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Electrical Level 3 and 4 > AC Machines > Flashcards

AC Machines Flashcards

1
Q

List the three main types of three-phase AC motors

A
  1. Squirrel cage induction motor
  2. Wound-rotor induction motor
  3. The synchronous motor
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2
Q

What are the main constructional parts of a three-phase induction motor?

A

Stator
Rotor
End bells
Bearings

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3
Q

How are the two circuits of an induction motor similar to those of a transformer?

A 
Stator = primary winding
Rotor = secondary winding
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4
Q

What is meant by the term “synchronous speed”?

A

The speed at which the magnetic field rotates produced by the stator winding (poles) moves.

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5
Q

What three basic factors allow the development of a rotating magnetic field in a three phase induction motor?

A
  1. The fact that the voltages in a 3 phase system are 120 degrees out of phase w/ each other
  2. The fact that the three voltages change polarity at regular intervals
  3. The arrangements of the stator windings around the inside of the motor
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6
Q

What factors determine the synchronous speed of an induction motor?

A

Speed in RPM
Frequency in Hertz
Number of stator poles per phase

S = 120 x F (frequency)
———–
P (stator poles)

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7
Q

What is meant by the term “slip”?

A

Difference in RPM between the rotor speed and synchronous speed of the stator

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8
Q

Why cant the rotor of an induction motor turn at sync speed?

A

There will be no cutting action (relative motion) of stator flux on rotor bars to induce a rotor current necessary for torque

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9
Q

How would you reverse the direction of rotation for a three-phase induction motor?

A

Interchange any 2 line leads to the stator

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10
Q

What is the purpose of “skewing” rotor bars in induction motors?

A

To provide for a more equal distance of rotor flux allowing for smoother torque development and quieter operation.

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11
Q

Why is the starting current of an induction motor so much higher than its rated running current?

A

100% slip = maximum cutting rate of stator flux on rotor bars (no CEMF)

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12
Q

State three fundamental reasons that cause the development of a rotating magnetic field within a three-phase induction motor.

A
  1. Physical arrangement of coil groups
  2. Each coil group energized with AC
  3. Each phase current time displaced by 120 electrical degrees
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13
Q

List the three main factors that determine the torque developed by an induction motor.

A
  1. Stator Flux
  2. Rotor current (flux)
  3. Rotor power factor
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14
Q

What effect does slip have on rotor reactance?

A

Rotor frequency is proportionate to % slip: Fr = Fs x %slip
Rotor reactance is proportionate to frequency: XL = 2(pie)fL

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15
Q

What is meant by the term “speed regulation”?

A

Difference between no load speed and full load speed of the rotor.

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16
Q

What constitutes the “power losses” for induction motors?

A 
Copper losses (I2R)
Iron losses (eddy current and hysteresis)
Rotational losses (friction, windage)
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17
Q

Why would increasing the rotor circuit resistance of an induction motor increase its starting torque?

A

Rotor PF is improved

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18
Q

List one advantage and one disadvantage of starting a large induction motor with “reduced voltage” across the stator windings.

A

Advantage: Reduced in rush current will cut load shock and system disturbances (lowers O/C setting)
Disadvantage: Reduced starting torque, longer accelerating times, more elaborate control and higher cost.

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19
Q

For a given value of slip, the T output of an induction motor varies as the square of the aplied voltage, explain.

A

Change in applied voltage to the stator results in the change of stator flux; this also change rotor current.
The combo results in the T being proportionate to the square of the applied voltage.

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20
Q

Explain starting torque

A

Rotational force developed at the instant of starting

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21
Q

Explain full-load torque

A

Rotational force required to keep rated load (HP) turning

Developed at about 2% - 5% slip for most SCIMs

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22
Q

Explain breakdown (pull out) torque

A

Amount of overload torque which will stall the motor

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23
Q

What are the objections to building a squirrel-cage induction motor with high enough rotor R to provide an excellent starting torque?

