4204h - Appliance Motors And Circuits
This quiz is designed for students who will be conducting an external exam on Appliance Motors and Circuits. Most questions in this exam are from past external exam papers.
- 1.
The angle of electrical displacement in a three phase electrical system is
- A.
90 electrical degrees
- B.
120 electrical degrees
- C.
180 electrical degrees
- D.
360 electrical degrees
Correct Answer
B. 120 electrical degreesExplanation
In a three-phase electrical system, the angle of electrical displacement refers to the phase difference between the voltages or currents in each phase. The electrical displacement angle is measured in degrees and represents the time delay between the waveforms. In a balanced three-phase system, the electrical displacement angle is typically 120 electrical degrees. This means that each phase is offset by 120 degrees from the previous phase, creating a balanced and efficient distribution of power.Rate this question:
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- 2.
The earth conductor between the earth pin of a plug and the metal frame of a motor is to be tested for continuity using a ohmmeter. Which of the following results would be acceptable?
- A.
0.1 ohms
- B.
10000 ohms
- C.
1 megohm
- D.
Infinite ohm
Correct Answer
A. 0.1 ohmsExplanation
A reading of 0.1 ohms would be an acceptable result when testing the continuity of the earth conductor. This indicates that there is a low resistance or impedance in the earth connection, which is desirable for safety purposes. A low resistance ensures that any fault currents are able to flow easily to the earth, preventing the risk of electric shock or damage to the motor. Therefore, a reading of 0.1 ohms indicates a good and reliable earth connection.Rate this question:
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- 3.
A 415 V squirrel cage motor on a 50Hz supply should have a synchronous speed of:
- A.
750 rpm
- B.
800 rpm
- C.
1000 rpm
- D.
1500 rpm
Correct Answer
D. 1500 rpmExplanation
A squirrel cage motor operates at a speed slightly less than its synchronous speed. The synchronous speed of a motor is determined by the frequency of the power supply and the number of poles in the motor. In this case, the motor is connected to a 50Hz supply, which means that it should have a synchronous speed of 1500 rpm.Rate this question:
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- 4.
Fleming Left Hand Motor Rule is used to identify the rotation direction of a motor, when the(i) Thumb identifiies the :(ii) Pointer finger identifies the:(iii) Middle finger identifies the:
- A.
(i) Direction (ii) Magnetic Field (iii) Current flow
- B.
(i) Direction (ii) Current flow (iii) Magnetic Field
- C.
(i) Current Flow (ii) Magnetic Field (iii) Direction
- D.
(i) Current flow (ii) Direction (iii) Magnetic Field
Correct Answer
A. (i) Direction (ii) Magnetic Field (iii) Current flowExplanation
The Fleming Left Hand Motor Rule is used to determine the direction of rotation of a motor. According to the rule, when the thumb represents the direction, the pointer finger represents the magnetic field, and the middle finger represents the current flow. Therefore, the correct answer is (i) Direction, (ii) Magnetic Field, and (iii) Current flow.Rate this question:
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- 5.
A motor name plate shows a LRA rating which represents the:
- A.
Typical operating current of the motor
- B.
Maximum operating current that the motor can sustain under load
- C.
Maximum allowable starting current
- D.
Operating current with a 10% safety margin, as identified by the manufacturer
Correct Answer
C. Maximum allowable starting currentExplanation
The LRA rating on a motor name plate represents the maximum allowable starting current. This means that it indicates the highest current that the motor can sustain when starting up without causing any damage. It is important to know this value in order to properly size and protect the motor, as exceeding the LRA rating during startup can lead to overheating and premature failure.Rate this question:
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- 6.
D.O.L starters are used as they provide:
- A.
Non-automatic restart function and fail safe operation
- B.
The use of a three phase switch only for on/off control
- C.
Control over starting current and/or starting torque
- D.
Reduced voltage and torque at start
Correct Answer
A. Non-automatic restart function and fail safe operationExplanation
D.O.L starters are used because they provide a non-automatic restart function and fail-safe operation. This means that in case of a power failure or any other interruption, the starter will not automatically restart, ensuring the safety of the equipment and preventing any potential damage. Additionally, D.O.L starters allow for control over the starting current and/or starting torque, which is important for proper functioning and protection of the motor. They also provide reduced voltage and torque at start, which helps in preventing any sudden jerks or overloads on the motor.Rate this question:
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- 7.
Variable Frequency Drives:
- A.
Are only used on a.c motors
- B.
Are only used on d.c motors
- C.
Can be retrofitted to most three phase motors
- D.
Are typically used for constant load applications
Correct Answer
C. Can be retrofitted to most three phase motorsExplanation
Variable Frequency Drives (VFDs) are electronic devices that control the speed and torque of an electric motor. They are commonly used to regulate the speed of three-phase motors, making them suitable for a wide range of applications. VFDs can be retrofitted to most three-phase motors, meaning they can be installed on existing motors without the need for significant modifications or replacements. This versatility makes VFDs a cost-effective solution for upgrading and improving the performance of various industrial and commercial motor systems.Rate this question:
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- 8.
The inverse time characteristics of a thermal overload means the overload will react:
- A.
To any overload current regardless of current value
- B.
Faster to a larger overload current that to a small overload current
- C.
Shower to a larger overload current that to a small overload current
- D.
Faster to a smaller overload current that a larger overload current
Correct Answer
B. Faster to a larger overload current that to a small overload currentExplanation
The inverse time characteristics of a thermal overload mean that it will react faster to a larger overload current compared to a smaller overload current. This means that as the magnitude of the overload current increases, the thermal overload will respond more quickly to protect the system from damage. This characteristic is designed to ensure that larger overloads are addressed promptly, reducing the risk of overheating and potential equipment failure.Rate this question:
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- 9.
