All About Electric Induction Motors Tutorials

All About Induction Motor Facts, Tutorials and Important Information

Advantages of a three phase induction motor:
This is simple in construction. It is robust. It si almost unbreakable.
- It requires minimum care and maintenance.
- For a given horse power cost of induction motor is less.
- It has high efficiency
- It has a reasonably good power factor.
- It is self starting

Disadvantages of a three phase induction motor:
- Its speed can not be varied without loss of efficiency.
- Its speed decrease with the increase of load.
- It has inferior starting torque

Principle of Induction Motor:
When a three phase supply is given to the stator, a rotating field produces induced e.m.f. in rotor windings which cause induced currents tend to propose the action, producing them and therefore they circulate in such a manner that a torque is produced in the rotor tending it to cause it to flow the rotating field and thus reduce the relative motion which is producing the induced currents.


Key facts on Induction Motor:
- Squirrel cage induction motor has very small starting torque.
- Just like a D.C. shunt motor, the speed of an induction motor operating in stable region decreases with increase in load.
- Magnetic flux produced due to the stator winding with three phase current of frequency and which is of constant value, rotates at synchonous speed. If the number of pole of the motor. the synchronous speed is given by Ns = 120f/P.
- An induction motor rotor runs at a speed which is always less than the speed of the stator field.
- In a three phase induction motor the torque increases with the increase in supply voltage.
- Improvement in the power factor in an induction motor increase the torque and decrease the current due to increased impedance.
- Two of the power supply terminals to a three phase induction motor got interchanged during recondition after maintenance of the motor, when put back into service, the motor will rotate in the reverse direction to that prior to maintenance.
- If the air gap of the induction motor is increased the magnetizing current will increse.
- In a slip ring induction motor resistance is connected in rotor phases to increase the starting current.
- For a three phase wound rotor induction motor, and increase in rotor resistance effects the motor performance in the starting current decrease.
- It is advisable to avoid line starting of induction motor and use starter because motor takes five to seven times its full load current.
- A change of five percent in the supply voltage of a three phase induction motor will produce to its torque a change of approximately ten percent.