3- Phase Induction Motor

Electric Motors

DC Motors AC Motors

Induction Synchronous
PMDC Motors
Motors Motors

Separately Excited Motors

3-ɸ Induction 1-ɸ Induction
Motors Motors
Series Motors

Shunt Motors

Compound Motors
Advantages

Simple and Rugged Construction

Absence of commutator

Good Power Factor

High efficiency

Good Speed Regulation

High Torque & Low Cost

 3-Phase Induction Motor

3-Phase supply

Terminal Box

Shaft
 Lifting Eye

Cooling-Fan

Stator
Rotor

Shaft

End-bell

Base
Terminal-box
 Frame

• It is the outer body of the motor.

• Made up of Silicon-Aluminium Alloy.

• Provides a housing for Stator, Protects the inner parts and also leads the path
for ventilation.
 Stator Core
• It carries Alternating Current.

• Made up of laminated Silicon Steel.

• Thickness of each lamination is 0.35mm-0.65 mm, insulated by Varnish.

• Slots are punched in the inner periphery.

• Ventilating ducts are provided.

 Stator Winding

• These can be connected in star or delta depending on the starting

method.
 Rotor Core

• Laminated cylindrical core.

• Made up of Silicon-Steel.

• Slots are present on the outer periphery.

• Ventilating ducts are provided.

• It has smaller no of slots.

• Slots are non-integral multiples as that of stator.

 Squirrel Cage Rotor
• It looks like a squirrel cage.

• Cu, Al or Brass bars are used.

• Rotor bars are skewed to avoid magnetic locking and humming.

• The bars are short circuited by end rings so, external resistance can not be
added.
 Wound-Rotor
• The winding is always three phase.

• Phosphor-Bronze or Brass slip rings are used.

• Here, the no. of stator poles are equal to no. of rotor
poles.

• They require high maintenance and cost.

• These are used if high torque and speed control is
required.
 Shaft and Bearing

• A short and stiff shaft is used as air gap is less.

• Air gap is 0.3-0.35 mm in large machines and 1.0-1.5 mm in large
machines.

• Shaft is made up of mild steel.

• Ball and Roller bearings are used.

Induction Motor Operation

For an Induction Motor to Rotate

3-phase voltages produce rotating magnetic field in stator

Current is induced in rotor by moving magnetic field

Induced current in rotor produces a magnetic field in

rotor

Field in rotor interacts with the field in the stator to

produce torque (rotor "chases" stator field)
 Production of rotating magnetic field

ɸR ɸY ɸB
ɸm ɸB
1200 ɸR
2400
ωt
1200
ɸY

ɸR= ɸmSin ωt
ɸY= ɸmSin (ωt-1200)
ɸB= ɸmSin (ωt-2400) or ɸmSin (ωt+1200)
ɸR= ɸmSin ωt
ɸY= ɸmSin (ωt-1200)
ɸB= ɸmSin (ωt-2400) or ɸmSin (ωt+1200) -ɸY ɸB
ɸRes
at ωt= 00
ɸR= ɸmSin 00=0
ɸB
ɸY= ɸmSin (00-1200)=− 3
ɸ -ɸY
2 m 600
ɸB= ɸmSin (00-2400)= 3
ɸ 1200
2 m

3 60
ɸRes=2 x ɸ x cos ( 2 )
2 m ɸY

=2 x 3
ɸm x 3
= 3ɸm= 1.5 ɸm
2 2 2
Note: If two vectors are having equal magnitude A and the
angle between them is θ, then their resultant is given by the
θ
expression 2ACos( )
2
ɸR= ɸmSin ωt
ɸY= ɸmSin (ωt-1200)
ɸB= ɸmSin (ωt-2400) or ɸmSin (ωt+1200)

at ωt= 600
ɸR= ɸmSin 600= ɸm
3 ɸR
2 ɸB -ɸY
ɸY= ɸmSin (600-1200)=−
3
ɸm ɸRes
2
ɸB= ɸmSin (600-2400)= 0 -ɸY
600
ɸR
3 60
ɸRes=2 x ɸ x cos ( 2 )
2 m

=2 x 3ɸ
mx
3 = 3ɸm= 1.5 ɸm ɸY 1200
2 2 2
ɸR= ɸmSin ωt
ɸY= ɸmSin (ωt-1200)
ɸB= ɸmSin (ωt-2400) or ɸmSin (ωt+1200)

