Three Phase Induction Machines Practice Questions

A 3-phase, $400 \mathrm{~V}, 4$ pole, 50 Hz star connected induction motor has the following parameters referred to the stator: $\mathrm{R}_{\mathrm{r}}^{\prime}=1 \Omega, \mathrm{X}_{\mathrm{s}}=\mathrm{X}_{\mathrm{r}}^{\prime}=2 \Omega$ Stator resistance, magnetizing reactance and core loss of the motor are neglected. The motor is run with constant $V/f$ control from a drive. For maximum starting torque, the voltage and frequency output, respectively, from the drive, is closest to,

A three phase, $50 \mathrm{~Hz}, 6$ pole induction motor runs at $960 \mathrm{rpm}$ . The stator copper loss, core loss, and the rotational loss of the motor can be neglected. the percentage efficiency of the motor is

A 3-phase star connected slip ring induction motor has the following parameters referred to the stator: $R_{s}=3 \Omega, X_{s}=2 \Omega, X_{r}^{\prime}=2 \Omega, R_{r}^{\prime}=2.5 \Omega,$ The per phase stator to rotor effective turns ratio is $3: 1$ . The rotor winding is also star connected. The magnetizing reactance and core loss of the motor can be neglected. To have maximum torque at starting, the value of the extra resistance in ohms (referred to the rotor side) to be connected in series with each phase of the rotor winding is _____ (rounded off to 2 decimal places).

A three phase $415 \mathrm{~V}, 50 \mathrm{~Hz}, 6-pole, 960 RPM, 4 HP$ squirrel cage induction motor drives a constant torque load at rated speed operating from rated supply and delivering rated output. If the supply voltage and frequency are reduced by $20 \%$ the resultant speed of the motor in RPM (neglecting the stator leakage impednace and rotational losses) is ____ (Round off to the nearest integer)

A 3-phase, 415 V, 4-pole, 50 Hz induction motor draws 5 times the rated current at rated voltage at starting. It is required to bring down the starting current from the supply to 2 times of the rated current using a 3-phase autotransformer. If the magnetizing impedance of the induction motor and no load current of the autotransformer is neglected, then the transformation ratio of the autotransformer is given by _______. (round off to two decimal places).

The frequencies of the stator and rotor currents flowing in a three-phase 8-pole induction motor are 40 Hz and 1 Hz, respectively. The motor speed, in rpm, is _______. (round off to nearest integer)

The power input to a $\text{500 V, 50 Hz, 6-pole, 3-phase}$ induction motor running at $975 \text{ RPM}$ is $40 \text{ kW}$ . The total stator losses are $1 \text{ kW}$ . If the total friction and windage losses are $2.025\text{~ kW}$ , then the efficiency is _____________%.

An 8-pole, $50 \:Hz$ , three-phase, slip-ring induction motor has an effective rotor resistance of $0.08\:\Omega$ per phase. Its speed at maximum torque is $650 \text{ RPM}$ . The additional resistance per phase that must be inserted in the rotor to achieve maximum torque at start is _____________ $\Omega$ . (Round off to 2 decimal places.) Neglect magnetizing current and stator leakage impedance. Consider equivalent circuit parameters referred to stator.

A delta-connected, 3.7 kW, 400 V(line), three-phase, 4-pole, 50-Hz squirrel-cage induction motor has the following equivalent circuit parameters per phase referred to the stator: $R_1=5.39\Omega,R_2=5.72\Omega,X_1=X_2=8.22\Omega$ . Neglect shunt branch in the equivalent circuit. The starting line current in amperes (round off to two decimal places) when it is connected to a 100 V (line), 10 Hz, three-phase AC source is ______

The parameter of an equivalent circuit of a three-phase induction motor affected by reducing the rms value of the supply voltage at the rated frequency is

A star-connected, 12.5 kW, 208 V (line), 3-phase, 60 Hz squirrel cage induction motor has following equivalent circuit parameters per phase referred to the stator: $R_{1}=0.3\Omega ,R_{2}=0.3\Omega ,X_{1}=0.41\Omega ,X_{2}=0.41\Omega$ . Neglect shunt branch in the equivalent circuit. The starting current (in Ampere) for this motor when connected to an 80 V (line), 20 Hz, 3-phase AC source is __________.

A 3-phase, 4-pole, 400 V, 50 Hz squirrel-cage induction motor is operating at a slip of 0.02. The speed of the rotor flux in mechanical rad/sec, sensed by a stationary observer, is closest to

A 4 pole induction machine is working as an induction generator. The generator supply frequency is 60 Hz. The rotor current frequency is 5 Hz. The mechanical speed of the rotor in RPM is

The starting line current of a 415 V, 3-phase, delta connected induction motor is 120 A, when the rated voltage is applied to its stator winding. The starting line current at a reduced voltage of 110 V, in ampere, is _________.

In a constant V/f induction motor drive, the slip at the maximum torque

A 3-phase 50 Hz square wave (6-step) VSI feeds a 3-phase, 4 pole induction motor. The VSI line voltage has a dominant $5^{th}$ harmonic component. If the operating slip of the motor with respect to fundamental component voltage is 0.04, the slip of the motor with respect to $5^{th}$ harmonic component of voltage is _________.

The figure shows the per-phase equivalent circuit of a two-pole three-phase induction motor operating at 50 Hz. The "air-gap" voltage, $V_{g}$ across the magnetizing inductance, is 210 V rms, and the slip, s, is 0.05. The torque (in Nm) produced by the motor is ________.

A three-phase, 4-pole, self excited induction generator is feeding power to a load at a frequency $f_1$ . If the load is partially removed, the frequency becomes $f_2$ . If the speed of the generator is maintained at 1500 rpm in both the cases, then

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 ?