efficiency
aluminum effi-
thewas
effi-
ciency
ciency was 76%. The utilization of steel and copper in an effective manner, improving the
76%. Thewas 76%.
utilization The of utilization
steel and of steel
copper inand
an copper
effective in an effective
manner, manner,
improving the improving
topology and
the
topology
the geometry
topology and
andofthe
thethegeometry
motor, and
geometry of
of the motor,
motor, and
enhancing
the the enhancing
and design
enhancing along the design
thewith
designthe along
cooling
along with
with the
the cooling
properties are
cooling
properties
the
properties are
are the
main aspects theformain aspects
achieving
main aspects thefor achieving
optimum
for achieving the
the optimum
design design
of an induction
optimum designmotor,of
of an induction
an which
induction motor,
ultimately
motor,
leads
which to an increase
ultimately in
leads the
to efficiency
an increase of the
in thewhole
efficiencysystem
which ultimately leads to an increase in the efficiency of the whole system [32]. of [32].
the whole system [32].
The
Theefficiency
The efficiencyofof
efficiency ofaaasquirrel-cage
squirrel-cage
squirrel-cage induction
induction
induction motor
motor
motor with
with
witha copper
aa copper
copper rotor andand
rotor
rotor oneone
and with
one an
with
with
aluminum
an aluminum rotor depends
rotor depends on several
on factors,
several including
factors, the
including
an aluminum rotor depends on several factors, including the torque and speed, as men- torque
the and
torque speed,
and as
speed, mentioned
as men-
tioned
tioned inin the
the above
above experimental
experimental statistics.
statistics. The The efficiency
efficiency in in an
an electric
electric motor
motor is is aa measure
measure
of
of how effectively it converts electrical power into mechanical power. In general, the
how effectively it converts electrical power into mechanical power. In general, the effi-
effi-
ciency
ciency cancan be
be influenced
influenced by by losses
losses in in the
the motor,
motor, including
including copper
copper losses losses (also
(also known
known as as
Computation 2024, 12, 6 10 of 21

in the above experimental statistics. The efficiency in an electric motor is a measure of how
effectively it converts electrical power into mechanical power. In general, the efficiency can
be influenced by losses in the motor, including copper losses (also known as I2 R losses) and
core losses. As the torque and speed increase, the mechanical power output increases, and
the copper losses also increase due to the higher current, reducing the motor’s efficiency.
However, the effect of the torque and speed on the efficiency depends on the load conditions
and the design of the motor. Different motors may have different efficiency profiles.
Comparing copper and aluminum rotors:
− Copper rotors typically have a lower resistance than aluminum rotors, which can lead to
lower copper losses and a potentially higher efficiency under heavy loads.
− Aluminum rotors are lighter, which can reduce the moment of inertia and improve the
acceleration performance. This can be advantageous in certain applications.
In summary, the efficiency of squirrel-cage induction motors, whether they have
copper or aluminum rotors, is influenced by various factors, including the load conditions,
design, and materials used. Copper rotors may have an advantage in terms of lower copper
losses, but other factors, like the motor size and application, also play a significant role in
determining the overall efficiency.

2.4. Experimental Analysis of the Stator of an Induction Motor

Table 2 shows the temperatures corresponding to the slot windings, end windings,
and the yoke of the iron core of the stator at a 2880 RPM rotor speed and a torque of 10 N-m.
The temperature of the end windings of the water-cooled motor is represented by WaterEW,
the temperature of the slot windings is represented by WaterSW, and the temperature at
the yoke of the iron core of the stator is represented by WaterYoke. For the air-cooled motor,
the temperature of the end windings is represented by WindEW, the temperature of the
slot windings is represented by WindSW, and the temperature at the yoke of the iron core
of the stator is represented by WaterYoke [31].

Table 2. Temperatures corresponding to the parts of the stator.

Temperature, ◦ C Temperature, ◦ C Temperature, ◦ C Temperature, ◦ C Temperature, ◦ C Temperature, ◦ C

t (min)
(WindEW) (WaterEW) (WindSW) (WaterSW) (WindYoke) (WaterYoke)
0 20 20 20 20 20 20
10 90 61 91.5 59 50 30
20 105 69 107 66 66 31
30 119 72 120 69 72 31
40 122 73 124 70 80 33
50 129 75 130 72 84 33
60 132 77 133 75 89 34
70 134 77 136 75 91 34
80 137 78 139 77 91 34
90 139 79 139 78 92 34

