Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

2nd 2020 > Three Phase Motors > Flashcards

Three Phase Motors Flashcards

1
Q

A four pole three phase motor has 12 coils.

A

True

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Formula for synchronous speed.

A

S= 120f/P.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

The speed at which a magnetic field rotates is called?

A

Synchronous speed . The difference between the rotors speed and the stairs magnetic field is called the slip speed. They spin at the same time there would be no flux lines cut

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the slip when the motor first starts?

A

The slip is 100% because the rotor is at a standstill.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

How does an induction motor reach maximum power factor?

A

In general, the power factor increases with the load until the maximum power factor is reached at full load. If load is increased further there is an increase in slip and decreases PF rather than increasing current.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Torque of an induction motor depends on what?

A

The strength of the magnetic field
The current in the rotor conductors
The phase angle between the magnetic field and the rotor current.
Maximum torque is produced when the phase angle is 45* therefore the power factor of the rotor is cosin 45*. This occurs whenever the rotor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What are the main parts of a squirrel cage rotor?

A

Core
End rings
Conductors
Shaft

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Why should records of air gap measurements be kept

A

Keeping record of these numbers periodically enables you to detect bearing wear before it causes serious damage. Measurements should be taken 90ﾟ apart.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Describe torque in a standard squirrel cage motor.

A

It has low starting torque as a result of the low resistance and high reactance of the rotor. Current lags voltage by a large amount resulting in a low power factor.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

How is a wound rotor motor started in regards to the rotor?

A

It is always started with maximum resistance in the rotor circuit. The high resistance matches the initial reactance, so that the starting torque is as high as possible.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What are some important factors in the life of a motor?

A

Life of a motor is determined largely by the life of its insulation. Proper care of the insulation can greatly improve a motors service life. Insulation must be kept clean and dry providing an adequate supply of cool dry air for ventilation and running the motor within its limits.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How many sources of power does a synchronous motor require?

A

It requires AC for the motor, and DC for the excitation.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What happens when DC power is applied to the rotor of a synchronous motor?

A

A set of North and South Poles is produced which locks into position the rotor with the Rotating magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

How does a synchronous motor with a low DC field current act?

A

The AC current in the armature lags the applied ac potential difference. The motor has the same effect as an inductor in the AC circuit.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What are the effects of slipping a pole?

A

Slipping a pole causes a surge of current in the DC line. The surge causes the magnetic field of the rotor to oppose the magnetic field of the stator. The stator then draws a large current to counteract the change in the rotor field.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Described power factor of a synchronous motor.

A

Power factor depends on the load and on the field current. Any increase in the load causes the phase angle to increase in the lagging direction which reduces power factor. Increasing the DC current in the rotor winding increases the strength of the rotors magnetic field. It increases the phase angle In the leading direction.

Decreasing the current in the rotor winding decreases the strength of the roaders magnetic field.The weaker field causes the AC current to lag the ac potential difference.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Describe a consequent Pole motor that has a single winding.

A

It’s can be connected for either 4 Pole or 8 pole operation if the line frequency is 60 Hz These motors would have synchronous speeds of 1800 and 900 RPM

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Describe variable torque motors?

A

They are designed to give an increase in torque with an increase in speed.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Describe a constant horsepower motor.

A

Torque varies inversely with the speed. A low speed connection allows more current to flow in the coils, thereby increasing the torque. The Torque doubles when the speed is cut in half.

20
Q

How to make a voltage change on a dual voltage motor?

A

You must 1st determine whether or not the insulation of the winding is sufficient for the voltage. Then you must determine whether the number of coils in series in the winding is sufficient so there is the same potential difference across each coil when the motor operates.

21
Q

What is the standard temperature for measuring insulation resistance?

A

It is measured at 40ﾟC or 104ﾟF.

22
Q

What are symptoms of a wound rotor motor not functioning correctly?

