DC Motors
A series motor will over speed when

The load is increase
The field is opened
Load is removed
The armature circuit is opened.

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DC Motors
As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit

A high value resistor is connected across the field winding
A resistance is connected in series with armature
Armature is temporarily open circuited
A resistance is connected parallel to the armature

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DC Motors
In variable speed motor

None of these
A weaker commutating field is needed at low speed than at high speed
Same commutating field is needed at low speed than at high speed
A stronger commutating field is needed at low speed than at high speed

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