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Machines > Direct Current (dc) Machines > Flashcards

Direct Current (dc) Machines Flashcards

1
Q

What 3 basic parts make up a DC generator?

A
  1. A set of permanent magnets or electromagnets
  2. An armature winding that is rotated within the field by a mechanical prime mover
  3. A commutator and brush set
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2
Q

What are magentic lines of force called?

A

Flux

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3
Q

Which direction are lines of force oriented?

A

From North pole to South pole of a magnet

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4
Q

Do magnetic lines of force always form complete loops?

A

Yes

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5
Q

Can magnetic lines of force cross?

A

No, they repel each other instead.

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6
Q

What path do magnetic lines of force follow?

A

That of least reluctance.

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7
Q

What is reluctance?

A

Reluctance is a common magnetic measurement. It is the opposition to flux in a magnetic circuit. Flux is measured in webers (Wb).

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8
Q

What is flux density?

A

Flux density describes the intensity of a magnetic field at a given point. The usual definition is flux per unit area. The quantity symbol for flux density is the Greek letter beta, and the unit of measurement is the tesla (T). Points closer to the poles of a magnet have a higher flux density than points that are further away.

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9
Q

What is the left hand rule for conductors?

A

The left hand rule for conductors states that if a ccc is grasped by the left hand with the thumb pointed in the direction of electron flow, the fingers indicate the direction of the magnetic lines of force.

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10
Q

What is the left hand rule for polarity?

A

The left hand rule for polarity states that if you were to hold a coil in your left hand with your fingers pointed in the direction of electron flow, your thumb would be pointed at the north pole of the electromagnet.

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11
Q

What factors effect the flux density of a coil?

A
  1. How much current is passing through the coil (more current more flux density)
  2. The number of turns of conductor in the coil’s construction (more turns, stronger field)
  3. The material composition of the coil’s core. (iron core makes a greater flux density than an air core)
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12
Q

What is Magnetomotive force? (mmf)

A

The measure of a coil’s ability to produce flux.

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13
Q

What is magnetomotive force measured in and how do you find it?

A

mmf is measured in ampere-turns and it is found by using the product of those two values. (amperes x turns)

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14
Q

What is the magnetomotive force of a 50 turn coil with 10 amperes of current running through it?

A

500 ampere-turns

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15
Q

What causes magnetic saturation?

A

Magnetic saturation occurs when all of the magnetic domains in the material become aligned. The limit of magnetic flux that the material can contain has been reached.

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16
Q

What items comprise the magnetic circuit of a dc generator?

A
  • The pole pieces (pole core and pole shoe)
  • The yoke (frame)
  • The armature core (rotor)
  • The air gap
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17
Q

What determines flux density in a generator?

A
  1. Number of turns in the coil of the electromagnet

2. Value of current in the field winding coils.

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18
Q

Does increasing or decreasing the field current produce a proportional change in the field strength as long a s the field poles are not saturated?

A

Yes

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19
Q

What is a Commutator?

A

A commutator is where the windings connect to the electrical load via the brushes. It is made up of bars and the bars are connected to the windings.

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20
Q

What is commutation?

A

Commutation is where a dc voltage output is extracted from an armature that has an ac voltage induced in it. The commutator reverses the armature loop connections to the external circuit. This happens at the same instant that the voltage polarity reverses in the armature loop.

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21
Q

When does the brush contact two commutator segments at once?

A

When the coil passes the neutral plane.

If the brush contacted the two commutator segments at once when not in the neutral plane, a significant amount of current flows from commutator segment to the brush, to the next commutator segment. This would cause severe arcing and overheating.

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22
Q

Are Interpoles used in most dc dynamos?

A

Yes!

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23
Q

What is an interpole?

A

Interpoles are narrow poles placed exactly halfway between the main poles and directly in line with the no-load magnetic neutral plane.

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24
Q

What kind of windings do interpoles have?

A

Commutating windings.

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25
Q

In a dc generator, the brushes should commute between commutator segments when the coil connected to those segments is traveling (parallel/perpendicular) to the field flux.

A

parallel.

Neutral plane

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26
Q

An interpole must have the same polarity as the main pole (ahead of/behind) it in the direction of rotation

A

Ahead of it!

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27
Q

Commutating windings are placed in slots or holes in the main pole faces. True or false?

A

False. Commutating windings are the interpoles.

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28
Q

What are the windings called that correct the problems of armature reaction in the polar zone?

A

Compensating windings.

29
Q

What is counter-rotational force or counter-torque?

A

Counter-torque is a mechanical force applied to the rotor that opposes torque.

30
Q

What is the Lorentz Force?

A

The Lorentz Force is the force that tends to push a ccc at right angles to a magnetic field.

31
Q

How do you determine the direction of Lorentz force on armature conductors?

A

By using the right-hand rule for motors.

32
Q

What is the right hand rule for motors?

A
  • The first finger points in the direction of the main field flux.
  • The middle finger indicates the direction or polarity of emf induced into the conductor (armature conductor)
  • The thumb points in the direction of the Lorentz force.
33
Q

What is generator action in a motor?

A

Generator action in a motor is when a counter electromagnetic force (cemf) is produced because the rotor is turning and the armature conductors are cutting lines of force. Cemf is opposite in polarity to applied voltage.

34
Q

Is the left hand rule for generators the same as the right hand rule for motors?

