DC Motor Principles

Question 1

What is a motor? What principle does it operate on?

Answer 1

Motors are devices that are used to convert electrical energy into mechanical energy. They operate on the principle of repulsion and attraction of magnetism.

Question 2

What is an armature?

Answer 2

The rotating coils of an electric motor

Question 3

What is torque?

Answer 3

The turning force developed by the motor

Question 4

What are two factors that determine the amount of torque produced by a DC motor?

Answer 4

1. The strength of the magnetic field of the pole pieces
2. The strength of the magnetic field of the armature

Question 5

Increasing the number of _______ or _______ will increase the torque

Answer 5

Loops or turns

Question 6

What are the three basic types of DC motors?

Answer 6

Shunt, series, and compound

Question 7

What is a shunt motor?

Answer 7

A motor that contains only a shunt field winding

Question 8

What is a good characteristic of shunt motors?

Answer 8

Shunt motors run at approximately the same speed from no load to full load

Question 9

What is counter-electromotive force (CEMF)?

Answer 9

An induced voltage in the armature that opposes the applied voltage and limits the armature current.

Question 10

CEMF is proportional to what?

Answer 10

1. The number of turns of wire in the armature
2. The strength of the magnetic field of the pole pieces
3. The speed of the armature

Question 11

What are four losses that a DC motor may experience while running?

Answer 11

1. I^2R loss in the armature windings
2. Windage loss
3. Bearing friction
4. Brush friction

Question 12

What is speed regulation? What is it proportional to?

Answer 12

Speed regulation is the amount of speed that decreases as a load is added. It is proportional to the resistance of the armature.

Question 13

What are series motors?

Answer 13

Series motors are motors that only contain a series field winding.

Question 14

What are the characteristics of a series motor?

Answer 14

Series motors have no natural speed limit, therefore, they can have extremely high speeds if not connected to a load.

They also can develop extremely high starting torques.

Question 15

What is a compound motor?

Answer 15

A motor that has both a series and a shunt motor operating characteristics. They can produce high torque and permit speed control and regulation.

Question 16

You can decrease the motor speed by reducing the amount of ____________ _________ in the motor.

Answer 16

Armature current

Question 17

What is field loss relay?

Answer 17

It is a protective device connected in series with the shunt field. The purpose of it is to disconnect power from the armature in the event that field current drops below a certain level.

Question 18

What are brushless DC motors?

Answer 18

They are DC motors that do not contain a wound armature, commutator, or brushes.

Question 19

What are the differences between a brush-type and a brushless DC motor?

Answer 19

Brushless motors are smaller, cost less, require less maintenance, and are more efficient than brush-type motors.

Question 20

What are permanent magnetic motors?

Answer 20

DC Motors contain a wound armature and brushes like a conventional DC motor but the pole pieces are permanent magnets. They have a higher efficiency than conventional field motors.