# Questions & Answers of Three Phase Induction Motors: Principle of Operation, Types, Performance, Torque-Speed Characteristics, No-Load and Blocked Rotor Tests, Equivalent Circuit, Starting and Speed Control

An 8-pole, 3-phase, 50 Hz induction motor is operating at a speed of 700 rpm. The frequency of the rotor current of the motor in Hz is ____________.

A 3 phase, 50 Hz, six pole induction motor has a rotor resistance of 0.1 Ω and reactance of 0.92 Ω. Neglect the voltage drop in stator and assume that the rotor resistance is constant. Given that the full load slip is 3%, the ratio of maximum torque to full load torque is

A three-phase, 4-pole, self excited induction generator is feeding power to a load at a frequency f1. If the load is partially removed, the frequency becomes f2. If the speed of the generator is maintained at 1500 rpm in both the cases, then

A single phase induction motor draws 12 MW power at 0.6 lagging power. A capacitor is connected in parallel to the motor to improve the power factor of the combination of motor and capacitor to 0.8 lagging. Assuming that the real and reactive power drawn by the motor remains same as before, the reactive power delivered by the capacitor in MVAR is __________.

In a constant V/f control of induction motor, the ratio V/f is maintained constant from 0 to base frequency, where V is the voltage applied to the motor at fundamental frequency f. Which of the following statements relating to low frequency operation of the motor is TRUE?

A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.

If the rotor winding is open circuited and the system is made to run at rotational speed fr with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is f1 and frequency of voltage across commutator brushes is f2. The values of f1 and f2 respectively are

A single phase induction motor is provided with capacitor and centrifugal switch in series with auxiliary winding. The switch is expected to operate at a speed of 0.7 Ns, but due to malfunctioning the switch fails to operate. The torque-speed characteristic of the motor is represented by

The torque-speed characteristics of motor (TM) and load (TL) for two cases are shown in the figures (a) and (b). The load torque is equal to motor torque at points P, Q, R and S

The stable operating points are

Leakage flux in an induction motor is

A 4-pole induction motor, supplied by a slightly unbalanced three-phase 50 Hz source, is rotating at 1440 rpm. The electrical frequency in Hz of the induced negative sequence current in the rotor is

The slip of an induction motor normally does not depend on

The locked rotor current in a 3-phase, star connected 15 kW, 4-pole, 230 V, 50 Hz induction motor at rated conditions is 50 A. Neglecting losses and magnetizing current, the approximate locked rotor line current drawn when the motor is connected to a 236 V, 57 Hz supply is

A three-phase 440 V, 6 pole, 50 Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field to rotor magnetic field and speed of rotor with respect of stator magnetic field are

A balanced three-phase voltage is applied to a star-connected induction motor, the phase to neutral voltage being V. The stator resistance, rotor resistance referred to the stator, stator leakage reactance, rotor leakage reactance referred to the stator, and the magnetizing reactance are denoted by rs, rr, xs , xr and Xm , respectively. The magnitude of the starting current of the motor is given by

A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source drives a mechanical  load. The torque-speed characteristics of the motor (solid curve) and of the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the following options correctly describes the stability of A and B?

A 400 V, 50 Hz 30 hp, three-phase induction motor is drawing 50 A current at 0.8 power factor lagging. The stator and rotor copper losses are 1.5 kW and 900 W respectively. The friction and windage losses are 1050 W and the core losses are 1200 W. The air-gap power of the motor will be

A 400 V, 50 Hz, 4-pole, 1400 rpm, star connected squirrel cage induction motor has the following parameters referred to the stator:

${R}_{r}^{\text{'}}=1.0\Omega ,{X}_{r}^{\text{'}}=1.5\Omega$

Neglect stator resistance and core and rotational losses of the motor. The motor is controlled from a 3-phase voltage source inverter with constant V/f control. The stator line-to-line voltage(rms) and frequency to obtain the maximum torque at starting will be :

A 3-phase, 440 V, 50 Hz, 4-pole slip ring induction motor is fed from the rotor side through an auto-transformer and the stator is connected to a variable resistance as shown in the figure.

The motor is coupled to a 220 V, separately excited dc generator feeding power to fixed resistance of 10 Ω. Two-wattmeter method is used to measure the input power to induction motor. The variable resistance is adjusted such the motor runs at 1410 rpm and the following readings were recorded W1 = 1800 W, W2 =− 200 W.

The speed of rotation of stator magnetic field with respect to rotor structure will be

A 3-phase, 440 V, 50 Hz, 4-pole slip ring induction motor is fed from the rotor side through an auto-transformer and the stator is connected to a variable resistance as shown in the figure.

The motor is coupled to a 220 V, separately excited dc generator feeding power to fixed resistance of 10 Ω. Two-wattmeter method is used to measure the input power to induction motor. The variable resistance is adjusted such the motor runs at 1410 rpm and the following readings were recorded W1 = 1800 W, W2 =− 200 W.

Neglecting all losses of both the machines, the dc generator power output and the current through resistance (Rex) will respectively be

In a transformer, zero voltage regulation at full load is

The electromagnetic torque Te of a drive and its connected load torque TL are as shown below. Out of the operating points A, B, C and D, the stable ones are

A three-phase squirrel cage induction motor has a starting torque of 150% and a maximum torque of 300% with respect to rated torque at rated voltage and rated frequency. Neglect the stator resistance and rotational losses. The value of slip for maximum torque is

A three phase squirrel cage induction motor has a starting current of seven times the full load current and full load slip of 5%

If an auto transformer is used for reduced voltage starting to provide 1.5 per unit starting torque, the auto transformer ratio(%) should be