UNIT – I



DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

EC2201-ELECTRICAL ENGINEERING

QUESTION BANK

UNIT I - D.C. MACHINES

PART A

1. Define electric motor.

The electric motor is a machine that converts electrical energy into mechanical energy or motion.

2. Define electric generator.

The electric generator is machine that converts mechanical energy into electrical energy.

3. What is a prime mover?

The basic source of mechanical power which drives the armature of the generator is called prime mover.

4. What are the different types of Motors?

1. Self Excited Motors

2. Seperately Excited Motors

a. Series Motor

b. Shunt Motor

c. Compound Motor.

i. Cumulative Compound

ii. Differential Compound

5. State the principle of working of D.C. motor.

An electric motor is a machine which converts electrical energy into mechanical energy. Its action is based on principle that when a current carrying conductor is placed in a magnetic field it experiences a mechanical force whose direction is given by Fleming’s left hand rule.

6. State the various types of D.C. machines based on excitation.

1) Separately excited machine.

2) Self exited machine.

(a)Shunt wound

(b)Series wound.

(c)Compound wound.

7. What is the purpose of Yoke in a d.c. machine?

(1)It provides mechanical support for the poles and acts as a protecting cover for whole machine.

(2)It carries the magnetic flux produced by the poles.

8. What is the need for laminated poles and armature?

The purpose of using lamination is to reduce the loss due to eddy current.

9. Explain the function of commutator in D.C. generator and D.C. Motor.

The function of the commutator is to facilitate the collection of current from the armature conductors. It converts the alternating current induced in the armature conductors into unidirectional current in the external load circuit.

10. What are the effects of armature reaction?

The effect of magnetic field set up by the armature current on the distribution of flux under main poles of a generator is known as armature reaction. It has two effects

(1) it demagnetizes the main flux

(2) it cross magnetizes or distorts it.

11. What are the methods of neutralizing the effects of armature reaction?

(1) By connecting compensating winding in series with the armature.

(2) By having a small poles fixed to the yoke and spaced in between the main poles (Inter poles).

12. What is commutation and what is the effect of poor commutation?

The process by which current in the short circuited coil is reversed while it crosses the m.n.a. (magnetic neutral axis) is called as Commutation. The effect of poor commutation is sparking at the brushes.

13. What are the methods to improve commutation?

(1) Resistance commutation.

(2) E.m.f. commutation.

14. What are Interpoles and state their use.

Interpoles small poles fixed to the yoke and spaced in between the main poles. Interpoles are used to neutralize the cross magnetizing effect of armature reaction.

15. How can the voltage in a D.C. generator be increased?

The voltage in a D.C generator can be increased by increasing the main field flux and the speed of armature.

16. What is critical resistance of a D.C. shunt generator?

The value of resistance of shunt field winding beyond which the shunt generator fails to build up its voltage is known as “critical resistance”.

17. What do you mean by residual flux in D.C. Generator?

The magnetic flux retained in the poles of the machine even without field supply is called the residual flux.

18. What do you mean by external characteristic of D.C. generator?

It is the relationship between the load current and the terminal voltage.

19. What is the effect of armature reaction in D.C. generator?

Due to the demagnetizing effect of armature reaction field pole flux is weakened and so the induced EMF in the armature is decreased.

20. What is the use of compensating winding in D.C. generator?

The function of compensating winding is to neutralize the cross magnetizing effect of armature reaction.

21. Why D.C. series generator is called as rising characteristic machine?

In D.C. series generator, with increase in load, it’ terminal voltage is also increased. So it is called as rising characteristic machine.

22. What do you mean by saturation of field pole?

If we are going on increasing the field current, after certain limit, any more rise in current will not have a rise in flux. This condition is called saturation of field poles.

23. Name the parts of a D.C. machine in which iron losses occur.

Iron losses occur in yoke, field pole and armature core.

24. What are the losses of a shunt machine assumed as constant?

Core losses, mechanical losses and shunt field copper loss assumed as constant in shunt machine.

25. What is the condition for maximum efficiency of a D.C. machine?

Efficiency of a D.C. machine will be maximum when variable losses are equal to constant losses.

26. What are the applications of D.C Series generator?

These are used for series arc lighting, series incandescent lighting and as a series booster for increasing the voltage in D.C. transmission lines.