A

They have 3 phase windings, They will not run properly unless all are working correctly. The 1st symptom of open rotor circuit is a lack of torque. The speed decreases, and the motor makes a growling noise. The motor may fail to start under load

23
Q

What are angular loads?

A

Angular loads are a combination of radial and thrust loans

24
Q

What is the operating temperatures for bearings?

A

They may be safely operated at up to 200ﾟF

25
Q

What does the capacity of the power supply limit?

A

It determines the size of a motor that may be started across the line, as well as the type and amount of other equipment on the line.

26
Q

Describe a motor at full load.

A

It will develop its rated power, draw it’s rated full load current, and run with its rated power factor.there will be a specific rise in the motors temp.

27
Q

What is the maximum torque most motors generate?

A

It is usually 150%-225% of the full load torque.

28
Q

What are common methods of reducing starting currents?

A

They are referred to as reduced-voltage starters and they decrease the starting force the motor exerts on components of the load.
Autotransformer
Resistor
Reactor
Y delta transformer
Can be made for manual or auto operation.

29
Q

What happens when you connect an inductor in series with motor winding?

A

It will reduce starting current. It adds inductive reactance to the circuit, which is the basis for the term reactor starter.

30
Q

What is low voltage release (no voltage release)?

A

The starter contacts (holding coil/contact) drop out when the potential difference across the line falls too low. It will trip an OL relay tripping the unit. In a two wire automatic system there is no need to reset the circuit by pressing stop.

31
Q

What is plugging?

A

It involves reversing the rotation of the stator magnetic field to slow the rotor. The current is higher at this time and the loss is 3x during normal operation.

32
Q

Describe a control circuit with both surge and backspin protection.

A

With the use of on and off timing relays. The on delay relay provides surge protection when the motor starts. The off delay relay provides both back spin protection and surge protection when the motor stops.

33
Q

When should reactance starting not be used?

A

Although it produces the smoothest starting, when the amount of steps in starting is greater than two, resistance starting should be used.

34
Q

The armature winding in an alternator is defined as?

A

Winding that supplies the potential difference for the load. This supply winding winding can be part of either the rotor or the stator in the alternator.

35
Q

The armature of an alternator always has the same number of poles as the field windings.

A

True

36
Q

What effect does current have in the armature?

A

There is current in the armature of an alternator when the field is excited, the rotor is turning, and the armature is connected to a load. The magnitude of this current depends on the potential difference produced in the armature and on the impedance of the alternator and the load. The power factor is determined almost entirely by the load.

37
Q

How do you determine how much current in alternator can conduct without overheating?

A

It is determined by the size of the conductors in the winding. If the current exceeds the rated value, the power consumed in the winding may damage the alternator.

38
Q

What are the 3 types of loads in alternator can drive?

A

The load maybe resistive, inductive, or capacitive. If the load is resistive the current is in phase with voltage. If the load is inductive the current lags the voltage. The lag causes the armature reaction to reduce the strength of the magnetic field, decreasing voltage at the terminals.

39
Q

At what percentage will power failure release act on the moto

A

Power failure relays act if the voltage across any phase drops to 70% or less of the rated value. Automatic transfer back to the normal source occurs when the potential difference across all phases reaches 90% or more of rated value

40
Q

How long should an auxiliary generator run once started?

A

It should run until all parts reache their normal operating temperature

41
Q

Crank control and time delay relays provide at least one cranking period. When only one cranking period is provided, its duration should be at least 15 seconds.

A

True

42
Q

When establishing a schedule for inspecting generators, match the schedule of the maintenance of the prime mover around the same time.

A

True

43
Q

What is service voltage?

A

It is the potential difference at which the electric system of the utility company and the plant are connected.

44
Q

What are the major benefits of system grounding?

A

Benefits include longer insulation life for motors, Transformers, and other components. Improvement in protective relaying, because ground faults result in better fault current control. Extra protection for the equipment.

45
Q

What is an inverse time relay?

A

Inverse time relays operate faster as the current increases.

46
Q

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

A

True

2nd 2020 (10 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions