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Motor & Generator Theory > Motor Final Pract. ?'s > Flashcards

Motor Final Pract. ?'s Flashcards

1
Q
  1. In most DC generators the ________ is the rotating member and the _____ is the stationary member.
A

armature

field

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2
Q
  1. The magnitude of the generated EMF in a DC generator is determined by what 3 important factors?
A
  1. The number, size, & length of the conductors
  2. The strength of the magnetic field
  3. The speed of rotation
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3
Q
  1. For a DC generator what is the commutator?
A

The device that turns or directs the flow of current.

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4
Q
  1. What is the difference between a self-excited and a separately excited DC generator?
A

Self-excited = supply their own field current

Separately excited = get their field current from an outside source

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5
Q
  1. The field current build-up in a self-excited DC generator is dependent upon…
A

Residual magnetism

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6
Q
  1. What is the difference between a self-excited and a separately excited DC generator?
A

Self-excited = supply their own field current

Separately excited = get their field current from an outside source

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7
Q
  1. What are the 3 general types of self-excited DC generator field coil windings?
A
  1. Series
  2. Shunt
  3. Compound
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8
Q
  1. How does the armature current of a DC generator affect the field flux and commutator operation?
A

It displaces the field flux in the direction of the rotation of the generator.
This causes the neutral zone of the brushes to shift with the load and excessive arcing at the brushes.

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9
Q
  1. In a DC generator it is necessary that the brushes be kept ahead of or in advance of the neutral load plane in order to obtain satisfactory commutation under load conditions.
    True or False
A

True

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10
Q
  1. How do most commercial DC generators correct for the shifting of the brush neutral zone?
A

They narrow interpoles to reduce the flux density to zero in the neutral zone.

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11
Q
  1. How does load affect the efficiency of a DC generator?
A

Efficiency increases with the load to full load rating and then drops off somewhat as additional load is picked up.

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12
Q
  1. What are the essential parts of a commutators on a DC generator?
A
  1. Steel spider or sleeve
  2. Steel V ring
  3. Copper commutator bars
  4. Mica insulating plates & rings
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13
Q
  1. What is used to insulate the bars from the steel spider & V-ring in a DC generator?
A

Mica rings

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14
Q
  1. What is the function of the brushes in a DC generator?
A

To carry the rotor current from the commutator.

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15
Q
  1. In a DC generator, what is added to natural graphite as a polishing agent?
A

Silica

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16
Q
  1. What does insufficient brush pressure and too much brush pressure cause a DC commutator?
A

Insufficient = arcing

Too much = excessive mechanical wear

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17
Q
  1. What causes the rotor in a DC motor to revolve?
A

The interaction of magnetic field of the armature and the magnetic field of the stator

18
Q
  1. What is the back EMF of a DC motor and how does it affect motor operation?
A

Back EMF is produced when flux cuts a conductor & causes induction. It works against or limits the applied voltage in accordance with rotor speed and load.

19
Q
  1. Large DC motors are usually provided with some type of _______ ________ ______ which prevents excessive starting currents. They are called _______ _________.
    When the motor reaches full speed, this device is _____________ removed from the circuit.
A

current limiting device
motor starters
automatically

20
Q
  1. What are the characteristics of a DC shunt wound motor?
A
  1. Good speed regulation under varying loads
  2. Independent of what happens to motor as long as line voltage consistant
  3. If field is lost it will run away & destroy itself.
21
Q
  1. A DC series motors’ characteristics are…
A

Wide variation of torque & speed are allowable.

Torque increases with load.

22
Q
  1. How do you figure the efficiency of a DC motor?
A 
Efficiency = output / input
input = watts
output = horsepower.
23
Q
  1. The torque of a DC motor depends on what two things?
A

The forces exerted between the field and armature magnets.

24
Q
  1. What are the 3 common types of DC motor windins?
A
  1. Series
  2. Shunt
  3. Compound
25
26. All AC motors depend on a _________ magnetic field for their operation
revolving
26
27. What is the speed of the revolving field of an AC motor known as and what is the formula for finding the field RPM?
``` Synchronous speed N = 120f / P N = field RPM f = frequency in CPS (Hertz) P= # of poles produced by the 3-phase winding ```
27
28. What is one of the functions of the AC amortisseur (or damper) winding?
To act as a squirrel-cage to provide the torque for starting as an induction motor.
28
29. When the field strength of a synchronous AC motor is increased, the speed is not affected. True or False
True
29
30. What is the difference between the speed of the revolving field of the stator and the actual speed of the rotor of an induction AC motor called? What is the formula to find it?
Slip | Slip = Synchronous speed - Rotor speed / Synchronous speed
30
31. What do you have to do, in anAC motor, to offset the heating effects of starting currents and why?
Limit the number of motor starts. | Because the temperature rise will damage the insulation.
31
32. Is a commutator required for an AC generator?
No
32
33. On large AC generators the armature is always the... | and the field is always the...
stationary part of the generator | rotating part of the generator
33
34. The relationship between the number of AC field poles, frequency of the output voltage, and the RPM of the revolving field is shown by the equation:
F = number of pole PAIRS x RPM / 60
34
35. Loading an AC generator will affect its ________ _______. If the load consists of pure __________ units, the power factor will be _____.
terminal voltage resistance unity
35
36. Circulating currents, between paralleled AC generators with unequal excitation, tend to equalize the terminal voltages by...
Armature reaction
36
37. Except for the periodic or momentary circulation currents which keep the AC generators in synchronism, all other circulating currents should be avoided or reduced to a minimum because...
they cause unnecessary heating which reduces the load capacity of the generators
37
38. What are the name-plate ratings of load and reactive power carried by AC generators limited by?
the winding temperatures
38
39. What are the more common causes of heat in AC generators and where?
I (squared) R losses in the field and armature coils Heat created in the stator & rotor Windage Frictional heat of bearings
39
40. AC generator losses normally vary from...
1 to 5%
40
1. A DC generator is a machine which converts ____________ energy into __________ energy. A DC motor changes __________ into ___________ energy.
mechanical electrical electrical mechanical

Motor & Generator Theory (6 decks)

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