Tables 2 and 3 suggest that the temperature was decreased by 39.49% at the end
windings, 41.67% at the side windings, and by a huge margin of 56.95% at the yoke of
the induction motor with the usage of water as a coolant instead of air as a coolant. It is
also noted that almost equal temperatures (79 ◦ C) were recorded at the end windings and
the slot windings when water was used as a coolant. As far as the water-cooling motor is
concerned, the end windings had the highest temperature, and the yoke region experienced
the highest temperature change [31]. From the above analysis, it was deduced that for
greater efficiency, copper rotors should be used, and when it comes to achieving better
reliability, water-cooled motors are the best compared to air-cooled motors [33].
Computation 2024, 12, 6 11 of 21

Computation 2024, 12, x FOR PEER REVIEW 11 of 22

Table 3. Difference between temperatures for air and water cooling.

TableComponent cooling. (◦ C)
Temperature
3. Difference between temperatures for air and water

Component Air-Cooled Water-Cooled

Temperature (°C) % Decrease in Temp.
End windings 119
Air-Cooled Water-Cooled 72 % Decrease in39.49%
Temp.
End windings
Slot windings 119 120 72 70 39.49% 41.67%
Slot windings
Yoke 120 72 70 31 41.67%56.95%
Yoke 72 31 56.95%
2.5. Materials for Parts of Squirrel-Cage Motor
2.5. Materials for Parts of Squirrel-Cage Motor
Kim et al. (2002) designed and investigated a composite squirrel-cage rotor made
Kim et al. (2002) designed and investigated a composite squirrel-cage rotor made of
of composites. Fiber-reinforced composite materials were used for making the spindle
composites. Fiber-reinforced composite materials were used for making the spindle shaft,
shaft, and an epoxy composite containing magnetic powder was used for the squirrel-cage
and an epoxy composite containing magnetic powder was used for the squirrel-cage rotor.
rotor. The thermal coefficient, thermal conductivity, magnetization properties, and storage
The thermal coefficient, thermal conductivity, magnetization properties, and storage
modulus
moduluswere
weremeasured,
measured,and andthetheoptimal
optimaldesign conditions
design conditionsofof
thethe
squirrel-cage rotor
squirrel-cage were
rotor
proposed. Iron powders and ferrite powders were used in the composites
were proposed. Iron powders and ferrite powders were used in the composites and were and were tested.
Both powders
tested. were suitable
Both powders at low temperatures,
were suitable and the and
at low temperatures, ferrite
thepowder was notwas
ferrite powder suitable
not at
high temperatures. An air-cooling system had to be provided for high-temperature
suitable at high temperatures. An air-cooling system had to be provided for high-temper- driving
conditions, as shown
ature driving in Figure
conditions, 8 [34].
as shown in Figure 8 [34].

Figure8.8. Squirrel-cage
Figure Squirrel-cagerotor:
rotor:(a)(a)
conductor barsbars
conductor and and
end rings beforebefore
end rings assembly; (b) steel(b)
assembly; core, con-core,
steel
ductor bars, and rings after assembly; (c) sectional view of the squirrel-cage rotor [34].
conductor bars, and rings after assembly; (c) sectional view of the squirrel-cage rotor [34].

