Тест по Electric Propulsion, Electrical Machines (CES v5.0)

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Правильные ответы:
1

A 3 phase synchronous AC motor is supplied at 60 Hz. If the supply frequency falls by 3 % what will be the effect on the motor speed?

Ваш ответ правильный:

It will also fall by 3 %.
2

A DC motor is driving a load at constant torque. Which one of the following best describes how best to reduce the speed of the motor?

Правильный ответ:

Increase the magnetic field density.

Ваш ответ:

Increase the armature current.
3

If an electric induction motor is supplied at 50 Hz and contains 1 pair of poles what is its synchronous speed?

Ваш ответ правильный:

50 revs/s (3 000 RPM).
4

Some larger electric induction motors have the number of starting attempts limited during a set time period. Which one of the following best describes why this is done?

Ваш ответ правильный:

Too many high starting currents in a short time may overheat the motor and associated control equipment.
5

The most common AC motor has a cage type rotor. Which one of the following best describes this type of rotor?

Ваш ответ правильный:

It has a set of parallel conductors fixed around an iron core which are shorted together at each end.
6

Which of the following best describes how 3 phase electricity can be produced from a diesel generator?

Ваш ответ правильный:

We need at least 3 coils so there are 3 outputs.
7

Which of the following best describes how a solenoid works?

Ваш ответ правильный:

A current in the coil produces a magnetic field which moves an iron rod.
8

Which of the following best describes how “brushless rectification” is carried output to supply the field coils of a typical synchronous AC generator?

Ваш ответ правильный:

The output of an AC exciter is converted to DC by rectifiers mounted on the shaft.
9

Which of the following best describes the basic operation of the voltage control system of a typical large modern synchronous AC generator?

Ваш ответ правильный:

The voltage regulator changes the main field strength by controlling the output of an exciter.
10

Which of the following best describes the convention for the magnetic field produced when a conductor carries an electrical current flowing directly away from you?

Ваш ответ правильный:

The flux runs clockwise around the conductor.
11

Which of the following best describes the electrical output when a coil is rotated at a steady RPM in a fixed magnetic field?

Ваш ответ правильный:

The output is a sine wave.
12

Which of the following best describes the electrical output when a magnet is rotated at a steady RPM inside a fixed coil?

Ваш ответ правильный:

The output is a sine wave.
13

Which of the following best describes the main difference in the construction between a 3 phase and a single phase AC generator?

Ваш ответ правильный:

The 3 phase generator has 3 pairs of stator coils and the single phase generator only 1 pair.
14

Which of the following best describes the meaning of “synchronous” when applied to electrical generators?

Ваш ответ правильный:

The stator output frequency varies in direct proportion to the speed of the rotor.
15

Which of the following best describes the term “slip” when applied to an asynchronous electric motor?

Ваш ответ правильный:

It is the difference between the rotor speed and the synchronous speed.
16

Which of the following best describes what happens to an electric induction motor if more torque is needed to turn the pump it is driving?

Ваш ответ правильный:

The rotor will slow down and the supply current will increase.
17

Which of the following best describes when the back EMF produced in a DC machine will be higher than the terminal voltage?

Ваш ответ правильный:

When the machine is run as a generator.
18

Which of the following best describes why single phase AC generators are more likely to suffer from mechanical failure than 3 phase machines?

Ваш ответ правильный:

Single phase generators tend to have larger torsional vibrations.
19

Which of the following best describes why the rotor of an asynchronous electric motor can never rotate at synchronous speed?

Ваш ответ правильный:

At synchronous speed the rotor would feel no change in stator flux and so would produce no current to make its own field.
20

Which of the following is a type of DC motor which can give accurate positional control?

Ваш ответ правильный:

Stepper motor.
21

Which of the following is the level of starting current typically taken by an electric induction motor when first started?

Ваш ответ правильный:

6-8 times rated full load.
22

Which of the following is the most accurate description of the magnetic field of a synchronous AC generator?

Ваш ответ правильный:

The field is produced from an electromagnetic pole arrangement supplied with a DC current.
23

Which one of the following best describes how a synchronous AC motor can be used to reduce a lagging power factor on a network?

Ваш ответ правильный:

By increasing the strength of its rotor field.
24

Which one of the following best describes how the magnetic field is created in the cage rotor of an electric induction motor?

Ваш ответ правильный:

Current is produced in the rotor conductors by transformer action. This creates an electromagnetic field.
25

Which one of the following best describes the affect on a conductor passing from left to right through a magnetic field if its direction of travel is reversed?

Ваш ответ правильный:

The voltage produced in it will be reversed.
26

Which one of the following best describes the affect on a conductor when passed through a magnetic field at double the previous speed?

Ваш ответ правильный:

The voltage produced in it will be doubled.
27

Which one of the following best describes the affect on a conductor when passed a magnetic field if the strength of the field is doubled?

Ваш ответ правильный:

The voltage produced in it will be doubled.
28

Which one of the following best describes the difference between the 2 basic types of asynchronous AC electrical machines?

Ваш ответ правильный:

One has a wound rotor and slip rings, the other has a cage rotor without slip rings.
29

Which one of the following best describes the effect of a current carrying conductor when a magnetic field is moved across it?

Ваш ответ правильный:

The conductor will try to move.
30

Which one of the following best describes the main criteria when designing an electrical generator?

Ваш ответ правильный:

The conductors and magnetic field must move in relation to each other.
31

Which one of the following best describes the reason why the commutator of a DC machine has so many individual segments?

Ваш ответ правильный:

Each armature conductor on the rotor must have its polarity reversed twice per revolution.
32

Which one of the following best describes the usual meaning of “EMF” when talking about electrical machines?

Ваш ответ правильный:

Electromotive force.
33

Which one of the following best describes what happens when 2 magnets are placed next to each other with their North poles alongside each other?

Ваш ответ правильный:

Their fields will be distorted and the flux density between them will increase.
34

Which one of the following best describes what happens when the brushes of a DC motor reach a gap in the commutator?

Ваш ответ правильный:

The conductor not being supplied is moving parallel to the magnetic field and so would produce little force.
35

Which one of the following best describes what is between the segments of a DC machine’s commutator?

Ваш ответ правильный:

Insulation.
36

Which one of the following best describes why DC motors are normally fitted with brushes and commutators?

Ваш ответ правильный:

The armature is mounted on the rotor and must be supplied with electricity.
37

Which one of the following best describes why an electric induction motor takes a large current when starting?

Правильный ответ:

The low resistance of the rotor conductors and the high rate of change in the magnetic field passing over them causes the large starting current.

Ваш ответ:

The load needs to be accelerated, which takes more current.
38

Which one of the following best describes the forms of energy used on ships in the process of producing electrical energy?

Ваш ответ правильный:

Chemical energy, mechanical energy then electrical energy.
39

Which one of the following is a correct statement in relation to the speed of a DC motor?

Ваш ответ правильный:

The speed is proportional to the back EMF.
40

Which one of the following is a way of increasing the torque output of a DC motor without losing speed?

Ваш ответ правильный:

Increase the field density.
41

Which one of the following is an application where a DC motor is likely to be used?

Ваш ответ правильный:

A servo motor.
42

Which one of the following statements about synchronous and asynchronous AC electrical machines is correct?

Ваш ответ правильный:

The asynchronous machine has no separately controlled field supply.
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