# Objective Type question of Synchronous Motor With Explanation

Ques 41. The armature current of a synchronous motor has large values for

1. High excitation only
2. Both low and high excitation
3. Low excitation only
4. None of the above

Answer 2 .Both low and high excitation

Explanation:

The synchronous motor always adjusts its cosφ i.e., power factor nature so that Power component Ia cosφ remains constant when excitation of the motor is changed keeping the load constant. This is the reason why a synchronous motor reacts by changing its power factor to variable excitation conditions.

Under excitation condition: When the excitation is adjusted in such a way that the magnitude of induced emf is less than the applied voltage (Eb < V) the excitation is called under excitation.

Due to this, ER increases in magnitude. This means for constant Synchronous Impedance (Zs), the current drawn by the motor increases. But ER, the phase shift in such a way that, phasor Ia, also shifts (as ER Ia = θ) to keep the Power component of Ia i.e Ia cosφ components constant. So in under excited conditions, the current drawn by the motor increases. The power factor cosφ decreases and becomes more and more lagging in nature.

Over excitation condition: The excitation to the field winding for which the induced emf becomes greater than the applied voltage (Eb > V) is called overexcitation.

Due to the increased magnitude of Eb, ER also increases in magnitude. But the phase of ER also changes. Now (as ER Ia = θ) is constant, hence Ia also changes its phase, So φ changes. The Ia increases to keep Ia cosφ constant. The phase of ER changes so that Ia becomes leading with respect to Vph in over-excited conditions. So power factor of the motor becomes leading in nature. So overexcited synchronous motor works on leading power factor. So power factor decreases as over excitation increases but it becomes more and more leading in nature.

Two important points stand out clearly from the above discussion :

(i) The magnitude of armature current varies with excitation. The current has a large value both for low and high values of excitation (though it is lagging for low excitation and leading for higher excitation). In between, it has a minimum value corresponding to a certain excitation.

(ii) For the same input, armature current varies over a wide range and so causes the power factor also to vary accordingly. When over-excited, motor runs with leading p.f. and with lagging p.f. when under-excited. In between, the p.f. is unity.

1. When the motor is under excited, the armature current and power factor is lagging. In this case, the motor behaves like an inductive load.
2. When the motor is normally excited, the power factor is unity. In this case, the armature current is minimum and is in phase with the terminal voltage.
3. When the motor is over-excited, the power factor is leading. In this case, the motor behaves like a capacitive load.

Ques 42.  A synchronous motor is a useful industrial machine due to its property of

1. Improving the power factor
2. Speed is constant
3. Can always be adjusted to operate at the unity power factor
4. All of the above

Answer 4. All of the above

Explanation:

Application of Synchronous motor

1. A polyphase synchronous motor has an inherent disadvantage of low-starting torque and sensitivity to system disturbance when sometimes it falls out of step.
2. The advantages of using a synchronous motor for industrial applications are the ease of controlling the power factor and also the constant speed.
3. At lower speeds and higher frequencies, synchronous motors are cheaper than induction motors.
4. With an improved design of their damper windings, the synchronous motors do not encounter any difficulty in handling sudden applications of load, and this does not affect their steady-state operation.
5. Synchronous motors were mainly used in constant speed applications.
6. Synchronous motors are particularly attractive for low speed (< 300 rpm) because the power factor can always be adjusted to unity and efficiency is high.
7. Over excited synchronous motor can be used to improve the power factor.
8. Synchronous motors are used to improve the voltage regulation of transmission lines.
9. Synchronous motor only runs at synchronous speed, therefore it is used in textile, paper mill etc.
10. Due to constant speed characteristics, it is used in:

(a) Machine tools

(b) Motor-Generator set

(c) Synchronous clocks

(d) Timing devices

(e) Fans and blowers

(f) Cement industries

Ques 43. Synchronous capacitor is

1. An ordinary static capacitor bank
2. An over-excited synchronous motor driving mechanical load
3. An over-excited synchronous motor without mechanical load
4. All of the above

Explanation:

An overexcited synchronous motor acts as a power factor correction device and is also known as a synchronous condenser. The variation of armature current and power factor as a function of field current is plotted to give a better insight. In electrical engineering, a synchronous condenser(sometimes called a synchronous capacitor or synchronous compensator) is an Over-Excited Synchronous Motor, whose shaft is not connected to anything but spins freely.

We can state that an over-excited synchronous motor draws a leading power factor current from the mains. The synchronous motor, therefore, when over-excited, in addition to driving some load, will work as a capacitor or condenser. A capacitor draws a leading power factor current. An over-excited synchronous motor draws the leading power factor current from the mains.

An over-excited synchronous motor is also called a synchronous condenser. Synchronous motors are used as constant-speed drive motors. Over-excited synchronous motors are used to improve the power factor of electrical loads in industries. Generally, the motor is run on load, and by overexcitation, the system power factor is also improved.