Usha S.
Usha S et.al.
et.al.[35]
[35]investigated
investigated aluminum
aluminum for the
for casing, rotor rotor
the casing, bars, and
bars,endand ring ma-ring
end
terial, Nomex 430 for the stator liner, and silicon steel for the stator slot. It
material, Nomex 430 for the stator liner, and silicon steel for the stator slot. It was found that was found that
ironcould
iron couldmake
make the
the motor
motor light
lightandandhadhadan
animproved
improved efficiency
efficiency at high-speed
at high-speed conditions.
conditions.
Their experimental results indicated that the overall efficiency of the motor was increased
Their experimental results indicated that the overall efficiency of the motor was increased
by 5%.
by 5%.
Karnavas and Chasiotis (2017) studied the influence of the application of soft mag-
Karnavas and Chasiotis (2017) studied the influence of the application of soft magnetic
netic materials on the design and performance of a squirrel-cage induction motor. A total
materials on the design and performance of a squirrel-cage induction motor. A total of
of twenty-two different materials were examined, which included silicon steel, nickel-
twenty-two different materials were examined, which included silicon steel, nickel-iron,
iron, cobalt iron, amorphous metallic alloys, permalloy powder, sendust powder, and iron
cobalt iron, amorphous metallic alloys, permalloy powder, sendust powder, and iron
powder [33]. Marfoli et al. (2021) compared the design of squirrel-cages made of alumi-
powder [33]. Marfoli et al. (2021) compared the design of squirrel-cages made of aluminum
num and copper. Copper cages reduced the motor losses but required a higher starting
and copper.
torque. The Copper
advantages cagesandreduced the motor
disadvantages lossesabut
of using required
copper cage awere
higher starting
analyzed torque.
under
The advantages and disadvantages
different operating conditions [36]. of using a copper cage were analyzed under different
operating
Kim conditions
(2015) analyzed[36]. the speed characteristics and starting torque for a squirrel-cage
induction motor related tothe
Kim (2015) analyzed thespeed
material characteristics
properties of and starting
the rotor. Thetorque
resultsfor a squirrel-cage
of the transient
induction motor related to the material properties of the rotor. The results
characteristics were given to a three-phase, four-pole, 5 hp induction motor for calculating of the transient
characteristics were given
the speeds, starting torque,to and
a three-phase, four-pole,
rotation angle 5 hp induction
of the rotors. motoroffor
The materials thecalculating
model
the speeds,
were starting
changed torque,
to copper andand rotation
silicon copperangle
[37]. of the rotors.
Wijaya The materials
et al. (2021) studied the of effect
the model
of
were changed
different core to copper in
materials and silicon copper [37].
very-low-voltage Wijaya
induction et al. for
motors (2021) studied
electric the effect
vehicles. The of
different core materials
core materials affected the inperformance
very-low-voltage induction
of the motor motors for
significantly. Theelectric vehicles.
performance of theThe
core materials
motor affected
was studied withthe
theperformance
following three of the motor significantly.
materials: Arnon7, nickelThe performance
steel carpenter, and of the
motor was studied
M19_24G. with that
It was found the following
the productthree
cost materials:
was the lowest Arnon7,
whennickel
Arnon7 steel
wascarpenter,
used. Theand
M19_24G. It was found that the product cost was the lowest when Arnon7 was used. The
Computation 2024, 12, 6 12 of 21

efficiency of the motor was 83.27% when nickel steel carpenter was used and 83.10% when
M19-24G was used. A power density of 0.37 kW/kg was achieved with M19_24G, and a
power density of 0.38 kW/kg was achieved with Arnon7 [38].
The performance of the motor can also be improved by changing the material used for
casing the stator and rotor [22,39]. Aluminum 61% IACS was used to make the housing
of a motor and for the rotor bars and end ring instead of cold-rolled steel. The losses that
occur in various parts of the motor could be minimized by using aluminum, resulting in an
improvement in the cooling performance. Silicon steel has good electrical resistivity and
is very useful for reducing eddy currents, hence it being used for stator back iron instead
of M-19 Ga steel [20]. After modifying the material and comparing it to a standard motor,
there was an increase of up to 5% in terms of efficiency. A comparison of a selection of
materials that can be used for the components of SCIMs is listed in Table 4.

Table 4. A comparison of the effects of materials on the power density and efficiency of SCIMs.

Component Material Power Density Efficiency Increment Cooling Scheme Reference

Epoxy composite containing
Rotor Yes Yes Air cooling [34]
magnetic powder
Aluminum Yes Yes Air cooling [35]
Soft magnetic materials Yes Yes Air cooling [33]
Copper, silicon copper Yes Yes Air cooling [36,37]
Fiber-reinforced composite
Spindle shaft Yes Yes Air cooling [34]
materials
Iron powders and ferrite
Conductor frame Yes Yes Air cooling [34]
powders
Casing Aluminum Yes Yes Air cooling [35]
End ring Aluminum Yes Yes Air cooling [35]
Arnon7, nickel steel carpenter,
Stator core Yes Yes Air cooling [38]
and M19_24G
Stator liner Nomex 430430 Yes Yes Air cooling [35]
Stator slot Silicon steel Yes Yes Air cooling [20,35]