A

Poor speed regulation
Higher I2R losses
Poorer efficiency

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24
Q

What advantages do a squirrel-cage induction motor have over other types of induction motors?

A 
Relatively constant speed between no load and full load
Initial purchase cost
Ruggedness
Low maintenance 
High dependability
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25
Q

Explain the constructional differences between a “double” squirrel cage and a “standard” squirrel cage motor?

A

Two sets of rotor bars (high and low R) embedded within the rotor iron for a “double”

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26
Q

How is the operating characteristics between the “double” and “standard” squirrel cage motors different?

A

Double typically has higher starting T but slightly poorer speed regulation

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27
Q

What is an advantage of the “double” squirrel cage motor compared to other types?

A

Relatively good starting T

Good speed regulation

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28
Q

On what type of driven load would you specify a high R type SCI motor?

A

Higher torque loads (compressors, conveyors)

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29
Q

If a three phase motor is running at rated load, and one line opens, what happens?

A

Typically it will remain running at a decreased speed but will have excessive current and usually a humming noise.

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30
Q

What is it called when a three phase motor has one of its line open?

A

Single phasing

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31
Q

Equation for sync speed (rpm) ?

A

N = f x 60 f x 120
———— or —————
pairs of poles poles

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32
Q

Rotor frequency equation?

A

fr = supply frequency x % slip

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33
Q

Rotor frequency is dependent upon _____ ?

A

Slip

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34
Q

At standstill for a SCIM, the rotating field cuts the rotor bars at ______ which results in the induced voltage and current being ________.

A

Full frequency

At a maximum

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35
Q

At standstill for a SCIM, the rotating field cuts the rotor bars at ______ which results in the induced voltage and current being ________.

A

Full frequency

At a maximum

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36
Q

At standstill for a SCIM, the rotor PF is ______ ?

A

Low

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37
Q

At standstill for a SCIM, induced voltage is _______, rotor impedance is ______, and rotor reactance will be _______.?

A

Highest
Highest
Maximum

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38
Q

Torque is produced by the interaction of ______ and _______.?

A

Stator flux and rotor flux

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39
Q

Rotor flux is proportionate to rotor _______ and therefore torque is a function of rotor _____ and ______.?

A

Current

Current and Frequency

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40
Q

As the rotor on a SCIM accelerates, the rotating magnetic field will cut _____ of the rotor bars and ____ current will flow in the rotor and the stator.

A

Less

Less

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41
Q

As the SCIM rotor accelerates the magnetic field will cut _____ of the rotor bars and _____ current will flow in the rotor.

A

Less

Less

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42
Q

Breakdown torque varies with motor (rotor design) but typically is?

A

200% - 300% of full load torque.

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43
Q

Torque is maximized when?

A

Rotor R = Rotor XL

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44
Q

What does rotor design affect?

A

In rush current
Starting T
Speed regulation
Full load T

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45
Q

Torque characteristics of a low R rotor?

A

Poor starting torque

Better running torque

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46
Q

Torque characteristics of a high R rotor?

A

Good starting torque

Low running torque

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47
Q

For a 3 phase motor to be connected to high voltage, how must the phase windings be connected?

A

Wye or Delta and in series

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48
Q

For a 3 phase motor to be connected for low voltage, how must the phase windings be connected?

A

Either wye or delta and in parallel

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49
Q

How many leads are brought out if the motor is intended to be used for wye-delta starting and how must it be connected?

A

12 leads (also windings must be connected in delta)

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50
Q

When a motor is connected for high voltage, why is the current flow be half as much when compared to low voltage operation?

A

When windings are connected in series for high voltage, the impedance is 4 times greater

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51
Q

The amount of induced voltage for a SCIM is determined by?

A
  1. Strength of the magnetic field of stator
  2. Number of turns of wire cut by the magnetic field
  3. Speed of cutting action
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52
Q

The amount of T produced by an AC induction motor is determined by?

A
  1. Strength of the magnetic field of stator
  2. Strength of magnetic field of the rotor
  3. Phase angle difference between rotor and stator fields
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53
Q

When the SCIM speed decreases, the rotating magnetic field cuts the rotor bars at a _____ rate?

A

Faster

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54
Q

The speed performance of an induction motor is measured in?

A

Percent slip = sync speed - rotor speed
————————————– x 100%
sync speed

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55
Q

What is a common rotor slip for a SCIM?

A

2% - 5%

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56
Q

What determines the operating characteristics of a motor?

A

The type of rotor bars used in the construction of the rotor

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57
Q

The code letter on the nameplate of an AC SCIM indicates what?

A

The type of bars used in the rotor

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58
Q

A PF of about _____ is common for a SCIM at no load.

A

10%

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59
Q

The PF of an induction motor at full load is about?

A

85 - 90% lagging

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60
Q

When does single phasing occur?

A

When one line of a 3 phase system is open

61
Q

For a double SCIM, the outer SC winding has ?

A

High R bars to provide high starting T and low starting current

62
Q

For a double SCIM, the inner SC bars have?

A

Low R and a higher inductance (XL)

63
Q

NEMA rated motors are designed to operate at?

A

+/- 10% of their rated voltage

64
Q

What are the 3 main parts of a SCIM?

A

Stator
Frame
Laminated Core

65
Q

What is the purpose of the core on a SCIM?

A

Supports the stator coils

Provides a low reluctance path for the magnetic circuit

66
Q

Maximum T is produced when the flux and rotor current are?

A

In phase

67
Q

What is the most common motor design?

A

Design B

68
Q

Characteristics of a Design D rotor

A 
High R rotor design
Small rotor bars
High starting T
Low starting current 
Poor speed regulation
69
Q

Characteristics of a Design B rotor

A 
High reactance rotor design
Deep bars 
Low R
Relatively low starting current
Fair starting torque 
Better speed regulation
70
Q

Characteristics of a Design A rotor

A 
Standard rotor design
Bars are a compromise btwn small/deep
Relatively low R
Low inductive reactance
High starting current
Fair starting T
Best speed regulation
71
Q

Characteristics of a Design C rotor

A

Double squirrel cage rotor
Combo of small bars for high R and deep bars for low R
High starting T
Good speed regulation
Has the deep low R bars for high XL for running

72
Q

A linear induction motor moves in a _____?

A

Line

73
Q

A linear induction motor consists of a ?

A

Flat stator and a flat rotor

74
Q

The rotor for a linear motor is normally made out of ?

A

Conductive material, normally aluminum (inexpensive and light weight)

75
Q

Describe the difference between the conventional motor and the linear-induction motor.

A

Conventional motor, the stator is stationary and the rotor moves
Linear, the stator moves and the rotor is stationary

76
Q

How is the sync speed of a linear induction motor measured?

A

Meters per second

77
Q

What is the sync speed equation for a linear-induction motor?

A

Sync speed = 2 x pole pitch x frequency

78
Q

What are the main constructional differences btwn a “SCIM” and a “WRIM”?

A

Rotor construction: 3 separate insulated windings 120 electrical degrees displaced
Usually Y connected internally with ends brought out to slip rings for external brush connection to resistor grids.

79
Q

What 2 major advantages does a “WRIM” have that are not available with a “SCIM”

A

High starting torque
Low starting current with R in rotor circuit
May be operated at variable speed (torque)
Smooth acceleration under heavy loads

80
Q

How should a WRIM be started?

A

With full resistance in rotor circuit and rated voltage across the stator

81
Q

Would a WRIM have a higher operating eff. at 80% slip or 5% slip?

A

5% slip, lower rotor currents therefore smaller I2R losses

82
Q

List 2 disadvantages of a WRIM

A

High initial cost
More maintenance
More elaborate control

83
Q

List three examples of where a WRIM may be used

A

Elevators
Cranes
Compressors

84
Q

How many slip rings are on a WRIM?

