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AC & DC MOTORS > GREENIE > Flashcards

GREENIE Flashcards

1
Q

Explain how a commutator enables a single loop generator to produce a direct current

A

The rotation coil produces an alternating current. The commutator causes the direction of the current to reverse every half revolution, producing a direct current

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2
Q

Briefly explain how armature reaction causes an increase in sparking at the commutator of a DC machine. State two ways of overcoming this sparking

A

the main field is distorted by the field created in the armature, which moves the MNA. This can be corrected by moving the brushes or using inter-poles

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3
Q

Show, with the aid of two simple graphs, how the output voltages of a shunt and series DC generator change as the load increases:

A

The output of the shunt generator falls, while that of the series generator rises until saturation occurs. GRAPH 4.10b/c

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4
Q

Explain, with the aid of a suitable formula, why the starting current in a DC motor is higher than the running current

A

At zero speed, back emf=0v so current is at a maximum. As back emf (Eb) rises with speed, current falls. Ia=(Vt-Eb)/R

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5
Q

Explain, with the aid of a simple graph, why it is dangerous to operate a series DC motor under no load conditions. State the most common method used overcome this problem

A

At low load the current and therefore the flux is small. This means the motor can run at very high speeds

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6
Q

State three different methods of controlling speed in a DC motor

A

flux control, armature resistance, voltage control

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7
Q

Show, with the aid of two simple graphs, the three different types of compounding possible in a compound DC generator

A

DIAGRAM 4.6

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8
Q

Show, with the aid of a graph, the typical starting currents of a motor with and without using a face-plate starter

A

DIAGRAM 10.3.3

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9
Q

Briefly describe the three advantages of using a three-phase supply

A

generator more efficient, more efficient transmission of power. Able to produce a magnetic field

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10
Q

What type of three phase generator can be connected to a neutral wire and when is a neutral connection required

A

A start connected generator. The neutral wire allows unbalanced loads to be connected

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11
Q

Name and briefly describe the two types of rotor found in AC generators

A

Salient pole rotor: short in length, larger in diameter. Cylindrical rotor, long in length, small in diameter

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12
Q

State the four conditions to be met when paralleling 2 AC generators

A

same voltage, same frequency, same phase rotation, same phase

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13
Q

Describe, with the aid of two simple diagrams the production of a rotating magnetic field from a three-phase supply

A

DIAGRAM 17.2.2

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14
Q

A 6-pole synchronous motor is connected to a 50hz supply. What will its speed be? What would happen to the speed if the number of poles is doubled?

A

Ns = 60f/p. 6 poles = 3 pairs. N = 60*50/3=1000rpm. If number of poles doubled, speed halves

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15
Q

Briefly describe two methods which can be used to start a synchronous motor

A

use a pony motor (external motor) to bring the rotor up to speed. Use the variable frequency supply, starting at a low frequency

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16
Q

Draw a fully labelled torque/speed curve for a 3-phase induction motor:

A

DRAW GRAPH 18.4.1. Pg 126

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17
Q

Briefly explain the difference between stator switching and pole changing in an induction motor and state which is normally used in 2-speed motors

A

in a stator switching, multiple stator windings are used, one for each speed. In a pole changing all of the windings are used at each speed, connected in a different way for each speed. Pole changing is the most common method used

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18
Q

State four common causes of faults from which a motor will need to be protected

A

short-circuit, overloading, stalling, under-voltage, single-phasing

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19
Q

Briefly explain the main construction differences between a universal motor and a single-phase induction motor. State the advantage of each type of motor

A

universal motors have brushes and a commutator (like DC motor) and induction motors have no brushes and use a squirrel cage rotor. Speed control is easy using a universal motor. Induction motors are quiet

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20
Q

Briefly explain the purpose of the capacitor in a capacitive start single phase induction motor

A

the capacitor is used to provide a phase shift (90 degree) for one of the motor windings

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21
Q

Explain, with the aid of a simple diagram, the operation of an isolation transformer. Give an example of the use of such a transformer

A

the primary and the secondary are not physically connected to an earth fault on one side is not seen by the other. Used to separate zones in a supply system or in the isolation of power tools

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22
Q

How does an auto-transformer differ in construction from other transformer types? Give one advantage and disadvantage to the auto-transformer

A

it only has one winding, which is tapped at two points. It is lighter/cheaper/more efficient but cannot provide isolation

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23
Q

State the four types of three phase transformers

A

Star/Star, Delta/Delta, Star/Delta, Delta/Star

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24
Q

What type of three phase transformer is normally used as a final distribution transformer and why?

