Single phase Induction Motor MCQ | Objective Type Question of Single phase Induction Motor MCQ

Ques 11. The direction of rotation of a hysteresis motor is determined by

  1. Retentivity of the rotor material
  2. Position of shaded Pole with respect to the main pole
  3. Interchanging the supply leads
  4. None of the above
Answer 2. Position of shaded pole with respect to the main pole

Single-phase cylindrical (non-salient-pole) synchronous-induction or shaded-pole motors are classed as hysteresis motors. Hysteresis motor is a single-phase synchronous motor without any projected poles and without DC excitation. Such motors start by virtue of the hysteresis losses induced in the hardened steel rotor by the rotating magnetic field produced by the stator winding and operate at synchronous speed due to the retentivity of the rotor core material.

SSC JE Electrical Previous Year Question Paper 2018-SET1|SSC JE 2018 1

Principle and Working

When a single-phase supply is given to the stator (split-phase type or shaded pole type), a revolving magnetic field is set up by it. Eddy currents are induced in the rotor. These eddy currents set up the rotor magnetic field which causes the rotor to rotate. A high starting torque is produced as a result of the high rotor resistance (proportional to the hysteresis loss). As the motor approaches synchronous speed, the frequency of current reversal in the crossbars decreases, and the rotor becomes permanently magnetized in one direction.

The direction of rotation of a hysteresis motor is determined by the orientation of the main poles and the shaded poles.

Shaded-poles direction of rotation

Explanation:

 

Ques 12. Direction of rotation of a split-phase motor can be reversed by reversing the connection of

  1. Starting winding
  2. Running winding
  3. Either 1 or 2
  4. None of the above

Answer 2. Either 1 or 2

Explanation:

  • Split-phase motors receive their name from the manner in which they operate, based on the principle of the rotating magnetic field.
  • A rotating magnetic field cannot be produced with only one phase. The current is split through two separate windings with a phase displacement, which acts like a 2-phase system.
  • A rotating magnetic field can be produced with a 2-phase system. Split-phase motors start as 2-phase motors by producing an out-of-phase condition for the current in the run winding and the start winding.

Direction of rotation of the split-phase motor

  • Rotation, clockwise or counter-clockwise, is always determined by the direction of current flowing through the start winding.
  • The direction of rotation of a split-phase motor depends on the magnetic polarity of the start winding. Reversing either the polarity of the “Start” winding, in relationship to the “Run” winding, reverses the direction of rotation of all single-phase alternating current (AC) motors.
  • This reverses current flow through the start winding, causing opposite polarity and rotation.
  • In open motors, the electrical connections are located at one end of the motor, called the lead end, and the motor shaft exits the opposite end, called the shaft end.
  • Rotation direction is usually determined by viewing the shaft end. However, rotation for Genera Electric motors is determined by viewing the lead end.
  • When recording rotation direction, always indicate which end of the motor is being viewed.
  • If the shaft is turning to the right, rotation is clockwise (CW).
  • If the shaft is turning to the left, rotation is counterclockwise (CCW). Not all motors have wiring connections that allow the motor to be reversed in the field.

 

Ques 13. In a capacitor start a single-phase motor, when the capacitor is replaced by a resistance

  1. Motor will consume less power
  2. Motor will continue to run in the same direction
  3. Motor may get burn
  4. None of the above

Answer 3. Motor may get burn

Explanation:

  • When the capacitor is replaced by a resistor it will behave as the split-phase resistance start the motor.
  • In a resistance split-phase induction motor, the starting winding has a relatively small number of tums of fine wire.
  • Consequently, its resistance is higher than that of the main winding.
  • The resistance starts motor has poor starting torque because the phase angle between the running winding and start winding current is 30 to 40 degrees.
  • Owing to the fine wire used on the starting winding. the current density is high and the winding heats up quickly.
  • If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in thermal relay.

 

Ques 14. A capacitor-start single phase induction motor is switched on to supply with its capacitor replaced by an inductor of equivalent reactance value. It will

  1. Motor will not start
  2. Start and run
  3. Small hp motor can start but large hp motor will not start
  4. None of the above

Answer 3. Small hp motor can start but large hp motor will not start Explanation:

Explanation:

  • The Single-phase Induction motor is not self-starting because of phase shift between main winding and starting winding is practically zero and currently lags the voltage.
  • To make it self-start there should be sufficient phase displacement between current and voltage.

    capacitor start phase difference copy
    Capacitor Start Phase Difference
  • The start winding and run winding are already inductors, so adding inductance in series with the start winding does not produce much phase difference between the start and run windings hence the torque is not developed until there is some phase shift between these two windings.
  • The winding inductance creates a phase delay. It is only used on small motors because of low efficiency, but it is smaller and more reliable than designs that require a start capacitor on a separate winding.

    Split phase phase difference copy
    Split phase phase difference
  • The capacitor is used in a single-phase Induction motor for creating a phase shift. The addition of a capacitor creates a phase shift of 90 degrees between the two windings which makes it self-starting.
  • The addition of an inductor for a high Hp motor won’t make the motor self-starting ( even if it starts it will rotate at very less speed) and inherently will lead to an increase in losses only.

