Revised Study Deck

Front 1

Define RMS value

Back 1

root mean square (aka effective value) is the square root of the mean value of the squares of the instantaneous values of a current, volltage, or other periodic quantity during one complete cycle

Front 2

RMS is used for

Back 2

the measurement of average voltage of an AC system

Front 3

The voltage applied to a purely inductive circuit:

Back 3

leads a quarter of a cycle in front of the current
OR
current lags a quarter cycle behind the applied voltage

Front 4

Reactance is the ratio of the _______ to the _______

Back 4

voltage
current

Front 5

The volatge applied to a purely capacitive circuit:

Back 5

lags behind the current by a quarter cycle
OR
the current is leading in front of the voltage by a quarter cycle

Front 6

Define true power

Back 6

the power that is actually used in a circuit
power associated with the total resistance in the circuit

Front 7

Define apparent power

Back 7

the power that appears to the cource because of the ccircuit's impedance
product of effective values of voltage and current in an AC circuit

Front 8

Define power factor

Back 8

the ratio of actual power in watts to the voltamperes of an AC circuit

Front 9

Power factor is dependant on

Back 9

how much the current and voltage are out of phase

Front 10

The power factor of an AC circuit is:

Back 10

equal to the cosine of the phase angle

Front 11

With three phase star connected windings, the phase _____ is equal to line

Back 11

current

Front 12

With three phase delta windings, the phase _______ is equal to line

Back 12

voltage

Front 13

Define armature reaction

Back 13

the action of the armature curents in establishing a field that distorts and weakens the stator field

Front 14

Each coil segment in a DC generator sets up magnetic fields that are proportional to the amount of current being supplied to the DC load. Therefore:

Back 14

the amount of distortion of the stator field is proportional to the generator load

Front 15

What can be used to reduce the effects of armature reaction?

Back 15

Compensating windings or pole faced windings

Front 16

How do compensating windings work?

Back 16

the winding is connected in series with the armature winding so that the full armature current flows through it but in the opposite direction to that of the adjacent armature conductors

Front 17

The flux produced by the _____ neutrallizes the flux produced by the ______ which corrects armature reaction.

Back 17

compensating winding
armature current

Front 18

Lap wound armatures are well suited for:

Back 18

high current DC generator applications

Front 19

Wave wound armatures are well suited for:

Back 19

high voltage applications

Front 20

Series wound generators use:

Back 20

very low resistance field coils

Front 21

A characteristic of compound wound generators is:

Back 21

the two fields are proportioned so that the decrease in the shunt field is compensated by the increase in the series field
output voltage remains constant

Front 22

The combination of series winding and shunt winding in a compound generator

Back 22

overcomes the disadvantages of both the series wound and shunt wound generators

Front 23

With respect to a DC generator, more turns in the series field produces:

Back 23

higher voltage with the same current

Front 24

Define differential compounding

Back 24

the series field opposes the shunt field
an increase in current causes a decrease in generated emf

Front 25

_____ and _____ generators are more stable for parallel operations than _______ and _______ generators

Back 25

shunt
undercompounded
flat
overcompounded

Front 26

Differentially compounded generators are used for:

Back 26

when overload protection is more desirable than fixed voltage

Front 27

A voltage characteristic of a separately excited generator is:

Back 27

particularily flat

Front 28

Define torque:

Back 28

the measurement of the action of a force on a body that tends to cause the body to rotate

Front 29

Increasing the ______ of a machine causes it to take more load. This causes the machine to _______ and the _______ admits more energy to the prime mover

Back 29

field excitation
slow down
govenor

Front 30

Without an equalized connection, any attempt to transfer some load between compounded generators results in:

Back 30

a complete load swing to one generator and motorization of the other

Front 31

With compounded machines of different kW ratings, the equalizer ensures:

Back 31

a division of load in proportion to the kW ratings of each machine

Front 32

The force developed on each conductor of a motor armature is due to:

Back 32

the combined action of the main field and the field around the conductor

Front 33

Cemf acts as:

Back 33

a load for the DC power supply feeding the motor so that the low resisteance motor windings do not draw excessive amounts of current

Front 34

What is the most common method of speed control for a shunt motor and why?

Back 34

a rheostat in series with the field
field current is less than armature surrent and results in less power loss across the rheostat

Front 35

Never open the ______ of a ______ that is running unloaded.
Why?

