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问题 1 从 42
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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?
It will rise by 3 %.
It will be unchanged.
It will fall by 9 %.
It will also fall by 3 %.
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?
Increase the magnetic field density.
Switch in the starting resistors.
Decrease the field coil current.
Increase the armature current.
If an electric induction motor is supplied at 50 Hz and contains 1 pair of poles what is its synchronous speed?
50 revs/s (3 000 RPM).
25 revs/s (1 500 RPM).
30 revs/s (1 800 RPM).
60 revs/s (3 600 RPM).
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?
Too many high starting currents in a short time may overheat the motor and associated control equipment.
Too many high starting currents in a short time mean that there is a problem with the equipment the motor is driving.
Too many high starting currents in a short time may cause damage to the motor drive coupling and shaft.
Too many high motor starting currents in a short time may overload the generator.
The most common AC motor has a cage type rotor. Which one of the following best describes this type of rotor?
It has a set of parallel conductors fixed around a copper core which are not connected at the ends.
It has a set of parallel conductors fixed around an iron core which are shorted together at each end.
It is made up of 3 conductor “cages” fitted one inside the others.
It has a set of conductors wound around a ring of iron cores.
Which of the following best describes how 3 phase electricity can be produced from a diesel generator?
We need at least 3 coils so there are 3 outputs.
We need at least 6 coils, so there are 6 outputs.
We need at least 6 coils, so there are 12 terminals.
We need only 1 coil, and the output can be split using thyristors.
Which of the following best describes how a solenoid works?
A current in the central rod overcomes the permanent magnetic field around it.
A current in the coil induces an opposing current in an iron rod.
A current in the coil produces a magnetic field which moves an iron rod.
A current in the coil produces a magnetic field which moves a copper rod.
Which of the following best describes how “brushless rectification” is carried output to supply the field coils of a typical synchronous AC generator?
A DC exciter is used with a commutator instead of slip rings.
A DC output is taken from the rotor of a DC exciter.
The output of an AC exciter is converted to DC by rectifiers mounted on the shaft.
An external DC supply is fed to the rotor through slip rings.
Which of the following best describes the basic operation of the voltage control system of a typical large modern synchronous AC generator?
The voltage regulator controls the back EMF through a set of brushes and slip rings.
The voltage regulator changes the driving engine speed by controlling its governor.
The voltage regulator changes the main field strength by controlling the output of an exciter.
The voltage regulator changes the main field strength by means of a sliding permanent magnet.
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?
The flux runs clockwise around the conductor.
The flux runs in the same direction as the current.
The flux runs in the opposite direction to the current.
The flux runs anticlockwise around the conductor.
Which of the following best describes the electrical output when a coil is rotated at a steady RPM in a fixed magnetic field?
The output is a steady DC voltage which changes direction.
The output is a steady DC voltage.
The output is a sine wave.
The output is a square wave.
Which of the following best describes the electrical output when a magnet is rotated at a steady RPM inside a fixed coil?
The output is a steady DC voltage.
The output is a sine wave.
The output is a square wave.
The output is a steady DC voltage which alternates in direction.
Which of the following best describes the main difference in the construction between a 3 phase and a single phase AC generator?
The single phase generator has no commutator and the 3 phase generator does.
The single phase generator has 3 pairs of rotor coils and the 3 phase generator only 1 pair.
The 3 phase generator has no commutator and the single phase generator does.
The 3 phase generator has 3 pairs of stator coils and the single phase generator only 1 pair.
Which of the following best describes the meaning of “synchronous” when applied to electrical generators?
The stator output frequency is always the same as the main switchboard frequency.
The stator output frequency is fixed.
The stator output voltage varies in direct proportion to the speed of the rotor.
The stator output frequency varies in direct proportion to the speed of the rotor.
Which of the following best describes the term “slip” when applied to an asynchronous electric motor?
It is the difference between speed of the load and the speed of the rotor.
It is the difference between the rotor speed and the synchronous speed.
It is the difference between the rotor speed and the synchronous speed only when measured during start up acceleration.
It is the difference between speed of the load and the speed of the rotor during start up acceleration.
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?
The rotor will slow down but the supply current will stay the same.
The supply current will increase but the rotor will stay at the same speed.
The rotor will slow down and the supply current will increase.
The rotor will speed up, which reduces the torque needed.
Which of the following best describes when the back EMF produced in a DC machine will be higher than the terminal voltage?
When the machine is run as a synchronous compensator.
When the machine is run as a generator.
When the machine is run as a motor.
When a supply is first connected to the machine.
Which of the following best describes why single phase AC generators are more likely to suffer from mechanical failure than 3 phase machines?
Single phase generators tend to be lower quality.
Single phase generators tend to be smaller.
Single phase generators tend to have larger torsional vibrations.
Single phase generators tend to be used for larger capacities.
Which of the following best describes why the rotor of an asynchronous electric motor can never rotate at synchronous speed?
The rotor construction is not strong enough to stand running at synchronous speed.
The number of pole pairs in an asynchronous motor is different in the rotor and stator so their synchronous speeds are always different.
The rotor can reach synchronous speed but will lag behind the stator position.
At synchronous speed the rotor would feel no change in stator flux and so would produce no current to make its own field.
Which of the following is a type of DC motor which can give accurate positional control?
Sensor motor.
Stepper motor.
Staged motor.
Synchronous motor.
Which of the following is the level of starting current typically taken by an electric induction motor when first started?
