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A cumulative dc compound motor runs at 1500 rpm. on full load. If its series field is short-circuited, its speed

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A cumulative dc compound motor runs at 1500 rpm. on full load. If its series field is short-circuited, its speed 

(1) becomes zero (2) remains unchanged (3) increases (4) decreases
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Increases.


As it's cumulative compound motor, both series and shunt field both assist each other. If series field is short circuited, the overall field strength will decrease and atop of that it will act as a shunt motor.

Now for shunt motor the speed is inversely proportional to field strength. 

So, the speed will increase overall.
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