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4th Year Electrical > Machines; Synchronous Motors > Flashcards

Machines; Synchronous Motors Flashcards

1
Q

What rate does a rotor turn in a Synchronous motor?

A

Same speed as the rotating magnetic field of the stator (synchronous speed)

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2
Q

In order to rotate synchronous Motors require a magnetic field in the rotor, how can this field be supplied?

A

Brushed DC excitation
Brushless excitation
Permanent magnet

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3
Q

What is the starting current and torque like on a synchronous motor?

A

Low starting current per horsepower and high starting torque

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4
Q

What is the speed regulation like on a synchronous motor?

A

They operate at a constant speed from no load to full load and maintain maximum efficiency

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5
Q

What is a synchronous motor called when it’s operated at no load and used for power factor correction only

A

Synchronous condenser

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6
Q

What kind of synchronous motors do not require an excitation system for the rotor?

A

Synchronous motors with Permanent magnet Pole

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7
Q

What is one disadvantage of using a permanent magnet pool synchronous motor?

A

Cannot alter the power factor

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8
Q

What advantages does a synchronous motor have over a squirrel cage induction motor?

A

1 can be operated at a leading power factor
2 gives constant speed from no load to full load
3 operate at higher efficiency especially when operating at Unity power factor

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9
Q

What are some disadvantages of a synchronous motor compared to an induction motor?

A

1 requires DC excitation supplied from a separate source to run
2 may I followed of synchronism or stop rotating if overloaded.
3 small starting torque, synchronous motor needs assistance to get to operating speed
4 doesn’t work well in applications that require frequent starting or Direction changes

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10
Q

What are the rotor and stator like in a synchronous motor construction compared to AC induction motor?

A

Stator is similar to the stator of an AC rotating field alternator. Stator supports the Armature windings which are connected to the supply voltage.

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11
Q

Where are the amortisseur windings in a synchronous motor located?

A

Within the rotor

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12
Q

What sort of Supply is applied to the Armature windings of a synchronous motor?

A

Three-phase AC Supply to produce a rotating magnetic field

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13
Q

Where are the field windings in a synchronous motor?

A

On the rotor

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14
Q

How many numbers of poles and field windings make up the rotor?

A

As many as there are poles and Armature windings on the stator

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15
Q

What mechanical connection device allows DC voltage to develop the rotor magnetic fields?

A

Slip rings and brushes

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16
Q

Why is DC supply to the rotor?

A

To develop a constant magnetic field that interacts with the rotating magnetic field of the stator

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17
Q

What type of construction are most synchronous motor rotors?

A

Salient Pole

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18
Q

What does it mean to be doubly excited?

A

Synchronous motors have to electrical inputs the stator and the rotor

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19
Q

What kind of supply is used for the stator and what kind of supply is used for the rotor?

A 
Stator= 3 phase AC
Rotor= DC
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20
Q

When a synchronous motor is a two-pole machine how many pole pairs would you expect to see?

A

1 pole pair for each phase. (Ie. 6)

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21
Q

What stator connection is most commonly found for synchronous Motors?

A

Wye connected but they can also be Delta connected

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22
Q

What direction is each phase winding wound on the laminated steel cores?

A

All in the same direction

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23
Q

How does the magnetic field rotate through the stator?

A

Each phase is at a maximum 120 electrical degrees apart.
The magnetic field produced by the field coils in one particular phase depends on the magnitude and direction of current through that phase.

From one instant to the next the magnetic field smoothly shift its position and rotates as the current Rises and falls at different times in the windings

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24
Q

When the Wye connected point is the South Pole in a given phase that phase is that a max

A

** see figure 4 pg. 8 030401e**

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25
Q

What is the speed of the stator’s rotating magnetic field dependant on?

A

The frequency of the supply voltage. directly proportional

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26
Q

What is the relationship between rotational frequency and the number of magnetic poles produced in the stator?

A

Inversely proportional to each other. The greater the number of magnetic poles produced in the stator the slower the speed of the stator’s rotating magnetic field

27
Q

What is the equation for synchronous speed?

A

N(s) = (120 × f) ÷ p

28
Q

How do you control the intensity of the magnetic field produced by the rotor field windings?

A

Vary the field excitation

29
Q

Why do you start a synchronous motor with little or no load?

A

So the rotor can build inertia enough to rotate. Too much counter torque wood prevent this

30
Q

What are the three common methods to start a synchronous motor?