 

Ques 15. A hysteresis motor works on the principle of

  1. Eddy current loss
  2. Magnetization of rotor
  3. Hysteresis loss
  4. Electromagnetic induction
Answer 3. Hysteresis loss

Explanation:

Working of Hysteresis Motor

The operation of this motor depends on the presence of continuously revolving magnetic flux. The rotor having high retentivity, so the hysteresis loss is high. It has no winding and work on the principle of hysteresis loss. Hysteresis motor free from mechanical and magnetic vibration.

(a) The stator is wound with main and auxiliary windings. Usually, the auxiliary winding is connected in series with a permanent capacitor for the split-phase operation.

(b) The rotor is a smooth solid cylinder of hardened high-retentivity steel whose hysteresis loss is very high. It does not contain any winding.

  • The rotating magnetic field produced by phase-splitting induces eddy currents in the steel of the rotor.
  • High-retentivity steel produces a high hysteresis loss, thus consuming an appreciable amount of energy from the rotating field when the reversal of flux direction in the rotor takes place.
  • Simultaneously, the rotor begins to rotate on account of the magnetic field set up by the eddy currents in the rotor.
  • A high starting torque is developed as a result of high rotor resistance which is proportional to hysteresis loss.
  • As the rotor approaches synchronous speed, the frequency of flux reversals in the rotor decreases, and the rotor becomes permanently magnetized in one direction as a result of the high retentivity of the steel rotor.
  • The rotor revolves at synchronous speed because the rotor poles magnetically lock up with the revolving stator poles of opposite polarity.

 

Ques 16. Which of the following single-phase motors is suitable for timing and control purposes?

  1. Universal Motor
  2. Reluctance Motor
  3. Series Motor
  4. Split phase motor
Answer 2. Reluctance motor Explanation:

Explanation:

Reluctance Motor

Single-phase induction motors built with a variable air gap reluctance with no DC supply on the rotor are called reluctance motors. Such motors start like induction motors but are pulled into synchronous speed because of the variation in air gap reluctance. This pull-in force on such motors is based on the magnetic reluctance principle.

  • The single-phase reluctance motor can either be of split-phase type or capacitor-type.
  • A single-phase supply is therefore provided to the main and auxiliary winding spaced 90° apart on the stator.
  • The stator produces the revolving field. The rotor of any uniformly distributed single-phase induction motor is modified or altered as shown in Figure, where it is shown that some iron portions of the rotor have been chipped or cut away so as to produce salient rotor – poles.
  • In this way, the reluctance of the air-gap flux path will be greater where there are no conductors embedded in slots.
  • Thus the rotor flux offers variable magnetic reluctance to the stator flux, as the reluctance of the air-gap flux path varies with the position of the rotor. The salient poles have lower-reluctance air gaps.

Application of Reluctance Motor

These motors have a constant speed, rugged construction, do not need DC excitation, and need minimum maintenance. The constant speed characteristics of the reluctance motor for application such as

  • Analog electric meters.
  • Some washing machine designs.
  • Control rod drive mechanisms of nuclear reactors.
  • Hard disk drive motor.
  • Signaling device
  • Recording Instrument
  • Phonograph

 

Ques 17. In the A.C. series, motor compensating winding is employed to

  1. Increase the torque
  2. Reduce the effect of armature reaction
  3. Reduce sparking in brushes
  4. Both 2 and 3
Answer 4. Both 2 and 3

Explanation:

Function of Compensating Winding

In the case of a single-phase ac series motor, two EMFs are induced in the armature conductors.

(a) Rotating emf

(b) Transformer emf

Due to this reason, the short-circuited coils across the brushes may experience both the above emf according to the position of the brushes. Thus, the outcome appears as excessive sparking at the brushes and poor power factor of the motor. So, the compensating winding to reduce this increased armature reaction is a must in the case of ac series motors. The two types of compensated ac series motors. These are as follows:

(a) Conductively compensated ac series motor

(b) Inductively compensated ac series motor

The performances of both the conductive and inductive compensations are similar. In the case of conductive compensation, the same current flows through the compensating winding and the armature winding.

conductivley compensated

But in the case of inductive compensation, the current developed in the compensating winding is totally by the method of induction. So, if properly designed and constructed, the same performance characteristics can be obtained both for conductively compensated and inductively compensated ac series motors.

Inductively compensated

To reduce sparking at the brushes on account of the transformer emf, high resistance leads to reduce circulating currents that can be connected between the armature coils and the commutator segments.

Hence to reduce the armature reaction and sparking at brushes the compensating winding is used.

 

Ques 18. The repulsion-start induction-run motor is used because of

  1. Good Power factor
  2. High efficiency
  3. High starting torque
  4. Low cost

Answer 3. High starting Torque

Explanation:

The repulsion-start induction-run motor starts as a repulsion motor but runs like a squirrel-cage motor. There are two types of repulsion-start induction-run motors:

  1. The brush-riding type
  2. The brush-lifting type

Brush Riding type Repulsion-start-induction Run Motor

The brush-riding type uses an axial commutator. The brushes ride against the commutator segments at all times when the motor is in operation. After the motor has accelerated to approximately 75% of its full-load speed, centrifugal force causes copper segments of a short-circuiting ring to overcome the force of a spring. The segments sling out and make contact with the segments of the commutator. This effectively short-circuits all the commutator segments together, and the motor operates in the same manner as a squirrel-cage motor.