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Ques 44.  In a synchronous motor which loss does not vary with the load?

1. Windage losses
2. Copper losses
3. Hysteresis losses
4. None of the above

Explanation:

## Losses in Synchronous machine

The losses in synchronous machines are as follows:

(a) Fixed losses:-  Core loss, bearing, friction, and windage loss, brush friction loss. These losses are obtained from the no-load test. Core loss occurs because of the eddy currents and hysteresis caused by the main magnetic field. It is the difference between the power required to drive the synchronous machine with or without field excitation. This is taken at the rated voltage and speed.

### Windage losses in the synchronous motor

• These losses occur during the circulation of moving air around inside the machine.
• The rotor “whips” air around and the air resistance cause losses.
• Sometimes the rotor fan losses are included in windage and sometimes they are calculated separately; however, the physics of both are the same.
• Windage losses vary with the airspeed relative to the motor surfaces squared. In low-speed machines, these are often neglected.
• But in large high-speed machines (like flywheels and some 400Hz generators) windage losses are a dominant loss.
• To reduce windage losses and improve cooling, large power generators are sometimes sealed and cooled with hydrogen rather than air.
• Windage losses are not dissipated by the stator core surface in a synchronous motor.

(b) I2R loss in armature winding, stray loss in iron and conductors:- Armature I2R loss is current2 × dc resistance R corrected at 70°C and not the effective resistance. This can be calculated when I and R are measured. Stray load losses are caused due to changes in the flux distribution due to load. This can be found by a short-circuit test. The short-circuit current is adjusted to the value of load current at which the loss is to be determined, then the stray load loss = mechanical power input friction and windage loss I2R loss. The synchronous machine is run at the rated speed. Ventilation loss is the power required to circulate cooling air in addition to the windage loss.

(c) Excitation circuit losses:- These include field copper loss, rheostat loss. brush contact loss, exciter losses. Field copper loss = I2fRf, where If is the field current and Rf is the resistance of field winding. Rheostat loss is I2fRr, where Rr, is the resistance of the rheostat. Brush contact loss is taken as slip ring current. Exciter loss is considered when it is driven by a Synchronous machine and is part of the whole machine. Otherwise, it is changed to the plant and not to the alternator.

Note:- If the machine is not excited (zero field current) and running on no-load, the core loss will be zero and only windage and friction loss takes place. If the machine is excited (field current is supplied), both windage and friction and core losses take place. Thus, the core loss can be computed by taking the difference of the power consumed by machines with excitation and without excitation. It is common practice to consider core loss under load and no-load conditions the same.

Ques 45. A synchronous motor is said to be ‘floating‘ when it operates at

1. High load and variable supply voltage
4. No load and without losses

Explanation:

### Floating in Synchronous machine

Suppose that a synchronous machine is driven by a prime-mover and its stator windings are connected to a three-phase ac supply. The operation of the machine is synchronized such that no current flows through its stator windings. Under this condition, the excitation emf E in the armature windings is exactly equal to the bus-bar voltage V. The machine is neither receiving nor delivering any power to the bus bar. It is the prime-mover that supplies power to meet the losses in the machine, Under this condition, we say that the synchronous machine is floating on the bus bar.

or

When synchronized, the generated emf of the incoming machine is just equal to the bus-bar voltage. The synchronous machine will be just Floating on the bus bar, i.e. it will neither deliver nor receive any power. The prime mover driving the machine will be supplying the no-load losses only.

Ques 46. A 3 phase synchronous motor is running clockwise. In case the direction of its field current is reversed then

1. Winding of the motor will burn
2. Motor will run in the reverse direction
3. Motor will continue to run in the same direction
4. Motor will stop

Answer 3. Motor will continue to run in the same direction

Explanation:

### Reversing a Synchronous Motor

• The direction of rotation of a synchronous motor is determined by its starting direction, as initiated by induction-motor action.
• Thus, to reverse the direction of a three-phase synchronous motor, it is necessary to first stop the motor and then reverse the phase sequence of the three-phase connections at the stator like an induction motor.
• The direction of rotation of a 3-phase synchronous motor can be changed by altering the phase sequence of the supply. I.e from RBY to RYB. Doing so will change the direction of rotation from clockwise to anticlockwise.
• Reversing the current to the field windings will not affect the direction of rotation. If the current in the field winding is reversed the motor will run in the same direction. The field side will only slip through a pole-pitch due to the reversal of the polarities of the field poles.

Ques 47. The rotor copper losses of the synchronous motor are met by

1. Armature input
2. Dc source
3. Ac supply mains
4. None of the above

Explanation:

A synchronous machine is used to convert mechanical energy into electrical energy or vice-versa. While doing so, the whole of input energy does not appear at the output but a part of it is lost in the form of heat in the surroundings. This wasted energy is called losses in the machine. These losses affect the efficiency of the machine.

