Induction Machine
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induction machine class slides is made for my friend who are studying bachelor in electrical engineering
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2017/2018
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Induction Machine
Basic Construction
Electromagnetics
Stator
Rotor
Squirrel cage rotor and Wound rotor
Squirrel Cage Rotor
Working principle
- Three phase supply provided to stator winding
- Rotating magnetic field is produced.
- Emf induced in rotor due to Faraday’s law.
- Current circulates in rotor as rotor is short
circuited by slip rings and brushes or end
rings.
- Thus rotor rotates due to BLI law.
Speed of rotating magnetic field and
rotor
- Ns = 120 f / P RPM
- If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed
- As rotor always rotates at a speed below synchronous speed, this type of motor is aka asynchronous motor.
Equivalent circuit
Torque-Slip Characteristics
Power flow in Induction Motor
Speed Control
- The induction motor is basically a constant speed motor given a constant voltage source operating at a constant frequency. As the load torque increases, the motor speed varies by only a small percentage of the rated speed. There are some techniques that allow for control of the induction motor speed.
- Pole Changing - By changing the stator winding connections, the total number of poles can be modified, only discrete speed changes are available.
- Line frequency variation- the synchronous speed of the motor, and thus the machine speed, can be changed by simply varying the line frequency.
- Line voltage control - the speed of the induction motor can be changed over a small range for a given load by varying the line voltage.
No Load Test
Blocked Rotor Test
Starting Methods
Star Delta Starter
Starting Methods
Autotransformer Starting