SSC JE Single Phase Induction Motor Solved Question (2021-2009)

Answer.4. Zero

Explanation:-

The working principle of an ac machine is primarily “one field following another field”. In the case of a multiphase induction motor, there will be a virtual rotating magnetic field. But considering the case of a single-phase induction motor, it’s only a pulsating field that is produced and not a rotating one. This can also be explained on the basis of ‘DOUBLE REVOLVING FIELD THEORY‘, which is based on Ferraris Principle

As per Ferrari’s principle, the alternating magnetic field produced by the stator can be split into two rotating magnetic fields of half the magnitude and rotating at the synchronous speed in opposite directions. When the alternating supply is fed to the stator winding, an alternating flux is developed This flux rotates and cuts the rotor conductors. Due to this, an EMF is induced. As the rotor circuit is closed the current flows through the rotor conductor. This rotor current will cause rotor flux and at any instant, its magnitude is given by,

φ_s = φ_m COS ωt

where φ_m is the maximum flux developed in the motor. According to this theory, the alternating flux φ_s can be resolved into two components of and φ_f  & φ_b such that the magnitude of by φ_f  & φ_b is equal to half the magnitude of φ_s. Let us assume that φ_f rotates in the clockwise direction and φ_b rotates in the anti-clockwise direction.

An emf is induced in the rotor circuit due to each rotating field. If the polarity of the induced emf in the rotor due to φ_f  is taken as positive then emf induced in the rotor due to φ_b is negative (i.e. in phase opposition). As, at standstill, the slip in either direction is the same (i.e. s = 1), the rotor impedance will also be same. Thus, the rotor currents are equal, but opposite in phase that is the starting torque developed by each revolving field is the same, with one acting in forwarding direction and other acting in the backward direction. Thus, the net torque developed by the motor is zero.

Based on the double-revolving field theory, a single phase induction motor can be visualized as having two rotors revolving in opposite directions with a common stator winding. At standstill, the two rotors develop equal torques in opposite directions and hence the net torque developed is zero.

Answer.4. Shading coils

Explanation:-

Shaded-pole motor has a salient pole stator similar to the stator of dc machine. The pole is laminated to reduce the core losses. The pole is physically divided into two sections as shown in the figure.  A heavy, short-circuited copper ring, called the shading coil, is placed on the smaller section of the pole. This section covers around one-third of the pole arc and is called the shaded portion of the pole. The remaining two-thirds section of the pole is referred to as the unshaded portion. The main single-phase winding is wound on the entire pole section. The rotor used is similar to the rotor of any other single-phase induction motor.

When a single-phase supply is fed to the main winding, an alternating flux is produced in the pole. A portion of this flux links with the shading coil and induces a voltage in it. As shading coil is a short-circuited coil, a large current flows in it. The current in the shading band causes the flux in the shaded portion of the pole to lag the flux in the unshaded portion of the pole. Thus the flux in the shaded portion reaches its maximum value after the flux in the unshaded portion reaches its maximum. This is equivalent to a progressive shift of the flux from the unshaded to the shaded portion of the pole, that is it is similar to a rotating field moving from the unshaded to the shaded portion of the Pole. Hence. the motor reproduces a starting torque.

Because of the small phase of displacement of the currents in the main and the auxiliary winding and because of the winding misalignment lower than π/2 the start torque is very low.

The shaded pole motor efficiency suffers greatly due to the presence of winding harmonic content, particularly the third harmonic which produces a dip in the speed-torque curve at approximately 1/3 synchronous speed. In addition, there are losses in the shading coils. These factors combine to make the shaded pole the least efficient and noisiest of the single-phase designs. It is used mostly in air moving applications where its low starting torque and the third harmonic dip can be tolerated.

Some important Points of shaded Poles Motor

• The peak efficiency of the shaded pole motor is about 20% to 40%.
• The power factor of the shaded pole motor is around 50% to 60%
• The starting torque of the shaded pole motor is 40% to 50% of the full load torque.
• The shaded pole motor is the cheapest of all single-phase motor.

