# Objective Type question of Synchronous Motor With Explanation

Ques 31. The speed of a synchronous motor

3. Always remains constant

Explanation:

As the load on the synchronous motor increases, there is no change in its speed. But what gets affected is the load angle δ i.e., the angle by which the rotor axis retards with respect to the stator axis. Torque produced in synchronous motor depends on the load angle δ.

Hence as load increases, δ increases but speed remains synchronous.

As δ increases, though Ebph and Vph magnitudes are the same, displacement of Ebph from its ideal position increases. Hence the vector difference Vph − Ebph increase. As synchronous impedance is constant, the magnitude of Iaph drawn by the motor increases as load increases. This current produces the necessary torque which satisfies the increased load demand. The magnetic locking still exists between the rotor and stator.

Step by step Procedure

• Whenever the load changes the torque angle δ changes the and hence the armature current increases irrespective of its excitation.
• When load increases, basically you are increasing the mass of the rotating objects which is coupled to the motor’s rotor. This asks for a higher torque supply this will result in torque angle getting increased, which means to say the rotor lags a bit.
• Because of magnetic locking, the motor again runs at sync speed. ( motor running slow is reflected as an increase in torque angle )
• Thus synchronous motor has the ability to adjust itself to achieve synchronous speed by changing the load angle.

Ques 32. In a synchronous motor if the saturation is neglected, then the short circuit ratio (SCR) will be related to the synchronous reactance (xd) as

1. SCR = 1/ (xd)
2. SCR = 1/ (xd)2
3. SCR = xd
4. SCR = (xd)2

Answer 1 . SCR = 1/ (xd)

Explanation:

Short Circuit Test

In the short circuit test of the synchronous machine, the armature terminals are short-circuited through suitable ammeters and the field current is set at zero. While the synchronous machine is driven at synchronous speed, its armature current Ia is measured as the field current gradually increases until the armature current is about 150% of the rated current. The plot of the average armature current, versus the field current If, gives the short-circuit characteristic (SCC) of the machine. The SCC is a linear line since the magnetic-circuit iron is unsaturated.”

Synchronous Reactance

A synchronous machine can be represented as an emf source in series with the internal impedance of the machine. The internal impedance, Zs is called synchronous impedance. The synchronous reactance takes into account the effect of armature reactions and the flux produced by the armature current.

The ratio of phase voltage obtained in the open-circuit test, to the phase current, obtain in the short circuit test determines the synchronous impedance per phase.

${Z_s} = \frac{{{V_{OC}}{._{Phase}}}}{{{I_{SC.Phase}}}}$

Short circuit Ratio

The short-circuit ratio of a synchronous machine is the ratio of field current required to produce a rated voltage on the open circuit to the field current required to produce the rated armature current with the armature terminals shorted when the machine is being run mechanically at synchronous speed.

It is also equal to the reciprocal of the pu synchronous reactance, i.e.

SCR = 1/Xpu

The short circuit ratio is a measure of the alternator’s sensitivity to load changes. Machines with higher SCRs are larger in size, weigh more, and also cost more. Their voltage regulation is smaller than those of machines with smaller SCRs. Conversely, alternators with lower SCRs have larger synchronous reactances and higher voltage regulations. Hence, they require quick-acting field control devices to maintain fairly constant output voltage.

A synchronous machine with the high value of SCR had a better voltage regulation and improved steady-state stability limit, but the short circuit fault current in the armature is high.

Ques 33. Which of the following devices can be used as a phase advancer?

1. Synchronous motor working at lagging power factor
2. Synchronous motor working at leading power factor
3. Squirrel cage induction motor
4. Slip ring induction motor

Explanation:

This is one of the methods of power factor improvement it is a simple AC exciter that is connected to the main shaft of the motor and operates with the motor’s rotor circuit for power factor improvement. A synchronous motor working at the leading power factor can be used for power factor improvement of the induction motors. Hence the synchronous motor is working as a phase advancer.

