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2nd 6 Month Final > TPC 208 Three Phase Systems > Flashcards

TPC 208 Three Phase Systems Flashcards

1
Q

A four-pole, three-phase motor has _____ coils.

A

12

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2
Q

The speed at which the magnetic field of an inducton motor rotates is called the:

A

Synchronous speed

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3
Q

On starting, the slip of the induction motor is _____% of the synchronous speed.

A

100

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4
Q

In general, the power factor of an induction motor increases with the load until the maximum power factor is reached at:

A

Full load

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5
Q

In an induction motor, maximum torque is always developed when the

A

Reactance and the resistance of the rotor are equal

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6
Q

Specifically , the core, the conductors the end rings, and the shaft are the main parts of the squirrel-cage

A

Rotor

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7
Q

When taking air-gap measurements on large motors, you should

A

ALL OF THE ABOVE

  1. keep a record of the measurements and compare them regularly
  2. Take measurements, about 90% apart
  3. take one measurement in the direction in which the load bears on the bearing
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8
Q

In a squirrel-cage motor, low starting torque is a result of

A

Low resistance and high reactance

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9
Q

A wound-rotor motor is always started with _____ in the rotor circuit.

A

Maximum resistance

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10
Q

To improve a motors service life, you should do all of the following EXCEPT

A

Remove the winding insulation

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11
Q

A synchronous motor requires:

A

Both AC and DC power

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12
Q

When dc power is applied to the rotor of a synchronous motor…

A

A set of north and south poles is produced

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13
Q

With a low dc field current, the ac current:

A

Lags the potential difference

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14
Q

Slipping a pole causes a(n)

A

Surge of current in the DC line

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15
Q

The power factor of a synchronous motor will be less than 100% if the DC current in the rotor winding is….

A

Either too high or too low

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16
Q

A motor with a single winding can be connected for

A

either four-pole or eight-pole operation

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17
Q

Variable torque motors are designed to give

A

an increase in torque with an increase in speed

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18
Q

In a constant horsepower motor, the torque

A

Doubles when the speed is cut in half

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19
Q

When you reconnect the coils of an induction motor designed to run on a potential difference of 240V so that it will run on 480V

A

You reconnect the coils in series

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20
Q

A two-parallel-Y connection:

A

requires a lower potential difference than a series-Y connection

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21
Q

When removing grease and oil from a motor, you should avoid using too much solvent because it

A

Can damage the insulation

22
Q

40° C is the

A

standard temperature for measuring insulation resistance

23
Q

A lack of torque, a decrease in speed, and growling noises are all indications of a(n)

A

Open rotor circuit

24
Q

Angular loads are caused by forces applied to the bearing

A

Both parallel and perpendicular to the shaft

25
Q

200°F (95°C) is the

A

maximum temperature at which bearings may be safely operated

26
Q

The capaciy of the power supply does NOT limit the

A

Normal running speed of the motor

27
Q

At full load a motor connected to the line will do all of the following EXCEPT

A

Decrease its internal temperature

28
Q

A motor rated at 2 hp at full load can develop _____ hp if it delivers at maximum torque.

A

3 to 4.5

29
Q

An inductor connected in series with the motor winding is found in a(n)

A

Reactor starter

30
Q

To ensure that three phase motor control equipment is in good operating condition, you should…

A

Maintain a regular plan of inspections

31
Q

Which of the following stops a motor when the potential difference falls too low and automatically restarts the motor when the potential difference returns to normal?

A

Low-voltage release

32
Q

During plugging, the current in an induction motor is higher than the
locked-rotor current causing a(n)

A

Power loss

33
Q

A controller that provides both surge and backspin protection for a motor that drives a pump has a(n)

A

on-delay relay plus an off-delay relay

34
Q

Reduced voltage starters for three phase motors change the potential difference across the motor by means of:

A

Autotransformers or resistors

35
Q

When smooth starting is necessary but more than two steps are required, _______ is used.

A

Resistance starting

36
Q

The supply winding in an alternator

A

can be part of either the rotor or the stator

37
Q

If the field winding has 4 poles, how many poles does the armature of the alternator have?

A

4

38
Q

There is no current in the armature of an alternator when the

A

Regulator is disconnected

39
Q

The size of the conductors in the winding determines the

A

current an alternator can conduct without overheating

40
Q

If the load driven by an alternator is purely inductive, the current:

A

Lags the potential difference

41
Q

When interlocks are used in motor-starting circuits to overcome voltage drop problems, which motor in the circuit is started first?

A

The largest

42
Q

In most automatic installations, power failure relays act if the potential difference drops to _______ of the rated value

A

70% or less

43
Q

Whenever an engine starts, it should run until all parts reach:

A

Their normal operating temperatures

44
Q

In order to avoid damaging an engine when only one cranking period is provided, the cranking attempt should last at least ______ seconds.

A

15

45
Q

In establishing for inspecting generators you should match the schedule to the maintenance schedule of the:

A

Prime mover

46
Q

The potential difference where the electric system of the utility company and the plant are connected is called the _____ voltage

A

Service

47
Q

All of the following are benefits of system grounding, EXCEPT

A

allows service to be maintained in entire circuit until the fault is located

48
Q

A relay with inverse-time characteristics operates:

A

Faster as the current increases

49
Q

In a cascade system having large air-gap circuit-breakers, there should not be more than ______ cascade step(s)

A

2

50
Q

In general, three phase motors will give satisfactory service when operated at what percent of their rated potential difference ?

A

90%

2nd 6 Month Final (10 decks)

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