Construction and Types of Three-phase Induction Motor
1. Introduction:
Induction motor (Also called asynchronous motor) is an A.C. motor. The
motor line current flows into the stator windings to set up a flux called
the main flux or the stator flux, which passes through the air gap to be
cut by the conductors of the rotor windings. Consequently, an
electromotive force to be induced in the rotor windings and produces
currents flow in the rotor windings and producing flux called the rotor
flux. The interact between the two fluxes (stator and rotor fluxes)
producing rotation of the rotating part of the motor (rotor). The rotor
receives electrical power in the same way as the secondary winding of
the electrical transformer receiving its power from the primary winding
by means of the electrical induction.
Induction Motor Types:
Depending on the construction of the rotor circuit there are two types of
induction motors:
I. Squirrel cage induction motor:
Rotors is very simple and consist of bars of aluminum (or copper)
with shorting rings at the ends.
II. Wound rotor induction motor:
Rotor consists of three phase windings (star connected) with
terminals brought out to slip rings for external connections.
Squirrel cage type is more common compared to the wound rotor type due to:
a. Robust, as no brushes, no contacts on the rotor shaft.
b. Simple in construction and easy to manufacture.
c. Almost maintenance-free, except for bearing and other mechanical
parts.
d. High efficiency as rotor has very low resistance and thus low copper
loss.
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3. Construction:
There are two main types of components which are used in induction
motor manufacturing as follows:
a) Active components: which are classified into two categories:
i. Magnetic materials (0.5 mm electrical steel).
ii. Electrical materials (copper wires, insulations, bars,
end rings, slip rings, brushes, and lead wires).
b) Constructional components: like frame, end shields, shaft, bearings, and
fan. These components are shown in figure 1.
Figure 1 Parts of Squirrel Cage Induction Motor
3.1 Stator construction:
The stator is made up of several thin laminations (0.5 mm) of electrical
steel (silicon steel), they are punched and clamped together to form a
hollow cylinder (stator core) with slots, as shown in Figure 2.
Coils of insulated wires are inserted into these slots. Each group of coils,
together with the core that it surrounds, forms an electromagnet, forms
an electromagnet (a pair of poles). The number of poles of an induction
motor depends on the internal connection of the stator windings.
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Figure 2 Typical Stator of Three-phase Induction Motor
3.2 Rotor construction:
The squirrel cage rotor is made up of several thin electrical steel
lamination (0.5mm) with evenly spaced bars , which are made up of
aluminum or copper , along the periphery .In the most popular type of
rotor (squirrel cage rotor), +these bars are connected at ends
mechanically and electrically by the use of end rings as in Figure 3 (A).
Almost 90 % of induction motors have squirrel cage rotors. The rotor
slots are not exactly parallel to the shaft. Instead, they are given a skew
for two main reasons, firstly to make the motor run quietly by reducing
magnetic hum and to decrease slot harmonics, secondly to help
reducing the locking tendency of the rotor (the rotor teeth tend to
remain locked under the stator teeth due to direct magnetic attraction
between the two). The rotor is mounted on the shaft using bearings on
both ends.
The wound rotor has a set of windings on the rotor slots which are not
short circuited, but they are terminated to a set of slip rings. These are
helpful in adding external resistors and contactors, as in Figure 3 (B). The
typical squirrel cage rotor circuit is shown in figure 4 (A), while the typical
wound rotor circuit with an external rotor resistor circuit is shown in
figure 4 (B).
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Figure 3 (A) Squirrel Cage Rotor Type (B) Wound Rotor Type
4. Typical name plate of induction motor:
A typical name plate of induction motor is shown in Figure 5, and table 1.
Figure 5 Typical Name Plate of Induction Motor
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State the difference between squirrel cage Induction motor and slip ring
Induction motor?
Parameters Squirrel cage IM Slip ring IM
Rotor winding Copper bars are short Three phase winding
circuited at both
ends
End ring present Absent
. Efficiency high low
External resistance in Not possible Possible
rotor
Starting torque Moderate High
Construction Simple & robust Complicated
Maintenance less High
Industrial use 95% 5%
Speed control from rotor Not possible Possible
side
cost Economical Expensive
Applications Lathes, drilling machine, Lifts, compressors,
water pumps, blowers, hoists, cranes, elevators,
fans, printing machine belt, conveyor
Refrence:
https://
www.lkouniv.ac.in/site/writereaddata/siteContent/202005171759549041wazid-3%20phase%20induction%20motor.pd
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https://testbook.com/amp/electrical-engineering/3-phase-induction-motor
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EQUEVALENT circuit OF A THREE PHASE INDUCTION MOTOR
Here, R1 is the winding resistance of the stator.
1) X1 is the inductance of the stator winding.
2) Rc is the core loss component.
3) XM is the magnetizing reactance of the winding.
4) R2/s is the power of the rotor, which includes output mechanical
power and copper loss of the rotor.
The complete equivalent circuit of an induction motor includes all the
parameters that affect the motor's performance:
Stator resistance (R1) and leakage reactance (X1): Represent the
resistance and reactance of the stator windings.
Rotor resistance (R2) and leakage reactance (X2): Represent the
resistance and reactance of the rotor windings, referred to as the stator
side.
Magnetizing branch (Rm and Xm): Represents the core losses and
magnetizing inductance.
Rotor slip (s): Indicates the difference between synchronous speed and
actual rotor speed.
exact equevalent circuit of three phase induction motor
Thivenin equivalent circuit for three
phase induction motor
Key Concepts:
1. Thevenin’s Theorem:
o Any linear circuit with voltage and current sources can be replaced by a single voltage source
(VthV_{th}Vth) in series with a resistance (RthR_{th}Rth).
2. Application:
For three-phase induction motors, this equivalent circuit simplifies analysis at the stator
terminals.
Steps to Derive Thevenin’s Equivalent:
3. Identify Terminals:
o Focus on the stator terminals where the load connects.
4. Calculate Thevenin Voltage (VthV_{th}Vth):
o Open-circuit the terminals (remove the load) and measure the voltage across them.
5. Calculate Thevenin Resistance (RthR_{th}Rth):
o Deactivate all sources (short voltage sources and open current sources) and find the equivalent
resistance from the terminals.
Key Features:
Thevenin Voltage (VthV_{th}Vth): Voltage across the terminals when the load is disconnected.
Thevenin Resistance (RthR_{th}Rth): Resistance seen from the load's perspective when sources are
deactivated.
Advantages:
Simplifies circuit analysis.
Helps study how different loads affect motor performance.
Useful for calculating current and voltage across the load.