• Phase advancer is used to improve the power factor of induction motors in industries.
• As the stator windings of the induction motor take a lagging current of 90° out of phase with Voltage, therefore the power factor of the induction motor is low.
• If the exciting ampere-turns are excited by an external AC source, then there would be no effect of exciting current on stator windings. Therefore the power factor of the induction motor will be improved. This process is done by Phase advancer.

1. The phase advancer can be easily used where the use of synchronous motors is Unacceptable.
2. Lagging kVAR (Reactive component of Power or reactive power) drawn by the motor is sufficiently reduced because the exciting ampere-turns are supplied at slip frequency.

Using Phase advancer is not economical for motors below 200 H.P. (about 150kW)

Ques 34.  In a synchronous machine is called a doubly excited machine because

1. It can be over-excited
2. It needs twice the normal exciting current
3. It has two sets of rotor poles
4. Both its rotor and stator are excited

Answer 4. Both its rotor and stator are excited.

Explanation:

Synchronous machines:- A three-phase synchronous machine is a doubly excited machine because its field winding is energized from dc source and its armature winding is connected to an ac source.

When this is done, torque can only be developed at one speed-the synchronous speed. At any other speed, the torque is zero. In fact, it is this characteristic (i.e doubly-fed) that enables the synchronous motor to develop non-zero torque at ~only one speed (i.e. synchronous speed) and hence the name synchronous motor.

when working as a motor, the synchronous machine takes active power from an ac source. During its working as a generator, a synchronous machine delivers, or export AC power.

When this is done, torque can only be developed at one speed-the synchronous speed. At any other speed, the torque is zero. In fact, it is this characteristic (i.e doubly-fed) that enables the synchronous motor to develop non-zero torque at ~only one speed (i.e. synchronous speed) and hence the name synchronous motor.

Ques 35. The maximum speed variation in a 3-phase synchronous motor is

1. 5%
2. Zero
3. 10%
4. 3%

Explanation:

• A synchronous motor runs at synchronous speed or not at all. Its speed is constant at all loads.
• The maximum speed variation in the synchronous motor is zero. The speed of operation remains constant from NO load to FULL load in the motor operating at constant frequency bus bars.
• A synchronous motor runs at one speed only i.e. synchronous speed. Consequently, slip is zero.

Ques 36. If the field of a synchronous motor is underexcited, the power factor will be

1. Lagging
3. Zero
4. None of the above

When the synchronous machine is operating as an under-excited motor, it has a lagging power factor due to a low field current. Here, the maximum power Pmax is small and hence the machine operation is less stable. In this case, the motor behaves like an inductive load and the motor absorbs reactive power from the 3-phase line.

Note:- The power factor at which a synchronous machine operates and its stator (armature) current can be controlled by changing its field excitation.

Ques 37. Which of the following is an unexcited single-phase synchronous motor?

1. Reluctance motor
2. Repulsion motor
3. A.C. series motor
4. Universal motor

Explanation:

Single-Phase Synchronous Motors

Very small single-phase motors have been developed which run at true synchronous speed. They do not require d.c. excitation for the rotor. Because of these characteristics, they are called unexcited single-phase synchronous motors.

The most commonly used types are :

(i) Reluctance motors

(ii) Hysteresis motors

The efficiency and torque-developing ability of these motors are low. The output of most of the commercial motors is only a few watts.

## Reluctance Motor

It is a single-phase synchronous motor that does not require d.c. excitation to the rotor. Its operation is based upon the following principle:

Whenever a piece of ferromagnetic material is located in a magnetic field, a force is exerted on the material, tending to align the material so that reluctance of the magnetic path that passes through the material is minimized.

Working Principle

The stator consists of a single winding called the main winding. But single winding cannot produce a rotating magnetic field. So for the production of a rotating magnetic field, the must be at least two windings separated by a certain phase angle. Hence staler consists of an additional winding called auxiliary winding which consists of the capacitor in series with it. Thus there exists a phase difference between the currents carried by the two winding and corresponding fluxes. Such two fluxes react to produce the rotating magnetic field. The technique is called the split phase technique of the production of a rotating magnetic field. The speed of this field is the synchronous speed which is decided by the number of poles for which stator winding is wound.