27. What is the use of shunt generator?

Shunt wound generator with field regulations are used for light and power supply purposes. These are also used for charging of batteries on account of its constant terminal voltage.

28. What causes sparking at the brushes?

It is either due to self-induction of the coil undergoing commutation or due to improper pressing of brush over the commutator surface.

29. Explain how you would reverse the direction of rotation of a D.C shunt motor.

The direction of rotation of a D.C. shunt motor can be reversed either by changing the direction of field current or armature current.

30. What are the factors that govern controlling of motor speed?

• flux per pole

• Voltage.

31. Suggest, with reason, a method of speed control of D.C motor above its rated speed.

The speed of a D.C motor can be obtained more than the rated value by field control method, because the flux per pole can be decreased to any value by decreasing the field current. (Field current can be decreased by inserting a field regulator)

32. How can the speed of a D.C shunt motor be controlled?

By varying the field current as well as armature voltage speed can controlled.

33. What will be the effect of adding resistance in the field circuit of a D.C shunt motor?

When the motor is running on no-load, the speed will increase, if additional resistance is connected in the field circuit. But the speed will decrease if it runs with load as torque produced decreases.

34. What do you mean by “commutation’ and commutation period?

The process by which current in the short circuited coil is reversed while it crosses the magnetic neutral axis is called “commutation”. The brief period during which coil remains short circuited is known as commutation period.

35. What happens when a D.C. Shunt motor is directly connected to the supply mains?

When the motor is at rest, there is no back EMF developed in the armature. If now full supply voltage is applied across the stationary armature, it will draw a very large current because armature resistance is very small ( I = V/R ) this excessive current will blow out the fuses and prior to that, it will damage the commutator, brushes etc.,

36. Name the starter usage for D.C Motors.

Two point starter for D.C series motor. Three point and four point starters for D.C. shunt and compound motors.

37. What are the protective devices used in 3 point starter?

Over – load release, No-volt release, Spiral spring used in the handle to bring it to the off position the time of failure of supply.

38. What are the various losses in a D.C. motor?

Armature copper loss, Field copper loss, Core losses (hysteresis loss and eddy current loss), Mechanical losses (frictional loss and windage loss).

39. Explain the variation of hysteresis and eddy current losses with speed of a D.C motor.

Hysteresis loss is proportional to the speed of the motor and the eddy current loss is proportional to the square of the speed so long as excitation is constant.

40. State the advantage of Swinburne’s test.

It is convenient and economical method of testing of D.C machines, since power required testing a large machine is very small. The efficiency of the machine can be pre-determined at any load, since stray losses (constant losses) are known.

41. Mention the effects of differential compounding and cumulatively compound on the performance of D.C compound motor?

Cumulative compound motor is used for variable loads with occasion over loads. But differential compound motor is used for running constant speed from no load to full load.

42. What happen if the direction of current at the terminals of a D.C. series motor is reversed?

It does not reverse the direction of rotation of motor because current flows through the armature in the same direction as through the field.

43. What is the function of holding coil in a 3-point starter?

When the supply fails or gets disconnected, the holding coil demagnetizes and so release the starting arm which goes back to the OFF position by the spring action.

44. What is the function of overload release coil in a 3-point starter?

Overload release coil is provided for the protection of the motor against the flow of excessive current due to over load. This coil is connected in series with the motor so carries full load current. When the motor is over loaded it draws heavy current and magnetizes the over load release coil to such an extent that it pulls its armature upwards and so short circuit the no-volt release coil, so it demagnetizes and release the arm to the OFF position.

45. Under what circumstance a dc shunt generator does fails to generate voltage?

• Absence of residual flux

• Initial flux setup by field may be opposite in direction to residual flux

• Shunt field circuit resistance may be higher than its critical field resistance

• Load circuit resistance may be less than its critical field resistance

46. What is the function of carbon brushes used in DC generator?

The function of carbon brushes is to collect current from the commutator and supply to external load circuit and to load.

47. Write down the application of D.C series motor

• Electric Trains

• Cranes, hoists, elevators and conveyors

• Fans and air compressors

• Hair driers, Vacuum cleaners, Sewing machines

• Traction drives

• Trolleys

Part-B

1. With a neat diagram explain the construction of a D.C Machine.

2. Deduce the emf equation of dc generator.

3. Deduce the expression for torque developed in a D.C. Motor. What is back emf and state its importance.

4. Deduce the equation for speed of a D.C. motor and hence suggest various methods of speed control and compare their merits and demerits.