Back 35

ield circuit
shunt motor
the motor will run away (residual magnetism of the pole pieces maintains a weak field: armature can still draw current)

Front 36

The speed of these motors are inversly proportional to the field flux

Back 36

shunt motor and series motor

Front 37

The load should never be removed from a series motor. Why?

Back 37

the motor has a tendancy to run away since the torque required to move the armature allows the motor to overspeed in an attempt to develop a cemf

Front 38

The speed of a series motor is controlled by:

Back 38

inserting resistance in the armature circuit

Front 39

When calculating losses for a motor or a generator, what factors need to be considered?

Back 39

the direction of the current and the wiring of the motor or generator

Front 40

A stator core is usually constructed of:

Back 40

laminated high quality magnetic silicon steel sheets varnished on each side to reduce magnetic and eddy current losses

Front 41

Stator windings are usually made of:

Back 41

copper bars insulated with mica on glass fibre tape

Front 42

What is the purpose of an exciter?

Back 42

supplies direct current to the alternator field windings to magnetize the rotating poles

Front 43

What is self-excitation of an AC generator?

Back 43

a small DC generator instaled on the alternator shaft
the output of the DC generator is supplied to the main rotor winding of the AC generator

Front 44

What is brushless excitation?

Back 44

a small AC generator on the main shaft supplies AC voltage to a 3 phase rectifier assembly also on the shaft. the DC voltage is then applied to the main generator field

Front 45

How does a diode work?

Back 45

a diode consists of an anode and a cathode. when AC voltage is applied to the diode, it only conducts half of the AC sinusoid.
(a single diode provides DC voltage with significant ripples)

Front 46

What is a rectifier bridge?

Back 46

a diode arrangement that produces a DC voltage with very little ripple

Front 47

The operations of a direct acting rheostat regulator depends on:

Back 47

the adjustment of variable resistances in the field circuit of the main exciter

Front 48

What is the purpose of the alternator rotor?

Back 48

to produce the magnetic field necessary to induc voltage in the stator winding

Front 49

What is a salient rotor?

Back 49

several seperately wound pole pieces bolted to the fframe of the rotor

Front 50

Why are salient rotors typically used in applications less than 1200 rpm?

Back 50

because of the large centrifugal forces created by the protruding poles

Front 51

What is a cylindrical rotor?

Back 51

a rotor made of solid steel forging where the windings are arranged to form two iof four distinct poles and are firmly embedded in slots

Front 52

In a cylindrical rotor, the windings are ______ insullated with _____ and held into the rotor slots against centrifugal force by _____.

Back 52

copper strip
micanite
steel wedges

Front 53

Define saturation of a coil

Back 53

the point of magnetization where increased ampere-turns produces smaller and smaller increases in flux in the core.

Front 54

Define induced electromotive force

Back 54

the voltage developed across the terminals of a voltmeter when the conductor is moved through a magneticc field

Front 55

Define induced current

Back 55

the current caused to flow in the conductor by the induced emf

Front 56

Define electromagnetic induction

Back 56

an emf is induced in a conductor when the conductor cuts (or is cut by) a magnetic field

Front 57

Define Faraday's law of induction

Back 57

whenever the number of lines of force threading through a coil is changed, an emf is induced in that coil. the amount of emf induced is proportional to the rate at which the number of lines of force through the coil is changing

Front 58

Define Lenz's law of induction

Back 58

an induced emf will cause a current to flow in a closed circuit in such a direction that its magnetic effect will oppose the change that produces it

Front 59

Define emf of self induction

Back 59

when an emf is induced in a circuit because of a change in the current in the circuit itself

Front 60

What is the unit for inductance and what does it represent?

Back 60

henry
one henry is equal to an emf of one volt induced in the circuit when the current changes at the rate of one ampere per second

Front 61

A circuit that has inductance

Back 61

opposes any CHANGE in current through the cicuit
(when current is unchanging in value, the inductance of the circuit has no effect on its flow)

Front 62

As pairs of poles are added to a DC generator:

Back 62

the alternations of the induced emf increase in proportion

Front 63

What is a separately excited generator?

Back 63

a generator that is excited from a storage battery of a separate DC source

Front 64

What is a short shunt compound generator?

Back 64

A self excited generator where both series and shunt fields are used. the shunt field is parallel to the armature only

Front 65

What is a field rheostat used for?

Back 65

used to provide means of verying the field flux and, thereby, the amount of emf generated by the generator

Front 66

A regulation curve represents what?
What is another name for it?

Back 66

the value of the terminal voltage of a generator (DC) for the different values of load current
load-voltage characteristic

Front 67

List the copper losses

Back 67

Armature losses
field losses
-shunt field
-series field
-interpole/compensating field

Front 68

List possible mechanical losses

Back 68

Iron losses -eddy currents -hysteresis
friction losses -bearing friction
brush friction
-windage or air friction

Front 69

What is the purpose of a discharge resistor used in conjunction with opening a highly inductive circuit?

Back 69

When a highly inducttive circuit is opened, the self induced emf may be high enough to damage insulation or endanger life. when the discharge resistor is used, the induced emf flows through the resistor and permits the field to collapse gradually, limiting the value of the self induced emf.

Front 70

How is force developed on a conductor?

Back 70

The current flowing in a conductor produces a magnetic field around that conductor. This conductor field distorts the main magnetic field and strengthens the magnetic field on one side of the conductor and weakens it on the other. the lines of force, like an elastic band, will try to shorten. this exerts the force on the conductor.

Front 71

Define starting torque

Back 71

the turning or twisting effort that a motor develops at the instant of starting

Front 72

What is the fundamental motor equation?

Back 72

V = E + IR
V= motor terminal voltage
E= counter emf
I= armature surrent
R= armature ciruit resistance

Front 73

What is the fundamental generator equation?

Back 73

V = E- IR
V= terminal voltage
E= generated emf
I= armature current
R= armature circuit resistance

Front 74

DC motor speed is controlled by what three methods?

Back 74

control of field flux
control of armature circuit resistance
control of terminal voltage

Front 75

Increasing resistance in the field circuit of a DC motor causes:

Back 75

a decrease in field flux and, therefore, an increase in speed

Front 76

Increasing resistance in the series circuit of a DC motor causes:

Back 76

a reduction in the voltage across the motor armature and the motor speed decreases.

Front 77

Varying the voltage applied to the armature circuit of a DC motor will:

Back 77

alter the speed

Front 78

A very small change in the field flux of a DC shunt motor will:

Back 78

bring about a very large change in armature current

Front 79

The speed of a series motor depends almost entirely on the flux because:

Back 79

the stronger the field flux is, the lower the speed will be.

Front 80

The effetive value of any sine-wave current is always:

Back 80

0.707 times the maximum value

Front 81

Both inductance and capacitance affect the flow of an alternating current because:

Back 81

the alternating currents and voltages are continually changing in magnitude and direction

Front 82

Any circuit that is capable of producing flux has:

Back 82

inductance

Front 83

Define capacitance

Back 83

capacitance of a capacitor is the amount of charge that a capcitor receives for each volt of applied potential

Front 84

What does a farad represent?

Back 84

a capacitor has a capacitance of one farad when an applied potential of one volt causes the capacitor to take a charge of one coloumb

Front 85

The capacitance of a capacitor is _______ proportional to the area of the plates and _______ proportional to their separation.

Back 85

directly
inversly

Front 86

If the frequency or the capacitance of a given circuit is _______, the current flow ________.

Back 86

increased
increases

Front 87

When inductive reactance and capacitive reactance are connected in series:

Back 87

their effects neutralize each other and their combined effect is then their difference

Front 88

Define apparent power

Back 88

the product of the effective values of current and voltge in a circuit

Front 89

Define actual power

Back 89

the value obtained when the voltamperes (apparent power) of the circuit are multiplied by the power factor of the circuit

Front 90

What units are used for apparent power?

Back 90

voltamperes (VA) or kilovoltamperes (kVA)

Front 91

Define power factor

Back 91

the ratio of the actual power in watts to the voltamperes of an AC circuit

Front 92

Define reactive power

Back 92

the product of the reactive component of the curent and the voltage

Front 93

How is the negative element of power referred to?

Back 93

reactive power (vars)

Front 94

how is the positive element of power referred to?

Back 94

true power (watts)

Front 95

When parallel resonance is considered, what happens with the inductive and capacitive reactive currents?

