Synchronous Motors — Part 1 MCQs with Explanations

Understanding Synchronous Motors Synchronous motors are special because their speed is directly tied to the frequency of the AC power supply and the number of poles in the motor. The Key Formula The synchronous speed (Ns) is calculated using the formula: Ns = (120 * f) / P, where: * Ns is the synchronous speed in RPM (revolutions per minute) * f is the frequency of the AC power supply in Hertz (Hz) * P is the number of poles What the Question Asks The question states that the number of poles (P) is made *half* of its original value. Let's See the Impact If you decrease P to half its original value in the formula Ns = (120 * f) / P, the overall speed (Ns) will *increase*. For example: * if initially P=4 then Ns = (120 * f) / 4 = 30f * if P becomes half i.e. P=2 then Ns = (120 * f) / 2 = 60f * So from above you can see that speed becomes double. Why the Other Options are Wrong * A: remains same as the original value: This is wrong because changing the number of poles *directly* impacts the speed. * B: decreases to half the original value: This is wrong because reducing the number of poles *increases* the speed. * C: tends to becomes zero: This is not correct synchronous motor will run at a particular speed which depends upon frequency and number of poles. The Answer Therefore, the correct answer is: D: increases to two times the original value .

The correct option is "it runs as a reluctance motor at the same speed". In a salient pole synchronous motor, torque is normally produced by the interaction between the stator rotating magnetic field and the rotor field created by DC excitation. If the excitation is suddenly disconnected while the motor is running at no load, the rotor loses its field excitation. However, due to the salient pole construction , the rotor still experiences a torque known as reluctance torque , which arises because the rotor tends to align itself along the path of minimum reluctance of the stator field. This reluctance torque is enough to keep the motor running in synchronism with the stator field, but only under light or no-load conditions. The machine in this case operates as a reluctance motor and continues to run at the same synchronous speed. Why not the others? "The motor stops" is incorrect, because reluctance torque prevents it from stopping immediately when excitation is lost at no load. "It runs as a reluctance motor at a lower speed" is incorrect, because synchronous machines always run at synchronous speed, not below it. "None of the above" is incorrect, because Option B correctly explains the behavior. Therefore, the correct answer is it runs as a reluctance motor at the same speed .

The correct answer is (C) load angle . Let's break down why: Option A: Speed: Synchronous motors operate at a fixed speed determined by the frequency of the power supply and the number of poles. Therefore, the back EMF isn't *directly* dependent on speed in a *variable* way. Option B: Load: While the motor adjusts to the load, the load itself doesn't *directly* dictate the back EMF's magnitude. The load affects the load angle . Option C: Load Angle: The back EMF (Eb) and the applied voltage (V) are related by the equation and are also affected by the load angle (δ) . As the load increases , the rotor lags behind the rotating magnetic field, which increases the load angle (δ) . This change in load angle directly affects the phase relationship between the applied voltage and the back EMF, thus influencing the magnitude of the back EMF. A larger load angle generally results in a lower effective back EMF. Option D: All of the above: While load and speed have related effects, the primary and most direct influence on the back EMF's magnitude is the load angle . Therefore, the back EMF is most directly dependent on the load angle .

To make a synchronous motor self-starting, a damper winding on the rotor poles is provided. The damper winding consists of bars embedded in the rotor poles and short-circuited at both ends. When the motor is started, the relative motion between the rotating magnetic field of the stator and the rotor induces currents in the damper winding. These currents produce a magnetic field that interacts with the stator's magnetic field, generating torque and assisting in the motor's starting process. Hence, option A is the correct choice.

90° is the correct answer to “The maximum power developed in a synchronous motor occurs at a coupling angle of”. This question is standard in competitive examination syllabi for Synchronous Motors.

always remains constant is the correct answer to “The speed of a synchronous motor”. This question is standard in competitive examination syllabi for Synchronous Motors.

Synchronous motor is the correct answer to “In which of the following motors the stator and rotor magnetic field rotate at the same speed”. This question is standard in competitive examination syllabi for Synchronous Motors.

directly proportional to applied voltage is the correct answer to “In a synchronous motor, the breakdown torque is”. This question is standard in competitive examination syllabi for Synchronous Motors.

torque is the correct answer to “The angle between the rotating stator flux and rotor poles is called _____ angle.”. This question is standard in competitive examination syllabi for Synchronous Motors.

the motor continue to run in the same direction is the correct answer to “A 3-phase synchronous motor is running clockwise. If the direction of its field current is reversed”. This question is standard in competitive examination syllabi for Synchronous Motors.