Repetitive starting of a motor can cause increased heating of the motor and protective devices, components stress, etc. This can be avoided using:
- A.
A timer
- B.
Motor start fuses
- C.
A contactor fitted with an inverse time characteristic overload
- D.
A lock out circuit
Correct Answer
A. A timerExplanation
A timer can be used to avoid the repetitive starting of a motor, which can cause increased heating of the motor and protective devices, as well as stress on components. By setting a timer, the motor can be prevented from starting too frequently, allowing it to cool down and reducing the risk of damage or overheating. This helps to prolong the lifespan of the motor and ensure its efficient operation.Rate this question:
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- 10.
What is the function of a centifugal switch in a single phase motor?
- A.
To act as an overload device
- B.
To act as a starting device
- C.
To disable the run winding
- D.
To disable the start winding
Correct Answer
D. To disable the start windingExplanation
The function of a centrifugal switch in a single phase motor is to disable the start winding. This switch is activated by the rotating motion of the motor's rotor, and once it reaches a certain speed, it opens the circuit to the start winding. This is necessary because the start winding is only needed during the initial startup of the motor, and once it reaches a certain speed, it can be disconnected to prevent damage to the motor.Rate this question:
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- 11.
A capacitor start, capacitor run compressor motor would have the following capacitors fitted:
- A.
Start 50-80 uf @ 240V : Run 4-20 uf @ 440V
- B.
Start 50-80 uf @ 440V : Run 4-20 uf @ 240V
- C.
Start 50 -80 uf @ 240V : Run 30-50 uf @ 440 V
- D.
Start 30-50 uf @ 440V : Run 30-50 uf @ 240V
Correct Answer
A. Start 50-80 uf @ 240V : Run 4-20 uf @ 440VExplanation
A capacitor start, capacitor run compressor motor requires a higher capacitance during the starting phase to provide the necessary torque to start the motor. Therefore, the start capacitor should have a capacitance range of 50-80 uf. Additionally, the start capacitor should be rated at 240V to match the voltage supply. During the running phase, a lower capacitance is sufficient to maintain the motor's operation, so the run capacitor should have a capacitance range of 4-20 uf. The run capacitor should be rated at 440V to handle the voltage during the running phase.Rate this question:
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- 12.
The most common fault that is found on a shaded pole motor is:
- A.
Burnt out start windings
- B.
Dry, sticking bearings
- C.
Stator is inserted the wrong way around
- D.
It is not plugged in
Correct Answer
C. Stator is inserted the wrong way aroundExplanation
The most common fault found on a shaded pole motor is when the stator is inserted the wrong way around. This means that the stator, which is responsible for producing the rotating magnetic field, is not aligned correctly with the rotor. As a result, the motor will not function properly and may not start or run efficiently. This mistake can occur during installation or maintenance of the motor.Rate this question:
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- 13.
The field core of a series universal motor is liminated to reduce:
- A.
The overall weight of the motor
- B.
Copper losses
- C.
Eddy current losses
- D.
Magnetisation losses
Correct Answer
C. Eddy current lossesExplanation
The field core of a series universal motor is eliminated to reduce eddy current losses. Eddy currents are induced in the core due to the changing magnetic field, which leads to energy losses and heating. By eliminating the field core, the motor can minimize the formation of eddy currents and reduce the associated losses. This helps to improve the efficiency and performance of the motor.Rate this question:
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- 14.
Typical faults of an electronically commutated motor can include:
- A.
Electronic equipment failure, worn brushes
- B.
Bearing failure, arching brushes
- C.
Electronic equipment failure, open circuit electromagnet
- D.
Open or short circuit in the windings, bearing failure
Correct Answer
D. Open or short circuit in the windings, bearing failureExplanation
The typical faults of an electronically commutated motor can include open or short circuit in the windings and bearing failure. Open or short circuit in the windings can cause a loss of electrical connectivity, resulting in the motor not functioning properly or not working at all. Bearing failure can occur due to wear and tear over time, causing the motor to produce noise, vibrations, and eventually stop working. These faults are common in electronically commutated motors and can affect their performance and lifespan.Rate this question:
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- 15.
Typical applications for synchonous motors are:
- A.
Recording devices and washing machine agitator motors
- B.
Bottle cabinet fan motors
- C.
Delay timers and cordless drills
- D.
Clocks and timers
Correct Answer
D. Clocks and timersExplanation
Synchronous motors are commonly used in clocks and timers because they can maintain a constant speed and keep accurate time. These motors are designed to rotate at a specific speed that corresponds to the desired time measurement. They are also used in applications where precise timing is required, such as industrial timers and control systems. Therefore, clocks and timers are a typical application for synchronous motors.Rate this question:
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- 16.
The speed of an induction motor can be varied by:
- A.
Increasing the load current
- B.
Decreasing the line current
- C.
Changing the number of poles in the motor
- D.
Connecting a capacitor in the starting circuit of the motor
Correct Answer
C. Changing the number of poles in the motorExplanation
The speed of an induction motor can be varied by changing the number of poles in the motor. This is because the number of poles determines the synchronous speed of the motor. By changing the number of poles, the synchronous speed can be altered, which in turn affects the actual speed of the motor. Therefore, changing the number of poles allows for control and variation of the motor's speed.Rate this question:
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Mar 03, 2023Quiz Edited by
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Sep 24, 2008Quiz Created by
Ihai
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