Back 95

they cancel each other out since they are equal and opposite in phase

Front 96

The term distribution tranformer is applied to transformers rated:

Back 96

`500kVA and smaller with high voltage ratings of 67000 V and below and low voltage ratings of 15000 V and below

Front 97

the term power transformer is applied to transformers rated:

Back 97

higher than 500kVA
high voltage ratings higher than 67000V
low voltage ratings higher than 15000V

Front 98

What are the two classes of tranformer losses?

Back 98

load losses (I^2R)
no-load losses (core losses - hysteresis, eddy currents in the iron core)

Front 99

Auto-transformers are used when:

Back 99

voltage transformations of near unity are required

Front 100

What two functions do instrument transformers perform?

Back 100

they act as ratio devices (making possible the use of low voltage and low current meters and instruments)
act as insulating devices (to protect the apparatus and the operating personell from high voltages

Front 101

What two kinds of instrument transformers are there?

Back 101

potential transformers
current transformers

Front 102

Potential tranformers are commonly designed to have:

Back 102

a rated voltage of 115 or 120V at the secondary terminals when rated voltage is applied to the primary winding

Front 103

Current transformers are usually designed so that:

Back 103

when rated current flows in the primary, 5A will flow in the secondary

Front 104

A delta-wye connected transformer is well-adapted for:

Back 104

stepping up voltages

Front 105

The wye-delta connected transformer is used for:

Back 105

stepping down voltages

Front 106

Regulation of an AC generator is defined as:

Back 106

the percentage rise in terminal voltage as load is reduced from the rated full load current to zero (the speed and excitation being constant)

Front 107

In an AC generator, the voltage drop due to the ___________ of the winding must also be taken into account

Back 107

inductive reactance

Front 108

The generated emf of an AC generator is equal to:

Back 108

the terminal voltage plus both the IR and IXL drops in the armature winding

Front 109

Before two synchronous generators may be paralleled, the following four conditions must be fulfilled:

Back 109

1. their phase sequence must be the same
2. their terminal voltages must be equal
3. their voltages must be in phase
4. their frequencies must be equal

Front 110

Why is it desireable to operate generators in parallel at the same power factor?

Back 110

to keep circulating current to a minimum

Front 111

What is a rotating magnetic field?

Back 111

three phase motor
owing to the spacing of the windings and the phase difference of the currents in the windings, the flux produced by each phase combines to form a resultant flux that moves around the stator surface at a constant

Front 112

Why is it impossible for an induction motor to run at synchronous speed?

Back 112

the rotor would be standing still with respect to the rotating flux. no emf would be induced in the rotor, no rotor current would flow, and therefore, no torque would be developed.

Front 113

Define slip

Back 113

the difference between rotor speed and syncronous speed

Front 114

Rotor frequency depends on:

Back 114

the slip - the greater the slip, the greater the rotor frequency

Front 115

Rotor frequency is significant because as it varies, the rotor ______ also varies, thus affecting both ____ and _____ characteristics of the motor

Back 115

reactance
starting
running

Front 116

The torque of an induction motor being due to the interaction of the rotor and stator fields is dependant on:

Back 116

the strength of those fields and the phase relations between them

Front 117

The value of slip at which the breakdown torque occurs depends on:

Back 117

the value of rotor resistance, the higher the resistance, the higher the value of slip at breakdown.

Front 118

With respect to a wound-rotor motor, the rotor rheostat provides a means of increasing the rotor-circuit resistance during the starting period, resulting in a:

Back 118

high value of starting torque

Front 119

As a wound-rotor motor accelerates, the rheostat is gradually cut out, thereby lowering the ______ as the _______ decreases.

Back 119

rotor resistance
rotor reactance

Front 120

The effects of operating a motor with external resistance in the rotor circuit on the motor performance are:

Back 120

a decreased efficiency due to the I^2R losses in the rheostat and a lowered value of speed regulation

Front 121

Multispeed squirrel-cage motors are provided with:

Back 121

stator windings that may be reconnected to form different numbers of poles

Front 122

Across-the-line starters are usually equipped with:

Back 122

thermal overload relays
undervoltage protection for the motor

Front 123

What is a primary-resistor starter?

Back 123

A starter that uses resistances connected in series with each stator lead to reduce the voltage during the starting period.

Front 124

When a primary-resistor starter is used, how is the full voltage applied to the motor?