The rotor carries the short-circuited copper or aluminum bars and acts as the squirrel cage rotor of an induction motor. If an iron piece is placed in a magnetic field, it aligns itself in a minimum reluctance position and gets locked magnetically. Similarly, in the reluctance motor, the rotor tries to align itself with the axis of the rotating magnetic field in a minimum reluctance position. But due to rotor inertia, it is not possible when the rotor is at standstill. So the rotor starts rotating near synchronous speed as a squirrel cage induction motor. When the rotor speed is about synchronous, the stator magnetic field pulls the rotor into synchronism i.e. minimum reluctance position, and keeps it magnetically locked. Then the rotor continues to rotate with a speed equal to synchronous speed. Such a torque exerted on the rotor is called the reluctance torque. Thus finally the reluctance motor runs as a synchronous motor. The resistance of the rotor must be very small and the combined inertia of the rotor and the load should be small to run the motor as a synchronous motor.

The reluctance motor has the following advantages

1. No d.c. supply is necessary for the rotor
2. Constant speed characteristics
3. Robust construction
4. Less maintenance

Limitations

The reluctance motor has the following limitation

1. Less efficiency.
2. Poor power factor.
3. The Need for a Very low inertia rotor.
4. Less capacity to drive the loads.

Ques 38. The damper winding in a synchronous motor is provided for

1. Starting torque only
2. Reduce eddy currents
3. Prevent hunting and provide the starting torque.
4. Reduce the noise level

Answer 3. Prevent hunting and provide starting torque

Explanation:

“Hunting is the phenomenon of the synchronous machines which oscillates at about its new equilibrium position” which means the synchronous machines rotate at other than synchronous speed.

### Use of Damper winding to prevent hunting

• Hunting can be reduced by using damper winding.
• When hunting occurs, the difference in the speed of stator and rotor poles develops and induces emf in the damper winding, which acts in such a way to suppress the rotor oscillation.
• When the rotation at constant load is uniform, there is no relative motion between the rotor and stator forward rotating fields and hence no current is induced in these windings.
• When the rotor speed is more than synchronous speed, induction generator torque will be produced through damper bars in the opposite direction to synchronous machine torque such that the above synchronous speed becomes nearer to synchronous speed. The relative motion of the rotor sets up eddy currents in these windings which flow such as to suppress the oscillations (as per Lenz’s Law).
• When the rotor speed is less than synchronous speed, induction motor torque will be produced through damper bars in the same direction of synchronous machine torque such that below synchronous speed becomes nearer to synchronous speed.
• The dampers should have low resistance . be more effective. However, this method cannot eliminate – completely.

Ques 39. When the field of a synchronous motor is over excited, the power factor will be

2. Lagging
3. Zero
4. Unity

An overexcited synchronous motor draws current at the leading power factor. If d.c. field excitation of a synchronous motor is such that the back emf Eb is greater than applied voltage V, then the motor is said to be over excited.

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.

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.

Ques 40. A Synchronous motor can operate at

1. Unity power factor
3. Lagging power factor
4. Leading as well as lagging power factor

Explanation:

For constant power output, a synchronous motor can be made to operate at either a lagging power factor, unity power factor or leading power factor by varying the field excitation.

Normal Excitation:- When the machine is operating as a normal excited motor, it has a unity power factor due to a normal field current. Under such conditions, the motor draws the minimum stator current.

Underexcited:- When the synchronous machine is operating as an under-excited motor, it has a lagging power factor due to a low field current. Here, the maximum power Pmax is small and hence the machine operation is less stable. In this case, the motor behaves like an inductive load and the motor absorbs reactive power from the 3-phase line.

Note:- The power factor at which a synchronous machine operates and its stator (armature) current can be controlled by changing its field excitation.

Overexcited:-When the machine is operating as an overexcited motor, it has a leading power factor due to a high field current. Therefore, the maximum power Pmax is large and the machine operation is stable. An overexcited synchronous motor acts as a power factor correction device and is also known as a synchronous condenser.

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