Two Marks Questions with Answers

Two Marks Questions with Answers

 Q. 1 List the inherent disadvantages of synchronous motor ? AU : May-06,16, Dec.-05.09.10

Ans. :

1. Higher cost.

2. Necessity of frequent maintenance

3. Not self starting hence addition starting device required.

4. Separate d.c. source required for exciting the rotor.

5. Sensitive to sudden load changes causing hunting.

6. Requires slip ring and brushes.

7. Speed control not possible hence cannot be used for variable loads.

 

Q. 2 When is a synchronous motor said to receive 100% excitation ? AU : May-07, Dec.-10, 15

Ans. :

An excitation for which induced e.m.f. E_b is equal to the input voltage V of a synchronous motor is called normal excitation or 100 % excitation.

 

Q. 3 What is hunting ?

Ans. :

If the load connected to the motor is suddenly changed by a large amount, then rotor tries to retard to take its new equilibrium position. But due to inertia of the rotor, it cannot achieve its final position instantaneously. While achieving its new position, due to inertia it passes beyond its final position corresponding to new load. This will produce more torque than what is demanded. This will try to reduce the load angle and rotor swings in other direction. So there is periodic swinging of the rotor on both sides of the new equilibrium position, corresponding to the load. Such a rotor swing is called hunting.

 

Q. 4 Enlist the advantages of synchronous motor.

Ans. :

1. By changing excitation, power factor can be varied.

2. Can be used as synchronous condenser for improving power factor.

3. Speed is always constant, irrespective of load.

4. Electromagnetic power varies linearly with voltage.

5. Operate at higher efficiency than other motors.

 

Q. 5 Mention four applications of synchronous motor.

Ans. :

Machine tools, clocks, stroboscopic devices, timing devices, belt driven reciprocating compressors, centrifugal pumps, power factor correction devices, phase advancers and phase modifiers.

 

Q. 6 What is synchronous condenser ? State its application. AU : May-07, 17,18, Dec.-ll, 12, 14

Ans. :

When the synchronous motor is on no load and it is overexcited then the power factor angle of synchronous motor increases almost upto 90° and motor rims with almost zero leading power factor condition. This is exactly identical to the behaviour of a capacitor. Hence synchronous motor on no load which is overexcited is called synchronous capacitor. It is used as a phase advancer or as a power factor correction device.

 

Q. 7 Write down the equation for power developed by a synchronous motor.

Ans. :

The equation for power developed by a synchronous motor is,

where δ = Load angle, θ = Internal machine angle, V_ph = Phase voltage and E_b is the induced e.m.f. i.e. excitation.

 

Q. 8 What is meant by torque angle ?

Ans. :

When the synchronous motor is loaded, the rotor field axis falls back with respect to the stator axis by an angle called torque angle denoted as δ. The torque produced by the synchronous motor depends on this angle δ.

 

Q. 9 What are the causes of Hunting ?

Ans. :

1. Sudden change in the load. 

2. A fault in the supply system.

3. A sudden change in the field current.

4. A load containing harmonic torques.

 

Q. 10 What is meant by break out torque ?

Ans. :

The torque developed by the motor when full voltage is applied to its stator at start is called starting torque which is sometimes called breakout torque or break away torque.

 

Q. 11 Define pull-out torque in synchronous motor.

Ans. :

When the synchronous motor is loaded, the rotor falls back with respect to stator by an angle called load angle δ .As δ increases, magnetic locking between stator and rotor decreases. At δ = 90°, the torque developed is maximum by the motor and magnetic locking is very weak. Any further increase in the load pulls motor out of synchronism and motor stops. Thus the maximum torque developed by the synchronous motor without pulling out of synchronism is called pull out torque.

 

Q. 12 What does synchronous phase modifier mean ?

Ans. :

A synchronous motor without any mechanical load, provided with the means of controlling its power factor, used for simulating capacitive reactance and inductive reactance, for controlling the regulation of alternating current power systems is called synchronous phase modifier.

 

Q. 13 What are V curves ?

Ans. :

The graph of armature current drawn by the synchronous motor plotted against field current for various excitations at constant load gives a shape like alphabet V. If such graphs are plotted at different load conditions we get a family of such curves all looking like V which are called V curves.

 

Q. 14 What are the main parts of synchronous motor ?

Ans. :

The main parts of synchronous motor are stator which carries a three phase star or delta connected winding and a rotor which is generally salient pole type carrying field winding. On the slots provided on pole faces, short circuited damper winding is provided.

 

Q. 15 Mention the need for starters in synchronous motors ?

Ans. :

The synchronous motors are not self starting hence the starters are necessary in synchronous motors. The starters are necessary to bring the rotor of the synchronous motor near the synchronous speed so that it can be conveniently synchronized.

 

Q. 16 Mention the advantages of salient pole in synchronous motor.

Ans. :

The advantages of salient pole are,

1) It allows better ventilation.

2) Due to the shape of pole faces, the radial airgap length increases from pole center to the pole tips hence flux distribution in the air gap is sinusoidal in shape.

3) It is cheaper in cost.

4) Salient poles have more residual magnetism which helps to synchronize the synchronous motor.

5) Smaller size rotor is sufficient to obtain the necessary exciting ampere-turns than a wound rotor.

 

Q. 17 What are the causes of faulty starting of a synchronous motor ? AU : May-10, Dec.-14

Ans. :

The causes of faulty starting of synchronous motor are,

i) The rotor is not driven up to required speed by external device.

ii) The d.c. supply to the rotor is not given.

 

Q. 18 Specify the role of damper winding in synchronous motor.

Ans. :

When the load is changed the synchronous motor oscillates about the final position before setting down. This is called hunting. The damper winding is short circuited winding provided in synchronous motor and when motor oscillates there is relative motion between damper winding and the rotating magnetic filed due to which e.m.f. is induced in the damper winding. According to Lenz's law this e.m.f. circulates the current so as to oppose the cause producing it which is hunting. Thus the damper winding opposes the hunting and minimizes the hunting effect. Damper winding also helps to start the synchronous motor as an induction motor. It brings synchronous motor at subsynchronous speed by acting as rotor winding of an induction motor. 

 

Q. 19 What are the starting methods of synchronous motor ?

Ans. :

A synchronous motor can be made self starting by,

1. Using pony motor    

2. Using damper winding

3. As a slip ring induction motor

4. Using small d.c. machine coupled to it.

 

Q. 20 State the characteristic features of synchronous motor.

Ans. :

Features of Synchronous Motor

1. The synchronous motors run only at synchronous seed.

2. By varying its excitation, its power factor can be varied.

3. As it can be operated at leading power factor, it is used as a power factor correction device.

4. They are not self starting and requires an additional facility to make it self starting.

5. Under no load and over excited condition it can be used as a synchronous consdenser.

 

Q. 21 What are the different excitations of a synchronous motor ?

Ans. :

1. When the excitation is adjusted to get E_b = V i.e. induced e.m.f. is equal to applied voltage. Such an excitation is called Normal Excitation of the motor.

2. When the excitation is adjusted in such a way that the magnitude of induced e.m.f. is less than the applied voltage (E_b < V) the excitation is called Under excitation.

3. The excitation to the field winding for which the induced e.m.f. becomes greater than applied voltage (Eb > V), is called over excitation.

4. When the excitation is changed, the power factor changes. The excitation for which the power factor of the motor is unity (cos ϕ = 1) is called critical excitation.

 

Q. 22 Comment on whether excitation affects the power factor of the synchronous motor.

Ans. :

The excitation affects the power factor of the synchronous motor. Then refer the answer of the Q.21 above.

 

Q. 23 State the principle of working of synchronous motor.

Ans. :

The synchronous motor works on the principle of magnetic locking. When the stator is excited by 3 phase supply, it produces rotating magnetic field. So it creates the effect of rotating magnet in space at synchronous speed. When the field winding on rotor is excited by a d.c. supply, it also produces magnet. Thus when the unlike poles of stator and rotor face to each other, the magnetic locking is established between stator and rotor magnetic fields and the rotor gets pulled into synchronism. Once this happens, rotor keeps on rotating along with stator magnetic field at synchronous speed.

 

Q. 24 Why the synchronous motor is not self starting ?

Ans. :

The stator produces rotating magnetic field which rotates at synchronous speed. When the field winding on rotor is excited by a d.c. supply, it also produces magnet. When the unlike poles of stator and rotor face to each other, there is force of attraction between them but due to inertia of rotor, before rotor hardly rotates in one particular direction, the position of stator poles changes. Hence before rotor rotates, it gets subjected to oppositely directed torque. As a result, the average torque exerted on the rotor is zero. And hence the synchronous motor is not self starting.

 

Q. 25 Explain a general procedure to start a synchronous motor.

Ans. :

1. Give a three phase a.c. supply to a three phase winding. This will produce rotating magnetic field rotating at synchronous speed Ns r.p.m.

2. Then drive the rotor by some external means like diesel engine in the direction of rotating magnetic field, at a speed very near or equal to synchronous speed.

3. Switch on the d.c. supply given to the rotor which will produce rotor poles. Now there are two fields one is rotating magnetic field produced by stator while the other is produced by rotor which is physically rotated almost at the same speed as that of rotating magnetic field.

4. At a particular instant, both the fields get magnetically locked. The stator field pulls rotor field into synchronism. Then the external device used to rotate rotor can be removed. But rotor will continue to rotate at the same speed as that of rotating magnetic field i.e. N_s due to magnetic locking.

 

Q. 26 Define pull in torque incase of synchronous motor.

Ans. :

Initially synchronous motor is rotated at a speed slightly less than the synchronous speed. When speed is near to synchronous, excitation is switched on and motor gets pulled into synchronism and starts rotating at the synchronous speed. The amount of torque developed by the motor at the time of pulling into synchronism is called pull in torque.

 

Q. 27 State the general expression for an induced e.m.f. in a synchronous motor.

Ans. :

Thus general expression for the induced e.m.f. is,

(E_bph)² = (V_ph ) ² + (E_Rph ) ² - 2 V_ph E_Rph cos (θ ± ϕ)

+ sign for leading p.f. while - sign for lagging p.f. 

 

Q. 28 State the disadvantages of low power factor.

Ans. :

1. For low power factor current drawn is high.

2. For the higher current, conductor size required is more which increases the cost.

3. For fixed active power P, low p.f. demands large kVA rating alternators and transformers. This increases the cost.

4. Large current means more copper losses and poor efficiency.

5. Large current causes large voltage drops in transmission lines, alternators and

other equipments. This results into poor regulation.

 

Q. 29 Why synchronous motor always runs at synchronous speed ? or Why does synchronous motor develop torque only at synchronous speed ?

Ans. :

While starting, the rotor is brought to the speed slightly less than the synchronous speed and then the supply is given to the field. Due to this there is magnetic locking between the stator rotating magnetic field and the rotor poles. As the stator rotating magnetic field is rotating at synchronous speed, due to the magnetic locking the rotor also rotates at the same speed which is synchronous speed. Due to magnetic locking, it can not be rotated at any other speed.

 

Q. 30 Name the important characteristics of a synchronous motor not found in an induction motor.

Ans. :

1. The rotor speed is synchronous with respect to stator rotating field.

2. The power factor can be easily varied by varying its field current.

3. It is used for constant speed operation, as speed is always constant.

 

Q. 31 Draw the phasor diagram of synchronous motor under leading power factor operation.

 

Q. 32 State the different methods used to minimise hunting.

Ans. :

1. Using the damper winding.

2. Using a flywheel.

3. By designing the synchronous machine with suitable synchronizing power coefficient or stiffness factor.

 

Q. 33 What is a phase advancer ?

Ans. :

A machine which supplies the reactive volt-amperes to the system to which it is connected is called phase advancer. Thus an overexcited synchronous motor on no load connected to the system supplies reactive volt-amperes acts as a phase advancer.

 

Q. 34 Why a synchronous motor is a constant speed motor ?

Ans. :

The synchronous motor works on the principle of magnetic locking. The supply given to the stator produces rotating magnetic field which rotates at the synchronous speed. The rotor is magnetically locked with the rotating magnetic field of the stator. Due to this rotor rotates at the same speed as that of rotating magnetic field. Due to magnetic locking, it cannot rotate at any other speed. Hence a synchronous motor is a constant speed motor.

 

Q. 35 Mention two methods used to prevent hunting.

Ans. :

1) Use of damper winding.     

2) Use of fly wheels.

 

Q. 36 How can we change the operating speed of synchronous motor  AU : May-16

Ans. :

The speed of synchronous motor is given by, N_s = 120 f/P. Hence to change the operating speed, it is necessary to change the supply frequency or to change the number of stator poles. It is difficult to change the number of stator poles but with special arrangements supply frequency can be varied to change the speed of synchronous motor. 

 

Q. 37 A 3-phase synchronous motor driving a constant load torque draws power from infinite bus at leading power factor. How power angle and power factor will change if the excitation is increased ?

Ans. :

As excitation increases, its power angle increases and power factor decreases.