AC Motor Questions

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Very Short Answers Questions [1 Mark Questions]

Ques. Which of the following motors is most suitable for use in a domestic food mixer?

1. Three-phase synchronous motor
2. Inductor motor
3. DC shunt motor
4. Universal motor

Ans. The correct answer is d. Universal motor

Explanation: Universal motor is required where high speed and light weight are desirable, for example, food mixers, blenders, vacuum cleaners, etc.

Ques. What is a DC motor?

Ans. A DC motor is a mechanical device that uses direct current (DC) to drive electricity.

Ques. A synchronous motor can develop synchronous torque

1. when under loaded
2. when overexcited
3. only at synchronous speed
4. below or above the synchronous speed

Ans. The correct answer is c. only at synchronous speed

Explanation: A torque can only be produced by synchronous motors when they are running at synchronous speed.

Ques. In the case of AC motor circuits, static frequency changers are used for

1. speed regulation
2. improvement in cooling
3. power factor improvement
4. reversal of direction of rotation

Ans. The correct answer is a. speed regulation

Explanation: Static frequency changers are a particular kind of frequency changer used to regulate the speed of AC motors like those used in fans and pumps.

Ques. In a double squirrel cage induction motor, the outer cage winding has

1. High inductance
2. Low resistance
3. High resistance
4. None of the above

Ans. The correct answer is c. High resistance

Explanation: Most of the starting current is carried by the outside cage winding, which has a low impedance for current flow. Therefore, a strong starting torque is produced by the outer cage winding's high resistance.

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Short Answers Questions [2 Marks Questions]

Ques. What is an AC motor?

Ans. An alternating current (AC) motor is an electric motor that is powered by alternating current (AC). It typically consists of two fundamental components: an inner rotor that receives a torque from the rotating field, and an outside stator coil that produces a rotating magnetic field with coils supplied with alternating current.

Ques. What are the main parts of an AC motor?

Ans. The rotor and stator are the two major components of an AC motor. 

• The rotor revolving components of an AC motor.
• The stator is the stationary component of an AC motor.

Ques. What is the function of slip rings in an AC motor?

Ans. A slip ring works as an electrical junction for power transfer in an AC motor. It acts as a connection between the rotating part and a stationary part.

Ques. What is a rotor?

Ans. The rotor is the moving component of a rotary electric motor. Due to the configuration of the wires and the magnetic field of the motor, which creates a torque at the rotor's axis, the rotor turns.

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Long Answers Questions [3 Marks Questions]

Ques. On what principle does an electric motor work?

Ans. The working principle of an electric motor is

• A force perpendicular to itself and the external magnetic field is experienced by a current-carrying conductor when it is placed in an outer magnetic field that is perpendicular to the conductor. 
• This is how the relationship between an electric current and a magnetic field produces mechanical force.

Ques. Explain Fleming’s left-hand rule.

Ans. According to Fleming's left-hand rule, if you stretch your left hand's thumb, middle finger, and forefinger so that they are perpendicular to one another, the forefinger will stand in for the direction of the magnetic field, the middle finger will specify the direction of the current, and the thumb will point out the direction of the force. For motors, Fleming's Left-Hand Rule applies.

Ques. What is a universal motor?

Ans. A universal motor is a type of electric motor that can run on either an AC or DC source. 

• Similar to a DC series motor, an AC series motor or universal motor has armature field winds linked in series. 
• The direction of the torque is constant regardless of the polarity of the armature current since the flux of the field and the armature current converse concurrently.

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Very Long Answers Questions [5 Marks Questions]

Ques. State and explain the working principle of the AC motor.

Ans. The fundamental operation of an alternating current motor is based on the magnetic principle. The basic AC motor consists of a wire coil and two fixed magnets around a shaft. 

• When an alternating current (AC) charge is applied to a wire coil, it transforms into an electromagnet. 
• An electromagnet produces a magnetic field. A solid metal axle, a wire loop, a coil, a squirrel cage formed of metal bars, and some other freely revolving metal item that may conduct electricity are all included within the stator. 
• Power is delivered to the outer coils of the stator in an AC motor. The coils were activated in pairs, one after the other, creating a magnetic field that rotated around the outermost part of the motor. 
• The rotor is suspended inside a magnetic field. Because the magnetic field is continually changing because of rotation, the magnetic field creates an electric current inside the rotor, according to the law of electromagnetism. 
• If the conductor is a ring or a wire, the current runs in a loop around it. Eddy currents circulate around the conductor if it is merely a solid piece of metal.
• The induced current generates its own magnetic field, and the spinning magnetic field rotates as well, according to another rule of electromagnetism. 
• Simply said, when the magnets contact, the shaft, and the wire coil begin to revolve, which drives the motor.

Ques. Explain the construction of an AC motor briefly.

Ans. It has a frame or yoke, a stator, a rotor, bearings, a fan, a shaft, and slip rings. The components of an AC machine are described below. 

• The frame acts as an outside protective layer, protecting against environmental conditions. 
• The frame also functions as an outside boundary, allowing the interior pieces to be readily accommodated. The steady-state part of the equipment is the stator, which houses the stator winding.
• The rotor is the moving part that rotates either clockwise or anticlockwise depending on the thrust applied to it. 
• The bearings offer enough friction to allow the rotor to spin smoothly. A fan is employed to remove the unwanted heat that is gained during the running of the rotor. 
• It is expelled through the ventilation that is provided behind the machine. The mechanical output is delivered by a shaft while the rotor revolves.
• Slip rings are used in a standard ac machine with revolving armature stationary field winding. In this case, the slip rings allow the alternating current input to the coils to vary continually.

Ques. What are the types of 3-phase induction motors?

Ans. There are mainly two types of 3-phase induction motors. They are

• The squirrel cage rotor: It consists of a cylindrical coated core rotor with parallel copper-filled slots in each slot. With end rings permanently shorting out all of the bars, the rotor conductors cannot be made more resistant from the outside.
• Wound or Slip Ring Rotor: It has uniformly spaced windings around a covered cylindrical core. Three slip rings that may be connected to external resistance through brushes attached to each slip ring are connected to the end of the rotor windings. This external resistance may be used to calculate the beginning torque.

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