Direct Current Motors

What determines the direction of rotation of a dc motor?

• The type of armature

• The method of excitation

• The number of armature coils

• The polarity of armature current and direction of magnetic flux

In a dc motor, what causes counter emf?

• Improper commutation

• Armature reaction

• Generator action

• Excessive speed

In a dc motor, how, if at all, does counter emf affect speed?

• It causes the speed to increase

• It causes the speed to decrease

• It causes rapid fluctuations of the speed

• It does not affect speed

What is the load on a dc motor?

• The field current

• The armature current

• The mechanical device the motor moves

• The total current drawn from the source

When a series dc motor is operated without a load, which of the following conditions occurs?

• The armature draws excessive current

• The voltage requirement increases

• The armature will not turn

• The armature speeds out of control

A dc series motor is best suited for which of the following applications?

• Steady load, low torque

• Variable load, low torque

• Steady load, high torque

• Variable load, high torque

What is the main advantage of a shunt motor over a series motor?

• A shunt motor develops higher torque at lower speeds than a series motor

• A shunt motor can be operated at higher speeds than a series motor

• A shunt motor draws less current from the source than a series motor

• A shunt motor maintains a more constant speed under varying load conditions than a series motor

How can the direction of rotation be changed in a dc motor?

• Only by reversing the field connections

• Only by reversing the armature connections

• By reversing both the armature connections and the field connections

• By reversing either the armature connections or the field connections

When the voltage applied to the armature of a dc shunt motor is decreased, what happens to the motor speed?

• It becomes uncontrollable

• It decreases

• It increases

• The motor stops

Which component has to be maintained most often in a small DC machine?

• Armature

• Brushes

• Permanent Magnets

• Commutator