3. Cooling Strategies of High-Efficiency Three-Phase Cage Induction Motors

A cooling system is essential for minimizing the energy losses due to heat spots formed
in all electric motors [40]. Cooling is mostly carried out based on the slot conduction and
end windings in the motor. A cooling jacket is also helpful for the cooling process. A system
with a good efficiency is crucial to minimizing the working temperature of the hotspots in
a motor, especially in the copper windings. Therefore, reliable and precise modeling of the
cooling system and the design of the motor is very important for the optimization of the
thermal management of the motor [13,41].
Different types of cooling methods are available, but the water-jacket-cooling scheme
dissipates >99% of the heat formed. The residual heat is dissipated in the form of a
convection process, where the heat converts solids to either gas or liquids, which can be
emitted naturally or by force. In a liquid-cooling system, a mixture of water and glycol in
an equal ratio is generally used, and the air formed after absorbing the heat can be forcibly
removed employing a fan. Oil cooling takes place by a spraying method, and it is used for
cooling the magnets that are inside the rotor [42].
Liquid cooling is required to keep the temperature in the windings under the per-
missible limits in greater-power-density motors [43]. Experiments were conducted on
the prototype of an electric motor to calculate the effectiveness of a heat pipe thermal
management scheme. The temperature of the exterior surface of the IM was reduced to
84.05 ◦ C, or from 162.11 ◦ C to 78.06 ◦ C, with a thermal resistance of 0.21 ◦ C/W and at a
heat load of 120 W. The best pulsating heat pipe effectiveness was indicated at a heat load
above 60W [44].
 missible limits in greater-power-density motors [43]. Experiments were conducted on the
prototype of an electric motor to calculate the effectiveness of a heat pipe thermal man-
agement scheme. The temperature of the exterior surface of the IM was reduced to 84.05
°C, or from 162.11 °C to 78.06 °C, with a thermal resistance of 0.21 °C/W and at a heat load
Computation 2024, 12, 6 of 120 W. The best pulsating heat pipe effectiveness was indicated at a heat load 13 above
of 21
60W [44].
In general, the most commonly used cooling methods applied in motors are air cool-
ing and water the
In general, cooling. Air coolingused
most commonly generally
coolinginvolves
methodsaapplied
forced-convection
in motors aremethod,
air coolingin
which
and waterair cooling.
is forcedAir
into the motor
cooling to cool
generally the rotor
involves and stator. When
a forced-convection the air
method, is forced
in which air
through
is the rotor,
forced into the air
the motor to gets
coolheated
the rotorupand
(thestator.
temperature would
When the air isbe higher
forced than the
through thestator,
rotor,
thusair
the providing
gets heatedextra
upheat
(thetotemperature
the stator) resulting
would beinhigher
a poorthan
motor theperformance
stator, thus [42].
providing
extra heat to the stator) resulting in a poor motor performance [42].
3.1. Water-Cooling System
3.1. Water-Cooling System
The basic principle of a water-cooling system is conduction. Water flows in a pattern
that isThe basic principle
combined with the ofhousing
a water-cooling system ismotor.
of the induction conduction. Waterwater
The flowing flowsabsorbs
in a pattern
heat
that is combined with the housing of the induction motor. The flowing
from the stator, thus enabling the cooling of the motor. Two types of water flow systemswater absorbs heat
from
have thebeenstator, thus enabling
examined, namely axialthe cooling
water flow of the motor.
and Two types
tangential waterof water
flow, flow systems
as shown in Fig-
have
ures 9 and 10. The water quantity and velocity influence the heat transfer as
been examined, namely axial water flow and tangential water flow, shownthe
between in
Figures 9 and 10. The water quantity and velocity influence the heat
frame and the water. Under simulation conditions, heat spots generated by water turbu- transfer between
the
lenceframe
can beand the water.
calculated [45].Under simulation conditions, heat spots generated by water
turbulence can be calculated [45].

Figure 9.
Figure 9. Axial water flowing through the
the induction
induction motor
motor housing
housing [45].
[45].

Figure 10. Tangential water flowing through the induction motor housing [45].

Reynold’s average Navier–Stokes equation is used to describe the flow of a non-

compressible liquid [46].

∂U
ρ − ρ(U.∇)U = −∇ρ + 2∇(η∇U) + f (15)
∂t
 Computation 2024, 12, x FOR PEER REVIEW 14 of 22

