# SSC JE Single Phase Induction Motor Solved Question (2021 – 2020)

1. Starting current of a straight type repulsion motor is about ________ its full load value. (2021 – Shift-1)

1. 3 to 4 times
2. Half
3. The same as
4. 7 to 10 times

Explanation

The starting torque of repulsion motor is about 2.5-3 times the full-load torque and its starting current is about 3-4 times the full-load current. At normal speed, both of the windings develop torque and the output of the motor is the combined output of both of the rotor windings. The speed-torque characteristics of the repulsion induction motor resemble the characteristics of the DC compound motor.

2. Which of the following motors is operated either on direct or single-phase AC supply and produces approximately the same speed and output? (2021 – Shift-1)

1. Universal motor
2. Capacitor start and capacitor run induction motor
3. Capacitor start induction run motor
4. 1-phase series motor

Explanation

The motors which can be used with a single-phase AC source, as well as a DC source of supply and voltages, are called Universal Motor. It is basically a single-phase DC series motor. A universal motor is a commutation type motor. The universal motor produces approximately the same speed and output when operated either on a direct or single-phase AC source.

3. A two-value capacitor-run motor starts with a ________ capacitor and runs with a ________ capacitor. (2021 – Shift-1)

1. High; High
2. Low; High
3. Low; Low
4. High; Low

Explanation

A two-value capacitor-run motor starts with a High capacitor and runs with a Low capacitor.

### Capacitor start capacitor run motors

• This type of motor is designed to operate with the start winding and it is a series capacitor permanently connected to the supply.
• Two capacitors are used in these types of motors.
• One is used at the time of the starting and is known as the starting capacitor.
• The other one is used for continuous running of the motor and is known as the run capacitor.
• lt has two capacitors connected in parallel in the start winding circuit for starting purposes.
• A start capacitor stays in the circuit long enough to rapidly bring the motor up to a predetermined speed, which is usually about 75% of the full speed, and is then taken out of the circuit, often by a centrifugal switch that releases at that speed.
• In a capacitor start and run the motor the function of the running capacitor in series with the auxiliary winding is to Improve power factor.
• Run capacitors are designed for continuous duty while the motor is powered, which is why electrolytic capacitors are avoided.
• As a result, this motor has a higher starting torque as well as a higher efficiency. These motors are employed in sizes ranging from 375 W to 7.5 kW.  The power factor of these motors is upto 80% to 100%.

Note:-

• Inductive Loads consume Reactive Power and Capacitive Loads deliver Reactive Power.
• The Power Factor Angle is inversely proportional to the Power Factor (reducing the angled gap between the Voltage vector and the Current Vector improved power factor).
• So in a single-phase Induction Motor, a Capacitor Bank is connected, then the Reactive power demand by the Inductive load (Induction Motor) is supplied by the Capacitor Bank, not from the Source.
• Therefore the system reactive power demand is reduced and the Power factor is improved.

4. The main and auxiliary winding impedance of a 50-Hz, capacitor-start single-phase induction motor is Zm = (3 + j3) Ω and Za = (7 + j3) Ω. Determine the value of the capacitor to be connected in series with the auxiliary winding to achieve a phase difference of 90° between the currents of the two windings at the start. (2021 – Shift-1)

1. 225 μF
2. 22.5 μF
3. 318 μF
4. 31.8 μF

Explanation

The phasor diagram of main and auxiliary winding current is shown in figure.

Given

Main winding impedance Zm = (3 + j3) Ω

Auxiliary winding Za = (7 + j3) Ω

Phase angle φ= tan-1(3/3)

φ= 45° (lagging)

The require leading phase angle provided for auxiliary winding will be

φa = (90° − 45°)

φa = 45°

or

Phase angle of RC circuit is given as

tanφa = (Xc − Xa)/Ra

tan45° = (Xc − 3)/7

7 = (Xc − 3)

Xc  = 10Ω

Capacitive Reactance Xc = 1/2πfC

C = 1/(10 × 2 × 3.14 × 50)

C = 318 μF

5. Reluctance motor is basically a (2021 – Shift-1)

1. DC shunt motor
2. Servo motor
3. DC series motor
4. Single-phase synchronous motor

Explanation

A single-phase reluctance motor is basically the same as the single-phase salient pole synchronous cage type induction motor, hence it is also called ​self-starting type synchronous motor.