Low field current is the correct answer to “Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current”. This question is standard in competitive examination syllabi for Synchronous Motors.

induction generator torque is the correct answer to “When the rotor speed, in a synchronous machine, becomes more than the synchronous speed during hunting, the damper bars develop”. This question is standard in competitive examination syllabi for Synchronous Motors.

transmission of mechancial power by shaft is the correct answer to “The working of a synchronous motor is similar to”. This question is standard in competitive examination syllabi for Synchronous Motors.

the armature current is minimum is the correct answer to “Power factor of a synchronous motor is unity when”. This question is standard in competitive examination syllabi for Synchronous Motors.

1500 r.p.m. is the correct answer to “In a 3-phase, 4-pole, 50 Hz synchronous motor, the frequency, pole number and load torque all are halved. The motor speed will be”. This question is standard in competitive examination syllabi for Synchronous Motors.

synchronous motor is the correct answer to “Which of the following motors is non-self starting”. This question is standard in competitive examination syllabi for Synchronous Motors.

Windage loss is the correct answer to “Which of the following losses, in a synchronous motor, does not vary with load”. This question is standard in competitive examination syllabi for Synchronous Motors.

increases with increase in its excitation is the correct answer to “Stability of a synchronous machine”. This question is standard in competitive examination syllabi for Synchronous Motors.

Damper winding in conjunction with star-delta starter is the correct answer to “Which of the following methods is used to start a synchronous motor”. This question is standard in competitive examination syllabi for Synchronous Motors.

All of the above is the correct answer to “Which of the following resistances can be measured by conducting insulation resistance test on a synchronous motor”. This question is standard in competitive examination syllabi for Synchronous Motors.

Windage losses is the correct answer to “Which of the following losses is not dissipated by the stator core surface in a synchronous motor”. This question is standard in competitive examination syllabi for Synchronous Motors.

five degrees is the correct answer to “The mechanical displacement of the rotor with respect to the stator, in polyphase multipolar synchronous motors running at full load, is of the order of”. This question is standard in competitive examination syllabi for Synchronous Motors.

phase advancer is the correct answer to “A synchronous motor working at leading power factor can be used as”. This question is standard in competitive examination syllabi for Synchronous Motors.

synchronous condenser is the correct answer to “A synchronous motor which works on a leading power factor and does not drive a mechanical load is called as”. This question is standard in competitive examination syllabi for Synchronous Motors.

field current and armature current is the correct answer to “For V-curves for a synchronous motor the graph is drawn between”. This question is standard in competitive examination syllabi for Synchronous Motors.

both (A) and (B) is the correct answer to “The armature current of the synchronous motor has higher values for”. This question is standard in competitive examination syllabi for Synchronous Motors.

Synchronous motor is the correct answer to “In which of the following motors the stator and rotor fields rotate simultaneously”. This question is standard in competitive examination syllabi for Synchronous Motors.

synchronous motor is the correct answer to “A rotory converter can also be run as a”. This question is standard in competitive examination syllabi for Synchronous Motors.

providing damper bars in the rotor pole faces is the correct answer to “The oscillations in a synchronous motor can be damped out by”. This question is standard in competitive examination syllabi for Synchronous Motors.

over-excited is the correct answer to “A synchronous motor can be used as a synchronous capacitor when it is”. This question is standard in competitive examination syllabi for Synchronous Motors.

direct current is the correct answer to “Slip-rings in a synchronous motor carry”. This question is standard in competitive examination syllabi for Synchronous Motors.

leading current is the correct answer to “An over-excited synchronous motor takes”. This question is standard in competitive examination syllabi for Synchronous Motors.

short-circuited by low resistance is the correct answer to “While starting a synchronous motor by induction motor action, field winding is usually”. This question is standard in competitive examination syllabi for Synchronous Motors.

power factor will decrease and armature current will increase is the correct answer to “A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased”. This question is standard in competitive examination syllabi for Synchronous Motors.

current rating is the correct answer to “To limit the operating temperature an electrical machine should have proper”. This question is standard in competitive examination syllabi for Synchronous Motors.

d.c. excitation only is the correct answer to “In a synchronous motor, the magnitude of stator back e.m.f. (E b ) depends on”. This question is standard in competitive examination syllabi for Synchronous Motors.

increase in both armature current and power factor angle is the correct answer to “If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to”. This question is standard in competitive examination syllabi for Synchronous Motors.

3 is the correct answer to “If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is”. This question is standard in competitive examination syllabi for Synchronous Motors.

Thrice is the correct answer to “In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately”. This question is standard in competitive examination syllabi for Synchronous Motors.

will become less is the correct answer to “If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor”. This question is standard in competitive examination syllabi for Synchronous Motors.