Computation 2024, 12, 6 Figure 10. Tangential water flowing through the induction motor housing [45].
14 of 21
Reynold’s average Navier–Stokes equation is used to describe the flow of a non-com-
pressible liquid [46].
η is the dynamic 𝜕𝑈 viscosity, U is the averaged velocity field, ρ is the density of the fluid,
𝜌 − 𝜌(U. ∇)U = −∇𝜌 + 2. η∇U + f (15)
U is the velocity vector, 𝜕𝑡 and f is the volumetric force vector. The physics of transient heat
conduction are given
η is the dynamic below:
viscosity, U is the averaged velocity field, ρ is the density of the
fluid, U is the velocity vector, and f is the volumetric
∂T force vector. The physics of transient
heat conduction are given below: ρc p = ∇.( λ ∇ T ) + P (16)
∂t
ρ is the mass density, c𝜌𝑐 𝜕𝑇 specific heat capacity, T is the temperature, λ is the heat
p is tie
= ∇. (𝜆∇𝑇) + 𝑃 (16)
𝜕𝑡
conductivity, and P is the heat generation rate.
ρ is the mass density, cp is tie specific heat capacity, T is the temperature, λ is the heat
The boundary conditions of convection and radiation for the water-cooled thermal
conductivity, and P is the heat generation rate.
field
Theare given conditions
boundary by: of convection and radiation for
∂T
  the water-cooled
 thermal
field are given by:
−λ = h T f − Tw (17)
𝑇 ∂t f
−𝜆 =ℎ 𝑇 −𝑇 (17)
𝑡
Tf is the frame temperature, Tw is the water temperature, h is the frame, and HTC is
Tf is the frame temperature, Tw is the water temperature, h is the frame, and HTC is
the fluid convective heat transfer coefficient.
the fluid convective heat transfer coefficient.
Submersible motor pumps are generally squirrel-cage motors that are submerged for
Submersible motor pumps are generally squirrel-cage motors that are submerged for
performing
performing cooling
cooling through
through the motorthe motor
housing, housing,
which results which
in a highresults in a high
power density but power density but
a lower
a lower efficiency,
efficiency, as shown
as shown in Figurein
11Figure
[47,48]. 11 [47,48].

Figure
Figure 11. Conceptual
11. Conceptual configuration
configuration of an
of an induction induction
motor motor with superconducting
with high-temperature high-temperature superconducting
(HTS) squirrel
(HTS) cage [48].
squirrel cage [48].
3.2. Oil-Cooling
3.2. Systems
Oil-Cooling Systems
The performance of an electric motor is essentially restricted by the magnitude of
heat thatThe
can performance
be successfully of an electric
dissipated. motor is essentially
The oil-cooling restricted
technique involves by the magnitude of heat
the spraying
ofthat canthe
oil onto becopper
successfully
coils that dissipated.
cannot be cooledTheusing
oil-cooling technique
water-cooling involves
techniques, as the the spraying of oil
ontocooling
direct the copper
method coils that cannot
is required beinside
to cool the cooled using
of the water-cooling
motor techniques, as the direct
according to research
carried
coolingout method
in the past. It helps to to
is required maintain
cool thetheinside
averageoftemperature
the motorofaccording
the coil. Even
to research carried out
though various research works have been conducted on this process, there is no exact
in the past. It helps to maintain the average temperature of the coil. Even though various
research works have been conducted on this process, there is no exact conclusion indicating
the ideal method to use [22,49]. Current signs of progress in IM thermal management
include using direct oil spraying or splash-based cooling to improve efficiency [50,51].
At present, a direct method is used where a groove is made between the stator and
the rotor and oil is sent to cool the coil. Experiments have been conducted where the coil
is immersed in the oil. This method results in friction losses between the cooling coil and
the rotor. Another method is an injection method where the oil is directly sprayed onto
the coil using injectors. Different shapes of injectors have been experimented with to find
better spraying characteristics, such as a full-cone nozzle, a flat jet nozzle, dripping, and
multi-jets. When a dripping injector is used, the best cooling performance is achieved.
Other methods, like optimizing the copper windings, have shown good improvements
in terms of heat generation and the efficiency of the motor itself. The hairpin winding
method is the latest modification, which has the benefit of a high efficiency and a high
 the rotor. Another method is an injection method where the oil is directly sprayed onto
the coil using injectors. Different shapes of injectors have been experimented with to find
better spraying characteristics, such as a full-cone nozzle, a flat jet nozzle, dripping, and
multi-jets. When a dripping injector is used, the best cooling performance is achieved.
Computation 2024, 12, 6
Other methods, like optimizing the copper windings, have shown good improve-
15 of 21
ments in terms of heat generation and the efficiency of the motor itself. The hairpin wind-
ing method is the latest modification, which has the benefit of a high efficiency and a high
power density in the IM due to the high space factor. The hairpin winding method is
power density in the IM due to the high space factor. The hairpin winding method is shown
shown in Figure 12, where hairpin-shaped square coils are inserted into the stator slots
in Figure 12, where hairpin-shaped square coils are inserted into the stator slots and are
and are welded at the end turns. This helps in decreasing the gap between the coil wind-
welded at the end turns. This helps in decreasing the gap between the coil windings and
ings and also the size of the motor. The cooling performance is also improved with this
also the size of the motor. The cooling performance is also improved with this method,
method, resulting in a high efficiency [52].
resulting in a high efficiency [52].

