Single Phase Motor Types MCQ Quiz in বাংলা - Objective Question with Answer for Single Phase Motor Types - বিনামূল্যে ডাউনলোড করুন [PDF]

The correct answer is option 2):(90 degree )

Concept:

• ​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 figure below shows the connection diagram of a capacitor start motor.

• 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 to the circuit. The phasor diagram of the capacitor start motor is shown below
• 
• I_M is the current in the main winding which is lagging the auxiliary current I_A by 90 degrees as shown in the phasor diagram above.
• The auxiliary current I_A leads the voltage. As the motor approaches its rated speed, the auxiliary winding and the starting capacitor are disconnected automatically by the centrifugal switch provided on the shaft of the motor.

Concept:

Synchronous speed (N_s) is given by

\(N_s = \frac{120f}{P}\)

Slip (s) is given by

\(s=\frac{N_s-N_r}{N_s}\)

Where N_r = Rotor speed in rpm

f = Supply frequency in Hz

P = Number of poles

The effective rotor resistance (R_f) in the forward branch of single phase induction motor is given by

\(R_f = \frac{R_2'}{2s}\)

The effective rotor resistance (Rb) in the backward branch of single phase induction motor is given by

\(R_b = \frac{R_2'}{2(2-s)}\)

Where R₂' = Standstill rotor resistance referred to as the main stator winding.

Calculation:

Given that

P = 4,

f = 50 Hz

N_r = 1000 rpm

R₂' = 1.7 Ω 

Now synchronous speed (N_s) is calculated as

\(N_s = \frac{120\times 50}{4}\)

N_s = 1500 rpm

∴ Slip can be calculated as

\(s=\frac{1500-1000}{1500} = \frac{500}{1500} \)

∴ s = 0.33 

Now effective rotor resistance (R_b) in the backward branch of single phase induction motor is calculated as

\(R_b = \frac{1.7}{2(2-0.33)}\)

R_b = \(\frac{1.7}{2\times1.67}\) ≈ 0.5 Ω 

Domestic ceiling fan is an example of single phase induction motor

Classification of 1-ϕ induction motor with their application is shown in table given below: 

A split-phase motor has no capacitance in the auxiliary circuit. A phase shift with respect to the main current is achieved by using narrow conductors to achieve a high resistance to reactance ratio.

Increasing the resistance means that the auxiliary winding can only be used during starting, otherwise, it would overheat.

A split-phase motor has significantly lower torque at starting than any of the capacitor motors due to the reduced phase angle between main and starting winding currents.

Torques developed by a split-phase motor is proportional to the sine of the angle between Im and Is

Single-phase permanent capacitor induction motor is used in ceiling fans.

Mistake Points 

Here, one may choose capacitor start capacitor run machine also as an answer for this question, as one model of capacitor start and capacitor run motor is similar to capacitor run or permanent capacitor motor.

But in another model of capacitor start and capacitor run the motor, one of the capacitors (starting capacitor) needs to be removed with the help of a centrifugal switch once the machine reaches about 75% of rated speed.

If the permanent capacitor or capacitor run motor is not there in the options, only the capacitor start and capacitor run is given in the options, in that case, we can go with capacitor start and capacitor run.

But if both are given, in that case, the best possible answer is permanent capacitor or capacitor run motor

The correct answer is option 3.

Split-phase induction motor

• It is a type of 1ϕ induction motor whose starting frequency is limited and is NOT used for drives that require more than 1 kW.
• It has a single cage rotor, and its stator has two windings known as main winding and starting winding. Both the windings are displaced 90° in space.
• The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance.
• A resistor is connected in series with the auxiliary winding. The current in the two windings is not equal as a result, the rotating field is not uniform. Hence, the starting torque is small, of the order of 1.5 to 2 times the full load torque. At the start of the motor both the windings are connected in parallel.
• As soon as the motor reaches the speed of about 70 to 80 % of the synchronous speed the starting winding is disconnected automatically from the supply mains.

Concept:

• 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. Both the windings are displaced 90 degrees in space.
• The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance.

​

• Split-phase induction motors have low starting current and moderate starting torque, these motors are used in fans, blowers, centrifugal pumps, washing machines, grinders, lathes, air conditioning fans, etc.
• These motors are available in the size ranging from 0.05 kW to 0.5 kW

The correct answer is option 2):High, Low

Concept:

• 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.
• Both the windings are displaced 90 degrees in space.
• The main winding has very low resistance and a high inductive reactance whereas the starting winding has high resistance and low inductive reactance.

• A resistor is connected in series with the auxiliary winding.
• The current in the two windings is not equal as a result the rotating field is not uniform.
• Hence, the starting torque is small, of the order of 1.5 to 2 times the start, running torque.
• At the start of the motor both the windings are connected in parallel.
• The phasor diagram of the Split Phase Induction Motor is shown below.

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 \(\frac{{{\phi _m}}}{2}\;and - \frac{{{\phi _m}}}{2}\)
• Each of these components rotates in the opposite direction i.e. if one \(\frac{{{\phi _m}}}{2}\) is rotating in a clockwise direction then the other \(\frac{{{\phi _m}}}{2}\) rotates in an anticlockwise direction.
• In a single-phase induction motor, let us call these two components of flux as forwarding component of flux \({\phi _f}\) and the backward component of flux \({\phi _b}\).
• The resultant of these two components of flux at any instant of time gives the value of instantaneous stator flux at that particular instant.
  \({\phi _r} = {\phi _f} + {\phi _b}\)

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

sf = s, and sb = 2 - s

\(s = \frac{{N_s - N_r}}{{N_s}} \)

\({N_s} = \frac{{120 \times f}}{P}\)

Where, Ns = Synchronous speed in rpm

Nr = Rotor speed in rpm

 f = Supply frequency in Hz

P = Number of poles

The correct option is 4

Concept:

• A capacitor in a single-phase induction motor serves a vital role in creating a phase shift to enable the motor to start. It generates a second phase of power to produce a rotating magnetic field that allows the motor to start and run. If this capacitor gets short-circuited, it won't fulfill its essential function, which will interfere with the motor's performance.
• In case of a short-circuited (or failed) capacitor, the single-phase induction motor generally Will not run
• The reason it won't run, especially at the start, is because the necessary rotating magnetic field won't be created due to the absence of the phase shift caused by the capacitor. A single-phase induction motor relies on an auxiliary winding and a start capacitor to create an initial rotating field and start the rotor. Without the phase shift provided by the capacitor, the motor will simply hum or buzz and may cause the windings to get overheated if left in an attempted start condition for too long.
• In theory, if the motor was already running when the capacitor failed, it could continue to run at a lower torque but will not start again after it's stopped. However, this is not generally a safe state for the motor to be in, and it should be repaired as soon as possible to prevent further damage.