Polyphase Induction Motors — Part 1 MCQs with Explanations

Slip rings in polyphase induction motors are typically made of phosphor bronze. This material is chosen because it has excellent wear resistance and conductivity. Phosphor bronze also provides a good balance between conductivity and durability, which is essential for the efficient operation of the slip rings in the motor.

Slip in an induction motor is the difference between the synchronous speed (the speed of the rotating magnetic field) and the rotor speed (the actual speed of the motor shaft). It's usually expressed as a percentage. * Option A (0.04 %): This is too low. A slip this small would mean the rotor is almost running at the same speed as the rotating magnetic field, which isn't typical for medium-sized induction motors under load. * Option B (0.4 %): This is also quite low. It's more typical of very large induction motors or motors operating with very light loads. * Option C (4 %): This is the most common slip value for medium-sized induction motors under normal operating conditions. It provides a good balance between torque production and efficiency. * Option D (14 %): This is a very high slip value. Such high slip is not ideal because it causes more losses in the motor, creates excessive heat, and reduces efficiency. It might be found in specific applications requiring high starting torque. In summary: For medium-sized induction motors, a slip of around 4% is the most realistic and commonly encountered value.

The circle diagram for an induction motor is a useful tool for analyzing various performance parameters. It can be used to determine: Efficiency – By analyzing the power losses and output power on the diagram. Power factor – By examining the power factor on the diagram based on the reactive and real power components. Output – By evaluating the output power and losses. However, the circle diagram does not provide direct information about the frequency of the induction motor. The frequency is typically determined separately through other means or directly from the motor's specifications. Therefore, Option C is the correct answer as it indicates what cannot be determined using the circle diagram.

In an induction motor, the slip is defined as the difference between the synchronous speed and the actual rotor speed, expressed as a percentage of the synchronous speed. At no-load conditions, the slip of an induction motor is typically very small because the rotor speed is very close to the synchronous speed. Less than 1 % : At no-load, the slip is generally less than 1% because the rotor speed approaches the synchronous speed very closely when the motor is not under load. 1.5 % , 2 % , and 4 % : These values are higher and more characteristic of conditions when the motor is under load. At no-load, these slips would be too high, which does not align with the typical performance of induction motors at no-load conditions. Thus, Option A: Less than 1 % is the correct choice.

The slip in an induction motor is the difference between the synchronous speed and the actual rotor speed, expressed as a percentage of the synchronous speed. The frequency of the rotor emf is related to the slip and the supply frequency. For a 3-phase induction motor: Supply Frequency (f s ) : 50 Hz Slip (s) : 4% or 0.04 The frequency of the rotor emf (f r ) can be calculated using the formula: f r = s × f s Substituting the values: f r = 0.04 × 50 Hz = 2 Hz Therefore, the frequency of the rotor emf is 2 Hz.

Percentage slip in an induction motor is given by the formula: slip (%) = [(Ns - Nr) / Ns] * 100 where Ns is the synchronous speed and Nr is the rotor speed. Synchronous speed (Ns) is determined by the supply frequency (f) and the number of poles (P) of the motor, given by: Ns = (120 * f) / P Thus, slip depends on: 1. Supply frequency: Changes in supply frequency affect the synchronous speed, which in turn affects the slip percentage. Higher frequency results in higher synchronous speed and can reduce the slip if rotor speed remains constant. Other options are incorrect because: Option B: Supply voltage: While supply voltage affects the torque and current characteristics of the motor, it does not directly determine the slip percentage. Option C: Copper losses in motor: Copper losses affect the efficiency and heating of the motor but do not directly influence the slip percentage. Option D: None of the above: This is incorrect because supply frequency directly affects the slip percentage. Therefore, the correct answer is Option A: Supply frequency.

Power factor in an induction motor is influenced by the flux density in the air gap . A good power factor can be achieved when the average flux density in the air gap is large . This is because a larger flux density reduces the magnetizing current required, leading to a higher power factor . If the flux density is too small, more magnetizing current is needed, which lowers the power factor.

Inserting resistance into the rotor circuit of an induction motor increases the total impedance of the rotor circuit. This increased impedance causes the rotor current to decrease for a given torque since current is inversely proportional to impedance. The purpose of adding resistance is to control the torque and current characteristics during starting or at low speeds. Therefore, inserting resistance decreases the rotor current, making Option A the correct answer.

Block rotor test, no-load test and stator resistance test is the correct answer to “Which is of the following data is required to draw the circle diagram for an induction motor”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Both (A) and (B) is the correct answer to “The starting torque of a three phase induction motor can be increased by”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Slip ring induction motors only is the correct answer to “Rotor rheostat control method of speed control is used for”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

External resistance to the rotor is the correct answer to “The stating torque of the slip ring induction motor can be increased by adding”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Two parallel windings in rotor is the correct answer to “A double squirrel-cage induction motor has”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Induction motors is the correct answer to “The term 'cogging' is associated with”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

All of the above is the correct answer to “Blocked rotor test on induction motors is used to find out”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Will continue running burning its one phase is the correct answer to “A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Will stop and carry heavy current causing permanent damage to its winding is the correct answer to “A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Single phase motors is the correct answer to “Star-delta starting of motors is not possible in case of”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Stator slots are not exact multiple of rotor slots is the correct answer to “In an induction motor the relationship between stator slots and rotor slots is that”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Wound rotor is the correct answer to “A 3-phase slip ring motor has”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Any one of the above is the correct answer to “Low voltage at motor terminals is due to”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

0.04 is the correct answer to “A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6-poles. The slip of the machine will be”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Motor takes five to seven times its full load current is the correct answer to “It is advisable to avoid line starting of induction motor and use starter because”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

By changing the number of stator poles is the correct answer to “Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Rotor conductors are short-circuited through end rings is the correct answer to “In three-phase squirrel-cage induction motors”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

The power factor will be low is the correct answer to “In an induction motor if air-gap is increased”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Get heated and subsequently get damaged is the correct answer to “A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen? It will . . . . . . . . .”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Windage losses is the correct answer to “Short-circuit test on an induction motor cannot be used to determine”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

At the value of the slip which makes rotor reactance per phase equal to the resistance per phase is the correct answer to “The torque of a rotor in an induction motor under running condition is maximum”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

The rotor will not run is the correct answer to “What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Improve starting torque is the correct answer to “In three-phase induction motors sometimes copper bars are placed deep in the rotor to”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

D.C. shunt motor is the correct answer to “The speed characteristics of an induction motor closely resemble the speed-load characteristics of which of the following machines”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Changing the number of poles is the correct answer to “Step-less speed control of induction motor is possible by which of the following methods”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

1.5 times the full load torque is the correct answer to “The starting torque of a 3 phase squirrel cage induction motor is”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

In parallel is the correct answer to “When the equivalent circuit diagram of double squirrel-cage induction motor is constructed the two cages can be considered”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Decreases the starting torque is the correct answer to “Insertion of resistance in the stator of an induction motor”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Applied voltage per stator phase is 57.7% of the line voltage is the correct answer to “In a star-delta starter of an induction motor”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Rectangular parabola is the correct answer to “The shape of the torque/slip curve of induction motor is”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Both (A) and (B) is the correct answer to “The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.

Reduce magnetic hum is the correct answer to “In squirrel cage induction motors, the rotor slots are usually given slight skew in order to”. This question is standard in competitive examination syllabi for Polyphase Induction Motors.