6. In a single-phase, resistance split-phase motor, the phase difference between the currents in the auxiliary winding and the main winding is approximately _______. (2021 – Shift-1)

1. 60° mechanical
2. 30° electrical
3. 60° electrical
4. 30° mechanical

Explanation

The Split Phase Motor is also known as a Resistance Start Motor. Both the windings are displaced 90 degrees in space.

• The current in the main winding (IM) lag behind the supply voltage V almost by the 90∘
• The current in the auxiliary winding IA is approximately in phase with the line voltage.
• Thus, there exists a time difference between the currents of the two windings.
• The time phase difference ϕ is not 90 degrees, but of the order of 30° to 45° degrees.
• This phase difference is enough to produce a rotating magnetic field.

7. Which of the following machines has high power factor and efficiency under running conditions? (2021 – Shift-2)

1. Capacitor-start single-phase induction motor
2. Two-value capacitor single-phase induction motor
3. Resistance split-phase single-phase induction motor

Answer.2. Two-value capacitor single-phase induction motor

Explanation

This motor has a cage rotor, and its stator has two windings namely the main winding and the auxiliary winding. The two windings are displaced 90° in space.

1. During starting of motor CS, CR both working so, ϕ is greater than 90°
2. During the running of motor only CR is working so, ϕ equals 90°
3. Due to this, it has a high power factor and efficiency under running conditions.

8. The starting winding of a single-phase motor is placed in (2021 – Shift-2)

1. Rotor
2. Stator
3. Armature
4. Field

Explanation

The starting winding of a single-phase induction motor is placed in the stator by 90° apart from running winding. When AC is supplied to the stator of a single-phase induction motor, it produces a flux of magnitude ϕm.

9. A 220-V, 50-Hz, 6-pole, single-phase induction motor runs with 3% slip. Determine the rotor speed. (2021 – Shift-2)

1. 1455 rpm
2. 728 rpm
3. 970 rpm
4. 960 rpm

Explanation

Slip is defined as

s = (Ns − Nr)/Ns

where

Ns = 120f/P

= 120 × 50/6

Ns = 1000 RPM

From above concept

0.03 = (1000 − Nr)/1000

Nr = 970 RPM

10. Which type of motor could provide the given speed-torque characteristics? (2021 – Shift-2)

1. Capacitor-start capacitor-run single-phase induction motor
2. Permanently connected capacitor-run single phase induction motor
4. Single-phase hysteresis motor

Answer.1. Capacitor-start capacitor-run single-phase induction motor

Explanation

• Capacitor-start capacitor-run single-phase induction motor could provide the given speed-torque characteristics.
• The two capacitors in this motor are CS and CR
• At starting, the two capacitors are connected in parallel.
• The capacitor CS is starting capacitor is short time rated. It is almost electrolytic.
• Since a large amount of current is required to obtain the starting torque, therefore the value of the capacitor reactance X should be low in the starting winding.
• The run capacitor is long time rated and is made of oil-filled paper.
• This type of motor is quiet and smooth running.
• They have higher efficiency than the motors that run the main winding only
• They are used for leads for higher inertia requiring frequent starts where the maximum pull-out torque and efficiently required are higher.
• These two value capacitor motors are used in pumping equipment, refrigeration, air compressor, etc.

11. Shaded pole motors are built commercially with the capacity of _______. (2021 – Shift-2)

1. 3 W to 125 W
2. 5 kW and above
3. 1 kW to 5 kW
4. 500 to 1 kW

Answer.1. 3 W to 125 W

Explanation

• The shaded pole motor is simply a self-starting single-phase induction motor whose one of the poles is shaded by the copper ring.
• The copper ring is also called the shaded ring. This copper ring acts as a secondary winding for the motor.
• Shaded pole motors are small in size and built commercially with the capacity of 3 W to 125 W

12. Choose the INCORRECT statement with respect to the single-phase hysteresis motor. (2021 – Shift-2)

1. The rotor is a smooth solid cylinder of hard steel and does not carry any winding.
2. The stator is wound with only one winding without capacitor.
3. The hysteresis motor has a low noise figure compared to the single-phase induction motor.
4. The phenomenon of hysteresis causes the rotor magnetisation to lag behind the stator-created MMF wave.

Answer.2. The stator is wound with only one winding without capacitor.

