Review Questions DC Motors

Question 1

What is a commutator?

Answer 1

The device that turns or directs the flow of current.

Question 2

What are the three general types of self-excited generator field coil windings?

Answer 2

1. Series
2. Shunt
3. Compound

Question 3

What is the difference between a self-excited and a separately excited generator?

Answer 3

Self-excited = supply their own field current

Separately excited = get their field current from an outside source

Question 4

Why is it necessary to excite a machine field with DC?

Answer 4

Increases the strength of the magnetic field resulting in a larger magnitude of EMF

Question 5

What causes the voltage of a DC generator to drop as load is applied?

Answer 5

The combined effect of voltage drop and armature reaction on the field current flow.

Question 6

How does the armature current of a DC generator affect the field flux and commutator operation?

Answer 6

It causes the neutral zone of the brushes to shift with the load and excessive arcing at the brushes.

Question 7

How do most commercial DC generators correct for the shifting of the brush neutral zone?

Answer 7

They have narrow interpoles to reduce the flux density to zero.

Question 8

How does the direction of rotation of a DC generator affect the polarity of the commutating poles?

Answer 8

They must be reversed together.

Question 9

How does load affect the efficiency of a DC generator?

Answer 9

Efficiency increases at full load then drops off as additional load is picked up.

Question 10

What are the essential parts of a commutator?

Answer 10

1. Steel spider or sleeve
2. Steel V ring
3. Copper commutator bars
4. Mica insulating plates & rings

Question 11

How are the commutator bars and mica insulation held in place?

Answer 11

By means of a threaded ring or by bolts. Long bars may require shrink rings.

Question 12

Describe the care and appearance of a properly operating commutator.

Answer 12

1. Clean dirt and fumes to prevent excessive wear.
2. No high bars or flat spots.
3. Mica should be even or below the bars.
4. Bars should have a chocolate covered oxide coating where brushes
   make contact.

Question 13

What is the function of the brushes?

Answer 13

To carry the rotor current from the commutators.

Question 14

What materials are used in making brushes?

Answer 14

They are composed of carbon of varying hardness and in some cases of carbon impregnated with silver or copper.

Question 15

What is the difference between a DC generator and a DC motor?

Answer 15

The generator takes mechanical energy and converts it to electrical energy.
The motor converts electrical energy into mechanical energy.

Question 16

What causes the rotor in a DC motor to revolve?

Answer 16

The interaction of magnetic forces between the rotor poles and the field poles.
As the rotor revolves, the polarity of the rotor poles is reversed to produce continuous rotative motion.

Question 17

What is the EMF of a motor and how does it affect motor operation?

Answer 17

Back EMF works against or limits the applied voltage in accordance with rotor speed and load.

Question 18

How are motors protected from insulation damage?

Answer 18

1. Back EMF limits the current once the motor is up to speed.
2. Current limiting devices while starting.
3. Sustained current limiting relays.

Question 19

What determines the torque of a motor?

Answer 19

The magnetic forces exerted between the field and armature magnets.

Question 20

What determines the horsepower of a motor?

Answer 20

The torque and speed of a motor.

Question 21

What are the three types of DC motor windings?

Answer 21

1. Series.
2. Shunt.
3. Compound

Question 22

What are the characteristics of the Series winding.

Answer 22

Good load starting characteristics but poor speed regulation and if its load is lost it continues to increase speed.

Question 23

What are the characteristics of the shunt windings?

Answer 23

Good speed regulation but will runaway if its field is lost.

Question 24

What are the characteristics of the Compound winding?

Answer 24

Good starting characteristics and good speed regulation without the danger of running away.

Question 25

What is the function of a starting box?

Answer 25

Limits the amount of starting current. Also removes the resistance after the motor has come up to normal speed.

Question 26

What is the chief advantage of a DC motor?

Answer 26

Ease with which their speed can be varied.
High starting torque.
Low initial costs.

Question 27

What are 4 fundamental parts of a DC generator?

Answer 27

1. Field
2. Armature
3. Commutator
4. Brushes