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Motors Cards B55a > Knowledge Base Assignment #4, 5,6,7 > Flashcards

Knowledge Base Assignment #4, 5,6,7 Flashcards

Midterm Study tool

1
Q
  1. What is the purpose of the coils of wire inside the motor?
A

The coils inside the electric motor create magnetism when electrical current flows through them. The magnetism creates the rotation of the motor shaft.

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2
Q
  1. What is the purpose of the “pole pieces”?
A

They concentrate the magnetic fields inside the motor.

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3
Q
  1. What is the difference between the “rotor” and the “stator”?
A

The rotor is the rotating magnetic field. The stator is the stationary magnetic field. In other words, the rotor rotates (moves) while the stator stays stationary (does not move).

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4
Q
  1. What is the purpose of the “rotor” or “armature”?
A

It creates the magnetic field that interacts with the magnetic field from the “stator” or “field” part of the motor to create rotation.

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5
Q
  1. What is the purpose of the “commutator”?
A

It connects electrical power to the rotor/armature’s coils. It also provides the proper electrical polarity to the proper coil as the motor spins.

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6
Q
  1. What is the purpose of the “brushes”?
A

They slide along the commutator as the commutator spins, allowing an electrical connection to be made.

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7
Q

What is the difference between a “commutator” and “split rings”?

A

Split rings perform the much of the same functions as a commutator. However, rather than having the surface broken into segments, the surface consists of complete “rings” of metal. Each wire in the rotor/armature is connected to a ring. This does not change the electrical polarity of the coil as it rotates.

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8
Q
  1. Write a summary of what makes a motor spin:
A

The stator contains multiple magnetic poles—usually created by current flowing through the electromagnet coils. Each pole creates a magnetic field that is strongest between the poles. When current flows through the coils in the rotor, a magnetic field is also created around each of the coil “windings” (one turn of a coil of wire). This rotor magnetic field interacts with the magnetic field of the stator, and either is attracted or repelled

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9
Q
  1. Give the formula for calculating mechanical horsepower:
A

hp = 2(pi)(PRM)(T)/33,000

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10
Q
  1. List the ways the “brushless” DC motor is different from a “brushed” DC motor (besides not having brushes)
    Part 1
A
  1. The rotor is a permanent magnet
  2. The stator consists of a number of coils that are energized in a certain pattern, rather than being constantly energized like in a brushed DC motor.
  3. The stator coils are connected to an electronic circuit (the “driver”) that creates a pattern of voltage levels to energize the coils in the right sequence to cause the magnetic field in the stator to rotate. The permanent magnet rotor simply follows the magnetic fields in the stator.
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11
Q
  1. List the ways the “brushless” DC motor is different from a “brushed” DC motor (besides not having brushes)
    Part 2
A
  1. The permanent magnet rotor simply follows the magnetic fields in the stator. 4. They are more complex than AC induction motors.
  2. They have a higher cost for both the motor and the driver circuitry.
  3. They are more reliable because there are no brushes to wear out.
  4. They are often capable of producing much higher speeds than are usually made by other DC motor types.
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12
Q
  1. What is a “universal motor”?
A

It is a motor capable of operating on either DC or AC.

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13
Q
  1. What happens in the “squirrel cage rotor” that allows the motor to spin
A

The changing magnetic field in the stator (created by alternating current)
“induces” (generates) current flow in the squirrel cage. When current flows through a coil of wire, magnetism is created. Therefore, the rotor’s magnetic field is created by the induction of current in it by the stator.

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14
Q
  1. What are the differences between a three-phase “synchronous” and a three-phase “induction” motor?
A
  1. Three-phase synchronous motors need a separate voltage applied to the rotor, while an induction motor does not.
  2. True synchronous motors must be started with special synchronous motor starters.
  3. Synchronous motors are high efficiency, and have the additional advantage of correcting “power factor.”
  4. They operate at full synchronous speed, while induction motors cannot.
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15
Q
  1. Explain how a “stepper motor” is made to rotate:
A

Pulses of current need to energize the various coils of the stator in the proper sequence (pattern).

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16
Q
  1. Where are “shaded pole motors” used?
A

They are the most common motor type in appliances.

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17
Q
  1. What two factors determine the speed of rotation of an AC motor?
A

The frequency of the power source, and the number of poles in the stator.

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18
Q
  1. The magnetic field of a magnet is represented by
A

d. Lines of flux

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19
Q
  1. How is the speed of a DC motor controlled?
A

By applying a variable voltage either to the armature or field.