Explanation

• Hysteresis motor is a single-phase synchronous motor with a cylindrical rotor made up of ferromagnetic material and does not require any DC excitation to the rotor.Construction of Hysteresis motor: Hysteresis motor consists of a stator and rotor.
• Stator:  The stator is wound with main and auxiliary windings so as to produce the rotating magnetic field. In some hysteresis motor, the stator can also be shaded pole type. The stator is designed to produce synchronizing revolving field from a single-phase supply.
• Rotor:  The rotor is the smooth cylindrical type made up of hard magnetic material like chrome steel or alnico for high retentivity. The rotor does not carry any winding or teeth.

13. The rotor of a hysteresis motor is made of ceramic permanent magnet material, which causes: (2021 – Shift-2)

1. No hysteresis losses
2. Stationary flux
3. No eddy current losses
4. High eddy current losses

Explanation

The rotor of the hysteresis motor is made up of ceramic permanent magnetic material, therefore, it is difficult to set up an eddy current in such a rotor due to the high resistivity of magnetic materials like ceramic. Hence there is no eddy current loss but only hysteresis loss. Eddy currents are minimized  by selecting magnetic core materials that have low electrical conductivity,

14. In a single-phase, single-winding induction motor, let Ns be the synchronous speed and N be the rotor speed. What will be the rotor slip with respect to the backward rotating field? (2021 – Shift-2)

1. (Ns + N)/N
2. (Ns − N)/N
3. (Ns + N)/Ns
4. (Ns − N)/Ns

Explanation

• The speed of the rotor structure with respect to stator structure is Nr
• With the rotor running at Nr, the relative speed of the stator rotating magnetic field with respect to the rotor conductors is Ns – Nr in the direction of Ns
• This relative speed is referred to as slip speed.

Where Ns is the synchronous speed of the machine

Ns = 120 f / p

Where,

f is the frequency of the machine

p is the number of poles of the machine

Slip speed = Ns – Nr

Per unit slip or slip s = (Ns – Nr) / Ns

Range of slip (s) is 0 < s < 1

If the rotor is made to revolve in a direction opposite to the rotating magnetic field:

The relative speed between the rotor winding and the rotating magnetic flux becomes Ns + Nr

Backward slip sb = (Ns + Nr) / Ns = 2 – s

As slip varies from 0 < s < 1, backward slip varies from 2 < s < 1

∴ The range of slip variation for this mode is 2 > s > 1

15. A universal motor is said to be inductively compensated when compensating winding: (2020 – Shift-1)

1. Is short circuited on itself.
2. Is not used.
3. Has large reactance.
4. Is kept open.

Answer.1. Is short circuited on itself.

Explanation

To compensate for increased armature flux which produces severe armature reaction, it is necessary to use compensating winding. The flux produced by this winding is opposite to that produced by armature and effectively neutralizes the armature reaction.

If such a compensating winding is connected in series with the armature as shown in the Fig. The motor is said to be *conductively compensated’. For motors to be operated on a.c. and d.c. both, the compensation should be conductive. If compensating winding is short circuited on itself as shown in the Fig. the motor is said to be ‘inductively compensated’.

In this compensating winding acts as a secondary of transformer and armature as its primary. The ampere turns produced by compensating winding neutralise the armature ampere turns.

16. A single-phase induction motor with single stator winding inherently has _________ starting torque. (2020 – Shift-1)

1. Zero
2. Finite but moderate
3. Finite but large
4. Infinite

Explanation

• Single phase induction motor is not self-starting. Hence it requires starting circuit
• It will rotate in the direction of the magnetic rotation provided by starting or auxiliary winding and capacitor.
• When  AC is supplied to the stator of single phase induction motor, it produces flux of magnitude ϕm.
• According to the double field theory, this alternating flux ϕm is divided into two components of magnitude ϕm / 2 each and rotate in the opposite direction, with the synchronous speed NS.
• The resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at that particular instant.
• At starting both the flux components are exactly opposite to each other and are equal in magnitude. So, they cancel each other and hence net torque produces by the rotor at the starting condition is zero and single phase induction motor is not self-starting.
• Hence, to start the motor starting windings are used.

