The operating region of the developed torque   and speed () of an induction motor drive is given by the shaded region OQRE in the figure. The load torque  characteristic is also shown. The motor drive moves from the initial operating point O to the final operating point S. Which one of the following trajectories will take the shortest time?

A 3-phase,  pole, 50 Hz star connected induction motor has the following parameters referred to the stator:

Stator resistance, magnetizing reactance and core loss of the motor are neglected. The motor is run with constant  control from a drive. For maximum starting torque, the voltage and frequency output, respectively, from the drive, is closest to,

A three phase,  pole induction motor runs at . 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:

The per phase stator to rotor effective turns ratio is . 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).

The following columns present various modes of induction machine operation and the ranges of slip

The correct matching between the elements in column A with those of column B is

A 10-pole, , single phase induction motor runs at  while driving rated load. The frequency of induced rotor currents due to backward field is

A three phase , 6-pole,  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 , the resultant speed of the motor in RPM (neglecting the stator leakage impedance and rotational losses) is _________ (Round off to the nearest integer).

The type of single-phase induction motor, expected to have the maximum power factor during steady state running condition, is

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).

A 4-pole induction motor with inertia of  drives a constant load torque of 2 Nm. The speed of the motor is increased linearly from 1000 rpm to 1500 rpm in 4 seconds as shown in the figure below. Neglect losses in the motor. The energy, in joules, consumed by the motor during the speed change is ……… (round off to nearest integer).

A 3-phase, 415 V, 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).

Consider the table given:

The correct combination that relates the constructional feature, machine type and mitigation is

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

An 8-pole, 50 Hz. three-phase, slip-ring induction motor has an effective rotor resistance of 0.08 Ω per phase. Its speed at maximum torque is 650 RPM. The additional resistance per phase that must be inserted in the rotor to achieve maximum torque at start is …………… Ω. (Round off to 2 decimal places.)

Neglect magnetizing current and stator leakage impedance. Consider equivalent circuit parameters referred to stator.

A three-phase, 50 Hz, 4-pole induction motor runs at no-load with a slip of 1 %. With full load, the slip increases to 5 %. The % speed regulation of the motor (rounded off to 2 decimal places) 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 delta connected 3.7kW, 400V (line), three phase, 4-pole, 50-Hz squirrel-cage induction motor has following equivalent circuit parameters per phase referred to the stator:

,,

Neglect shunt branch in the equivalent circuit. The starting line current in amperes (round off to two decimal places) when it is connected to 100V (line), 10Hz, three-phase AC source is ___________.

The equivalent circuit of a single phase induction motor is shown in the figure, where the parameters are and S is the slip. At no-load, the motor speed can be approximated to be the synchronous speed. The no-load lagging power factor of the motor is ________ (up to 3 decimal places).  

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

A 375 W, 230 V, 50 Hz, capacitor start single-phase induction motor has the following constants for the main and auxiliary windings (at starting):  (main winding),  (auxiliary winding). Neglecting the magnetizing branch, the value of the capacitance  to be added in series with the auxiliary winding to obtain maximum torque at starting 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 star-connected, 12.5kW, 208 V (line), 3-phase, 60 Hz squirrel cage induction motor has following equivalent circuit parameters per phase referred to the stator: , , , . 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 ___________.

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

Consider a system consisting of a synchronous generator working at a lagging power factor, a synchronous motor working at an overexcited condition and a directly grid-connected induction generator. Consider capacitive VAr to be a source and inductive VAr to be a sink of reactive power. Which one of the following statements is TRUE?

The direction of rotation of a single-phase capacitor run induction motor is reversed by

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

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  harmonic component. If the operating slip of the motor with respect to fundamental component voltages is 0.04, the slip of the motor with respect to  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,  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 220 V, 3-phase, 4-pole, 50 Hz inductor motor of wound rotor type is supplied at rated voltage and frequency. The stator resistance, magnetizing reactance, and core loss are negligible. The maximum torque produced by the rotor is 225% of full load torque and it occurs at 15% slip. The actual rotor resistance is 0.03. The value of external resistance (in Ohm) which must be inserted in a rotor phase if the maximum torque is to occur at start is______.

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

A 3 phase, 50Hz, six pole induction motor has a rotor resistance of  and reactance of . Neglect the voltage drop in stator and assume that the rotor resistance is constant.

Given that the full load slip of 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 . If the load is partially removed, the frequency becomes . 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 operating 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 a rotational speed  with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is  and frequency of voltage across commutator brushes is . The values of  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 0.7Ns, but due to malfunctioning the switch fails to operate. The torque-speed characteristics of the motor is represented by

The torque-speed characteristics of motor  and load  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 50Hz source, is rotating at 1440rpm. 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

A two-phase load draws the following phase currents: , . These currents are balanced if  is equal to

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

A three-phase 440V, 6pole, 50Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field with respect to rotor magnetic field and speed of rotor with respect to state 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  and  respectively. The magnitude of the starting current of the motor is given by:

A separately excited dc machine is coupled to a 50Hz, three-phase. 4-pole induction machine as shown in the figure. The dc machine is energized first and the machines rotate at 1600rpm. Subsequently the induction machine is also connected to a 50Hz, three-phase source, the phase sequence being consistent with the direction of rotation, in steady state.

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 220V, 50 Hz, single-phase induction motor has the following connection diagram and winding orientations shown. MM’ is the axis of the main stator winding  and AA' is that of the auxiliary winding . Directions of the winding axes indicate direction of .flux when currents in the windings are in the directions shown. Parameters of each winding are indicated.  When switch S is closed, the motor.

A 230V. 50 Hz, 4-pole, single-phase induction motor is rotating in the clockwise (forward) direction at a speed of 1425 rpm. If the rotor resistance at standstill is 7.8Ω, then the effective rotor resistance in the backward branch of the equivalent circuit will be

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 1050W 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:

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 autotransformer and the stator is connected to a variable resistance as shown in the figure.

The motor is coupled to a 220V, separately excited, dc generator feeding power to fixed resistance of10Ω. Two-wattmeter method is used to measure the input power to induction motor. The variable resistance is adjusted such that the motor runs at 1410 rpm and the following readings were recorded:

,

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 autotransformer and the stator is connected to a variable resistance as shown in the figure.

The motor is coupled to a 220V, separately excited, dc generator feeding power to fixed resistance of10Ω. Two-wattmeter method is used to measure the input power to induction motor. The variable resistance is adjusted such that the motor runs at 1410 rpm and the following readings were recorded:

,

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

The electromagnetic torque  of a drive, and its connected load torque 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

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

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 a star-delta starter is used to start this induction motor, the per unit starting torque will be

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 a starting torque of 0.5 per unit is required then the per unit starting current should be

For a single phase capacitor start induction motor which of the following statements is valid?

The speed of a 3-phase, 440 V, 50 Hz induction motor is to be controlled over a wide range from zero speed to 1.5 times the rated speed using a 3-phase voltage source inverter.  It is desired to keep the flux in

the machine constant in the constant torque region by controlling the terminal voltage as the frequency changes.  The inverter output voltage vs frequency characteristics should be  

The speed of a 4-pole induction motor is controlled by varying the supply frequency while maintaining the ratio of supply voltage to supply frequency (V/fJ constant. At rated frequency of 50 Hz and rated voltage of 400 V its speed is 1440 rpm. Find the speed at 30 Hz, if the load torque is constant.         

A 3 phase, 4 poles, 400 V, 50 Hz, star connected induction motor has following circuit parameters.  The starting torque when the motor is started direct-on-line is (use approximate equivalent circuit model)

A 3 phase, 10 kW, 400 V, 4 pole, 50 Hz, star connected induction motor draws 20A on full load. It’s no load and blocked rotor test data are given below:

No Load test  : 400 V  6A 1002 W

Blocked Rotor test : 90 V  15A 762 W

Neglecting copper loss in no Load test and core loss in Blocked Rotor test, estimate motor’s full load efficiency

On the torque/speed curve of induction motor shown in Figure, four points of operation are market as W, X, Y and Z. Which one of them represents the operation at a slip greater than 1?

For an induction motor, operating at a slip s, the ratio of gross power output to air gap power is equal to:

Under no load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value,

A three-phase cage induction motor is started by direct-on-line (DOL) switching at the rated voltage. If the starting current drawn is 6 times the full load current, and the full load slip is 4%, the ratio of the starting developed torque to the full load torque is approximately equal to

In a single phase induction motor driving a fan load, the reason for having a high resistance rotor is to achieve

Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].

Assertion: Under V/f control of induction motor, the maximum value of the developed torque remains constant over a wide range of speed in the sub-synchronous region.

Reason: The magnetic flux is maintained almost constant at the rated value by keeping the ratio V/f constant over the considered speed range.

The type of single-phase induction motor having the highest power factor at full load is

The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with 3-phase sinusoidal voltage having phase sequence A-B-C. For counter clockwise rotation of the motor, the phase sequence of the power supply should be 

A single-phase, 230 V, 50 Hz, 4 pole, capacitor-start induction motor has the following stand still impedances

Main winding =6.0 + j4.0Ω

Auxiliary winding = 8.0 + j6.0Ω

The value of the starting capacitor required to produce 90° phase difference between the currents in the main and auxiliary windings will be

A 400V, 15 kW, 4 poles, 50 Hz, Y-connected induction motor has full load slip of 4%. The output torque of the machine at full load is

An ac induction motor is used for a speed control application. It is driven from an inverter with a constant V/f control. The motor nameplate details are as follows

V: 415 V       Ph: 3        f: 50Hz         N: 2850 rpm

The motor is run with the inverter output frequency set at 40 Hz, and with half the rated slip. The running speed of the motor is

No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields

Following are some of the properties of rotating electrical machines                

P        Stator winding current is dc, rotor-winding current is ac

Q         Stator winding current is ac, rotor-winding current is dc

R         Stator winding current is ac, rotor-winding current is ac

S         Stator has salient poles and rotor has         commutator

T         Rotor has salient poles and slip rings and stator is         cylindrical

U         Both stator and rotor have poly-phase windings

DC machines, Synchronous machines and Induction machines exhibit some of the above properties as given in the following table. Indicate the correct combination from this table

A 3-phase Inductor Motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, following statements related to the new condition if stator resistance, leakage reactance and core loss are ignored.

P        The difference between synchronous speed and actual         speed remains same

Q        The air-gap flux remains same

R        The stator current remains same

S        The p.u. slip remains same

Among the above, correct statements are

A single phase induction motor with only the main winding excited would exhibit the following response at synchronous speed         

If a 400V, 50 Hz, star connected, 3 phase squirrel cage induction motor is operated from a 400 V, 75 Hz supply, the torque that the motor can now provide while drawing rated current from the supply?

The rotor of a three phase, 5 kW, 400V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no load steady state speed when this motor is connected to 400V, 50 Hz supply is

A 440 V, 50 Hz, 6 poles and 960 rpm star connected induction machine has the following per phase parameters referred to the stator:

The magnetizing reactance is very high and is neglected. The machine is connected to the 440V, 50 Hz supply and a certain mechanical load is coupled to it. It is found that the magnitude of the stator current is equal to the rated current of the machine but the machine is running at a speed higher than its rated speed. Find the speed at which the machine is running. Also find the torque developed by the machine.

A 3-phase, 4-pole squirrel cage induction motor has 36 stator and 28 rotor slots. The number of phases in the rotor is:

The power input to a 415V, 50 Hz, 6 pole, 3-phase induction motor running at 975 rpm is 40 kW. The stator losses are 1 kW and friction and windage losses total 2kW. The efficiency of the motor is        

A three phase, wound rotor induction motor is to be operated with slip energy recovery in the constant torque mode, when it delivers an output power at slip s. Then theoretically, the maximum power that is available for recovery at the rotor terminals, is equal to

A 230V, 20 hp, 60 Hz, 6-pole, 3-phase induction motor driving a constant torque load at rated frequency, rated voltage and rated horse-power, has a speed of 1175 rpm and an efficiency of 92.1%. Determine the new operating speed if a system disturbance causes 10% drop in voltage and 6% drop in frequency. Assume that friction, windage and stray power losses remain constant.

Starting torque can be obtained in the case of a single phase induction motor with identical main and auxiliary windings by connecting

The following starting method for an induction motor is inferior view of the poor starting torque per ampere of the line current drawn:

A 3 kW, 400V/200V, Delta/Star, 50Hz, three phase, 6-pole induction motor is found to draw a line current of 25A at a power factor of 0.4, when a blocked rotor test is conducted at the rated voltage. The stator and rotor winding resistances in ohms per phase are A and B, if the torque developed by the motor under the above conditions is 25 N-m. Find A+B=

A 3 phase squirrel cage induction motor has a full load efficiency of 0.8 and a maximum efficiency of 0.9. it is operated at a slip of 0.6 by applying a reduced voltage. The efficiency of the motor at this operating point is:

Match the column         

                Test        Machine

(A)        No load and         (P) Transformer

                blocked rotor test

(B) Sumpner’s test         (Q) Induction machine

(C) Swinburne’s test         (R) Synchronous machine

                        (S) DC machine

A 50 hp, 6 pole, 50 Hz slip-ring induction motor runs at 960 rpm on full load with a rotor current of 40A. Allowing 300W for the copper loss in the short circuiting gear and 1200W for mechanical losses, find the resistance  per phase of the 3 phase rotor winding.

If an induction machine is run at above synchronous speed, it acts as

A 3-phase, 20kW, 400V, 1470rpm, 50Hz squirrel cage induction motor develops a torque of 100 N-m at a speed of 1400rpm. If the motor is connected to a 30Hz supply, for keeping the same air-gap flux, then supply voltage should be… V and for the same load torque, the new speed will be…. Rpm

A 50kVA, 400V, 3-phase, 50Hz squirrel cage induction motor has full load slip of 5%. Its standstill impedance is 0.866 ohms/phase. It is started using a tapped autotransformer. It the maximum allowable supply current at the time of starting is 100A, then calculate the tap position and the ratio of starting torque to full load torque

Unbalanced supply voltage given to a 3-phase, delta-connected induction motor will cause

When the supply voltage to an induction motor is reduced by 10%, the maximum torque will decrease by approximately

An induction motor having full load torque of 60Nm when delta-connected develops a starting torque of 120Nm. For the same supply voltage, if the motor is changed to star-connection, the starting torque developed will be

In case of a split phase motor, the phase shift between currents in the two windings is around

In an induction motor, if the air gap is increased

Out of the several characteristics shown in figure identify the appropriate ones to match the following for a 3-phase induction motor.

 x-axis                        y-axis

(a) Load         efficiency                (P) curve-I

(b) Speed        current                 (Q) curve-II

(c) Speed         power factor        (R) curve-III

                        (S) curve-IV

                                (T) curve-V

An induction motor is fed from a balanced three phase supply at rated voltage and frequency through a bank of three single phase transformers connected in delta-delta. One unit of the bank develops fault and is removed. Then,

An induction motor runs stably under constant torque load at 1250 rpm off a 50Hz supply. Its number of poles is _______________

Motor characteristics         

(a) Speed torque characteristics of induction machine under motoring operation.

(b) Current torque characteristics of a dc series motor

(c) Power factor variation with voltage of an induction motor under no-load operation

(d) Speed torque characteristics of induction machine under dc injection dynamic braking operation

(e) Speed torque characteristics of dc series motor.