Therefore shaded-pole motors are especially suited for small fans and pumps. Other applications are juice presses, clothe dryers, grills, simple butterfly control waves, massage apparatus, hot-air stoves and cabinet fans. Drives for reversing duties can be built with two motors assembled homologously. Shaded-pole motors are low-cost motors. Because of their low efficiency, they mostly need intensive cooling.

Answer.4. High starting torque

Explanation:-

A repulsion motor operates on the principle that like magnetic poles repel each other, not on the principle of a rotating magnetic field. The stator of a repulsion motor contains only a run winding very similar to that used in the split-phase motor. Start windings are not necessary. The rotor is actually called an armature because it contains a slotted metal-core with windings placed in the slots. The windings are connected to a commutator. A set of brushes makes contact with the surface of the commutator bars. The entire assembly looks very much like a DC armature and brush assembly. One difference, however, is that the brushes of the repulsion motor are shorted together. Their function is to provide a current path through certain parts of the armature, not to provide power to the armature from an external source.

Operation

Although the repulsion motor does not operate on the principle of a rotating magnetic field, it is an induction motor. When AC power is connected to the stator winding, a magnetic field with alternating polarities is produced in the poles. This alternating field induces a voltage into the windings of the armature. When the brushes are placed in the proper position, current Hows through the armature windings, producing a magnetic field of the same polarity in the armature. The armature magnetic field is repelled by the stator magnetic field, causing the armature to rotate. Repulsion motors will contain the same number of brushes as there are stator poles. Repulsion motors are commonly wound for four, six, or eight poles.

Repulsion-Start Induction-Run Motor

The repulsion-start induction motor which, exactly like a repulsion motor, is capable of a developing high starting torque At about 75 percent of synchronous speed, a centrifugally operated device short-circuits the entire commutator. From this speed onwards, the motor behaves like an induction motor. After the commutator is short-circuited, brushes do not carry any current, hence they may also be lifted from the commutator, in order to avoid unnecessary wear and tear and friction losses.

Repulsion-start motors are of two different designs:

1. Brush-lifting type in which the brushes are automatically lifted from the commutator when it is short-circuited. These motors generally employ the radial form of the commutator and are built both in small and large sizes.

2. Brush-riding type in which brushes ride on the commutator at all times. These motors use an axial form of the commutator and are always built-in small sizes.

The torque developed is proportional to the product of the stator flux and rotor current. Rotor current depends on the stator flux and stator flux is proportional to the stator current. Therefore, the torque developed by a repulsion motor is proportional to the square o the staler current. Since the torque developed is proportional to the square of the stator current, the torque-speed characteristics of a repulsion motor are high. The magnitude of the starting torque of a repulsion motor is high. The magnitude of starting torque depends on the position of the brush axis. Its speed regulation like a series motor is poor.

The starting torque of such a motor is in excess of 350 percent with moderate starting current. It is particularly useful where the starting period is of comparatively long duration, because of high inertia loads. Applications of such motors include machine tools, commercial refrigerators, compressors, pumps, hoists, floor-polishing and grinding devices etc.

The disadvantage is:-  noisy performance, poor speed regulation, and periodic commutator maintenance refinement. 

Because of the disadvantages mentioned above, repulsion motors have largely been replaced by capacitor-type motors and very few repulsion motors are manufactured.

Answer.3. Helps in reducing the tendency of the rotor teeth to remain under the stator teeth

Explanation:-

The rotor bars are not exactly parallel to the shaft but are given a slight skew. This skew helps to make the motor run quietly by reducing the magnetic hum. Another advantage is that this skew reduces the locking tendency of the rotor.

