DC Motor Questions #1

As there is no emf at the instant of starting a DC motor, in order to prevent a heavy current from flowing through the armature circuit

• a resistance is connected in series with armature armature

• a resistance is connected parallel to the armature

• armature is temporarily open circuited the field winding

• a high value resistor is connected across

In case the back emf and the speed of a DC motor are doubled, the torque developed by the motor will

• remain unchanged

• reduce to one fourth value

• increase four folds

• be doubled

If the speed of a DC shunt motor is increased the back emf of the motor will

• increase

• decrease

• remain the same

• become zero

Which motor should not be started on no load?

• series motor

• shunt motor

• cumulatively compound motor

• differently compound motor

Which of the following can be used to control the speed of a DC motor?

•  thermistor

• thyristor

• thyratron

• transistor

The rotating part of a dc motors is known as

• pole

• stator

•  armature

• carbon brush

What is the effect produced by the electric current in an electric motor?

• magnetic effect only

• magnetic as well as heating effect

• heating effect only

• heating as well as chemical effect

If a dc shunt motor is working at full load and if shunt field circuit suddenly opens

•  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

•  nothing will happen to the motor

• motor will come to stop

Carbon brushes are used in electric motors to

• brush off carbon deposits on the commutator

• provides a path for flow of current

• prevent overheating of armature windings

• prevent sparking during commutation

An external resistance is added in series with the field of a dc shunt motor. When the motor runs, the effect of resistance is

• to reduce the speed of the motor

• to increase the speed of the motor

•  to reduce the armature current drawn by the motor

• to reduce the losses