Final Review

Question 1

Define armature reaction

Answer 1

Distortion of the main field flux caused by current flowing from the armature to the conductors

Question 2

What is flat compound?

Answer 2

Where the terminal voltage is flat or constant with respect to the load

Question 3

What is over compound?

Answer 3

The rise in voltage at the terminals of the generator compensates for the line drop at the load

Question 4

The condition under which the magnetic flux of the series and shunt fields oppose each other is called __________

Answer 4

Differential compound

Question 5

What application is differential compound most commonly used in?

Answer 5

DC arc welding

Question 6

Heat produced by current flowing through the armature and field coils is called ________________

Answer 6

Copper losses

Question 7

Interpole winds consist of _______________

Answer 7

Small number of turns of large-gauge wire

Question 8

The effects of armature reaction can be minimized by placing windings called __________________ in the main pole faces

Answer 8

Compensating windings

Question 9

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

Answer 9

Solid-state drives

Question 10

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

Answer 10

Base speed

Question 11

Define torque

Answer 11

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

Question 12

Define the term slip speed

Answer 12

Slip speed is the difference between synchronous speed and rotor speed.

Question 13

How is the direction of rotation reversed in a squirrel-cage induction motor?

Answer 13

By reversing the direction of the rotating field. This is accomplished by interchanging any two of the stator line connections to the source

Question 14

How is the direction of rotation reversed in a split-phase motor?

Answer 14

By reversing the direction of current through the auxiliary winding

Question 15

How is the direction of rotation reversed in a series motor?

Answer 15

By interchanging the two armature leads, or the two series field leads

Question 16

How is the direction of rotation of a shaded-pole face motor reversed?

Answer 16

By removing the stator and turning it end-for-end before replacing it over the rotor

Question 17

Define the term breakdown torque

Answer 17

The breakdown torque of a motor is the maximum torque the motor will produce

Question 18

Why does a standard split-phase motor (without a starting capacitor) have a low starting torque compared to a capacitor-start, split-phase motor?

Answer 18

The standard split-phase motor has a phase shift between the main and auxiliary winding of 30–50 electrical degrees. The capacitor-start, split-phase motor has a phase shift of 90 electrical degrees, which corresponds better with the physical displacement of the poles in the motor

Question 19

Which type of split-phase motor combines good starting torque with good overload capacity?

Answer 19

The capacitor-start, capacitor-run split-phase motor

Question 20

What are the two types of windings in the stator of a shaded-pole motor?

Answer 20

Exciter winding, shading coil

Question 21

In what direction does the field move across the pole face in a shaded-pole motor?

Answer 21

From the unshaded portion to the shaded portion

Question 22

Is the starting torque of a shaded-pole motor high or low compared to a split-phase motor?

Answer 22

Low

Question 23

What is the purpose of the tertiary winding included in some wye-connected autotransformer banks?

Answer 23

It permits the flow of third-harmonic currents. This eliminates the third-harmonic voltages in the wye-connected windings

Question 24

What advantage does the extended-delta autotransformer have over the standard-delta autotransformer?

Answer 24

It permits transformations in ratios higher than 2:1

Question 25

Which autotransformer connection would likely be used to achieve a desired phase displacement between the primary and secondary systems?

Answer 25

The closed-delta connection

Question 26

What is the “form factor” commonly used for the three-phase half-wave rectifier

Answer 26

1.17 × AC phase (rms) value

Question 27

What is the “form factor” commonly used for the three-phase full-wave rectifier

Answer 27

1.35 × AC line (rms) value

Question 28

What advantage is offered by using SCRs instead of diodes in three-phase rectifier circuits?

Answer 28

SCRs can be phase-controlled to offer a variable DC output.

Question 29

How is an SCR normally turned on?

Answer 29

By applying a small amount of power to the gate terminal

Question 30

How does the power-handling ability of the SCR compare to that of a transistor?

Answer 30

SCRs can handle much more power.

Question 31

What happens if the SCR’s reverse-blocking voltage is exceeded?

Answer 31

The SCR will conduct in the reverse direction and will be permanently damaged.

Question 32

What happens if the SCR’s forward-blocking voltage is exceeded?

Answer 32

The SCR will be turned on without a gate signal.

Question 33

If resistor triggering is used, the SCR can be fired between ________ degrees up to ________ degrees in the positive half-cycle

Answer 33

0 to 90

Question 34

If resistor-capacitor triggering is used, the SCR can be fired from ________ degrees up to ________ degrees in the positive half-cycle

Answer 34

0 to 180

Question 35

Whereas the SCR is a unidirectional device, a triac is a ________ device

Answer 35

Bidirectional

Question 36

The action of a triac could be likened to two ________ connected back-to-back

Answer 36

SCR's

Question 37

Triacs are available in rms current ratings up to a maximum of about ________ A

Answer 37

45

Question 38

What is the difference between a reduced-voltage starter with an open transition and a reduced-voltage starter with a closed transition?

Answer 38

For open-transition starters, there is a momentary loss of voltage (power) to the motor as the control transfers from the starting to the running stage. But for closed-transition operation, there is no interruption of voltage

Question 39

What is anti-plugging protection?

Answer 39

Anti-plugging uses a zero-speed switch to prevent a motor from reversing until it has come to a complete stop

Question 40

Explain what is meant by the term plugging

Answer 40

Plugging is a control function that provides braking by reversing the motor line voltage polarity or phase sequence so that the motor develops a counter-torque that exerts a retarding force

Question 41

What conditions must be met in order that alternators may be paralleled?

Answer 41

Same: voltage, frequency, phase rotation. ( then phase opposition for sync. point.)

Question 42

What happens when a coil is rotated between a pair of opposite polarity field magnetic poles?

Answer 42

AC voltage is induced into the coil

Question 43

Brushes are positioned to short out a coil when it is ___________

Answer 43

Crossing the neutral plane

Question 44

Increasing the firing angle for an SCR from 0 75 degrees would result in _________

Answer 44

Less power supplied to the load

Question 45

The output of a revolving field 3 phase AC machine is taken from _____________

Answer 45

Slip rings and brushes

Question 46

Eddy currents and hysteresis losses are referred to as __________

Answer 46

Core losses

Question 47

The variable resistance placed into the rotor circuit is mainly for the purpose of __________

Answer 47

Varying the torque

Question 48

The maximum value of torque in a SCIM will ______

Answer 48

Occur when the rotor has XL = Resistance

Question 49

When a holding contact is wired into the control circuit, the motor is considered to have ________

Answer 49

Low voltage protection