A

Delta/star transformer. This type allows a neutral to be connected on the secondary side

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25
Q

Using a simple sketch describe the construction of a DC generator, labelling the main component parts

A

DIAGRAM 1.6.1

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26
Q

What effects does increase the armature speed have on the output of a DC generator

A

increase speed will increase the generated emf.

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27
Q

Explain how back torque is produced in a DC generator and state the two main parameters which affects its magnitude

A

the current produced by the generator creates its own magnetic field. This field interacts with the main field to produce a retarding force, the back torque. The back torque depends mainly on the armature current and main flux

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28
Q

What are the two main types of winding used in a DC generator armature? Which type is used for a higher current output?

A

Lap and wave windings. Lap windings are used for a higher current output machine

29
Q

With the aid of a simple diagram, explain how torque is produced in a DC motor. Identify the three main parameters which affect the amount of torque:

A

DIAGRAM FROM 7.2. the main parameter affecting torque are the current, strength of field and length of wire.

30
Q

With the aid of a graph, explain how the speed of DC shunt motor varies with load

A

DIAGRAM FROM 8.2. the speed of the motor falls with load, not by a very significant amount

31
Q

Why is the flux in a DC shunt motor normally unaffected by the speed or load of the motor

A

the field coil is connected directly across the supply voltage and is therefore independent of changes in armature current or load

32
Q

Draw a simple circuit diagram which shows the arrangements used for armature resistance speed control? State one advantage and disadvantage of this method

A

DIAGRAM FROM 9.4. full control speed can be achieved, but efficiency can be low

33
Q

Explain why two AC voltage at different frequencies cannot be connected in parallel

A

the instantaneous value of the two voltages is different even if the peak and RMS value are the same

34
Q

Identify three ways in which the output of a simple AC generator can be increased

A

increase the field strength, increase the speed of rotation of the armature, increase the length of the conductor.

35
Q

What AC frequency can be produced by an 8-pole generator rotating at 510rpm? If this rotation speed was double, what would the effect be on the frequency of the AC produced

A

P=8/4=2
F=Np/60=510x4/60=34Hz.
The frequency would also doubled.

36
Q

Explain why the line voltage is greater than the phase voltage in a start connected three phase system. What is the factor that connects line and phase quantities?

A

In a star connected system, the line is connected across two of the phases, so VL is always different to VP. The linking factor is root 3

37
Q

Explain with the aid of simple diagram the production of a rotating magnetic field given a three-phase supply

A

DIAGRAM FROM 17.2 pg117 three of them

38
Q

Using a simple diagram, explain how torque is produced in an AC induction motor:

A

DIAGRAM FROM 18.1. The rotating field induces a current in the rotor bars. This current produces its own magnetic field which interact with the rotating field to produce a torque

39
Q

With the aid of a labelled graph, explain why a 3-phase induction motor can stall

A

GRAPH FROM 18.4. As the load increases the motor slows and the torque delivered increases. However, as we can see from the graph, the torque has a maximum value. If the load demands more torque the motor will be unable to supply it, and it will stall

40
Q

Briefly explain why frequency controls provides a more flexible way to control the spend of an AC motor than pole changing. State one advantage of pole changing over frequency controls

A

frequency controls provides complete and smooth control of speed from zero to maximum. Pole changing only provide two discrete speeds, normally full speed, and half speed. Pole changing is simpler and cheaper to use than frequency control

41
Q

Why are universal motors normally used to power domestic appliances such as power tools and kitchen equipment?

A

Universal motors provide a wide range of simple speed control and are effective at load of up around 1-2kW

42
Q

Give an example of the domestic use of a single-phase induction motor

A

central heating pumps, tumble dryers, vacuum cleaners, fans

43
Q

Explain why a single-phase induction motor can switch off its auxiliary windings when it nears full speed. What type of automatic switch is often use d in such a motor

A

on starting a rotating field is required, which requires two out of phase windings. When running, the motor can single-phase and therefore the auxiliary winding can be switched off. A centrifugal switch is commonly used

44
Q

Explain why most transformer can provide isolation between their primary and secondary sides. Why type of transformer is unable to provide this type of protection

A

The primary and secondary windings are not connected so current cannot flow directly between them. The autotransformer cannot provide isolation

45
Q

What danger exists in the use of a current transformer and why

A

A high secondary voltage can be produced. This is due to the very high turns ratio of the transformer

46
Q

Describe, with the aid of a simple sketch, the construction of a three-phase transformer

A

DIAGRAM FROM 22.8. a three-phase transformer comprises three single-phase transformers wound onto a common magnetic core

47
Q

When is the connection of a neutral wire in three phase system desirable? What type of transformer winding can connect to a neutral line and since 2096 what colour used to identify the neutral?