1,25 times rated full load.
6-8 times rated full load.
2-3 times rated full load.
0,6-0,8 times rated full load.
Which of the following is the most accurate description of the magnetic field of a synchronous AC generator?
The field is produced from an electromagnet pole arrangement supplied with a AC current.
The field is produced from an electromagnetic pole arrangement supplied with a DC current.
The field is produced from an electromagnet pole arrangement with a fixed strength.
The field is produced from a molybdenum permanent magnet pole arrangement.
Which one of the following best describes how a synchronous AC motor can be used to reduce a lagging power factor on a network?
By increasing the strength of its rotor field.
By reversing its direction.
By running it above synchronous speed.
By arranging it for dynamic compensation.
Which one of the following best describes how the magnetic field is created in the cage rotor of an electric induction motor?
A magnetic field is induced in the rotor cage of opposite polarity to that in the nearest stator winding.
Current is produced in the rotor conductors by transformer action. This creates an electromagnetic field.
The cage of the rotor is made of highly magnetic materials.
Current is passed to the rotor conductors by brushes. This creates an electromagnetic field.
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?
The voltage produced in it will be increased.
The voltage produced in it will stay the same.
The voltage produced in it will be reduced.
The voltage produced in it will be reversed.
Which one of the following best describes the affect on a conductor when passed through a magnetic field at double the previous speed?
The voltage produced in it will be doubled.
The voltage produced in it will be halved.
The voltage produced in it will stay the same.
The voltage produced in it will be quadrupled.
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?
The voltage produced in it will stay the same.
The voltage produced in it will be quadrupled.
The voltage produced in it will be doubled.
The voltage produced in it will be halved.
Which one of the following best describes the difference between the 2 basic types of asynchronous AC electrical machines?
One has a wound rotor and a commutator, the other has a cage rotor without a commutator.
One has a wound rotor and the other has a cage rotor. Neither has slip rings.
One has a wound rotor without slip rings, the other has a cage rotor and slip rings.
One has a wound rotor and slip rings, the other has a cage rotor without slip rings.
Which one of the following best describes the effect of a current carrying conductor when a magnetic field is moved across it?
The current flow in the conductor will increase.
The current flow in the conductor will reduce.
The conductor will try to move.
There will be no effect.
Which one of the following best describes the main criteria when designing an electrical generator?
The conductors must be stationary and the magnetic field must rotate.
The magnetic field must be stationary and the conductors must rotate.
The conductors and magnetic field must move at the same speed.
The conductors and magnetic field must move in relation to each other.
Which one of the following best describes the reason why the commutator of a DC machine has so many individual segments?
Each armature conductor on the rotor must have its polarity reversed twice per revolution.
Each coil on the rotor must have its polarity reversed twice per revolution.
The number of commutator segments is determined by the need to convert the AC supply frequency to DC while the motor turns.
Each armature conductor on the rotor must have its polarity reversed four times per revolution.
Which one of the following best describes the usual meaning of “EMF” when talking about electrical machines?
Electro mechanical force.
Electromagnetic field.
Electromotive force.
Electromagnetic force.
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?
Their fields will be distorted and the flux density between them will decrease.
Their magnetic fields are in the same direction, so there will be no change in flux density.
Their fields will be distorted and the flux density between them will increase.
They will jump together.
Which one of the following best describes what happens when the brushes of a DC motor reach a gap in the commutator?
The conductor not being supplied would be subject to maximum current so time for cooling is created.
The change in current in the conductor not being supplied generates a boost to the rotation.
The conductor not being supplied is moving at right angles to the field and so would produce little force.
The conductor not being supplied is moving parallel to the magnetic field and so would produce little force.
Which one of the following best describes what is between the segments of a DC machine’s commutator?
Insulation.
Glue.
A wear down sensor.
A groove cut in the solid commutator.
Which one of the following best describes why DC motors are normally fitted with brushes and commutators?
The rotor is prone to dirt build up and must be cleaned.
The stator must be supplied via the commutator.
The frame is mounted on the rotor and must be supplied with electricity.
The armature is mounted on the rotor and must be supplied with electricity.
Which one of the following best describes why an electric induction motor takes a large current when starting?
The load needs to be accelerated, which takes more current.
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.
The slow rate of change in the magnetic field within the rotor causes the large starting current.
The low resistance of the stator conductors causes the large starting current.
Which one of the following best describes the forms of energy used on ships in the process of producing electrical energy?
Mechanical energy, chemical energy then electrical energy.
Chemical energy, mechanical energy then electrical energy.
Mechanical energy then electrical energy.
Electrostatic energy then electrical energy.
Which one of the following is a correct statement in relation to the speed of a DC motor?
The speed is inversely proportional to the back EMF.
The speed is fixed by the number of pole pairs.
The speed is proportional to the flux density.
The speed is proportional to the back EMF.
Which one of the following is a way of increasing the torque output of a DC motor without losing speed?
Increase the field density.
Increase the armature current.
Decrease the field density.
Decrease the armature current.
Which one of the following is an application where a DC motor is likely to be used?
A sea water pump motor.
An air compressor motor.
A servo motor.
A pump room fan motor.
Which one of the following statements about synchronous and asynchronous AC electrical machines is correct?
A synchronous machine has no separately controlled field supply.
The speed of the synchronous machine cannot be controlled.
The speed of the synchronous machine is dependent on its “slip”.
The asynchronous machine has no separately controlled field supply.
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