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20
Q
  1. What needs to be done to the voltage applied to the field and the armature to operate the motor at a speed that is lower than its RPM rating?
A

Full voltage needs to be applied to the field, and a lower voltage needs to be applied to the armature

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21
Q
  1. What needs to be done to the voltage applied to the field and the armature to operate the motor at a speed that is above its RPM rating?
A

Apply full voltage to the armature and a reduced voltage to the field

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22
Q
  1. Explain what “dynamic braking” is:
A

Dynamic braking keeps the field energized, but the armature is connected to a large resistor, turning the motor into a generator. The mechanical power of the rotating motor is converted to electrical power that is dissipated (used up) by the load resistor.

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23
Q
  1. Where are the “field windings” located
A

They are located in the non-moving part of the motor

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24
Q
  1. What is the purpose of the “interpoles?
A

They are auxiliary windings placed between the main field windings, and they help reduce the sparking between the brushes and the commutator, increasing the life of the brushes.

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25
Q
  1. Explain what the “armature” is:
A

The armature is the rotating part of the DC motor, and it consists of laminated steel slots connected to the shaft around which the coils are wound.

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26
Q
  1. What is the purpose of the “commutator”?
A

The commutator consists of a series of copper segments connected to the armature windings. It supplies current to the armature winding, and reverses the current flow as needed to make current flow in one direction through the armature windings

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27
Q
  1. What is the purpose of the “brushes”?
A

The brushes provide connection between the power source and the commutator.

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28
Q
  1. What are the two fields in our DC motor? (The “interpoles” are not considered one of the fields.)
A

The series field and the shunt field.

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29
Q
  1. What is the major advantage of a DC series motor
A

It produces a high starting torque.

30
Q
  1. What is the disadvantage of a DC series motor?
A

It must be operated under a load – if it is not loaded, it will continue to spin faster and faster until it destroys itself (‘runaway”)

31
Q
  1. Explain how the direction of rotation of a DC motor is reversed
A

A DC motor is reversed by reversing the polarity of either the armature or the field, but not both.

32
Q
  1. What are the advantages of a DC shunt motor configuration?
A

`They provide fairly constant speed (excellent speed regulation) and do not require a load to be connected to them (no chance of “runaway

33
Q
  1. What is the difference in the connections between a “cumulative compound motor” and a “differential compound motor”?
A

A cumulative compound motor has the series and shunt coils connected with the same magnetic polarity so that the magnetic energy adds up and increases torque.

34
Q
  1. Which one of the compound motor configurations results in a reduction in torque?
A

A differential compound motor.

35
Q
  1. Explain how torque is developed in an electric motor
A

Torque is developed in an electric motor because of the interactions of different magnetic fields. The attracting and repelling action of the magnetic fields creates the rotation and results in torque being produced.

36
Q
  1. In order to produce enough torque to handle an increase in load on the motor, what needs to happen?
A

More current is drawn by the motor to produce the torque necessary to handle the increased load.

37
Q
  1. Why is it not necessary to insulate the bars inside a squirrel cage rotor?
A

Because of the low voltages induced into the rotor bars.

38
Q
  1. Why does an induction motor (with a squirrel cage rotor) need “slip” - operating at less than the synchronous speed (the speed in which the magnetic field rotates in the stator)?
A

If there were no difference in speed between the synchronous and actual speed, there would be no voltage generated in the rotor, and therefore no torque or rotation would result.

39
Q
  1. What is the formula for calculating the synchronous speed of an AC motor
A

Synchronous speed = (120 x frequency) / number of pairs of magnetic poles

40
Q
  1. What is motor “slip”?
A

The percentage difference between the synchronous speed and actual speed

41
Q
  1. Can synchronous motors be connected “across the line” (directly connected to the power lines) to get them started? Why or why not?
A

No! If this happens, or if the DC power is connected to the motor not at exactly the right time, damage to the motor will occur.

42
Q
  1. Can synchronous motors run at synchronous speed (0% slip)?
A

Yes

43
Q
  1. List (5) advantages of a three-phase motor Part 1
A

1) They do not require brushes or internal switches so they have minimal maintenance needs.
2) They are less expensive to manufacture.
3) They are more efficient in power consumption than single-phase motors.

44
Q
  1. List (5) advantages of a three-phase motor Part 2
A

4) Their speed can be easily controlled with a VFD.
5) Three-phase power is generally available at industrial sites.
6) It is very simple to reverse the direction of rotation.

45
Q
  1. Which three-phase motor configuration has the three motor coils connected together on one side?
A

A “wye” (Y) configuration.

46
Q
  1. Which three-phase motor configuration has each side of a motor coil connected to one side of the coil adjacent (next) to it?
A

A “delta” configuration.