A

1 use a shaft mounted induction motor
2 use the load from the synchronous motor as a starting motor
3 use an armortisseur on the rotor

31
Q

What is the purpose of the amortisseur windings?

A

The voltage induced into these findings from the stators rotating magnetic field produces current in the windings which produces a flux that interacts with the stator flux this produces torque

32
Q

What’s the risky thing that happens drink start process?

A

Rotating stator magnetic field cuts the turns of the DC winding which induces High unwanted voltages in the DC field windings. This can damage the winding insulation slip Rings brushes and other components

33
Q

How do we reduce The Unwanted High voltages across the DC windings during the start process?

A

Connect a field discharge resistor across the field using two normally closed contacts of a synchronizing switch

34
Q

What type of switch is used in the field discharge system of a synchronous motor?

A

Make before break Style

35
Q

What is the relationship between the supply voltage applied to the stator Armature and the counter electromotive Force induced in the Armature?

A

Cemf opposes the supply voltage by 180 degrees

36
Q

True or false Supply phase voltage is fixed

A

True

37
Q

True or false cemf generated voltage (E(generated)) is fixed

A

False it can be varied by changing the field excitation

38
Q

What is induced voltage or cemf due to?

A

Field flux

39
Q

What is the value of current in the stator windings determined by?

A

The resultant voltage applied to the stator winding impedance

40
Q

What is the value of impedance?

A

The phasor some of:
Synchronous reactance
(due to inductance and armature reaction of the coils)
and resistance
(due to the resistance of the copper in the coils)

41
Q

What is the torque angle?

A

The angle between the centre of a stator pole and the centre of a corresponding rotor Pole

42
Q

What affects the torque angle?

A

Increased load

43
Q

What does a torque angle on a phasor diagram illustrate?

A

That generated voltage is not 180 degrees out of phase with phase voltage. When the motor is under load it legs the 180 degree reference line by the torque angle value

44
Q

What is the angle between the stator and rotor poles called?

A

Torque angle

45
Q

What is pull in torque

A

The torque the rotor must develop to pull into synchronism once the DC windings have been energized. It must be greater than the counter torque of the load

46
Q

What is pull out torque

A

The value of the load counter torque required to rip the rotor out of synchronism. The point where the load is sufficient to break the magnetic attraction between the two fields

47
Q

What degree angle represents pull out torque?

A

90° for cylindrical Motors

60 to 70 degrees for Salient pole Motors

48
Q

At what load would you find Unity power factor on the synchronous motor?

A

Rated load

49
Q

At what load would you find a legging power factor on a synchronous motor?

A

Overload

50
Q

At what load would you find a leading power factor on a synchronous motor?

A

Light load

51
Q

At what load would you find no losses on a synchronous motor?

A

No load

52
Q

What effect does changing the DC excitation have on torque angle?

A

Changing DC excitation is mainly to change power factor and its effect on torque angle is generally small enough to be ignored

53
Q

What is the excitation of a motor when it results in a lagging power factor (Iarm lagging Vphase)?

A

Under excited

54
Q

What is the excitation of a motor when it results in a leading power factor (Iarm leading Vphase)?

A

Over-excited

55
Q

What is a rotary capacitor?

A

Another word for synchronous condenser or a synchronous motor operating add a leading power factor for power factor correction purposes only

56
Q

What two values are compared to produce a v curve?

A

Armature current and DC field current of a synchronous motor

57
Q

What loading would cause a deeper Vee curve?

A

No load

58
Q

When referencing a v curve as field current increases what happens to the torque angle?

A

It becomes less lagging and more leading

59
Q

Why is a full load vee curve more shallow?

A

Armature current is on the x axis, and more Armature current flows to produce the power to drive the load. (I(arm) values stay higher as field current values increase)

60
Q

What values does a bell curve compare?

A

Power factor and field current

61
Q

At what point on a bell curve does a synchronous motor reach normal excitation?

A

At the peak or Unity power factor

62
Q

At what point on a bell curve does a synchronous motor reach normal excitation?

A

At the peak or Unity power factor

63
Q

As field current is increased from Unity power factor what happens to the torque angle?

A

Becomes more leading

64
Q

Which would take more field current to produce Unity power factor: a motor operating with a full load or with no load?

A

A motor operating with full load would take Moorefield current to reach Unity power factor

4th Year Electrical (24 decks)

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