ELEN 30104
Module 4
Polyphase Induction Motor
Objectives
After successful completion of this module, you should be able to:
Describe what is a Polyphase induction motor
Know how does Polyphase induction motor works
Identify the Difference between AC and DC induction motor
Describe the main use of AC Polyphase induction motor
Understand the equivalent circuit of an Induction Motor
Understand the Power Flow of Induction Motor and its Power Losses
Determine the parameters and equivalent circuit of an Induction Motor
using Test data
Understand the relationship between torque, speed, slip and voltage of
Induction motor.
Determine the different type of starting method for Induction motor.
Types of AC Machine
Two Types:
(1) Synchronous Machine.
(2) Asynchronous Machine.
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(1) Synchronous Machine:
The machine whose speed is always
constant with the increasing load.
Ex: Synchronous motor
(2) Asynchronous Machine:
The machine whose speed is not
always constant with the increasing
load.
Ex: Induction motor, Alternator.
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Poly-phase ( 3-φ) Induction Motor
Construction:
Induction motor consists
essentially of two main parts
1. Stator
2. Rotor 5
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3-φ induction motor: cross section
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Stator:
➢ The stator consists of a cylindrical laminated & slotted core placed in a
frame of rolled or cast steel.
➢ It carries a 3-phase winding and is fed from a 3-phase supply.
➢ It is wound for a definite number of poles (determined by the requirement
of speed).
➢ Greater the number of poles, lesser the speed and vice versa.
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Rotor:
The rotor consists of a laminated & slotted
core tightly pressed on the shaft.
There are two general types of rotors:
1. Squirrel-cage rotor
2. ‘Phase wound’ or ‘wound’ or ‘slip ring’
rotor.
Completely wound stator for an IM. Rotor for an induction motor
Squirrel-cage Rotor
➢ The rotor consists of a cylindrical laminated core with parallel slots for
carrying the rotor conductors which are not wires but consist of heavy bars of
copper, aluminium or alloys.
➢ One bar is placed in each slot.
➢ The rotor bars are brazed or electrically welded or bolted to two heavy and
stout short-circuited end-rings, thus giving us, what is so picturesquely called,
a squirrel-case construction.
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Q. Write down the significance of the name ‘squirrel-cage’
in case of squirrel-cage rotor.
Phase wound Rotor
(‘Phase wound’ or ‘wound’ or ‘slip ring’ rotor): This rotor is
provided with 3-φ, double-layer, distributed
winding.
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Contd.
The rotor is wound for as many poles as the number of stator poles and is
always wound 3-phase even when the stator is wound two-phase.
The three phases are starred internally. The other three winding terminals
are brought out and connected to three insulated slip rings mounted on
the shaft with brushes resting on them.
3 brushes of the slip ring are further externally connected to a 3 phase
star-connected rheostat.
Slip ring vs Commutator
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Starting Resistance of Slip ring motor
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Why does the Rotor Rotate?
➢ 3-φ stator winding is fed by 3-φ supply
➢ Rotating flux of const. magnitude produced
➢ Flux passes through air-gap & cuts rotor conductor
➢ An emf is induced in rotor conductor
➢ Since rotor bars or conductors from closed circuit, current flows
through rotor conductors whose direction, as given
by Lenz’s law, is such as to oppose the very cause producing it.
➢ In this case the cause of rotor current is the relative velocity between the
rotating stator flux & the stationary rotor conductors.
➢ Hence, to reduce the relative speed, the rotor starts running in the same
direction as that of the flux and tries to catch up with the rotating flux.
Thus rotor of induction motor rotates
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3-φ supply
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Contd.
Rotation of Rotor of an IM.
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Write down the significance of
the name “Induction Motor”.
In an induction motor, no current is conducted to
one of the motor element (field or armature).
The current in one of these elements results from
an induced voltage and for that reason it is called
Induction motor.
Induction motors are somewhat referred to as
asynchronous(meaning not synchronous) machines.
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Why induction motor treated as a
rotating transformer?
➢ Transformer has two sides: primary & secondary
➢ Transformer transforms energy from primary to
secondary by induction
➢ Similarly, Induction motor has primary (stator) &
secondary (rotor)
➢ Voltage is induced in secondary by rotating flux of
const. magnitude i.e the process of induction
Thus induction motor treated as a rotating
transformer.
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Slip
The difference between the synchronous speed Ns and the actual rotor speed
Nr is known as slip. It is usually expressed as a percentage of the synchronous
speed.
Sometimes, (Ns - Nr) is called the slip speed.
So, the rotor speed Nr = Ns(1-s)
Ns − Nr
%s = 100
Ns
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Frequency of Rotor Current
When the rotor is stationary, the frequency of rotor current is
the same as the supply frequency.
But when the rotor starts revolving, then the frequency
depends upon the relative speed or on slip-speed.
Let at any slip speed, the frequency of the rotor current be f ’.
Then,
Dividing one by the other, we get,
So rotor current frequency is fr =s (fs)
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EXAMPLE PROBLEMS
EXAMPLE PROBLEMS
EXAMPLE PROBLEMS
6. A 150 Hp, 3 phase, 6 pole, 460 V, 60 Hz induction motor draws 195 A line current
at full load speed of 1170 rpm and 0.85 pf. Solve for the percent slip at full load.
Given:
𝑃𝑜𝑢𝑡 = 510 ℎ𝑝 𝑃 = 6 𝑉𝑙𝑙 = 460 𝑉 𝑓𝑠 = 60 𝑐𝑝𝑠 𝑁𝑟 = 1170 𝑟𝑝𝑚 𝑝𝑓 = 0.85 𝑙𝑎𝑔
Solution:
𝑁𝑟 𝑃 1170 6
𝑠 =1− =1− = 0.025 𝑜𝑟 2.5%
120𝑓𝑠 120 60