Back 124

at a definite time after the motor is connected to the line through the resistors, accelerating contacts close which short-circuit the starting resistors and apply full voltage to the motor

Front 125

Autotranformer starters are used to:

Back 125

reduce the voltage applied to the motor during the starting period

Front 126

Solid-state motor starters are used to:

Back 126

regulate motor starting current electronically to provide a smooth, stepless motor acceleration

Front 127

What does a SCR do?

Back 127

it blocks the flow of current in one direction, but allows current to flow in the other direction after it has received a "turn on" signal called a gate pulse

Front 128

Two starters that are not strictly reduced-voltage starters but which are used to limit motor starting currents are:

Back 128

part-winding starter
wye-delta starter

Front 129

A part-winding starter is designed to:

Back 129

connect one of two three phase motor windings to the supply when the starting impulse is received. After a time delay, the starter automatically connects the second motor winding to the supply in parallel with the first

Front 130

Wye-delta starting involves:

Back 130

connecting a motor winding first in wye during the starting period and then in delta after the motor has begun to accelerate

Front 131

If, when two syncronous generators are operating in parallel, the prime mover is disconnected from one, what will happen?

Back 131

it will continue to rotate, drawing power from the line to supply its losses.

Front 132

A syncronous motor only develops torque when:

Back 132

it is operating at syncronous speed

Front 133

Most syncronous motors are started by use of:

Back 133

a squirrel cage winding which is embedded in the face of the rotor poles (damper winding)

Front 134

Define pull-in torque

Back 134

the amount of torque that a motor is able to exert when pulling into syncronism

Front 135

Motors may be designed to have a pull-in torque of:

Back 135

up to 150% of ful load torque

Front 136

If the starting period of a syncronous motor is prolonged:

Back 136

the squirrel cage winding may overheat since it is designed for starting duty only.

Front 137

In DC motors and induction motors, an addition of load causes:

Back 137

the motor speed to decrease

Front 138

When the speed of a DC motor or an induction motor decreases, then:

Back 138

the counter emf reduces enough so that additional current is drawn from the source to carry the increased load at a reduced speed

Front 139

When load is added to a syncronous motor, what happens?

Back 139

there is a backward shift of the rotor pole relative to the stator pole

Front 140

Define torque angle

Back 140

the angular displacement between the rotor and stator poles

Front 141

When a syncronous motor operates at no load or with a torque angle of practically 0, the counter emf of the motor is:

Back 141

equal and opposite to the applied terminal voltage (neglecting motor losses)

Front 142

With increasing loads and torque angles (syncronous motor), the phase position of the counter emf:

Back 142

changes with respect to the applied voltage, which allows more stator current to flow to carry the additional load.

Front 143

A syncronous motor is able to supply increasing mechanical loads, not by a decrease in speed but by:

Back 143

a shift in relative positions of the rotor and the rotating magnertic field

Front 144

What happens if too great of a load is placed on a syncronous motor?

Back 144

the rotor is pulled out of syncronism

Front 145

Define pull-out torque

Back 145

the maximum value of torque that a motor can develop without losing its syncronism

Front 146

A syncronous motor may be made to operate over a wide range of power factors by:

Back 146

adjustment of its field excitation

Front 147

What is an advantage of a syncronous motor?

Back 147

the fact that they operate at unity or leading power factor

Front 148

Why is it advantageous that a syncronous motor operates at unity or leading power factor?

Back 148

improves power factor on an electrical system with lagging power fator due to induction motors (or other devices)

Front 149

Define syncronous condenser

Back 149

a syncronous motor operated without mechanical load for the purpose of improving power factor

Front 150

Self-syncronous devices are used to:

Back 150

transmit motion between two points

Front 151

Define 'inductively compensated'

Back 151

when the compensating winding is short circuited on itself

Front 152

Define 'conductively compensated'

Back 152

when the compensating winding is connected in series with the armature and field

Front 153

What is a universal motor?