5. Explain the characteristics of DC shunt motor and from the nature of the curve explain the applications of DC shunt motor.

6. Explain the characteristics of DC series motor and from the nature of the curve explain the application DC series motor.

7. Explain the principle of operation of D.C motor.

8. What is the need for starters? With the help of a neat diagram explain the working of a 3 point starter & state its limitations.

9. With the help of neat diagram explain the working of four point starter.

10. Discuss various methods of speed control of D.C series motor.

11. Explain the various characteristics of dc generators.

12. Explain in detail the procedure of calculating the efficiency of dc motor and dc generator in Swinburne’s test.

13. Explain in detail about the ward-Leonard system of speed control of DC motor.

TUTORIALS

1. Calculate the e.m.f generated by a 4 – pole wave wound generator having 65 slots with 12 conductors per slot when driven at 1200 r.p.m. The flux per pole is 0.02 Wb.

2. The induced emf in a DC machine while running at 750 rpm is 220V. Calculate (i)the speed at which the induced emf is 250V, assuming constant flux .(ii)the percentage increase in field flux for an induced emf of 250V and speed of 750 rpm .

3. A 200V DC shunt motor having an armature circuit resistance of 0.02 ohm and a field circuit resistance of 100 ohms draws a line current of 50A at full load at a speed of 1500 rpm. Calculate its speed at half – load.

4. The input to a 220V D.C Shunt motor is 11 kW. Calculate the Torque developed the Efficiency, the speed at the load when the following particulars of the motor are given No-load current = 5A; No-load speed=1150rpm; Armature resistance = 0.5 ohm; Shunt field resistance =110 ohms.

5. A shunt motor has an armature resistance of 0.25ohms.The motor takes 125 A From a 400V supply and runs at 1000rpm.If the total torque developed remains unchanged, Calculate the speed and armature Current if the magnetic field is reduced to 80% of the initial value.

6. A 500 V D.C Series motor has a resistance of 0.25ohm and runs at 500rpm, when taking a current of 50A .Calculate the speed when taking a current of 30A if the flux is reduced to 60% of that at 50A.

7. A 500V Shunt motor takes a current of 5A on no-load. The resistance of the armature and field circuits is 0.5 ohm and 250 ohm respectively. Calculate the efficiency when the motor takes a current of 100A.

8. A DC series motor connected across 460V supply runs at 500 rpm when the current is 40 A. The total resistance of the armature & field circuit is 0.6 Ω. Calculate the torque developed.

9. A compound generator is to supply a load of 250 lamps, each rated at 100W, 250V. The armature, series and shunt windings have resistances of 0.06 Ω, 0.04 Ω and 50 Ω respectively. Determine the generated e.m.f. when the machine is connected in (i) Long shunt, (ii) Short shunt. Take the drop per brush as 1V

10. A 50 kW, 230V DC shunt motor when running light takes a current of 14A. If Ra = 0.15 Ω & Rf = 120 Ω, estimate the efficiency when it is drawing a current of 215A.

UNIT-II - TRANSFORMERS

Part-A

1. State the principle of working of a transformer.

Transformer consists of two inductive coils which are electrically separated but magnetically linked through a path of low reluctance. If one coil connected to a source of alternating voltage, an alternating flux is set up in the laminated core, most of which linked with the other coil in which it produces mutually induced emf. If the second coil is closed current flows in it and so electrical energy is transferred from the first coil to the second coil without change in frequency.

2. List the types of Transformer based on Construction.

• Core type transformer

• Shell type transformer

3. Mention the difference between core and shell type transformer.

In core type the winding surround the core considerably and in shell type the core surround the windings i.e windings is placed in central limb of the core.

4. Define Transformation Ratio and classify the Transformer based on Transformation ratio.

Transformation ratio is defined as the ratio of number of turns in the secondary winding to number of turns in primary winding.

