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Final > Three Phase > Flashcards

Three Phase Flashcards

1
Q

To reverse the rotation direction of the magnetic field

A

Just reverse any two leads of a three phase motor

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2
Q

A four pole three phase motor has how many coils

A

12

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3
Q

The torque on the rotor depends on how fast the rotor drifts…

A

Backwards through the rotating magnetic field

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4
Q

The speed at which the magnetic field rotates is called the

A

Synchronous speed of the motor

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5
Q

On starting, the slip of the motor is at …

A

100%

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6
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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7
Q

Maximum torque is always developed when the phase angle is at

A

45 degrees

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8
Q

The rotor of a squirrel cage induction motor has four basic parts…

A

Core, conductors, end rings, shaft

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9
Q

Low starting torque is a result of the low resistance and high…

A

Reactance of the rotor

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10
Q

The wound rotor motor is always started with blank blank in the rotor circuit

A

Maximum resistance

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11
Q

The life of a motor is determined largely by the

A

Life of its insulation

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12
Q

A synchronous motor requires two sources of electric power…

A

One ac and one D.C.

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13
Q

When the D.C. Power is applied to the rotor, a set of what is produced

A

North and south poles

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14
Q

With a low D.C. Field current the ac current in the armature blanks the applied ac potential difference

A

Lags

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15
Q

Slipping a pole causes a blank of current in the D.C. Line during the blank period

A

Surge, slipping

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16
Q

Increasing the D.C. Current in the rotor winding increases the phase angle in the

A

Lagging direction thus reducing the power factor

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17
Q

A motor with a single winding can be connected for either…

A

Four pole or eight pole operation

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18
Q

Variable torque motors do what

A

Designed to give an increase in torque with an increase in speed

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19
Q

In a constant horsepower motor the torque…

A

Doubles when the speed is cut in half so the power delivered by the motor remains unchanged

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20
Q

A two parallel Y connection requires a blank potential difference than a series Y connection

A

Lower

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21
Q

Avoid using too much solvent because it can

A

Flow into places too difficult to remove, and if left too long could damage insulation

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22
Q

The standard temp for measuring insulation resistance is

A

40 degrees C, 104 degrees F

23
Q

The first symptom of an open rotor circuit is a lack of

A

Torque, the speed decreases and the motor makes a growling noise

24
Q

Angular loads are a combination of

A

Radial and thrust loads

25
Q

For the average service conditions bearings may be safely operated at temperatures up to…

A

95 degrees C, 200 degrees F

26
Q

The capacity of the power supply limits the size of a blank that may be started across the line

A

Motor

27
Q

At full load a motor will develop its rated power, draw its rated current, and run with its rated power factor, but with that there will be a

A

Rise in the motors internal temperature

28
Q

The max torque a motor can deliver is usually

A

150 to 225% of the rated full load torque

Ex. Rated at 2HP full load can make 3 to 4.5 HP

29
Q

Connecting an inductor in series with the motor windings is another method of

A

Reducing the starting current

30
Q

The inductor adds inductive reactance to the circuit which is the basis for the name

A

Reactor starter

31
Q

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

A

Maintain a regular and systematic plan of inspections

32
Q

When the starter contacts drop out when the potential difference across the line falls too low, is called…

A

Low voltage release

33
Q

During plugging the current in an induction motor is higher than the locked rotor current. Blank blank is about three times the loss during normal starting

A

Power loss

34
Q

What provides both backspin protection and surge protection when the motor stops

A

Off delay relay

35
Q

What provides surge protection when the motor starts

A

On delay relay

36
Q

The blank provide a reduced potential difference across the motor windings for starting

A

Autotransformers

37
Q

Which method of starting produces the smoothest starting conditions of any of the methods of starting

A

Reactance starting

38
Q

When a large number of steps is required to start what must be used

A

Resistance starting

39
Q

What kind of winding can be a part of either the rotor or the stator in the alternator

A

Supply winding

40
Q

The armature of an alternator always has the blank number of poles as the blank winding

A

Same, field

41
Q

The current an alternator can conduct without overheating is determined by the size of the

A

Conductors in the winding

42
Q

If the load is inductive the current blanks the voltage

A

Lags

43
Q

If the armature of an alternator has four poles, how many poles does the field winding have

A

Four

44
Q

Interlocks can be used in motor starting circuits so the motors can be started in a sertain sequence, the largest motor is started…

A

First

45
Q

Power failure relays act if the potential difference across any phase drops below blank or less of the rated value

A

70%

46
Q

Whenever an auxiliary generator starts it should run until when

A

All parts reach their normal operating temperature

47
Q

When only one cranking period is provided it duration should be how long

A

At least 15 seconds

48
Q

If more than one cranking period is provided each period should last how long

A

At least 7 seconds

49
Q

When making a schedule for inspecting generators match the schedule to the maintenance schedule for the

A

Prime mover

50
Q

The potential difference at which the electric systems of the utility company and the plant are connected

A

Service voltage

51
Q

The major benefits of system grounding are

A

Longer insulation life, improvement in protective relaying, extra protection

52
Q

A relay that operates faster as the current increases is said to have…

A

Inverse time characteristics

53
Q

In a cascade system of large air gap circuit breakers the number of cascades steps should not exceed…

A

Two

54
Q

In general three phase motors will give satisfactory service when operated at blank or more of their rated potential difference

A

90%

Final (12 decks)

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