47
Q
  1. On a dual-voltage, wye configuration motor, how should the motor coils in each phase be connected for low voltage operation?
A

In parallel.

48
Q
  1. On a dual-voltage, wye configuration motor, how should the motor coils in each phase be connected for high voltage operation?
A

In series.

49
Q
  1. On a dual-voltage, delta configuration motor, how should the motor coils in each phase be connected for low voltage operation?
A

In parallel.

50
Q
  1. On a dual-voltage, delta configuration motor, how should the motor coils in each phase be connected for high voltage operation?
A

In series.

51
Q
  1. On a wye configuration motor, how does the voltage across each coil compare to the voltage between two of the power lines feeding the motor?
A

The voltage across the coil is equal to the voltage between the phases divided by the square root of 3 (or 1.73).

52
Q
  1. On a delta configuration motor, how does the voltage across each coil compare to the voltage between two of the power lines feeding the motor?
A

The voltage across the coil is equal to the voltage between two of the power lines (phase voltage).

53
Q
  1. What would happen if a single phase AC motor were to be constructed with only one set of permanently-connected stator coils?
A

The motor would not spin at all unless it was started by hand in a particular direction.

54
Q
  1. What is the principle of operation of the two sets of coils in a split-phase motor?
A

There is a natural phase shift between the two sets of coils because there are two different inductive reactances and resistances between the two coils because of the different number of turns and gauges of wire between the two.

55
Q
  1. What is the effect on phase shift when the amount of resistance in an AC circuit (in our case, the coils within a motor stator) is increased?
A

More resistance means a lower phase shift angle.

56
Q
  1. Why are the “start” coils (windings) and the “run” coils (windings) constructed differently rather than being identical?
A

Because if they were identical, there would be no phase shift, and therefore the motor would not spin in a particular direction.

57
Q
  1. How are the “start” windings constructed?
A

They consist of fewer turns of wire, and have less resistance.

58
Q

23.How are the “run” windings constructed?

A

They consist of more turns of wire, and have more resistance.

59
Q
  1. What is the purpose of the “centrifugal switch” in a split-phase motor?
A

It keeps the start windings energized until the motor reaches about 80% of its full speed.

60
Q
  1. Why is it not a good practice to leave the “start” windings connected as long as the motor is energized?
A

Because the start windings have lower resistance and this creates heat. They are not built to handle the amount of current that would flow through them continuously. Plus, it’s very inefficient if the start windings remained energized

61
Q
  1. What could be the problem if a split-phase AC motor does not spin, but simply
    “hums” and/or gets really hot?
A

Either it has a bad set of contacts on the centrifugal switch or a burned out start winding.

62
Q
  1. How should the direction of rotation of a split-phase motor be reversed?
A

By reversing the start winding (the preferred method) or the run winding, but not both.

63
Q

Can single phase AC induction motors operate at full synchronous speed? Why or why not?

A

No. A certain speed difference (“slip”) is required in order for voltage to be induced in the squirrel cage rotor.

64
Q
  1. What is “slip” when referring to split-phase motors? Is it the same concept as “slip” in a three-phase motor?
A

It is the percentage difference between the synchronous peed of the motor and the actual speed. It is the same concept in both split-phase and threephase motors.

65
Q
  1. Is a split-phase motor a good choice for equipment that needs to be started under a serious load? Why or why not?
A

It is not. Its starting torque is 75% to 200% of operating torque. Other types have a better starting torque, like capacitor-start motors.

66
Q
  1. Are the “start” and “run” windings in a capacitor start motor constructed the same as with split-phase motors?
A

No

67
Q
  1. What are the advantages of permanent capacitor motors?
A

They create less vibration and noise than other types.

68
Q
  1. What are the two main factors in determining the speed of an AC motor (not using speed control)?
A

Frequency of the AC input voltage and the number of pairs of magnetic poles.

69
Q
  1. How do multiple speed motors allow multiple speed operation?
A

By changing the number of poles that are connected to power.

70
Q

What is the purpose of the centrifugal switch in a capacitor start motor?

A

To allow the starting capacitor and the start winding to be connected to the power source until the motor has reached almost full speed.

71
Q
  1. What are the differences between “start” and “run” capacitors as used in AC motors
A

Start capacitors are designed to stay in the circuit only a small amount of time, so they have a large capacitance in a small package. Run capacitors have much less capacitance, but can stay in the circuit continually. They are also oil-filled.

72
Q
  1. Which of these motors have a greater phase shift between the “start” and “run” windings: split-phase motor or capacitor-start motor?
A

The capacitor-start motor comes a s close as possible to the desired 90 degrees of phase shift.

Motors Cards B55a (1 decks)

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