Figure
Figure 12.
12. Toroidal
Toroidal and hairpin winding [52].

In general,
In general, induction
induction motors
motors are cooled by airflow around the housing. Even Even though
though
this method
this method is is cheap,
cheap, the
the cooling
cooling performance
performance is is not
not good
good enough
enough forfor motors
motors being
being used
used
at high
at high speeds.
speeds. Therefore,
Therefore, some
some modifications
modifications werewere made
made toto the
the existing
existing totally
totally enclosed
enclosed
fan-cooled (TEFC)
fan-cooled (TEFC) prototype
prototype byby inserting
inserting holes
holes into
into the
the machine,
machine, where
where andand oil-cooling
oil-cooling
circuit is used, as shown in Figure 13. These holes let the oil flow inside
circuit is used, as shown in Figure 13. These holes let the oil flow inside the motor the motor on
on the
the active hotspots of the motor such as the stator end-windings and rotor
active hotspots of the motor such as the stator end-windings and rotor end-rings [46]. An end-rings [46].
An enclosed
enclosed water-cooled
water-cooled motor motor thermal
thermal modelmodel wasdeveloped
was also also developed and analyzed.
and analyzed. A water- A
water-cooling
cooling
Computation 2024, 12, x FOR PEER REVIEW jacket jacket
was was
also also investigated
investigated for for
radial radial
and and
axial axial
water water
flow flow directions.
directions. The The
meas-
16 of 22
measurement
urement and simulation
and simulation resultsresults indicated
indicated thattemperatures
that the the temperatures
of theofstator
the stator winding
winding and
androtor
the the rotor
werewere compared
compared and showed
and showed goodgood improvement
improvement [53].[53].

Figure 13.
Figure 13. Oil-cooling
Oil-cooling circuit
circuit of
of the
the motor
motor housing
housing [54].
[54].

3.3.
3.3. Natural
Natural Water-Cooling
Water-Cooling Capillaries (NWCC)
(NWCC) Method
Method
Cooling
Cooling through the NWCC approach does not
through the NWCC approach does not need
need aa large
large external
external heat
heat exchanger
exchanger
and
and external
external energy.
energy. In
In this
this method,
method, water
water is
is absorbed
absorbed by
by capillaries,
capillaries, which
which are
are usually
usually
made
made of of cotton
cotton or
or jute
jute natural
natural ventilation systems and are mounted to reduce the sur-
rounding
rounding temperature
temperature of of the
the motor,
motor, asas shown
shownin inFigure
Figure14.
14.TheTheNWCC
NWCC scheme
scheme is is used
used to
to decrease the temperature and to protect the shielding of the insulation material.
decrease the temperature and to protect the shielding of the insulation material. This in This
in turn
turn increases
increases thethe thermal
thermal resisting
resisting capacity
capacity of motor,
of the the motor,
which which results
results in a long
in a long life
life span
of the motor. This technique is useful in reducing the neighboring temperatures of the
motor [55].
 Cooling through the NWCC approach does not need a large external heat exchanger
and external energy. In this method, water is absorbed by capillaries, which are usually
made of cotton or jute natural ventilation systems and are mounted to reduce the sur-
Computation 2024, 12, 6 rounding temperature of the motor, as shown in Figure 14. The NWCC scheme is used to
16 of 21
decrease the temperature and to protect the shielding of the insulation material. This in
turn increases the thermal resisting capacity of the motor, which results in a long life span
of theofmotor.
span ThisThis
the motor. technique is useful
technique in in
is useful reducing
reducingthe
theneighboring
neighboringtemperatures
temperatures of the
of the
motor [55].
motor [55].

Figure 14.
Figure 14. Structure
Structure of
of capillary
capillary jacket
jacket and
and capillary
capillarytube
tube[55].
[55].

