DC Motors
The rheostatic speed control method is very

Suitable for getting speeds above the normal
Unsuitable for rapidly changing loads
Efficient
Economical

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DC Motors
If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

Motor will come to stop
Nothing will happen to motor
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
This will make armature to take heavy current, possibly burning it

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