DC Motors
The iron loss in dc motor depends on

Speed only
Flux only
Both flux and speed
None of the listed here

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DC Motors
If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

Nothing will happen to the motor
Motor will run at very slow speed
This will make armature to take heavy current, possibly burning it
This will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

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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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