Page 202 - Solutions Manual to accompany Electric Machinery Fundamentals

ylabel('\bf\tau_{ind}');
title ('\bfInduction Motor Torque-Speed Characteristic');
grid on;
The resulting plot is shown below:

6-30. Answer the following questions about a 460-V -connected two-pole 100-hp 60-Hz starting code letter F
induction motor:
(a) What is the maximum current starting current that this machine’s controller must be designed to
handle?

(b) If the controller is designed to switch the stator windings from a  connection to a Y connection
during starting, what is the maximum starting current that the controller must be designed to handle?
(c) If a 1.25:1 step-down autotransformer starter is used during starting, what is the maximum
starting current that it must be designed to handle?

SOLUTION
(a) Starting code letter F corresponds to a 5.00 – 5.60 kVA/hp, so the maximum starting kVA of this
motor is
  S 100 hp 5.60    560 kVA
start
Therefore,
S 560 kVA
I start    703 A
3 T 3  V V 460

(b) The line voltage will still be 460 V when the motor is switched to the Y-connection, but now the
phase voltage will be 460 / 3 = 266 V.

Before (in ):

196