Ac Motor

1.   STARTING CHARACTERISTICS OF INDUCTION SQUIRREL CAGE MOTORS

General Operating Principle

1.   A revolving magnetic field is set up when a 3 phase voltage is applied to the stator of an induction motor.

2.   The revolving field induces a voltage in the rotor bars

3.   The induced voltage creates large currents which flow in the rotor bars and end rings

4.   The current carrying rotor bars are immersed in the magnetic flux created by the stator. Because of that current carrying rotor bars are subjected to strong mechanical force.

5.   The sum of mechanical forces on all the rotor bars produce torque which tends to drag the rotor along in the same direction as the revolving field.

6.   As the rotor picks up speed, the relative velocity of the field with respect to rotor diminishes progressively

7.   This causes magnitude and frequency of induced voltage to decrease as the rotor bars are cut more slowly.

8.   Thus the rotor current is very large initially but decreases rapidly as the motor picks up speed.

9.   But the rotor will never be able to catch up with rotating field as this will reduce induced voltage to zero and consequently mechanical force acting on rotor will also become zero.

10. Difference in rotor magnetic field speed and rotor speed is called slip, its normal value is less than 0.1% of synchronous speed.

11.

2.0  Motor Under Load

2.1  When motor is running at no load and then a load is applied, its speed will begin to fall. This cause the rotating field to cut rotor bars at much higher rate and consequently magnitude of induced voltage will rise to produce more torque.

2.2  This will continue until the motor torque equals the mechanical torque of the load being driven

3.0  Locked Rotor Characteristics

3.1  The locked rotor current is 5-6 times the full load current which makes the I² R losses 25-36 times higher than normal. The rotor therefore, must not remain locked for more than a few seconds.

For an induction motor-

                        Frequency of induced voltage/current, f₂ = s f

            Induced voltage  E₂ = s E_oc

magnitude of induced Current in rotor= 600 P / E

            P = Output Power ( hp)

            E = Rated Line voltage

 

           

4.0  Torque Speed Characteristics

Fig 13. 17 shows torque-speed characteristics of a 3 phase induction motor. The  full load torque is T. The starting torque is 1.5 T. The maximum torque , called as breakdown torque is 2.5 T. The minimum torque produced by the motor while accelerating from rest to breakdown torque is called pull up torque.

Fig 13.17

5.0  Effect of Resistance on torque speed charactersistics