Shunt-wound motor: Maximum start-up torque

(a) A shunt-wound motor set was designed to run from a 250 V d.c. supply. It had an effective armature resistance of 3 m Ohm and when run at 3000 rev/min it drew 10 kA.

Find:
(i) The maximum permissible supply voltage at start-up if the motor set is not to draw more than 20 kA when started from rest.
(ii) The maximum start-up torque available.

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...rad/s.

Since , in this start-up case, Ea =0, then we have

If it is required that the motor set is not to draw more than 20000A, that is, the maximum voltage is ...