PPT ON INDUCTION MOTOR STARTING

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Induction Motor Starting  Presentation Transcript

1.Induction-Motor Starting
Connect directly across full-voltage using a contactor for each phase.

2.Full-Voltage Starting
Major disadvantage is the high in-rush current.  Large voltage drops occur, dimming lights and forcing computers off-line.

3.Reduced-Voltage Starting
Autotransformer starting

4.“S”, or “start” contactor – close to start the motor at a reduced voltage – the autotransformers are set to 50, 60, or 80 percent of full-voltage.
At rated speed, open the S contactors and close the “R”, or run contactors – apply full voltage.

5.After coming up to speed, “Running”

6.A three-phase, 15-hp, 460-V, 150-A, 60-Hz, six-pole, 1141 r/min, design B motor with a code letter H is to be started at reduced voltage using an autotransformer with a 65-% tap.  Determine
a) locked-rotor torque and expected average in-rush current to the stator if the motor is started at rated voltage

7.b)  repeat part (a) assuming the motor is started at reduced voltage using an autotransformer with a 65% tap
c)  the in-rush current when starting at reduced voltage.

8.Check Table 5.1 for the minimum locked-rotor torque for a 125-hp, design B, six-pole motor

9.With the 65% tap, the voltage across the stator at locked-rotor is

10.The average in-rush current to the stator will be proportional to the voltage applied to the stator since the locked-rotor input impedance is constant.

11.The locked-rotor torque is proportional to the square of the voltage.

12.Current Curves

13.Locked-Rotor Torque Curves

14.The in-rush line current