Line Y-3

Question 1

Define torque

Answer 1

Torque is a twisting or turning force that is capable of producing rotation about an axis

Question 2

Define the motor effect principle of magnetism

Answer 2

Whenever a conductor carries current in the presence of a magnetic field, a force will act on the conductor at right angles to the field

Question 3

Fleming’s right-hand motor rule is used to determine _____________________

Answer 3

The direction of force acting on a conductor

Question 4

How does the operation of the armature differ between a DC generator and a DC motor?

Answer 4

In a generator, the armature is a source of emf. In a motor, the armature is a load

Question 5

State two ways in which the direction of rotation of a DC motor can be reversed

Answer 5

Reverse the connection of the armature leads.
Reverse the connection of both the series and shunt field leads

Question 6

What is torque directly proportional to?

Answer 6

The product of flux and armature current

Question 7

The motion of armature conductors cutting a field results in an induced emf that ____________________ the applied emf

Answer 7

Opposes

Question 8

The emf induced in armature conductors as they cut the magnetic field is called _____________

Answer 8

Counter emf

Question 9

What is the counter emf of a motor proportional to?

Answer 9

Flux and speed

Question 10

What is effective armature voltage the product of?

Answer 10

Armature current and armature resistance

Question 11

The difference between applied voltage and counter emf is called ___________

Answer 11

Effective voltage

Question 12

At standstill, the counter emf of a motor is _______________

Answer 12

Zero

Question 13

At start-up, the current drawn by a DC motor is limited only by ____________

Answer 13

Armature resistance

Question 14

How is the starting current limited to a safe value in large DC motors?

Answer 14

Resistance is placed in series with the armature

Question 15

A locked rotor usually means that the motor will ________________

Answer 15

Fail to start when voltage is applied

Question 16

Why is a locked rotor condition undesirable?

Answer 16

The persistence of high starting current may damage the motor

Question 17

An increase in mechanical load on the shaft of a motor causes motor speed to ___________

Answer 17

Decrease

Question 18

When the speed of a motor decreases due to an increase in mechanical load, the counter emf ___________

Answer 18

Decreases

Question 19

A reduction in counter emf causes armature current to ______________

Answer 19

Increase

Question 20

What would happen if an increase in mechanical load exceeded the ability of the motor to develop additional torque?

Answer 20

The motor would stall

Question 21

In a motor, power is proportional to ________________ and ________________

Answer 21

torque and speed

Question 22

The variation in motor speed with load is called ___________

Answer 22

Speed regulation

Question 23

The speed at which a DC motor operates without speed control devices is called __________

Answer 23

Base speed

Question 24

Why does increasing the resistance in the field circuit of a DC motor cause the speed to increase?

Answer 24

The increase in armature current (due to a decrease in counter emf ) is greater than the decrease in excitation

Question 25

To operate a motor below base speed, it is necessary to decrease __________

Answer 25

Armature current

Question 26

A practical method of operating below base speed involves controlling ______________

Answer 26

The voltage applied to the armature

Question 27

List two adverse effects of armature reaction if it remains uncorrected

Answer 27

Weakening of the main field Excessive brush sparking

Question 28

The most common solution for minimizing the effects of armature reaction involves the use of __________________

Answer 28

Interpoles

Question 29

Why are interpoles connected in series with the armature?

Answer 29

So that the strength of the interpole matches the strength of the armature field

Question 30

Shunt field windings consist of a:

Answer 30

Large number of turns of small-gauge wire

Question 31

T/F Excitation of a shunt motor is provided by field coils connected in series with the armature

Answer 31

False

Question 32

When using NEMA subscripts, the convention of A1 positive indicates (CCW or CW) rotation?

Answer 32

CCW

Question 33

T/F When a DC generator and a DC motor are operated with the same line voltage, polarity and direction of rotation, the respective armature currents will be in opposite directions

Answer 33

True

Question 34

State two ways to reverse the direction of the force acting on the armature conductors of a shunt motor.

Answer 34

Reverse the direction of current in the armature Reverse the direction of current in the shunt field windings

Question 35

What is the most practical method of reversing the direction of rotation of a shunt motor?

Answer 35

Reverse the armature leads

Question 36

What happens to the direction of rotation if the polarity of the line terminals is reversed?

Answer 36

The rotation remains the same

Question 37

A shunt motor is commonly called a _____________________ motor

Answer 37

Constant speed

Question 38

Why must the field circuit of a shunt motor never be allowed to become open while the motor is in operation?

Answer 38

The motor can accelerate to unsafe speeds

Question 39

To prevent runaway in a shunt motor, an _______________ is used

Answer 39

Over-speed sensor (or loss of field relay)

Question 40

List three typical uses for a shunt motor

Answer 40

Drives for paper machines Printing presses Drill presses

Question 41

Series field windings consist of a:

Answer 41

Small number of turns of large-gauge wire

Question 42

What is the main difference between series and shunt excitation?

Answer 42

The magnetic strength of series field coils varies with load current

Question 43

State two ways to reverse the direction of rotation of a series motor

Answer 43

Reverse the connection of the armature leads Reverse the connection of the series field leads

Question 44

Will changing the polarity of the line leads change the direction of rotation of a series motor? Why?

Answer 44

No. Reversing line polarity reverses both armature current and field polarity, maintaining torque in the same direction as before

Question 45

Why is the series motor unsuitable for constant-speed applications?

Answer 45

Since the torque varies with the square of the armature current, changes in mechanical load are accompanied by large changes in operating speed

Question 46

In practice, speed control of series motors is accomplished using _____________

Answer 46

Solid-state drives

Question 47

T/F The main advantage of the series motor is its very high starting torque

Answer 47

True

Question 48

Why should a series motor always be mechanically coupled to the driven load?

Answer 48

The sudden removal of load may cause the motor to accelerate to an unsafe speed

Question 49

The main characteristics of the series motor are _______________ and _______________

Answer 49

High starting torque and poor speed regulation

Question 50

List three common uses for series motors

Answer 50

Cranes, hoists, locomotives

Question 51

A compound motor has both ___________________ and ___________________ field coils

Answer 51

Shunt and series

Question 52

In most compound motors, the motor characteristics are predominantly like the ___________________ motor

Answer 52

Shunt

Question 53

When the flux of the series field aids the flux of the shunt field, the connection is called a ___________________ compound

Answer 53

Cumulative

Question 54

When the flux of the series field opposes the flux of the shunt field, the connection is called a ___________________ compound

Answer 54

Differential

Question 55

Explain the difference between long shunt and short shunt connections

Answer 55

In the long shunt connection, the shunt field is connected across the line (armature and series field). In the short shunt connection, the shunt field is connected directly across the armature

Question 56

State two ways of reversing the direction of rotation of a compound motor

Answer 56

Reverse the connection of the armature leads Reverse the connection of both the series and shunt field leads

Question 57

T/F The speed of a compound motor can be increased by increasing resistance in the shunt field circuit

Answer 57

True

Question 58

Is it necessary to take precautions to prevent runaway of a compound motor?

Answer 58

Yes. The degree of compounding may not be sufficient to prevent dangerous over-speed in the event of an open in the shunt field circuit

Question 59

Is it acceptable to have belt-driven mechanical loads with a compound motor?

Answer 59

Yes. The no-load speed hazard of the series motor is not present

Question 60

When selecting a compound motor, the _______________ and _______________ characteristics must be matched to the specific application

Answer 60

Torque and speed

Question 61

Compound motors are more suitable than shunt motors for handling ______________

Answer 61

Sudden increases in mechanical load

Question 62

List three applications for compound motors

Answer 62

Presses, shears, compressors