DC Motors

Question 1

What is the rotor of a DC motor made from?

Answer 1

Coils of wire.

Question 2

What does Maxwell’s Right Hand Grip Rule indicate?

Answer 2

The direction of a magnetic field around a conductor.

Question 3

What will lines of magnetic flux never do?

Answer 3

They will never cross (they will either bunch together or cancel each other out)

Question 4

What does each finger represent in the left hand rule?

Answer 4

Thumb = Motion of conductor
First finger = Field
Second finger = Current

Question 5

What is back EMF?

Answer 5

It’s an induced EMF that is opposite in polarity to the EMF that is being applied to the conductor

Question 6

The amount of torque produced by a motor is dependent upon what two things?

Answer 6

The strength of the magnetic field

Magnitude of the armature current

Question 7

What is torque?

Answer 7

The turning force that a motor produces.

Question 8

List the 3 classifications of DC motors.

Answer 8

Shunt
Series
Compound

Question 9

What must the carbon brushes be fixed in the same position as? And why?

Answer 9

Magnetic neutral axis (MNA)

To stop sparking at the brushes (when the gap in the two halves of the split ring passes the brushes)

Question 10

What happens when an armature has a current flowing through it?

Answer 10

A magnetic field is established.

Question 11

What does the magnetic field of the armature do?

Answer 11

Distort the magnetic field of the of the field windings and move the magnetic neutral axis (MNA)

Question 12

What does distortion of the MNA cause?

Answer 12

Causes the brushed to be misaligned, which causes sparking at the brushes.

Question 13

What are inter-poles, where are they fitted and what do they do?

Answer 13

Poles similar to the main poles but smaller.
Situated in between main poles.
They assist in giving spark-less commutation by neutralising the effect of armature reaction.

Question 14

What is geometric neutral axis?

Answer 14

Physical position centered between the two opposite field windings. This position cannot be changed.

Question 15

What is magnetic neutral axis?

Answer 15

The magnetic position that is centered in the magnetic field between the opposite field windings. This can be changed and is done so by distortion of the field winding magnetic field.

Question 16

What are the characteristics of a series motor?

Answer 16

Field coils connected in series. Only a few turns of heavy gauge wire.
Good starting torque but poor speed control.
Runs very slowly with heavy loads and very rapidly with light loads.
If the load is removed completely, the motor will speed dangerously and disintegrate.
Drives machines such as fans, hoists, and traction where loads cant be removed.

Question 17

What are the characteristics of shunt motors?

Answer 17

Field coils connected directly across the voltage source and in parallel with the armature. Many turns of thin gauge wire.
Poor starting torque, good regulation under all load conditions.
Torque developed varies directly with armature current.
The field current is practically constant so the shunt motor moves at almost constant speed.Used to drive workshop machinery and constant speed pumps.

Question 18

What are the characteristics of compound motors?

Answer 18

Series winding and shunt winding.

Torque characteristics of the series motor and regulated speed characteristics of the shunt motor.