17. Which of the following is NOT a characteristic of hysteresis motor? (2020 – Shift-1)

1. Low noise
2. Unslotted rotor
3. Smooth running
4. High power

Explanation

• Hysteresis motor is a synchronous motor with a uniform air gap and without DC excitation.
• The stator winding remains continuously connected to the single-phase supply both at the starting as well as the running of the motor.
• The rotor of the motor is made up of a magnetically smooth chrome steel cylinder and it has no winding.
• As it’s rotor is smooth and also it operates at constant speed (synchronous motor), the noise level hysteresis motor is very low as compared to the induction motor.
• This type of motor is the smoothest running, quietest single phase motor and is used for quality sound reproduction equipment like record players, tape rewarded, etc. It is also employed electric clocks and other firming devices.
• High power is NOT a characteristic of hysteresis motor. It is used for low power applications.

18. In a single-phase repulsion motor, torque is developed on the rotor when the brush axis is fixed: (2020 – Shift-1)

1. At 90° electrical with the stator field axis
2. At an alignment with the stator field axis
3. At 90° mechanical with the stator field axis
4. At an angle with the stator field axis

Answer.4. At an angle with the stator field axis

Explanation

• Repulsion Motor is a special kind of single phase AC motor which works due to the repulsion of similar poles.
•  The stator of this motor is supplied with a single phase AC supply and the rotor circuit is shorted through a carbon brush. The main components of repulsion motor are stator, rotor and commutator brush assembly.

When the brush axis is parallel or perpendicular to the magnetic axis then no torque will be developed because the net voltage developed in the armature conductor is zero. When the brushes are at 90° electrical to the field axis or pole pieces then no torque is developed because the equal induced voltages in two halves of the armature winding will oppose each other hence there will be no induced current in the armature winding, therefore, no flux will be developed by the armature winding.

For the production of torque in the rotor, we have to place the short-circuited brushes between 0° to 90° say α with respect to the magnetic axis. The torque in repulsion motor is given as,

Armatur torque (Ta) = k sin (2α)

When α is 45° than maximum torque will be produced.

19. For a single-phase motor of 2 HP rating, supply voltage is 240 V ac. If the efficiency is 70% and power factor is 0.8, find the input current. (2020 – Shift-1)

1. 19.95 A
2. 15.95 A
3. 10.95 A
4. 17.95 A

Explanation

1 HP = 746 Watt.

Efficiency is the ratio of output power to the motor to input power to the motor.

Electrical input power P = V × I × cos ϕ

Where, P = Power

V = Supply Voltage

I = Input Current

cos ϕ = Power Factor (ϕ = angle between supply voltage and input current)

Note:-  If rating of any machine is given that means that is its output power rating.

Calculation:

Given: Output power rating of single phase Induction motor = 2 HP = 2 × 746 W = 1492 W.

Supply voltage (V) = 240 V

Efficiency (η) = 70 % = 0.7

Power factor (cos ϕ) = 0.8

η = Output Power/Input Power

Input power = Output Power/η

Input power = 1492/0.7 = 2131.43 W

Now P = V × I × cos ϕ

2131.43 = 240 × I × 0.8

I = 11.1 A ≅ 10.95 A.

20. A hybrid stepping motor has eight poles which have been castellated to have six teeth each. If the rotor has 60 teeth, calculate the stepping angle. (2020 – Shift-2)

1. 1.8°
2. 1.5°
3. 3.6°

Explanation

Step angle of the hybrid stepper motor is given by

β = (Ns − Nr) × 360/(Ns × Nr)

Where Ns = Number of stator teeth

Nr = Number of rotor teeth

Calculation:

Given that, Nr = 60

N= 8 × 6 = 48

∴ stepping angle can be calculated as

β = (60 − 48) × 360/(60 × 48)

β = 1.5°

21. A single-phase universal motor is operated with AC source. The torque of the motor during the negative half cycle of the input current: (2020 – Shift-2)

1. Follows the same pattern of the torque caused by the positive half-cycle of the input current
2. Is half of the torque caused by the positive half cycle of the input current
3. Is negative
4. Is zero

Answer.1. Follows the same pattern of the torque caused by the positive half-cycle of the input current

Explanation

In a single-phase universal motor the torque of the motor during the negative half cycle of the input current follows the same pattern of the torque caused by the positive half-cycle of the input current.

• When fed with AC supply, it still produces unidirectional torque. Because, armature winding and field winding are connected in series, they are in same phase.
•  Hence, as polarity of AC changes periodically, the direction of current in armature and field winding reverses at the same time.
• Thus, direction of magnetic field and the direction of armature current reverses in such a way that the direction of force experienced by armature conductors remains same.
• Thus, regardless of AC or DC supply, universal motor works on the same principle that DC series motor works.

