The induced emf in a  pole, 3-phase star connected synchronous motor is considered to be equal and in phase with the terminal voltage under no load condition. On application of a mechanical load, the induced emf phasor is deflected by an angle of  mechanical with respect to the terminal voltage phasor. If the synchronous reactance is , and stator resistance is negligible, then the motor armature current magnitude, in ampere, during loaded condition is closest to, _________ (round off to two decimal places).

A 3-phase, 11kV, 10 MVA synchronous generator is connected to an inductive load of power factor  via a lossless line with a per-phase inductive reactance of . The per-phase synchronous reactance of the generator is  with negligible armature resistance. If the generator is producing the rated current at the rated voltage, then the power factor at the terminal of the generator is

A three-phase synchronous motor with synchronous impedance of  per unit per phase has a static stability limit of 2.5 per unit. The corresponding excitation voltage in per unit is _________(Round off to 2 decimal places).

Two generating units rated for 250 MW and 400 MW have governor speed regulations of 6% and 6.4%, respectively, from no load to full load. Both the generating units are operating in parallel to share a load of 500 MW. Assuming free governor action, the load shared in MW, by the 250 MW generating unit is _________. (round off to nearest integer)

A star-connected 3-phase, 400 V, 50 kVA, 50 Hz synchronous motor has a synchronous reactance of 1 ohm per phase with negligible armature resistance. The shaft load on the motor is 10 kW while the power factor is 0.8 leading. The loss in the motor is 2 kW. The magnitude of the per phase excitation emf of the motor, in volts, is ___________ (round off to nearest integer).

A three-phase cylindrical rotor synchronous generator has a synchronous reactance  and a negligible armature resistance. The magnitude of per phase terminal voltage is and the magnitude of per phase induced emf is . Considering the following two statements, P and Q,

P: For any three phase balanced leading load connected across the terminals of this synchronous generator,  is always more than  

Q: For any three-phase balanced lagging load connected across the terminals of this synchronous generator, is always less than  

Which of the following options is correct?

A single 50 Hz synchronous generator on droop control was delivering 100 MW power to a system. Due to increase in load, generator power had to be increased by 10 MW, as a result of which, system frequency dropped to 49.75 Hz. Further increase in load in the system resulted in a frequency of 49.25 Hz. At this condition, the power in MW supplied by the generator is __________ (rounded off to 2 decimal pages).

The figure below shows the per-phase Open Circuit Characteristics (measured in V) and Short Circuit Characteristics (measured in A) of a 14 kVA, 400~V. 50~Hz, 4-pole, 3-phase, delta connected alternator, driven at 1500 rpm. The field current, If is measured in A. Readings taken are marked as respective (x, y) coordinates in the figure. Ratio of the unsaturated and saturated synchronous impedances  of the alternator is closest to

A cylindrical rotor synchronous generator with constant real power output and constant terminal voltage is supplying 100 A current to a 0.9 lagging power factor load. An ideal reactor is now connected in parallel with the load, as a result of which the total lagging reactive power requirement of the load is twice the previous value while the real power remains unchanged. The armature current is now _________ A (Rounded off to 2 decimal places).

A cylindrical rotor synchronous generator has steady state synchronous reactance of 0.7 pu and subtransient reactance of 0.2 pu. It is operating at (1 + j0) pu terminal voltage with an internal emf of (1 + j0.7) pu. Following a three-phase solid short circuit fault at the terminal of the generator, the magnitude of the subtransient internal emf (rounded off to 2 decimal places) is _____________ pu.

A Three-phase synchronous motor draws 200 A from the line at unity power factor at rated load. Considering the same line voltage and load, the line current at a power factor of 0.5 leading is.

A 220 V (line), three-phase, Y-connected, synchronous motor has a synchronous impedance of .

The motor draws the rated current of 10 A at 0.8 pf leading. The rms value of line-to-line internal voltage in volts (round off to two decimal places) is _________.

