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Here is a electrical question which i came across.A DC cummulative compound motor delivers rated load torque at rated
speed. if the series field is short circuited then what happens to armature
current and speed?
There are two types of compound motor.They are cummulative compound motor and Differential compound motor.In differential motor the series field flux and the shunt field flux are in opposite direction whereas in the cummulative compound motor, series field flux & shunt field flux are in same direction and they produce an aiding effect.Therefore the speed regulation is good in cummulative DC compound motor when compared to the Differential compound motor.
The cummulative compound motor running at rated speed and rated torque.If you cut the field resistance in series ,it would act as a shunt motor. Probably in the practical you might have done this.We won't cut the resistance abruptly,instead move the rheostat to the minimum resistance position and observe the readings.So if we cut the series field resistance,the armature current increases as the resistance is lesser when compared to the motor with series field winding.
From the formula ,speed is inversely proportional to the the flux.As a result of removing the series field winding resistance, the total flux is reduced.This reduced flux increases the speed.You might add some more load to reduce the speed but the motor would run beyond its rated load and the speed would get reduced.
Hope this answers the question.Please post your valuable comments in the comments section and post more questions to share the knowledge.Cheers !!!