When an AC voltage is applied, the direction of the current will alternate. Since the current reverses in both the rotor and stator at the same time, the magnetic fields around both windings also change simultaneously. The result is that the interaction of the two fields causes the direction of the developed torque to remain in the same direction. Therefore, the rotor turns one way, regardless of which direction the applied current flows.

22. In double-field revolving theory, slip with respect to forward flux and backward flux are: (2020 – Shift-2)

1. sf = 1 – s, and sb = s
2. sf = 2 – s, and sb = 1 – s
3. sf = s, and sb = 2 – s
4. sf = 1 – s, and sb = 2 – s

Answer.3. sf = s, and sb = 2 – s

Explanation

Double field revolving theory:

• According to the double field revolving theory, we can resolve any alternating quantity into two components.
• Each component has a magnitude equal to half of the maximum magnitude of the alternating quantity, and both these components rotate in the opposite direction to each other.

For example,

• A flux, φ can be resolved into two components ϕm/2 andϕm/2.
• Each of these components rotates in the opposite direction i.e. if one ϕm/2 is rotating in a clockwise direction then the other ϕm/2 rotates in an anticlockwise direction.
• In a single-phase induction motor, let us call these two components of flux as forwarding component of flux ϕf and the backward component of flux ϕb.
• The resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at that particular instant.
ϕr=ϕf + ϕb

The forward flux has a slip of s and the backward flux has a slip of 2-s.

23. In case of permanent-split capacitor one-phase induction motor, the starting torque is: (2020 – Shift-2)

1. 25% of the maximum torque
2. −25% of the maximum torque
3. 5% of the maximum torque
4. 0 Nm

Answer.1. 25% of the maximum torque

Explanation

Permanent-split Capacitor motor have a cage rotor, and its rotor consists of two windings namely, the main winding and the auxiliary winding. The single-phase induction motor has only one capacitor C which is connected in series with the starting winding. The capacitor C is permanently connected in series with the starting winding. The capacitor C is permanently connected in the circuit at starting and running conditions.

A single-value capacitor motor has the following advantages:

• In this type of motor, no centrifugal switch is required.
• This motor has higher efficiency.
• It has higher power-factor because of a permanently-connected capacitor.
• It has higher pull-out torque.

Limitations of permanent-split capacitor motor:

• Electrolytic capacitors cannot be used for continuous running. Therefore, paper-spaced oil-filled type capacitors are to be used. Paper capacitors of the same rating are larger in size and more costly.
• A single-value capacitor has a low starting torque usually less than full-load torque.
• At the instant of starting the current in the capacitive branch is very low. The result is that PSC motor has very low starting torque about 25% to 100% of  the rated torque depending on rotor resistance.

24. In a single-phase single-winding induction machine, single-phase AC supply is applied to the machine when the rotor is at rest. Choose the INCORRECT statement. (2020 – Shift-2)

1. MMF is stationary in space and varying in magnitude.
2. The nature of the MMF is pulsating
3. The machine produces a rotating MMF at synchronous speed.
4. The stator winding gives rise to an MMF whose axis is along with winding

Answer.3. The machine produces a rotating MMF at synchronous speed.

Explanation

Single-Phase Induction Motor:

A single-phase induction motor consists of a single-phase winding on the stator and a cage winding on the rotor. When a 1 phase supply is connected to the stator winding, a pulsating magnetic field is produced. In the pulsating field, the rotor does not rotate due to inertia.

Constructionally a single phase motor is similar to a 3 phase induction motor that the stator is provided with a single phase winding. When this winding is fed from a single phase supply, its stator produces a flux which is only alternating along the space axis.

Single-phase induction motors generally have a distributed stator winding and a squirrel-cage rotor. Figure 9.1 shows a schematic diagram of a single-phase induction motor. The ac supply voltage is applied to the stator winding, which in turn creates a nonrotating (i.e., stationary in position and pulsating with time) magnetic field.* The currents are induced in the squirrel-cage rotor windings by transformer action. These currents produce an mmf opposing the stator mmf. Since the axis of the rotor-mmf wave coincides with that of the stator field, the torque angle is zero and no starting torque develops. At standstill, therefore, the motor behaves like a single-phase nonrotating transformer with a short- circuited secondary.  This type of mmf field is sometimes referred to as a breathing field because it expands and contracts in the same place on the stator. In other words, the stator winding does not provide a rotating mmf field for the rotor mmf to chase. The fundamental mmf (flux) is stationary in space but its magnitude will alternate as the coil current alternate. A pulsating field (mmf or flux) can be considered as two revolving fields of equal magnitude but rotating in opposite direction at synchronous speed.