In a salient pole synchronous motor, the developed reluctance torque attains the maximum value when the load angle in electrical degrees is

A three-phase, 50 Hz, star-connected cylindrical-rotor synchronous machine is running as a motor. The machine is operated from a 6.6 kV grid and draws current at unity power factor (UPF). The synchronous reactance of the motor is  per phase. The load angle is 30 degrees. The power delivered to the motor in kW is __________.

(Give the answer up to one decimal place).

Two parallel connected, three-phase, 50 Hz, 11 kV, star-connected synchronous machines A and B, are operating as synchronous condensers. They together supply 50 MVAR to a 11 kV grid. Current supplied by both the machines are equal. Synchronous reactance’s of machine A and machine B are and , respectively. Assuming the magnetic circuit to be linear, the ratio of excitation current of machine A to that of machine B is ____________. (Given the answer up to two decimal places).

If a synchronous motor is running at a leading power factor, its excitation induced voltage () is

Two generating units rated 300 MW and 400 MW have governor speed regulation of 6% and 4% respectively from no load to full load. Both the generating units are operating in parallel to share a load of 600 MW. Assuming free governor action, the load shared by the larger unit is ___________ MW.

A 25 kVA, 400 V, –connected, 3-phase, cylindrical rotor synchronous generator requires a field current of 5 A to maintain the rated armature current under short-circuit condition. For the same field current, the open-circuit voltage is 360 V. Neglecting the armature resistance and magnetic saturation, its voltage regulation (in % with respect to terminal voltage), when the generator delivers the rated load at 0.8 pf leading, at rated terminal voltage is ___________.

A three-phase, 50Hz salient-pole synchronous motor has a per-phase direct-axis reactance  of 0.8pu and a per-phase quadrature-axis reactance of 0.6pu. Resistance of the machine is negligible. It is drawing full-load current at 0.8pf (leading). When the terminal voltage is 1 pu, per-phase induced voltage, in pu, is_______________.

A three-phase, 11 kV, 50 Hz, 2pole, star connected, and cylindrical rotor synchronous motor is connected to an 11kV, 50 Hz source. Its synchronous reactance is  per phase, and its stator resistance is negligible. The motor has a constant field excitation. At a particular load torque, its stator current is 100A at unity power factor. If the load torque is increased so that the stator current is 120 A, then the load angle (in degrees) at this load is __________.

A star connected 400V, 50Hz, 4 pole synchronous machine gave the following open circuit and short circuit test results:

Open circuit Test: at field current

Short circuit Test:  at field current

The value of per phase synchronous impedance in at rated voltage is______________.

A three phase synchronous generator is to be connected to the infinite bus. The lamps are connected as shown in the figure for the synchronization. The phase sequence of bus voltage is R-Y-B and that of incoming generator voltage is R-Y-B.

It was found that the lamps are becoming dark in the sequence . It means that the phase sequence of incoming generator is

A 20-pole alternator is having 180 identical stator slots with 6 conductors in each slot. All the coils of a phase are in series. If the coils are connected to realize single-phase winding, the generated voltage is . If the coils are reconnected to realize three-phase star-connected winding, the generated phase voltage is .

Assuming full pitch, single-layer winding, the ratio  is

A synchronous generator is connected to an infinite bus with excitation voltage . The generator has a synchronous reactance of 1.1pu and is delivering real power (P) of 0.6 pu of the bus. Assume the infinite bus voltage to be 1.0pu. Neglect stator reactance. The reactive power (Q) in pu supplied by the generator to the bus under this condition is _______________.

There are two generator in a power system. No-load frequencies of the generators are 51.5Hz and 51Hz, respectively. And both are having droop constant of 1 Hz/MW. Total load in the system is 2.5MW. Assuming that the generators are operating under their respective droop characteristics, the frequency of the power system in Hz in the steady state is__________.

In a synchronous machine, hunting is predominantly damped by

A non-salient pole synchronous generator having synchronous reactance of 0.8 pu is supplying 1 pu power to a unity power factor load at a terminal voltage of 1.1 pu. Neglecting the armature resistance, the angle of the voltage behind the synchronous reactance with respect to the angle of the terminal voltage in degrees is ______________.

A three-phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load at normal excitation. The field excitation of the motor is first reduced to zero and then increased in the reverse direction gradually. Then the armature current

The direct axis and quadrature axis reactance of a salient pole alternator are 1.2p.u. and 1.0 p.u. respectively.  The armature resistance is negligible. If this alternator is delivering rated kVA at upf and at rated voltage then its power angle is

A field excitation of 20 A in a certain alternator results in an armature current of 400 A in short Circuit and a terminal voltage of 2000 V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200 A is

Distributed winding and short chording employed in AC machines will result in

A synchronous motor is connected to an infinite bus at 1.0pu voltage and draws 0.6pu current at unity power factor. Its synchronous reactance is 1.0pu and resistance is negligible.

The excitation voltage (E) and load angle (δ) will respectively be

A synchronous motor is connected to an infinite bus at 1.0pu voltage and draws 0.6pu current at unity power factor. Its synchronous reactance is 1.0pu and resistance is negligible.

Keeping the excitation voltage same, the load on the motor is increased such that the motor current increases by 20%. The operating power factor will become

A three-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be

A 100 kVA, 415V (line), star-connected synchronous machine generates rated open circuit voltage of 415 V at a field current of 15 A. The short circuit armature current at a field current of 10A is equal to the rated armature current. The per unit saturated synchronous reactance is         

An isolated 50 Hz synchronous generator is rated at 15 MW which is also the maximum continuous power limit of its prime mover. It is equipped with a speed governor with 5% droop. Initially, the generator is feeding three loads of 4 MW each at 50 Hz. One of these loads is programmed to trip permanently if the frequency falls below 48 Hz. If an additional load of 3.5 MW is connected then the frequency will settle down to

A synchronous generator is feeding a zero power factor (lagging) load at rated current.  The armature reaction is

A 3 phase, 400 V, 5 kW, star connected synchronous motor having an internal reactance of 10Ω is operating at 50% load, unit pf. Now, the excitation is increased by 1%.  What will be the new load in percent, if the power factor is to be kept same? Neglect all losses and consider linear magnetic circuit.

A 4 pole, 50 Hz synchronous generator has 48 slots in which a double layer winding is housed.  Each coil has 10 turns and is short pitched by an angle to 36° electrical.  The fundamental flux per pole is 0.025Wb.

The line-to-line induced EMF (in volts). For a three phase star connection is approximately

A 4 pole, 50 Hz synchronous generator has 48 slots in which a double layer winding is housed.  Each coil has 10 turns and is short pitched by an angle to 36° electrical.  The fundamental flux per pole is 0.025Wb.

The line-to-line induced EMF (in volts), for a two phase connection is approximately

A 4 pole, 50 Hz synchronous generator has 48 slots in which a double layer winding is housed.  Each coil has 10 turns and is short pitched by an angle to 36° electrical.  The fundamental flux per pole is 0.025Wb.

The fifth harmonic component of phase emf
(in volts), for a three phase star connection is

In relation to the synchronous machines, which one of the following statements is false?

A 1000 kVA, 6.6 kV, 3-phase star connected cylindrical pole synchronous generator has a synchronous reactance of 20 Ω. Neglect the armature resistance and consider operation at full load and unity power factor.        

The induced emf (line-to-line) is close to

A 1000 kVA, 6.6 kV, 3-phase star connected cylindrical pole synchronous generator has a synchronous reactance of 20 Ω. Neglect the armature resistance and consider operation at full load and unity power factor.        

The power (or torque) angle is close to?

The synchronous speed for the seventh space harmonic mmf wave of a 3-phase, 8 pole, 50 Hz induction machine is

Two 3-phases, Y-connected alternators are to be paralleled to a set of common bus bars. The armature has a per phase synchronous reactance of 1.7Ω and negligible armature resistance. The line voltage of the first machines is adjusted to 3300 V and that of the second machine is adjusted to 3200 V. the machine voltages are in phase at the instant they are paralleled. Under this condition, the synchronizing current per phase will be

A 400V, 50 kVA, 0.8 pf leading Δ-connected, 50 Hz synchronous machine has a synchronous reactance of 2Ω and negligible armature resistance. The friction and windage losses are 2kW and the core loss is 0.8 kW. The shaft is supplying 9kW load at a power factor of 0.8 leading. The line current drawn is

A 500 MW 3-phase Y-connected synchronous generator has a rated voltage of 21.5 kV at 0.85 pf. The line current when operating at full load rated conditions will be

A hydraulic turbine having rated speed of 250 rpm is connected to a synchronous generator. In order to produce power at 50 Hz, the number of poles required in the generator are

A stand-alone engine driven Synchronous generator is feeding a partly inductive load. A capacitor is now connected across the load to completely nullify the inductive current. For this operating condition.

Curves X and Y in Figure denote open circuit and full-load zero power factor (zpf) characteristics of asynchronous generator. Q is a point on the zpf characteristics at 1.0 p.u. voltage. The vertical distance PQ in Figure gives the voltage drop across

A 4-pole, 3-phase, double layer winding is housed in a 36-slot stator for an ac machine with 60° phase spread. Coil span is 7 slot pitches. Number of slots in which top and bottom layers belong to different phases is

The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25mWb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20mWb while the motor is running on no load.

Assuming no load losses to be zero, the no load current down by the motor from the supply

A 415 V, 2 poles, 3 phases, 50 Hz, star connected, non-salient pole synchronous motor has synchronous reactance of 2 Ω per phase and negligible stator resistance. At a particular field excitation, it draws 20 A at unity power factor from a 415 V, 3 phase, 50 Hz supply. The mechanical load on the motor is now increased till the stator current is equal to 50A. The field excitation remains unchanged. Determine:

(a) The per phase open circuit voltage

(b) The developed power for the new operating condition and corresponding power factor

It is desirable to eliminate 5th harmonic voltage from the phase voltage of an alternator. The coils should be short-pitched by an electrical angle of

Figure shows the magnetization curves of an alternator at rated armature current, unity power factor and also at no load.  The magnetization curve for rated armature current, 0.8 power factor leading is given by

A star-connected 440 V, 50 Hz alternators has per phase synchronous reactance of 10Ω.  It supplies a balanced capacitive load current of 20A, as shown in the per phase equivalent circuit of Figure. It is desirable to have zero voltage regulation.  The load power factor should be

A 50 kW synchronous motor is tested by driving it by another motor.  When the excitation is not switched on, the driving motor takes 800W.  When the armature is short circuited and the rated armature current of 10A is passed through it, the driving motor required 2500 W.  On open-circuiting the armature with rated excitation, the driving motor takes 1800W.

Calculate the efficiency of the synchronous motor at 50% load.  Neglect the losses in the driving motor.

Two identical synchronous generators, each of 100 MVA, are working in parallel supplying 100 MVA at 0.8 lagging p.f. at rated voltage.  Initially the machines are sharing load equally.  If the field current of first generator is reduced by 5% and of the second generator increased by 5%, find the sharing of load (MW and MVAR) between the generators.  Assume 0.8 p.u., no field saturation and rated voltage across load.  Reasonable approximations may be made.

The phase sequence of a three-phase alternator will reverse if

A single-phase, 2000V alternator has armature resistance and reactance of 0.8 ohms and 4.94 ohms respectively. The voltage regulation of the alternator at 100A load at 0.8 leading power-factor is:

A 2300V, 3-phase synchronous motor driving a pump is provided with a line ammeter and a field rheostat. When the rheostat is adjusted such that the ac line current is minimum, the armature reads 8.8A.
(a) What is the power being delivered to the pump, neglecting losses?

(b) How should the rheostat be adjusted so that the motor operates at 0.8 leading power factor?
(c) How many KVARs is the motor supply to the system at this new power factor?

Higher synchronous reactance is preferred in the present day alternators, because one can have

A three phase alternator is wound with a 60 degree phase-spread armature windings and develops 300 kVA. If the armature is reconnected utilizing all the coils for single phase operation with a phase spread of 180 degrees, the new rating of the machine is

A 5 MVA, 11 kV, 3-phase star connected alternator is synchronized to the bus bars and is operating with an induced EMF of 125% of the rated voltage. If the load current is 500A, what is the power factor of operation? The machine has a synchronous reactance of 5Ω and negligible resistance per phase.

A synchronous generator connected to an infinite bus is overexcited. Considering only the reactive power, from the point of view of the system, the machine acts as

The armature of a single phase alternator is completely wound with T single turn coils distributed uniformly. The induced voltage in each turn is 2V (rms). The emf of the whole winding is

A single-phase alternator has a synchronous reactance of 2 ohms and negligible resistance. If it supplies 10A to a purely capacitive load at 200V, then generated e.m.f will be… V and the regulation will be…%

A cylindrical rotor synchronous motor is switched on the supply with its field winding shorted on themselves. It will

In figure the characteristic that corresponds to the variation of synchronous reactance of a synchronous motor with field current is

During hunting of synchronous motor

A synchronous motor on load draws a current at a leading power factor angle . If the internal power factor angle- which is the phase angle between the excitation e.m.f. and current in the time phasor diagram is  then the air gap excitation m.m.f lags the armature m.m.f by

The distribution factor for a 36 slot stator with three-phase, 8-pole winding, having 120° phase spread, is _____________

A three-phase, 50Hz, 415V, synchronous machine operates at rated voltage and at a leading power factor of 0.8. Shaft power is 15 kW and excitation emf is 400V. If per phase resistance is 0.5 ohm, find the synchronous reactance. Neglect mechanical losses of the system.

Match the items on the right hand side with those on left hand side

(a) Commutation                             (P) Resistive load

(b) V-curves                                     (Q) Inductive load

(c) Freewheeling diode                  (R) Capacitive load

(d) Overlap                                       (S) Inter pole

                                                           (T) Source Inductance

                                                           (U) Synchronous motor

A three phase alternator has negligible stator resistance. A short circuit test is conducted on this alternator. At a particular speed a field current of  is required to drive the rated armature current. If the speed of the alternator is reduced to half, the field current required to maintain rated armature current

A synchronous motor operates at 0.8 p.f. lagging. If the field current of the motor is continuously increased

The torque angle of a synchronous machine operating from a constant voltage bus, is usually defined as the space angle between

Figure depicts the load characteristics of an isolated three-phase alternator, running at constant speed. Match the following sets of operating conditions with the given characteristics. Disregard the effects of saliency, saturation and stator resistance.

(a) Constant excitation and non-zero leading power-factor
(b) Constant excitation and zero power-factor leading
(c) Constant terminal voltage and zero power-factor leading
(d) Constant terminal voltage and non-zero leading power-factor

Damper winding is provided in a poly-phase synchronous motor in order to

A 500 MVA, 11 KV synchronous generator has 0.2 p.u. synchronous reactance. The p.u. synchronous reactance on the base values of 100 MVA and 22 KV is:

A 10 kVA, 380V, 4-pole, 50 Hz, star-connected cyclindrical rotor alternator has a stator resistance and synchronous reactance of 1 ohm and 15 ohms respectively. It supplies a load of 8 kW at rated voltage and 0.8 power factor lagging.