Electrical Machines

Question 1

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

Answer 1

Shunt motor

Question 2

what type of motor would use a squirrel cage rotor?

Answer 2

three phase induction motor

Question 3

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

Answer 3

copper
aluminium
iron

Question 4

how are eddy currents minimised in a practical transformer?

Answer 4

use a laminated iron core

Question 5

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

Answer 5

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

Question 6

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

Answer 6

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

Question 7

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

Answer 7

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

Question 8

Identify the types of single phase AC motor

Answer 8

induction
universal

Question 9

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

Answer 9

it is cheaper and more efficient but provides no isolation

Question 10

list the four types of three phase transformers

Answer 10

• star star
• star delta
• delta star
• delta delta

Question 11

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

Answer 11

star

Question 12

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

Answer 12

to allow unbalanced currents to flow

Question 13

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

Answer 13

• same terminal voltage
• slightly higher oncoming frequency
• same phase rotation
• in phase

Question 14

what are two parameters measured by a synchroscope

Answer 14

• frequency difference
• phase difference

Question 15

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

Answer 15

• the governor setting
• The AVR output

Question 16

state the typical power factor or an RN warship electrical supply

Answer 16

0.8

Question 17

what is the purpose of a DC motor?

Answer 17

a machine that converts direct electrical energy to mechanical energy

Question 18

define motor effect

Answer 18

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

Flemings left hand rules gives the direction of the force

Question 19

what are the consequences of Armature reaction

Answer 19

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

Question 20

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

Answer 20

• Flux control, decreases
• Armature resistance, decreases
• Voltage control, increases

Question 21

what is field control used for in DC shunt motors?

Answer 21

varying the field current

Question 22

what is field control used for in DC series motors?

Answer 22

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

Question 23

describe a single loop AC generator

Answer 23

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.

Question 24

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

Answer 24

• 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.

Question 25

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

Answer 25

- 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

Question 26

what are the advantages of a 3-pahse system?

Answer 26

- 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

Question 27

what is a key characteristic of Delta configuration?

Answer 27

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

Question 28

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

Answer 28

- salient pole, low speed application - cylindrical rotor, highspeed operation, subs

Question 29

what are the requirements for voltage control in the RN?

Answer 29

- 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.

Question 30

what are the two reasons the RN parallel generators?

Answer 30

- 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

Question 31

what are the conditions for paralleling?

Answer 31

- 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

Question 32

give a brief description on the operation of a synchronous motor

Answer 32

- 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.

Question 33

what is motor slip?

Answer 33

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

Question 34

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

Answer 34

use a double cage rotor

Question 35

what are the advantages of double cage rotors?

Answer 35

- high starting torque - reduced starting current

Question 36

what are the disadvantages of double cage rotors?

Answer 36

- 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.

Question 37

identify methods of speed control

Answer 37

- frequency control - pole pairs control- stator switching or pole chaining

Question 38

list the common motor faults

Answer 38

- short circuits - overloading - stalling - under voltage

Question 39

what do short circuits cause?

Answer 39

high fault currents

Question 40

what do overloading cause?

Answer 40

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

Question 41

what do stalling cause?

Answer 41

severe overloading or mechanical failure

Question 42

what is single phasing?

Answer 42

when an open circuit in one supply line

Question 43

what happens when single phasing occurs ?

Answer 43

- if running, may continue to run, current will double - if stopped, cannot start

Question 44

describe universal motors

Answer 44

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

Question 45

List the advantages of universal motors

Answer 45

inexpensive effective speed control

Question 46

List the disadvantages of universal motors

Answer 46

commutator and brushes require maintenance noisy

Question 47

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

Answer 47

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