A

The neutral is desirable if the load is unbalanced. A start winding can accommodate a neutral connection. The neutral is blue post 2006 (pre-2006 is black)

48
Q

Explain how back emf is produced and its effect on the current in a DC motor:

A

an EMF is generated in the armature of a motor by the motion of the conductors through the magnetic field. Back EMF opposes the current driving the armature

49
Q

Sketch the torque/speed characteristics for a separately excited DC Shunt motor and using the formula for speed (N) explain shape:

A

the equation shows that as current increase speed decreases and it is the equation of a straight line

50
Q

Describe briefly two methods of speed controls for a DC motor

A

Flux control; increasing flux decreases speed. Armature resistance control; increasing resistance decrease speed. Voltage control; increasing the voltage increases speed

51
Q

State three requirements to be incorporated into a DC motor starter

A

limit starting current, prevent overload current, switch off if the supply fails

52
Q

Describe the difference and similarities between the three voltage in a three-phase supply

A

the voltages have the same amplitude and frequency but different phase (120degree apart)

53
Q

Describe briefly three methods of improving the output of a single loop AC generator

A

increase the field strength, increase the speed of rotation, add more loops to the armature

54
Q

The frequency of an AC supply varies between 48.5Hz and 51.5Hz. if a 4-pole motor is used, between what two speed will it run:

A

N=60f/p. 4 poles=2pairs. 6048.5/2=1455rpm, 6051.5/2=1545rpm

55
Q

State the advantages of placing the armature of an AC generator on the stator and using a rotating magnetic field:

A

higher speeds possible, limits centrifugal forces, heavy duty brush gear not required.

56
Q

List and explain the four conditions that must be met before two AC generators are connected in parallel

A

both generators to have the same voltage, both generators to have the same frequency, both generators to have the same rotation, both generators to have the same phase rotation

57
Q

Wat is measured by a synchroscope

A

the difference in frequency between two AC generators and their phase difference.

58
Q

Explain how a synchroscope is used when paralleling two AC generators

A

synchroscope should be resolving slowly in a clockwise direction, and that the breakers should be closed when the needle is between the ‘5 minutes to and 5 minutes past’ position.

59
Q

Describe with the aid of a diagram, the principles of operation of an AC synchroscope motor

A

a rotating magnetic field is produced by a three-phase stator. The rotor contains a magnet, whose field locks onto the rotating stator field, causing it to be dragged around the same speed. DIAGRAM 17.3 Pg. 118

60
Q

Define ‘slip’ and state an equation which links it to rotor and field speed:

A

slip is the difference between the speed of the rotating field and the squirrel cage rotor. Slip=(NS-Nr)/Ns.

61
Q

Draw and explain the torque/slip characteristic for a double cage rotor and state one advantage and one disadvantage of this type of motor

A

DRAW GRAPG 18.4. Pg. 128. The overall effect is the result of adding the two cages together. Advantage-high starting torque and reduced staring current. Disadvantages-max torque less than for a single cage rotor of the same size. I2R losses in the high resistance cage at start up may cause burnout if frequent starts attempted

62
Q

Define the term synchronous speed as it applies to a synchronous AC motor:

A

the rotor and the rotating field move at the same speed.

63
Q

Identify the two parameters that can be altered to change the synchronous speed of an AC motor and state the formula the links them

A

frequency and the number of pairs of poles. N=60f/p.

64
Q

State the effects of a three-phase induction motor when it is single phasing if it is;

A

-Running: little effect at low load, at medium load vibration, may stall at higher load, may overheat.
- Stopped: the motor cannot start.

65
Q

Explain how motor thermal protection unit (MTPU) also provide under voltage protection

A

if the supply voltage falls below 53% of its rated voltage for more than 3 seconds the MTPU will trip the motor out

66
Q

Explain with the aid of a labelled diagram, how a transformer may be used to provide tow outputs of the same amplitude but 180degree with each other:

A

if the centre tap is earthed the output of each of the secondary will be 1800 out of phase with the other. DIAGRAM 22.7. Pg167.

67
Q

Describe wit the aid of a diagram, the operation of a current transformer:

A

the busbar acts as the primary of a transformer with a very high turn ratio. This reduces the high current to a safe level which can be measured with an armature attached to the secondary. Care must be taken as the secondary voltage can be very high. DIAGRAM 22.6 Pg 165.

68
Q

State three types of three phase transformers and give an example of their use

A

Start/Star-HV transformer, unbalanced load. Delta/Delta-LV transformer. Start/Delta-power distribution system. Delta/Star-final distribution transformer.

AC & DC MOTORS (1 decks)

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