Back 153

a fractional horespower motor that operates satisfactorily on 50 or 60 Hz systems or on a DC at 115 or 230V

Front 154

A single phase induction motor is not:

Back 154

self starting

Front 155

Define 'cross field'

Back 155

when the rotor field is at right angles to the stator field

Front 156

In a single phase induction motor, because the cross field acts at right angles to the stator field and also lags the stator field by 90° in time phase, the two fields combine to form:

Back 156

a resultant rotating field that revolves at syncronous speed

Front 157

The cross field in a single phase induction motor is produced by generator action and, therefore:

Back 157

is present only when the rotor is turning

Front 158

The torque developed by a single phase induction motor is:

Back 158

irregular or pulsating

Front 159

Why are single phase induction motors set on rubber or spring mounts?

Back 159

to reduce the vibration and noise which are inherent in these motors due to the pulsating torque

Front 160

Define 'phase splitting'

Back 160

two windings connected in parallel to a single phase source with one winding having an impedance connected in series so that the time phase is made to shift by 90°, producing a rotating field like the field of a two phase motor

Front 161

A resistance start motor is also known as a:

Back 161

split-phase motor

Front 162

The high resistance winding of a split phase motor is called the:

Back 162

starting or auxilliary winding

Front 163

The low resistance winding of a split phase motor is called the:

Back 163

main winding

Front 164

The starting winding of a resistance start motor is disconnected when the motor reaches:

Back 164

a predetermined speed, usually 70 to 80 percent of syncronous speed

Front 165

What is a capacitor start motor?

Back 165

a split phase motor that uses a capacitor instead of a resistance in series with the starting winding

Front 166

A capacitor start motor has _________ and _________ than a resistance start motor

Back 166

relatively higher starting torque
significantly lower starting current

Front 167

The starting winding of a capacitor start motor is disconnected when the motor reaches:

Back 167

a predetermined speed, usually 70 to 80 percent of syncronous speed

Front 168

The single phase DC excited syncronous motor requires

Back 168

a DC supply to energize the rotor winding

Front 169

The reluctance type single phase synchronous motor have:

Back 169

modified squirrel cage rotors

Front 170

The rotor of the hysteresis motor is made of:

Back 170

permanent magnet steel

Front 171

Hysteresis motor rotors have no defined poles. Why?

Back 171

when the stator field is energized, eddy currents are induced in the rotor surface which induce poles on the rotor surface

Front 172

The pull out torque of the hysteresis motor is a measure of:

Back 172

the ability of the rotor steel to retain its magnetism

Front 173

The torque of a hysteresis motor is:

Back 173

constant, very smooth and uniform

Front 174

Short circuits are the greatest hazard to:

Back 174

continuity of service

Front 175

Define short circuit

Back 175

an insulation failure in an electric circuit that provides a low resistance path for the current of the circuit

Front 176

Insulation failures cause:

Back 176

abnormally large currents to flow.

Front 177

Circuit protective and switching devices in general have two ratings:

Back 177

a continuous rating
an interrupting rating

Front 178

Define continuous rating

Back 178

the amount of current that a device can carry continuously without overheating

Front 179

Define interrupting rating

Back 179

the ability of a device to interrupt the flow of short circuit current

Front 180

Define 'let through energy'

Back 180

the amount of current that will flow during a short circuit and the time that short circuit current is permitted to flow

Front 181

At each change in circuit capacity on an electric system, there is a:

Back 181

distribution centre at which is grouped circuit protective and switching equipment

Front 182

A fuse contains

Back 182

a current carrying element so sized that the heat created by the flow of normal current is not sufficient to fuse or melt through the fuse

Front 183

What happens to a fuse when short circuit current or overload current is applied to it?

Back 183

the fusible element of the fuse melts and opens the circuit

Front 184

Overcurrent protection is required at every point in a distribution system where

Back 184

wire sizes are reduced

Front 185

What is a GFCI?

Back 185

ground fault current interrupter
a current sensing device that monitors the quantity of current entering and leaving an electrical operated device

Front 186

What is a protective relay?

Back 186

detects the presence of short circuits on a system and are used to connect the breaker trip coils to the source of tripping power

Front 187

What are two types of protective relays?

Back 187

overcurrent
differential

Front 188

Overcurrent relays operate to:

Back 188

close their contacts when current through the relay operating coil exceeds a predetermined value

Front 189

What are differential relays used for:

Back 189

protection of rotating machines, transformers and buses

Front 190

How do differential relays work?

Back 190

they compare the currents entering and leaving each phase winding of the apparatus being protected

Front 191

What reduces armature reaction and improves commutation?

Back 191

interpoles (compensating windings)