The various losses occurring in a synchronous machine can be sub-divided as

1. Copper losses.
2. Iron losses.
3. Mechanical losses
4. Stray losses

1. Copper losses: The various windings of the synchronous machine such as armature and field winding are made of copper and have some resistance. When current flows through them, there will be power loss proportional to the square of their respective currents. These power losses are called copper losses.

In general, the various copper losses in a synchronous machine are:

(i) Armature copper loss = I2R

(ii) Field winding copper loss = I2fRf

(iii) Brush contact loss = I2Rb

The brush contact loss is generally included in the field winding copper losses.

Copper losses (both stator and field) depend on the load and they are met by a DC source.

Note:-

• Friction and windage losses do not depend upon the load condition of the machine; They are the function of the speed only, thus for the most synchronous machine they are constant
• The core losses (hysteresis and eddy current losses) depends upon the speed and magnitude of the rotating magnetic field.

Ques 48.  If the load angle of 4 poles synchronous motor is 8o electrical its value in mechanical degree is

1. 2
2. 8
3. 10
4. 4

Explanation:

Mechanical Degree: Mechanical degree or angle is the angle at which the rotor of a machine is displaced mechanically.

Electrical Degree: The degree or the cycle of emf induced in a single conductor in a synchronous machine. In one electrical cycle, the electrical angle varies from 0-360 degree

• Consider a 4 pole machine there are 2 North and 2 South poles. So the rotor had to rotate 180 degrees mechanically to reach from one North pole to another North pole.
• That is to generate a complete cycle(360 degrees) electrically it has to rotate half cycle(180 degrees)mechanically.
• So the relation between electrical and mechanical angle is dependent on the no of poles in the machine.

Electrical Degree = (No of poles/2) × Mechanical Degree

8 = (4/2) × Mechanical Degree

Mechanical Degree = 4°

Ques 49. In a synchronous motor, the synchronizing power comes into action when

1. Rotor speed is less than synchronous speed
2. Rotor speed is more than synchronous speed
3. Rotor speed is equal to synchronous speed
4. Rotor speed is either less or more than synchronous speed

Answer 4. Rotor speed is either less or more than synchronous speed

Explanation:

• The rate at which synchronous power, ‘P’ varies with load angle δ is called the synchronizing-power coefficient.
• It is also called stiffness of coupling, stability, or rigidity factor; It is represented as Psyn.

$\begin{array}{l} {P_{syn}} = \frac{{V{E_f}}}{{{X_s}}}cos\delta \\ \\ P \propto \frac{1}{{{X_s}}} \end{array}$

• Synchronizing power of synchronous machines is inversely proportional to the synchronous reactance
• A synchronous machine, when synchronized to infinite busbars has an inherent tendency to remain in synchronism
• At perfect synchronization, their synchronizing power is zero.
• Hence, an over-excited synchronous machine is more rigidly coupled than the one which is under-excited. A large air gap decreases the value of (synchronous reactance) Xs, thus a synchronous machine with a longer air gap is more stiffer than the one with smaller air-gap units of synchronizing power coefficient are watt per electrical radian.
• The variation of synchronous power with the change of load angle is called the synchronizing power. It exists only during the transient state, i.e. whenever there is a sudden disturbance in load (or steady-state operating conditions).
• Once the steady-state is reached, the synchronizing power reduces to zero.
• The synchronizing power flows from or to the bus in order to maintain the relative velocity between the interacting stator and rotor field, zero, once the equality is reached, the synchronizing power vanishes.

Ques 50. The magnitude of field flux in a 3-phase synchronous motor

1. Remains constant at all loads
2. Varies with speed
4. Varies with power factor

Explanation:

• In general, the counter emf or bac ef of any motor must be very nearly equal and opposite to the impressed terminal voltage so that if the latter is constant, the counter emf will be substantially constant.
• Now the counter emf is proportional to the speed and to the field flux, and since the speed is constant in the case of the synchronous motor, the field flux must likewise be substantially constant within the normal limits of operation.
• It follows, then, that if the field excitation is increased, thereby tending to increase the field flux that can vary only slightly, there must be an automatic change of the armature MMF in order to offset the effect of the increased field excitation
• The armature current must therefore contain a leading component, that a leading current in a synchronous motor exerts a demagnetizing effect.
• A weakening of the field excitation tends to draw a lagging current from the source of supply.

In short:- The magnitude of a field flux in a 3-phase synchronous machine remains constant at all loads because this motor runs at a constant speed for that The magnitude of field flux must be constant. The field is supplied from a d.c. source and the stator coils with a three-phase current.