ECE 4411 Induction-Motor Starting Connect directly across full-voltage using a contactor for each phase.

ECE 4412 Full-Voltage Starting Major disadvantage is the high in-rush current. Large voltage drops occur, dimming lights and forcing computers off-line.

ECE 4413 Reduced-Voltage Starting Autotransformer starting

ECE 4414 “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.

ECE 4415 After coming up to speed, “Running”

ECE 4416 Example 5.18 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

ECE 4417 Example 5.18 continued 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.

ECE 4418 Example 5.18 Solution Check Table 5.1 for the minimum locked- rotor torque for a 125-hp, design B, six- pole motor

ECE 4419 Example 5.18 Solution continued From Table 5.9, the average locked-rotor kVA/hp for a code H motor is

ECE Example 5.18 Solution continued The expected average in-rush current to the stator is

ECE Example 5.18 Solution continued With the 65% tap, the voltage across the stator at locked-rotor is

ECE Example 5.18 Solution continued 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.

ECE Example 5.18 Solution continued The locked-rotor torque is proportional to the square of the voltage.

ECE Current Curves Transition from low to rated voltage is at slip = shaft speed = n s (1-s) shaft speed = 1200( ) shaft speed = 1050 r/min

ECE Locked-Rotor Torque Curves Transition from low to rated voltage is at slip = shaft speed = n s (1-s) shaft speed = 1200( ) shaft speed = 1050 r/min

ECE The in-rush line current