A single-phase induction motor is inherently not a self-starting motor.

Working:

Consider a single-phase induction motor whose rotor is at rest. Let a single-phase a.c source be connected to the stator winding. Let the stator be wound for two poles.

When the power supply for the stator is switched on, an alternating current flows through the stator winding. This sets up an alternating flux. This flux crosses the air gap and links with the rotor conductors.

By electromagnetic induction, e.m.f’s are induced in the rotor conductors. Since the rotor forms a closed circuit, currents are induced in the rotor bars.

Due to the interaction between the rotor-induced currents and the stator flux, a torque is produced.

It is readily seen that if all rotor conductors in the upper half come under a stator N pole, all rotor conductors in the lower half come under a stator S pole. Hence the upper half of the rotor is subjected to a torque which tends to rotate it in one direction and the lower half of the rotor is acted upon by an equal torque which tends to rotate it in the opposite direction. The two equal and opposite torques cancel out, with the result that the net driving torque is zero. Hence the rotor remains stationary. Thus the single-phase motor fails to develop starting torque.

25. The stator of a split-phase induction motor has two windings, the main winding and the auxiliary winding. How are these windings displaced in space by electrical degrees? (2020 – Shift-3)

1. 60°
2. 30°
3. 45°
4. 90°

Explanation

The Split Phase Motor is also known as a Resistance Start Motor. Both the windings are displaced 90 degrees in space.

• The current in the main winding (IM) lag behind the supply voltage V almost by the 90∘
• The current in the auxiliary winding IA is approximately in phase with the line voltage.
• Thus, there exists a time difference between the currents of the two windings.
• The time phase difference ϕ is not 90 degrees, but of the order of 30° to 45° degrees.
• This phase difference is enough to produce a rotating magnetic field.

26. Which of the following motors is operated either on direct or single-phase AC supply and produces approximately the same speed and output? (2020 – Shift-3)

1. Universal motor
2. Capacitor start and capacitor run induction motor
3. Capacitor start induction run motor
4. 1-phase series motor

Explanation

The motors which can be used with a single-phase AC source, as well as a DC source of supply and voltages, are called Universal Motor. It is basically a single-phase DC series motor. A universal motor is a commutation type motor. The universal motor produces approximately the same speed and output when operated either on a direct or single-phase AC source.

27. The current in a starting winding of a capacitor-start induction-run single phase motor: (2020 – Shift-3)

1. Lags with respect to current of the main winding
2. Is in phase with the voltage of the main winding
3. Is in phase with the supply voltage
4. Is leading with respect to voltage of the main winding

Explanation

A capacitor-start motor is a single-phase induction motor that employs a capacitor in the auxiliary winding circuit to produce a greater phase difference between the current in the main and the auxiliary windings.

• The capacitor start motor has a cage rotor and has two windings on the stator. They are known as the main winding and the auxiliary or the starting winding. The two windings are placed 90 degrees apart.
• A capacitor CS is connected in series with the starting winding. A centrifugal switch SC is also connected in the circuit.
• IM is the current in the main winding which is lagging the auxiliary current IA by 90 degrees as shown in the phasor diagram above. The auxiliary current IA leads the voltage.
• As the motor approaches its rated speed, the auxiliary winding and the starting capacitor is disconnected automatically by the centrifugal switch provided on the shaft of the motor.
• A capacitor motor has a capacitor in the auxiliary winding of the split-phase motor. Because of the capacitor, the starting (or auxiliary) Winding current leads the main winding current approximately by 90° in time.

On starting, the switch is closed, placing the capacitor in series with the auxiliary winding. The capacitor is of such a value that the auxiliary winding is effectively a resistive capacitive circuit in which the current leads the line voltage by approximately 45°. The main winding has enough inductance to cause the current to lag the line voltage by approximately 45°. The two field currents are therefore approximately 90° out of phase. Consequently, the fields generated are also at an angle of 90°. The result is a revolving field that is sufficient to start the rotor turning.

28. The current in a starting winding of a split-phase single-phase motor is: (2020 – Shift-3)

1. Leading with respect to voltage of the main winding
2. In phase with the supply voltage
3. Leading with respect to current of the main winding
4. In phase with the voltage of the main winding

Explanation

The Split Phase Motor is also known as a Resistance Start Motor. It has a single cage rotor, and its stator has two windings known as main winding and starting winding.