Rotor conductors are skewed because of these following main reasons

• Cogging:- Cogging is magnetic locking. When an induction motor refuses to start even if the full voltage is applied to it, this is called cogging. This happens when the rotor slots and stator slots are the same in number or they are integer multiples of each other. due to this the opposite poles of the stator and rotor come in front of each other and get locked. Skewing the rotor bars prevents locking thus preventing Cogging.
• Starting torque of an induction motor depends on the product of the magnitude of stator and rotor current and sine of the angle between both.
• If the conductors remain linear, the angle between the stator and rotor current will be 180 degrees. As sin(180)=0, the starting resultant torque will be zero and thus motor will fail to start. This phenomenon is called cogging.
• Skewing help the motor run more quietly because the magnetic fields are slightly skewed to offset alignment with the rotor field coils. This feature tends to reduce vibration or magnetic hum as the rotor speed changes slightly every time the conductor bars align with the rotor magnetic field. As the speed is the function of frequency so the induction motor is unable to attend the resonance frequency, therefore, the magnetic vibration is reduced.
• Skewing makes the rotor conductor longer with the reduced cross area. This increases the rotor conductor resistance hence starting performance and the torque of an induction motor is improved.

Crawling:– Crawling is a phenomenon where harmonic components introduce oscillations in torque. With the bar skewed, the amount of the bar cutting the field line grows continuously and the next bar starts cutting the field lines as the first finishes. Due to this, we get Uniform Torque.

Answer.4. Any of the above

Explanation:-

Some important Points of shaded Poles Motor

• The peak efficiency of the shaded pole motor is about 20% to 40%.
• The power factor of the shaded pole motor is around 50% to 60%
• The starting torque of the shaded pole motor is 40% to 50% of the full load torque.
• The shaded pole motor is the cheapest of all single-phase motor.

Therefore shaded-pole motors are especially suited for small fans and pumps. Other applications are juice presses, clothe dryers, grills, simple butterfly control waves, massage apparatus, hot-air stoves, and cabinet fans. Drives for reversing duties can be built with two motors assembled homologously. Shaded-pole motors are low-cost motors. Because of their low efficiency, they mostly need intensive cooling.

Answer.3. Chrome Steel

Explanation:-

Hysteresis motor is the synchronous motor which does not require any d.c. excitation to the rotor and it uses non-projected poles. Hysteresis motors start by virtue of the hysteresis losses induced in the rotor by the rotating magnetic field produced by the stator windings.

It consists of a stator that carries main and auxiliary windings so as to produce the rotating magnetic field. The stator can also be shaded pole type. The rotor is a smooth cylindrical type made up of hard magnetic material like chrome steel or alnico for high retentivity (it is the capacity of an object to retain magnetism after the action of the magnetizing force has ceased. This requires selecting a material with a high hysteresis loop area. The rotor does not carry any winding.

Answer.2. Won’t Start

Explanation:

• When the capacitor is short-circuited high inrush current will flow in the winding which may burn the starting winding of a single-phase induction motor. Therefore the motor will not start at all.

Answer.1. Hysteresis motor

Explanation:

• Hysteresis motor develops hysteresis torque which is extremely steady both in amplitude and phase.

Answer.4. Series with Ia

Explanation:

• In order to neutralize the cross magnetizing effect of armature reaction, a compensating winding is used.
• The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature. The series-connected compensating windings produce a magnetic field, which varies directly with armature current.

Answer.1. Magnetic Relay

Explanation:-

The function of starting relay provided in the refrigerator is to start the split-phase induction motor by connecting the auxiliary winding or starting winding across the main supply in addition to the main winding at the time of starting. This helps to make the split-phase inflection motor as a self-induction motor is unable to start.

The first method of disconnecting the starting winding of a split-phase motor can be done by using a centrifugal switch.

Another method of disconnecting the auxiliary winding when the motor has picked up speed is by using an electromagnetic relay as has been shown in Fig

The torque required to start the motor is significantly more than needed in the running condition At the starting time of the motor, electrical power is given to start relay and winding of the motor. It also provides current to the starting winding of the motor. The starting winding provides sufficient torque so that the motor starts running. As the motor speed increases, the torque requirement decreases and thereby current required by the motor also decreases. The current in starting relay is not able to hold the relay and it gets released which opens the starting winding contacts. Therefore, the starting winding gets disconnected.