A

3

85
Q

How is the starting current in a large WRIM limited to a safe and reasonable value?

A

External resistors during starting

86
Q

What constructional features make a 3 phase Sync motor different from a SCIM?

A

Rotor construction: Salient DC field poles, 2 slip rings, brushes, ammortisseur winding

87
Q

What factors determine the rotor speed of a 3 phase sync motor?

A

Number of stator poles

Frequency of the applied voltage

88
Q

How is a 3 phase sync motor most commonly started?

A

By means of ammortisseur windings (cage rotor)

89
Q

What TWO important charac. are possessed only by the Sync motor and not by an other 3 phase motor?

A 
0% speed regulation (constant speed)
Variable PF (lead, lag or unity)
90
Q

For a constant mech output (load), how can the PF at which a sync motor operates change?

A

By varying the DC field excitation (increased: leadings vars, decreased: lagging vars)

91
Q

What is a sync condenser (sync capacitor/rotating caps) ?

A

Sync machine used solely for PF correction

92
Q

What is meant by the torque angle of a 3 phase sync motor?

A

The angular displacement between the centers of the rotor poles and the rotating stator poles.

93
Q

Why will a sync motor not operate properly if the field circuit is energized before the rotor speed approaches near sync speed?

A

Torque angle is too large to allow the rotor to pull in to synchronism

94
Q

Define pull-in torque

A

Rotational force required to pull the rotor into synchronism

95
Q

Define pull-out torque

A

Force required to pull rotor out of synchronism

96
Q

What is the purpose of the SCIM type winding on the face of the salient poles?

A

to allow the motor to be self starting (amortisseur winding)

97
Q

At what PF does a sync motor draw the least stator current for a given load?

A

Unity PF

98
Q

A “bump” when a sync motor pulls into synchronism would be caused by?

A

Rotor speed (torque angle) is too low at point of sync

99
Q

What would be considered as an acceptable percent slip on a sync motor at the moment of field application?

A

3 - 5 % slip

100
Q

For a sync motor at constant mech load, what is an indicator for unity PF?

A

Minimum line current

101
Q

Define under excited

A

Decreased field current (lagging PF)

102
Q

Define over excited

A

Increased field current (leading PF)

103
Q

What is the purpose of a “field discharge resistor” used with a 3 phase sync motor?

A

To reduce the level of induced voltage during starting and dissipate heat energy

104
Q

What type of 3 phase induction motor has the best speed regulation?

A

Sync motor

105
Q

What type of 3 phase induction motor has the best PF?

A

Sync motor

106
Q

What type of 3 phase induction motor has the best starting T?

A

WRIM

107
Q

What type of 3 phase induction motor has the lowest initial cost and maintenance cost?

A

SCIM

108
Q

What creates the CEMF in a 3 phase sync motor?

A

The magnetic field produced by magnetizing current called excitation current

109
Q

The rotor circuit in a WRIM is often referred to as what?

A

Secondary circuit of the motor

110
Q

What would happen to the rotor speed of a SCIM if the frequency of the applied voltage was increased?

A

Speed increases

111
Q

What happens to the rotor of a WRIM when the load is increased?

A

Increases

112
Q

A 3 phase sync motor may be used wherever a ______ motor is required?

A

NEMA Class B

113
Q

A Sync motor is particularly well-suited to loads that operate ?

A

Below speeds of 500 rpm

114
Q

Sync motors require a ?

A

Starting aid

115
Q

Two ways to start a sync motor?

A

Amortisseur windings embedded in the rotors pole face

WRIM called a “pony motor” for high inertia loads

116
Q

What are the 3 common methods of supplying the sync motor with DC field current?

A

External converters
Built-in exciters
Brushless exciters

117
Q

Typical angles at which pull-out T occurs?