Needless
Needless totosay,
say,thetheabove-mentioned
above-mentioned methods
methodshave their
have advantages
their advantagesandand
disadvan-
disad-
tages. Water
vantages. cooling
Water is advantageous
cooling is advantageousin some scenarios
in some because
scenarios it can
because it absorb a significant
can absorb a signif-
amount of heat
icant amount ofbefore reaching
heat before its boiling
reaching point,point,
its boiling making it a reliable
making cooling
it a reliable medium.
cooling me-
It also generally requires less maintenance compared to some other
dium. It also generally requires less maintenance compared to some other methods. methods. However,
there is a potential risk of water leakage, which could damage the motor and the
surrounding equipment.
Oil cooling exhibits better heat transfer properties and can efficiently cool the motor
windings. In addition, oil cooling offers improved dielectric properties, and the oil can help
to insulate the motor windings and protect them from contaminants. However, there is a
risk attached to it, as some cooling oils can be flammable, posing a safety risk in certain
environments. Setting up oil cooling is comparatively more expensive than air cooling,
which sometimes can be a limitation for economical scenarios.
The natural water-cooling capillaries method (NWCC) also has its advantages and
disadvantages. It perfectly suits situations where available energy is limited to an extent.
Since it relies on natural convection, it consumes less energy compared to forced water-
cooling systems. This method can efficiently cool the motor through natural convection
without the need for a pump. However, the downside is its limited applicability, as this
method may not be suitable for all motor configurations and applications. Designing
and implementing capillaries within the motor can be complex and may require custom
solutions. The cooling capacity of this method may be limited compared to other active
cooling methods like water cooling.
In summary, the choice of cooling method for a squirrel-cage induction motor depends
on various factors, including the specific application, operating conditions, and safety
considerations. Water and oil cooling are more common in industrial settings, while the
natural water-cooling capillaries method is more specialized and may be suitable for certain
niche applications.
Computation 2024, 12, 6 17 of 21

3.4. Energy Harvesting and Self-Powered Induction Motor with Thermal Analysis
The process of energy harvesting involves the collection and storage of several forms
of energy, including solar, thermal, mechanical, and electromagnetic [56]. Induction motors
that are self-powered generate electricity from the rotational energy they store. This is
accomplished with a motor-integrated generator [19]. The rotation of the motor causes the
generator to produce electricity, which may then be used to power the electronic devices.
The efficiency, dependability, and ease of maintenance are all improved with self-powered
induction motors [57]. Engineers can make accurate predictions about the thermal behavior
of the system under a variety of conditions of operation and locate hotspots as well as
thermal stress using these methods.
Self-powered induction motors and their surrounding environments are modeled
mathematically as part of the thermal modeling process [58]. This model takes into account
the heat produced by the motor as well as the heat that is transferred through the air
and the components. By testing the model under a variety of different operating situa-
tions, engineers can forecast the temperature distribution across the motor and locate any
hotspots. The thermal behavior of these motors can be simulated through computation
using appropriate computer software. Engineers can input several running situations
into the software to the test and discover any potential thermal problems [3]. Engineers
can evaluate their thermal models and simulations and find inconsistencies between the
measured and expected temperature distributions [32]. Testing can uncover any thermal
concerns that are not accounted for by the models and simulations. Both the dependability
and the efficiency of self-powered induction motors are dependent on thermal analysis [59].
The collection of energy is an intriguing potential source of renewable energy. In-
duction motors that are self-powered can obtain their power from energy harvesting [60].
Engineers can optimize the thermal behavior of these systems to ensure their continued ef-
fectiveness and dependability over the long term. As the need for environmentally friendly
power sources grows, technologies like energy harvesting and self-powered induction
motors may gain increasing traction [61]. Thermal analysis is necessary for designing,
developing, and perfecting these systems to achieve long-term success, such as lowering
both the energy consumption and operational expenses.

3.5. Miscellaneous Methods of Thermal Management

Various schemes of cooling have been researched. One of them involves using baffles
inside the coolant pipes as well as introducing nano-fluids into the coolant pipe, which
could improve the efficiency of an air-cooled motor [13]. This might help in achieving
effective cooling using a fan. Using different coolants or a mixture of coolants instead of
water might also help in achieving effective cooling. Experimenting and changing the
liquid flow pattern attached to the housing of the motor that is used for water cooling can
also present advantages. At present, the thermal management of mobile and electronic
devices is maintained by heat pipes, and there has been a drastic increase in the usage of
these pipes in current appliances and devices, and some have even received patent rights.
Different types of heat pipes are used for cooling different electronic gadgets. In certain IM
cooling devices, the evaporator part of the heat pipe is mounted inside the motor housing.
But in some other cases, it is placed within the shaft and the condenser, which is cooled
either by a liquid or air is placed outside the housing of the motor [62].
Numerical results have shown drastic effects of the estimations of the intricate thermal
physics occurring in the end region of a more porous end winding geometry; here, the
heat removal percentage through the outer frame surges by up to 35%. Furthermore, a
lessening of the total internal convective thermal resistance between the end windings
and the external frame of about 28% is realized [63]. A comparison of the existing cooling
schemes is listed in Table 5.
Computation 2024, 12, 6 18 of 21

Table 5. A comparison of cooling schemes in SCIMs.