The starting winding has been wound with thinner wires than the main winding. The auxiliary winding will have higher resistance than the main winding. If both the windings were identical with respect to their resistance and reactance, the current flowing through these windings would have been the same and the angle of lag with the voltage would also be the same. However, since the starting winding is more resistive, the angle of lag of Ia which is θa is less than the angle of lag of Im i.e. θm . Thus, the two currents Ia and Im flowing through the starting winding and the mam winding are split by an angle α which is equal to (θm — θa). This angle a can be increased by having variations in L and R ratio of the two windings.

• The current in a starting winding of a split-phase single-phase motor is leading with respect to the current of the main winding.
• The current in the main winding (IM) lag behind the supply voltage V almost by the 90∘
• The current in the auxiliary winding IA is approximately in phase with the line voltage.
• Thus, there exists a time difference between the currents of the two windings.
• The time phase difference ϕ is not 90 degrees, but of the order of 30 degrees.
• This phase difference is enough to produce a rotating magnetic field.

29. Select the INCORRECT option with respect to repulsion type motor. (2020 – Shift-3)

1. Commutator is used
2. One stator winding
3. Rotor similar to squirrel cage induction motor
4. A set of brushes used

Answer.3. Rotor similar to squirrel cage induction motor

Explanation

The rotor of the repulsion motor is same as the armature of a dc machine with a commutator and brushes assembly. The rotor also wound with distributed winding in the rotor core slots. The rotor of repulsion motor is of non-salient pole type, wound with single-phase winding for four, six, or eight poles. Non-Salient rotor have smaller diameter and longer axial length. They are used for High speed operation (typically speed will be 1500 and 3000 rpm).

30. The rotor of a single-phase induction motor is running at N rpm, what is the slip with respect to forward rotating flux? (2020 – Shift-3)

1. (NS – N) / NS
2. (N + NS) / NS
3. (N – NS) / NS
4. NS/NR

Answer.3. Rotor similar to squirrel cage induction motor

Explanation

The difference between the rotor speed (N) and the rotating magnetic flux speed (Ns) is called slip.

The induction motor slip is usually expressed as a percentage of synchronous speed (NS) and is represented by symbol s.

Mathematically, Percentage slip, %s=[(Ns − N)/Ns]×100

Or fractional slip, s = (Ns − N)/Ns

At start, N = 0, therefore, Slip, s = (Ns−0)/Ns=1

31. A 8-kW, 4-pole, 220-V, 50-Hz reluctance motor has a torque angle of 30° when operating under rated load conditions. Calculate the load torque. (2020 – Shift-3)

1. 51 Nm
2. 4.3 Nm
3. 43 Nm
4. 5.1 Nm

Explanation

A reluctance motor is a constant speed motor like a synchronous motor.

Speed (Ns) = 120 f / P

f = frequency, P = number of poles

Angular speed (ω) = (2 π Ns) / 60

Calculation:

Given

P = 4, f = 50 Hz, power = 8000 W

Ns = 120 × 50 / 4 = 1500 RPM

ω = (2 × π × 1500) / 60

Torque = Power / ω

Torque = 8000 / 157

Torque = 51 N-m

32. How to extract maximum torque in a single-phase repulsion type motor? (2020 – Shift-3)

1. The angle between brush axis and stator field axis must be equal to 180°.
2. The angle between brush axis and stator field axis must be equal to 0°.
3. The angle between brush axis and stator field axis must be equal to 90°.
4. The angle between brush axis and stator field axis must be equal to 45°.

Answer.4. The angle between brush axis and stator field axis must be equal to 45°.

Explanation

Repulsion Motor: It is a special kind of single-phase AC motor which works due to the repulsion of similar poles. The stator of this motor is supplied with a single-phase AC supply and the rotor circuit is shorted through a carbon brush.

The torque in the repulsion motor is given as

Te = 0.5k IsNs2 sin 2α

Where Is and Ns are the stator filed-current and an effective number of stator turns.

α is the angle between the brush axis or rotor field axis to the stator field axis.

Since a constant stator field motor extract maximum torque when

sin 2α = 1 ⇒ 2α = 90°  ⇒ α = 45°

Maximum torque in the repulsion motor is achieved when the stator and rotor field axis is 45° apart.

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