A

The only difference is the hand you’re using. All of the fingers are describing the same variables.

35
Q

What is output voltage dependent on in a dc generator?

A
  1. The rotational frequency

2. The intensity of the magnetic field.

36
Q

Which direction does the neutral plane shift in the case of a dc motor?

A

In the opposite direction of rotation.

37
Q

What 3 factors affect the speed of a dc motor?

A
  1. The amount of load on the motor
  2. The intensity of the magnetic field
  3. The amount of armature voltage.
38
Q

A dc motor operates at 220V and a current of 20A at rated speed. The resistance of the armature is 2ohms. Clculate the amount of cemf at the instant of start.

A

cemf at the instant of start is zero. cemf is generated by motion and the rotor is not moving at the instant of start.

39
Q

A dc motor operates at 220V and a current of 20A at rated speed. The resistance of the armature is 2ohms. Calculate the armature current at the instant of start.

A 
I = V/R
I = 220/2
I = 110A
40
Q

A dc motor operates at 220V and a current of 20A at rated speed. The resistance of the armature is 2ohms. Calculate the amount of cemf at rated speed.

*Hint, use the fundamental motor equation.

A 
V = cemf + (I x R)
cemf = V - (I x R)
cemf = 220 - (20 x 2)
cemf = 180V
41
Q

What are 2 methods used to help control armature reaction?

A
  1. Interpoles with commutating windings

2. Compensating windings in the face of the pole shoe.

42
Q

Motor action in a dc generator is the interaction of the magnetic fields produced by the field windings and the armature windings. True or false?

A

True

43
Q

Is a dc generator and a dc motor basically constructed the same?

A

Yes

44
Q

Does shunt wound mean that the windings are connected in series or parallel?

A

Parallel

45
Q

A shunt winding is made up of (few/many) turns of (thick/fine) wire.

A

Many turns of fine wire.

46
Q

How much current runs through the shunt vs the armature?

A

The shunt field current can be as low as 1% of the armature current.

47
Q

Is torque proportional to armature current in a shunt motor?

A

True!

48
Q

Shunt motor runaway will damage the motor, true or false?

A

True.

49
Q

How does shunt motor runaway occur?

A

If the field resistance is very high at the start, there will be little or no generated cemf. The condition of no cemf allows the armature current to increase to a high value, resulting in runaway.

50
Q

The field winding in a series motor is made up of (few/many) turns of (fine/thick) wire.

A

Few, thick. Remember that this winding is connected in series, so it must carry the same current as the armature and the wire must be thick enough for the current.

51
Q

Series motors produce (high/low) starting torque.

A

High.

52
Q

What is torque proportional to in a series motor?

A

The square of the current. This is because the same current flows through the series winding and the armature winding.

53
Q

What happens if you decrease the load on a series motor?

A

The motor speeds up.

54
Q

What happens when a series motor speeds up?

A

The cemf increases and causes the current and torque to stabilize at the new load condition.

55
Q

Can series motors be used under no load or for light loads? Why?

A

With no load, a series motor can accelerate to dangerous speeds. The series motor must not be used for light loads and must be directly connected to the load.

56
Q

How much starting torque can a series motor develop compared to full-load torque?

A

500% !

57
Q

Do compound motors follow the same rules for windings as series and shunt motors? Being that series windings are made of few turns of heavy wire and shunt windings are made of many turns of fine wire?

A

Yes, they do!

58
Q

Do magnetic fields aid or oppose each other in a cumulative compound motor?

A

They aid each other.

59
Q

Do magnetic fields aid or oppose each other in a differentially compounded motor?

A

They oppose each other.

60
Q

What is the difference between long shunt and short shunt compound motors?

A

In a long shunt, the shunt winding is connected in parallel to the series and armature winding.

In a short shunt, the shunt winding is connected in parallel to only the armature winding.

61
Q

What is motor speed regulation?

A

The variation of motor speed as the load on the motor changes. It compares no-load speed to full-load speed.

62
Q

What is the formula to calculate speed regulation?

A

Percent Speed Regulation =

[((no-load speed)-(full-load speed)) / (full-load speed)] x 100

63
Q

True or false:

The shunt winding is made of few turns of fine wire.

A

False. Shunt windings are made of many turns of fine wire.

64
Q

True or false:

The armature winding carries the same current as the shunt field winding.

A

False. The shunt winding is wired in parallel to the armature winding, therefore it draws its own current which would be calculated using I=E/R

65
Q

Why is a resistor connected in series with the armature needed for starting a shunt motor?

A

At start, there is no cemf to limit current. Armature resistance is low and therefore would allow high values of current which could damage the armature.

66
Q

Why does the series motor have such high starting torque?

A

The series winding is designed to carry the same high currents as the armature. At start, high current produces intense magnetic fields around the series and armature windings which results in high torque. As the motor speeds up, the cemf produced tends to limit current and the torque decreases by the square of the current.

67
Q

What happens to the speed of a series motor as the load is decreased?

A

The motor speed increases as load is decreased.

68
Q

How are cumulative compound and differential compound motors different from one another?

A

The direction of the fields is the only difference. If you took the shunt winding and reversed it on a cumulative, you’d have a differential.

69
Q

A 12hp dc motor runs at 1100 rpm at full load. If the no load speed is 1210 rpm, what is its % speed regulation?

A

%speed reg = ((1210-1100) / 1100) x 100

= 10%

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