Brush lifting type Repulsion-start-induction Run Motor

The brush-lifting-type motor uses a radial commutator. Weights are mounted at the front of the armature. When the motor reaches about 75% of full speed, these weights swing outward due to centrifugal force and cause two pushrods to act against a spring barrel and short-circuiting necklace.

Brush lifting Repulsion

The weights overcome the force of the spring and cause the entire spring barrel and brush holder assembly to move toward the back of the motor. The motor is so designed that the short-circuiting necklace will short-circuit the commutator bars before the brushes lift off the surface of the radial commutator. The motor will now operate as a squirrel-cage induction motor.

Operation of a Repulsion-Start, Induction-Run Motor

  • The starting torque is good for either the brush-lifting type or the brush-riding type of repulsion-start, induction-run motor.
  • The speed performance of both types of motors is very good because they operate as single-phase induction motors.
  • The repulsion-start, the induction-run motor is of the single-phase type, ranging in size from about 1/6 hp to as high as 20 hp.
  • The starting torque is 4 times higher than the running torque.
  • The starting torque of the repulsion start motor is 350 percent with a moderate starting current.
  • Because of the excellent starting and running characteristics for both types of repulsion- start, induction-run motors, they have been used for a variety of industrial applications, including commercial refrigerators, compressors, and pumps.

 

Ques 19. The speed of a universal motor is generally reduced by using

  1. Gear train
  2. V- belt
  3. Brakes
  4. Chains
Answer 1. Gear train

Explanation:

Speed Control of Universal Motor

Various methods are employed for speed control of universal motor

Gear train Method:- Generally in order to reduce the speed of a Universal motor, an output gear system is used. When the speed reduces, the torque increases and this helps in the movement of heavy loads. Normally, two types of gear, viz. worm gear and spur gear are common in the industry.  The worm gear is self-locking thus load cannot move downward by back-driving the motor. In the case of spur gear, the load can back drive the system but frictions are normally provided for protection. Worm gear uses more power than spur gear and is approximately one-half as efficient as a spur gear. However, speed reduction from 1800 rpm to 1 rpm is possible through worm gear only.

worm gear reduction

Resistance method:- In this method, a variable resistance is connected in series with the motor. The resistance in the circuit decides the speed of the motor. This method of speed control is adopted in sewing machines. The amount of resistance is controlled by the force applied by the foot on the foot pedal.

Auto-transformer method:- An auto-transformer is used across the supply. The auto-transformer has different tapping points. Depending on the speed requirement, supply from different tappings is given to the motor.

Tapping-field method:- In this method, the field winding is tapped (for one pole) at different points. By using this technique, the flux is controlled.

Centrifugal device mechanism:- This mechanism is employed in universal motors used in home food and drink mixers. They have a number speed selection facility. An external liver (knob) is used to set the required speed. A centrifugal device is placed inside the motor. The switch is adjustable. If the motor speed exceeds the set speed, the switch opens and the resistance is excluded in the motor circuit so that speed decreases. If speed decreases, the switch closes and removes the resistance from the circuit, so that speed increases. This process is very fast and hence motor runs at almost the set speed. The capacitor is to avoid sparking due to the switching operations.

universal motor speed contr

 

Ques 20. Which of the following motors can be used for the unity power factor?

  1. Hysteresis motor
  2. Capacitor motor
  3. Schrage motor
  4. Split phase motor

Answer 3. Schrage motor

Explanation:

Schrage Motor

The Schrage motor is virtually an induction motor and a frequency changer combined into one machine. The Schrage motor has characteristics similar to those of an induction motor with constant injected voltage. Only there is an increase in speed drop due to the addition of the resistance of the brush contact and regulating winding. At low speeds particularly, this drop is more than that of an induction motor but less than that of a shunt commutator motor.

In a Schrage motor with a 3:1 speed range, the full load speed is about 20 to 30% below the no-load speed. The figure shows the speed-torque curve of a Schrage motor working at super synchronous speed, normal speed, and subsynchronous speed.

scharge motor

The Schrage type motor is useful for drives needing shunt Speed-Torque characteristics and a continuously variable speed over a wide range.

The power factor of the Schrage motor approaches unity at speeds above synchronous as the negative slip results in a capacitive effect. At subsynchronous speeds the power factor falls; however, it can be raised in non-reversing motors by arranging that the brush movement is asymmetric with respect to the `in-line’ position, the axis bisecting a corresponding pair of brushes being progressively displaced in a direction opposite to that of the rotor rotation.

Note:- By varying the brush position the speed of the scharge motor can be obtained below and above synchronous speed and scharge motor can run on positive, negative, and unity slip therefore scharge motor can work on all power factors.

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