A

May be as high as 60-70 degrees

118
Q

How do you locate a shorted stator coil?

A

Growler

119
Q

How do you locate a grounded stator coil?

A

Smoking at the point of grounding

120
Q

How do you locate an open rotor bar?

A

Variations in the ammeter

121
Q

Another name for a sync shaded-pole motor?

A

Hysteresis Motor

122
Q

List the essential parts of a split phase induction motor?

A 
Stator
Rotor
Centrifugal switch
End bells/bearings
Nameplate
123
Q

What are the differences in the start and run windings on a single phase motor?

A 
Start = High R, smaller wire, more turns, top of stator slots
Run = Low R, larger wire, less turns, bottom of stator slots
124
Q

What would happen if the centrifugal switch on the single phase motor fails to close when the motor stops?

A

Will not start

125
Q

At what approx. percentage of full-load speed does the centrifugal switch for a phase motor open?

A

`75%

126
Q

How is the direction of rotation reversed on a split phase induction motor?

A

Interchange leads of either start or stop windings, typically start windings T5 and T8

127
Q

What is the main function of the capacitor in the capacitor start induction motor?

A

To improve the starting T

128
Q

List two methods of controlling the speed on a single phase universal motor?

A

Series resistor

Electronic control

129
Q

How is the direction of rotation changed on a universal motor?

A

Interchange armature leads

130
Q

The direction of flux on the stator for a shaded pole motor is from?

A

Main pole face to the shaded pole face

131
Q

Name 2 applications of shaded pole motors?

A

Bathroom/Hood fans

Small appliances

132
Q

Why is the starting T of a capacitor start single phase motor better than that of a resistance start motor?

A

Greater phase displacement between start and run winding currents, resulting in smoother rotating field

133
Q

What is another name commonly applied to the AC series motor?

A

Universal motor

134
Q

What are some common applications of a universal motor?

A

drills, vacuum cleaners, reciprocating saws and sanders

135
Q

What are the 2 general classes of alternators?

A

Rotating field type

Rotating armature type

136
Q

Which alternator is used more commonly?

A

Rotating field type

137
Q

What type of alternator rotor is commonly used for high speed machines?

A

Cylindrical (non-salient) type

138
Q

What type of alt. rotor is commonly used for low speed machines?

A

Salient pole rotors

139
Q

When paralleling alternators, what indicates wrong phase displacement and how is it corrected?

A

Lights flashing at different times

Interchange 2 leads of the incoming alternator or reverse prime mover

140
Q

When paralleling alternators, what indicates wrong frequency and how is it corrected?

A

Lights flashing

Changing speed of prime mover (varying DC field of DC motor)

141
Q

When paralleling alternators, what indicates wrong voltage and how is it corrected?

A 
Lights on (dim to bright), voltage across switch
Adjust field excitation of 3 phase alternator
142
Q

When paralleling alternators, what indicates wrong phase displacement and how is it corrected?

A

Lights on or some voltage across the switch

“tweak” the excitation of prime mover to change the phase displacement

143
Q

What is “hunting” when paralleling alternators and how is it reduced?

A

Surging/Oscillating of power between 2 alternators, not desired.
“damping” the prime mover governor controls, heavy fly wheels or amortisseur windings.

144
Q

Lamps have to be rated for _____ phase voltage.

A

Double

145
Q

When paralleling alts., a change in field excitation causes a change in?

A

Ampere load, KVARS of alt.

146
Q

When paralleling alts., a change in the prime mover governor controls causes a change in?

A

Kilo watts btwn sync gens.

147
Q

Operation of paralleling alts., a lagging F causes terminal voltage to ?

A

Drop on alts.

148
Q

Operation of paralleling alts., leading PF causes Vt to?

A

Go up on alts.

149
Q

When paralleling alts., the prime mover must have relatively ___________?

A

Similar speed-load characteristics

Electrical Level 3 and 4 (10 decks)

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