Motors test

Question 1

Counter-electromotive force (CEMF) or back-EM are terms describing voltage that is induced into the armature of a DC motor as it begins rotating. T or F

Answer 1

True

Question 2

A device that converts electrical energy into mechanical energy is called a(n)______

Answer 2

Electric motor

Question 3

A series motor may be operated with no load connected. T or F

Answer 3

False

Question 4

The compound motor connection that should generally be avoided is the _________.

Cumulative
Differential
Series
Shunt

Answer 4

Differential

Question 5

The speed regulation of a DC motor is proportional to the ________

Answer 5

Armature Resistance

Question 6

Motor with the best speed characteristics?

Answer 6

Series

Question 7

Motor with the best Torque characteristics?

Answer 7

Shunt

Question 8

A compound motor has ____ and ______ field windings.

Answer 8

Shunt and Series

Question 9

In a DC motor, terminals Ay and Az are connected to the shunt fields. T OR F

Answer 9

False

Question 10

Series field windings are connected in series with the armature. They are wound with ____ turns of fairly _____ wire.

Answer 10

Few, Large

Question 11

Shunt field windings are connected in parallel with the armature. They are wound with _____ turns of fairly ____ wire.

Answer 11

Many, small

Question 12

The __________
ride against the commutator
segments and are used to connect the armature to the external circuit of the DC machine.

Answer 12

Brushes

Question 13

Brushes are made from a material that is softer than the ________
bars of the
commutator.

Answer 13

Copper

Question 14

How does the speed vary as the mechanical load applied to the series motor increases, i.e., as the motor torque increases.

Answer 14

The speed decreases nonlinearly when the motor torque increases.

Question 15

The DC motor brush leads are generally labeled ____ and ____.

Answer 15

A1 and A2.

Question 16

When a motor has only one set of field windings mounted on a DC motor that is connected in parallel with the armature, the motor is called a _____ motor.

Answer 16

Shunt.

Question 17

Is the cumulative compound motor a good compromise of the characteristics of the series and shunt connected motors?

Answer 17

The characteristics of a cumulative compound motor is a good compromise of the series and shunt motor characteristics.

Question 18

Can a series connected motor be started with no mechanical load connected to it?

Answer 18

A series motor must never be started with no mechanical load because the speed increases to a very high value, which can damage the motor (motor runaway).

Question 19

What happens to the speed of a shunt connected motor as the load applied to it increases?

Answer 19

The motor speed varies little as the mechanical load varies

Question 20

Select types of overload protection devices available for motor starters.

Magnetic
Thermal
Electronic
All of the above

Answer 20

All of the above

Question 21

When an overload relay is added to a magnetic contactor, the combined device is now called a magnetic motor starter, provided that the contactor is HP rated. T or F

Answer 21

True

Question 22

The primary reasons for utilizing automated control circuits for motors is to reduce costs by automating processes and to improve safety and productivity. T or F

Answer 22

True

Question 23

What is the most common application for using one of the N.O. auxiliary contacts as part of a three-wire control circuit?

Answer 23

Holding/maintaining contact

Question 24

Upon applying power to a magnetic motor starter, the coil produces a humming sound.
The pole contact surfaces are clean (no carbon, dirt or debris) and the plunger appears to be moving freely.
What is the most likely cause of the noise and what corrective action should be taken in order to repair it?

Answer 24

Under voltage, correct voltage source problem.

Question 25

Upon applying power to a magnetic motor starter, the coil produces a humming sound. The pole contact surfaces are clean (no carbon, dirt or debris) and the plunger appears to be moving freely. What is the most likely cause of the noise and what corrective action should be taken in order to repair it?

Answer 25

Under voltage, correct voltage source problem.

Question 26

What problems are likely occurred with a DC motor if the brushes are NOT properly set up to the correct neutral plane position (Zero Point)? Arcing and sparking at brushes Damage to commutator Power loss Efficiency reduction All of these

Answer 26

All of these

Question 27

To reverse the direction of a shunt motor, the wire leads for both the armature and shunt field should be swapped. T or F

Answer 27

False

Question 28

What conditions can cause the contact's surface to be excessively worn or pitted? Loose contact pressure Debris/dirt on the contact surfaces Weak or damaged springs excessive current All of the above

Answer 28

All of the above

Question 29

What typically happens to the speed, CEMF and current of a shunt motor as load is added to the motor?

Answer 29

The speed drops, the CEMF reduces causing current to increase through the armature and strengthens its magnetic field which increases torque to meet the added load requirements.

Question 30

What advantages does Dynamic Braking have over mechanical braking methods? Dynamic Braking does not require additional equipment Provides very smooth deceleration without any wear and tear Less maintenance is required due to no mechanical brake parts very little extra electrical equipment is required All of these

Answer 30

All of these

Question 31

How does changing the resistance value in a Dynamic Braking circuit affect the length of time to stop the motor completely?

Answer 31

The counter torque produced during dynamic braking changes accordingly with changes in current affected by resistance.

Question 32

Which best describes the advantages of using a drum switch to reverse the direction of a DC motor? Easy to use, heavy duty, the handle returns to OFF (center) position when let go Maintains the selected direction and turns the motor off when changing directions, has heavy duty contacts Easy to operate, small and less expensive All of the above

Answer 32

All of the above

Question 33

What two purposes does the MCR serve in a control circuit?

Answer 33

The MCR is used to provide low voltage protection and safely shut down an entire section or all of a machine.

Question 34

Reduced voltage starting methods are used instead of full voltage starting for a motor control circuit is to reduce the high inrush current and/or reduce starting torque to reduce the mechanical stress on equipment being driven by the motor. T or F

Answer 34

True

Question 35

The advantage of reversing direction of a DC Shunt Motor through the armature instead of the field winding, is that smaller, less expensive devices can be used due to the lower current of the armature winding. T or F

Answer 35

False

Question 36

A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM. The motor is connected to a 250 VDC source and measures a current draw of 175 Amps. What is the Power output (watts) of the motor? 45532.51 34541.88 56254.25 43750.4

Answer 36

Power output = Output HP X 746 Power output = 46.303 × 746 Power output = 34541.88

Question 37

A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM. The motor is connected to a 250 VDC source and measures a current draw of 175 Amps. What is the Power input (watts) of the motor? 79500 68250 55650 43750

Answer 37

250 v x 175 amps = 43750 watts

Question 38

A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM. The motor is connected to a 250 VDC source and measures a current draw of 175 Amps. What is the efficiency of the motor? 57% 53% 79% 85%

Answer 38

Efficiency = Power output ÷ Power input X 100 Efficiency = 34541.88 watts ÷ 43750 X 100 Efficiency = 78.9 % or 79%

Question 39

A series-connected DC motor is found to produce a torque of 200 Nm when operated at a speed of 1650 RPM. The motor is connected to a 250 VDC source and measures a current draw of 175 Amps. What is the HP of the motor? 61.036 21.258 7127 46.303

Answer 39

Output Horsepower = T(Nm)x Speed (RPM) ÷ 7127 Output Horsepower = 200(Nm)x 1650 (RPM) ÷ 7127 Output Horsepower = 46.303 46.303 HP