Cooling Scheme The Component That Is Cooled Pattern of Cooling Reference

Axial water flow and tangential
Motor housing [45]
water flow
Water cooling
Axial water flow and tangential
Stator [45]
water flow
Immersed cooling Motor housing SCIM is submerged in the liquid [47,48]
Direct spraying of oil on the copper
Oil or splash-based cooling Copper coils [22,24,47]
coils
Totally enclosed fan-cooled
Motor housing Air flow around the housing [46]
(TEFC) scheme
Totally enclosed Radial and axial directions of water
Stator winding and the rotor [53]
water-cooled scheme flow
Natural water-cooling Natural ventilation systems
Shielding of insulation material [54]
capillaries (NWCC) surrounding the motor
Baffles in coolant pipes/air
Stator Coolant pipes with baffles [61]
cooling
Heat pipe/air cooling Motor housing Heat pipe mounted in the housing [13]
Nano-liquid/mixed Axial water flow and tangential
Motor housing [13]
liquid/air cooling water flow
Porous geometry/air
End winding Region of porous geometry [9]
cooling

4. Future Scope
Large-sized motors used in industry use a fan as their cooling system, and they face
problems in terms of cooling the stator due to the air already being heated up by the time it
reaches the stator. The area of modifying the end winding is also a vast area of research and
could be useful for the efficient functioning of electric motors. Numerous cooling system
methods are present in the market, and some are being explored by researchers across
the globe. Even though the existing ones are currently being used, they still have some
drawbacks based on the size and type of the motor being used. Instead of targeting one
method to cool a stator or motor, multiple methods or an integrated cooling method could
be examined, and research could be conducted on it.

5. Summary
Technologies like energy harvesting and self-powered induction motors may gain more
popularity as the demand for greener power sources rises. The thermal behavior of these
systems may be optimized to guarantee their long-term reliability and efficiency. The sole
objective could be to eliminate thermal issues with the use of modeling and testing, which
would enhance the system’s overall performance and reliability. Overheating due to motor
losses has a substantial impact on the power coefficient, efficiency, and the insulating glass
and components required for the magnetic materials. As a result, the thermal management
of an electrical motor is critical to the equipment’s safety and efficiency.
The thermal analysis could be carried out with two computation methods, the lumped-
parameter circuit method and the finite element analysis method. LPCM is used for
assemblies in which the components have no big temperature gradients, i.e., the temper-
ature is constant at every single point within a particular component. LPCM is fast and
needs experience in order to understand the direction of the flow of heat between compo-
nents. Finite element analysis is used for understanding bodies that have large temperature
gradients. FEA is slow and complicated to solve, and it is difficult to take the convective
and radiative heat transfer of the cooling fluids into account.
In this paper, aluminum and copper rotors were compared based on their efficiency.
From the experimental tests, it was quite evident that the copper rotors were more efficient
when compared to the aluminum rotors. At 2300 RPM and 15 Nm torque, the efficiency of
the copper rotor came out to be 87%, which was 11% greater than the aluminum rotor. So,
in all senses, copper rotors are more efficient compared to aluminum rotors.
Computation 2024, 12, 6 19 of 21

The analysis also indicated that the usage of water as a coolant helped the system
to decrease the working temperature by 39.49% at the end windings, 41.67% at the side
windings, and by a huge margin of 56.95% at the yoke of the induction motor. Cooling the
motor could be achieved using many techniques, but the effectiveness of the cooling system
depends on the type and size of the motor. After finding the hotspots, the cooling system
should be selected based on the part that needs to be cooled. Various cooling techniques
are adopted to cool different parts of motors individually.

Funding: This research received no external funding.

Data Availability Statement: Data may be shared through request.
Conflicts of Interest: Author Yashwanth Reddy Konda was employed by the company Infosys
Technologies Limited. Author Peram Venkata Sivarami Reddy was employed by the company
Cognizant Technology Solutions. Other authors declare no conflict of interest.

Nomenclature

CFD Computational fluid dynamics

SW Side windings
FD Flux density
EW End windings
SCIM Squirrel-cage induction motor
IM Induction motor
LPTN Lumped-parameter thermal network
LPCM Lumped-parameter circuit method
FEA Finite element analysis
HTC Heat transfer coefficient
RTD Resistance temperature detectors
TEFC Totally enclosed fan-cooled
NWCC Natural water-cooling capillaries

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