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151/23/005 > Electrical Machines > Flashcards

Electrical Machines Flashcards

1
Q

identify the main type of DC propulsion motor in use with the RN

A

Shunt motor

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2
Q

what type of motor would use a squirrel cage rotor?

A

three phase induction motor

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3
Q

what material would be used typically for the conducting parts of the squirrel cage rotor?

A

copper
aluminium
iron

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4
Q

how are eddy currents minimised in a practical transformer?

A

use a laminated iron core

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5
Q

describe the operation of a lightly loaded motor which develops a single phasing fault

A

it will continue to run, but will not start if stopped.

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6
Q

what is the purpose of a capacitor in a single-phase induction motor?

A

to change the phase of the current in one of the windings

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7
Q

what is produced in the motor when the switch is closed. What happens after the motor is switched on?

A
  • a rotating magnetic field
  • as the motor gains speed the switch opens, disconnecting the cable.
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8
Q

Identify the types of single phase AC motor

A

induction
universal

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9
Q

describe the characteristics of an autotransformer, comparing to other types.

A

it is cheaper and more efficient but provides no isolation

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10
Q

list the four types of three phase transformers

A
  • star star
  • star delta
  • delta star
  • delta delta
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11
Q

which type of three phase winding allows the connection of a neutral wire?

A

star

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12
Q

what is the purpose of the neutral wire when used in a three phase transformer?

A

to allow unbalanced currents to flow

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13
Q

state four conditions which must be satisfied before two AC generators can be paralleled

A
  • same terminal voltage
  • slightly higher oncoming frequency
  • same phase rotation
  • in phase
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14
Q

what are two parameters measured by a synchroscope

A
  • frequency difference
  • phase difference
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15
Q

complete the following sentence.
“True power is transferred by altering ___, while Reactive power is transferred by altering ___”

A
  • the governor setting
  • The AVR output
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16
Q

state the typical power factor or an RN warship electrical supply

A

0.8

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17
Q

what is the purpose of a DC motor?

A

a machine that converts direct electrical energy to mechanical energy

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18
Q

define motor effect

A

a current carrying conductor lies in a magnetic field it experiences a force.

Flemings left hand rules gives the direction of the force

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19
Q

what are the consequences of Armature reaction

A
  • increased sparking at the brushes
  • it reduces the maximum torque that the motor can produce
20
Q

what are the three methods of speed control? And how do they effect the speed when increased?

A
  • Flux control, decreases
  • Armature resistance, decreases
  • Voltage control, increases
21
Q

what is field control used for in DC shunt motors?

A

varying the field current

22
Q

what is field control used for in DC series motors?

A

uses of a series field diverter, allows armature current to by-pass the series field

23
Q

describe a single loop AC generator

A

A field component that produces a magnetic field and an armature where the EMF is generated. Generated current is taken form the rotating armature via a pair of slip rings and brushes.

24
Q

list the three ways the single loop generator can be improved to cut more flux and the limitations

A
  • increase the field strength, limited by magnetic saturation
  • increase the speed of rotation of the armature, limited by centrifugal forces
  • add more loops to the armature and put them in series with each other, will increase centrifugal forces.
25
Q

list the three advantages of stationary armature/rotating field configuration.

A
  • the stationary armature may consist of heavy loops of conductors because centrifugal forces is no longer a problem
  • rotating field is much lighter than a rotating armature so higher speeds are possible.
  • since the output current is supplied via stationary terminal, heavy duty brush gear gear and slip rings are not required
26
Q

what are the advantages of a 3-pahse system?

A
  • A 3-phase system generator will be able to produce more power than a single-phase
  • power can be transmitted with greater economy since only three or four conductors are required.
  • 3-phase system enable self-starting induction motor to be built because three-phase voltage can be used to provide a rotating magnetic field
27
Q

what is a key characteristic of Delta configuration?

A

no neutral point to connect a neutral wire, so must be balanced

28
Q

what are the two types of rotors fitted to AC generators? what are they used for?

A
  • salient pole, low speed application
  • cylindrical rotor, highspeed operation, subs
29
Q

what are the requirements for voltage control in the RN?

A
  • no load to full load, the max change in terminal voltage must be less than 1%
  • suddenly applied load, voltage not to be changed by more that 15% and must be back within 1% of the desired voltage within 1 second.
30
Q

what are the two reasons the RN parallel generators?

A
  • transfer from one gen to another or to shore supply without interrupting power to the ship.
  • 2 or more gens to share the ships electrical load increased reliability
31
Q

what are the conditions for paralleling?

A
  • both generators must have the same terminal voltage
  • the oncoming generators frequency must be slightly higher than the running generator frequency
  • both gens must have the same phase rotation
  • both gens must be in phase when circuit breaker is shut
32
Q

give a brief description on the operation of a synchronous motor

A
  • the stator causes a rotating magnetic field
  • rotor is energised with DC, acts like bar magnet.
  • magnetic field of magnetised rotor will align with rotating magnetic field set up in the stator.
  • rotor will turn at the same speed as the stator.
  • the rotor is said to be locked to the synchronous speed of the rotating magnetic feild.
33
Q

what is motor slip?

A

the difference between the speed of the rotating stator fields and the speed of the rotor

34
Q

how do you over come low staring torque with induction motors?

A

use a double cage rotor

35
Q

what are the advantages of double cage rotors?

A
  • high starting torque
  • reduced starting current
36
Q

what are the disadvantages of double cage rotors?

A
  • maximum torque less than for a single cage rotor of the same weight/size
  • high resistance start up may cause burnout if frequent starts are attempted.
37
Q

identify methods of speed control

A
  • frequency control
  • pole pairs control- stator switching or pole chaining
38
Q

list the common motor faults

A
  • short circuits
  • overloading
  • stalling
  • under voltage
39
Q

what do short circuits cause?

A

high fault currents

40
Q

what do overloading cause?

A

draw current above its rated, overheat the motor, causes stress to the insulation

41
Q

what do stalling cause?

A

severe overloading or mechanical failure

42
Q

what is single phasing?

A

when an open circuit in one supply line

43
Q

what happens when single phasing occurs ?

A
  • if running, may continue to run, current will double
  • if stopped, cannot start
44
Q

describe universal motors

A

constructed with a small fan permanently attached to the rotor shaft, provides cooling and a load to prevent excessive speeds

45
Q

List the advantages of universal motors

A

inexpensive
effective speed control

46
Q

List the disadvantages of universal motors

A

commutator and brushes require maintenance
noisy

47
Q

list the types of transformer cores, list some key points about each.

A

Core type- cheap, for low voltage transformers
shell type- less flux leakage, light current and high voltage application

151/23/005 (10 decks)

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