ɸB
at ωt= 1200
1200
3
ɸR= ɸmSin 1200= ɸm
2
ɸY= ɸmSin (1200-1200)=0 ɸR
600 -ɸB
ɸY ɸRes
ɸB= ɸmSin (1200-2400)=− 3ɸ
m
2
-ɸB
3 60
ɸRes=2 x ɸ x cos ( 2 )
2 m ɸR
=2 x 3
ɸm x 3
= 3ɸm= 1.5 ɸm
2 2 2
ɸR= ɸmSin ωt
ɸY= ɸmSin (ωt-1200)
ɸB= ɸmSin (ωt-2400) or ɸmSin (ωt+1200)
ɸB

at ωt= 1800
ɸR= ɸmSin 1800=0
3
1200 ɸR
ɸY= ɸmSin (1800-1200)= ɸm
2
600
ɸB= ɸmSin (1800-2400)=− 3
ɸm ɸY -ɸB
2

3 60 ɸRes
ɸRes=2 x 2
ɸm x cos ( 2 )
-ɸB
=2 x 3
ɸm x 3
= ɸm= 1.5 ɸm
3
ɸY
2 2 2
ωt= 00 ωt= 600

ωt= 1200 ωt= 1800

 NS

ωt= 00

ωt= 3600
ωt= 600
N or NR
ωt= 3000

ωt= 1200

ωt= 2400

ωt= 1800
 Working principle
• Stator of 3-ɸ Induction Motor receives 3-ɸ supply, that produces rotating
magnetic field (RMF) in the air-gap.
• The speed of stator magnetic field is called synchronous speed (Ns).
• With rotor being stationary, RMF causes the rotor conductors to cut the
magnetic flux.
• According to faraday’s law of electromagnetic induction, a rate of change of
flux causes an EMF in the rotor conductors.
𝐍𝐬
𝐈𝟐

𝐄𝟐
• If the rotor conductors are short circuited , a current will flow in the rotor
conductors.
• According to Lenz’s Law , the EMF will oppose it’s cause.
• The current so produced will develop a torque that will oppose the relative
motion between rotor and RMF.
• To reduce the relative motion, rotor tries to catch up to RMF and starts rotating
in the same direction as RMF.
• The EMF induced in the stator is expressed as .
= 4.44kw1N1ɸResf Volts
Where, kw1= stator winding factor
ɸRes= Resultant air-gap flux
 Reversing the supply

ɸR ɸB ɸY
ɸm ɸY
1200 ɸR
2400
ωt
1200
ɸB

ɸR= ɸmSin ωt
ɸB= ɸmSin (ωt-1200)
ɸY= ɸmSin (ωt-2400) or ɸmSin (ωt+1200)
 NS

ωt= 00
ωt= 3600

N or NR
ωt= 3000
ωt= 600

ωt= 2400

ωt= 1200

ωt= 1800
 Slip (s)
• The difference between synchronous speed and actual speed of the rotor is
called slip. And is denoted by ‘s’. Slip can be expressed in RPM or RPS or in
percentage.
𝑠 𝑅
% Slip= x 100
𝑠

Where, =120 x
Here, f= frequency of supply
P= No. of stator poles
• When the motor is about to start slip=1
• When motor runs with no-load or light load, slip≈0
• As motor is loaded, its speed decreases and slip increases from 0.
 Frequency of rotor current
• Frequency of rotor EMF or Current =
fR= S
120
P

=
= S

s
=s
=s

• The frequency of rotor EMF is the product of slip and supply frequency. So, it
is also called slip frequency.
 POWER FLOW DIAGRAM

Electrical power Power transferred

Mechanical power Mechanical power
input to stator= across the air-gap to
developed in the output at shaft
3V1I1Cos ɸ1 rotor= 3E2I2Cos ɸ2
rotor (about 88.5%)
(100%)

Rotor copper Friction and

loss (3.5%) windage loss (2%)

Stator copper loses Stator iron loss

2
(=3I1 R1) (3.5%) (2.5%)
 =

= x 100

= 𝑚𝑒 𝑐 ℎ
x 100
𝑚𝑒𝑐ℎ 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑣𝑎𝑟𝑖𝑎𝑏𝑙𝑒
Equivalent circuit of an induction motor: