# GATE Questions & Answers of Synchronous Machines: Cylindrical and Salient Pole Machines, Performance, Regulation and Parallel Operation of Generators, Starting of Synchronous Motor, Characteristics

## What is the Weightage of Synchronous Machines: Cylindrical and Salient Pole Machines, Performance, Regulation and Parallel Operation of Generators, Starting of Synchronous Motor, Characteristics in GATE Exam?

Total 23 Questions have been asked from Synchronous Machines: Cylindrical and Salient Pole Machines, Performance, Regulation and Parallel Operation of Generators, Starting of Synchronous Motor, Characteristics topic of Electrical Machines subject in previous GATE papers. Average marks 1.57.

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 draw current at unity power factor (UPF). The synchronous reactance of the motor is 30 $\Omega$ per phase. The load angle is 30°. The power delivered to the motor in kW is________. (Give the answer up to two decimal phase).

Two parallel connected, three-phase, 50Hz, 11 kV, star-connected synchronous machine 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 reactances of machine A and machine B are 1$\Omega$ and 3$\Omega$ , respectively. Assuming the magnetic circuit to be linear, the ratio of excitation current of machine A to that of machine B is  ______________. (Give the answer up to two decimal places.)

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

A 25 kVA, 400 V, $\triangle$-connected, 3-phase, cylinder 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_______________.

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?

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

The power consumption of an industry is 500 kVA, at 0.8 p.f. lagging. A synchronous motor is added to raise the power factor of the industry to unity. If the power intake of the motor is 100 kW, the p.f. of the motor is _____________

A three-phase, 11 kV, 50 Hz, 2 pole, star connected, cylindrical rotor synchronous motor is connected to an 11 kV, 50 Hz source, Its synchronous reactance is 50$\mathrm{\Omega }$ per phase, and its stator resistance is negligible. The motor has a constant field excitation. At a particular load torque, its stator current is 100 A 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 400 V, 50 Hz, 4 pole synchronous machine gave the following open circuit and short circuit test results:
Open circuit test: Voc = 400 V (rms, line-to-line) at field current, If = 2.3 A
Short circuit test: Isc = 10 A (rms, phase) at field current, If = 1.5 A

The value of per phase synchronous impedance in Ω 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 La-Lb-Lc. 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 V1. If the coils are reconnected to realize three-phase star-connected winding, the generated phase voltage is V2. Assuming full pitch, single-layer winding, the ratio V1/V2 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 ________.

The angle δ in the swing equation of a synchronous generator is the

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

The direct axis and quadrature axis reactances of a salient pole alternator are 1.2 p.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 400A in short circuit and a terminal voltage of 2000V on open circuit. The magnitude of the internal voltage drop within the machine at a load current of 200A is

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

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

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

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