K= N2/N1

= E2/E1

Types :

Step up transformer

Step down transformer

5. Define Voltage Regulation of a Transformer.

The change is secondary terminal voltage from no load condition to full load condition expressed as a percentage of no load or full load voltage is termed as voltage regulation

6. Draw the complete equivalent circuit diagram of a transformer.

[pic]

7. List the various losses in a Transformer and state the Condition for Maximum efficiency.

Losses: i. Core loss

ii. Copper loss

Condition for maximum efficiency:

Iron loss = copper loss

8. Draw the approximate equivalent circuit of a transformer.

[pic]

9. State why the transformer core is made of magnetic materials?

The transformer core is made of magnetic materials to provide a continuous magnetic path with the minimum air gap.

10. State the methods of reducing leakage flux:

To minimize the leakage flux, transformer core is sectionalized and by interleaving the primary and secondary windings.

11. State the methods to reduce hysteresis and eddy-current losses in a transformer:

To reduce hysteresis loss, core is made up of CRGO (Cold rolled Grain oriented)

Eddy current loss is minimized by laminating the core.

12. Draw the phasor diagram of a transformer in no load

[pic]

13. Draw the complete phasor diagram for transformer with lagging load:

[pic]

14. Why transformers are rated in KVA?

Copper Loss of transformer depends on current & its iron loss on voltage. Hence total losses depend on Volt-Ampere but not on power factor. That is why the rating of transformers is in KVA and not in KW.

15. State the test to be conducted to determine the efficiency of a transformer

OC test –iron losses

SC test – copper losses

16. What are the two advantages of three phase transformer over single phase transformer?

The main advantages of three phase transformer are

1 Occupies less floor space for equal rating

a. Weighs less

b. Costs about 15% less

c. Maintenance is easy.

17. State the various types of transformers depending on its voltage transformation ratio:

Step-up transformer

Step- down transformer

18. Write the emf equation of a transformer and explain the terms used:

E=4.44 (m f N (volts)

19. What information can be obtained by conducting a SC test on a transformer:

Copper loss

20. What is the function of transformer oil in transformer?

• It provides good insulation

• Cooling

21. What are the connections of 3-phase transformer?

Primary Secondary

Y - Y

Y - Δ

Δ - Y

Δ - Δ

22. What happens when D.C supply is given to a transformer instead of A.C?

Due to saturation of the magnetic core a large current flows through the windings, without inducing emf. This large current burns the windings of the transformer.

23. Define transformer

A transformer is a static piece of apparatus by means of which electric power in one circuit is transferred to another circuit without change in frequency.

PART-B

1. Explain the Principle of working of a single phase transformer & derive the EMF equation of a single-phase transformer.

2. Describe the Constructional details of a single phase transformer.

3. Develop the equivalent circuit of a single phase transformer starting from first principle.

4. Define Voltage regulation of a Transformer. Deduce the expression for Voltage regulation.

5. State the various losses in a Transformer .Define efficiency of a Transformer and hence deduce the Condition for Maximum efficiency.

6. Explain with neat circuit diagram the OC and SC tests on a transformer and explain how the efficiency and equivalent circuit of a transformer can be determined?

7. Explain with neat diagram the Load test on a single phase transformer.

8. With a neat phasor diagram, explain about the ideal Transformer under Load condition.

9. With a neat phasor diagram, explain about the Practical Transformer under Load condition.

TUTORIALS

1. The voltage/turn of a 1-Φ transformer is 1.1V, when the primary winding is connected to a 220V, 50Hz ac supply, the secondary voltage is found to be 660V.Find (1) Primary & secondary currents (2) the core area if the maximum flux density is 1.2 T.

2. The maximum flux density in the core of a 250/3000 V, 50 Hz, 1-Φ transformer is 1.2 Wb/m2.If the emf/turn is 8 V, determine the primary & secondary turns.

3. A 25-kVA transformer has 500 turns on the primary & 100 turns on the secondary winding. The primary is connected to 2000V. Determine the F.L primary & secondary currents & also N.L secondary voltage.

4. A 5-kVA transformer has iron loss & copper loss of 100 & 200W respectively. Determine the efficiency at half F.L & 0.8 power factor lagging.

5. The maximum flux density in the core of 250 / 3000V, 50 Hz 1-Φ transformer is 1.2 Wb /m2 if the emf /turn is 8V, determine the number of primary & secondary turns.

6. The required N.L voltage ratio in a 150kVA, 50 Hz 1-Φ transformer is 5000 / 250V. Find the number of turns in each winding for a maximum core flux of 0.06 Wb & efficiency at half rated KVA, u.p.f.

7. The efficiency of a 400 kVA, single – phase transformer is 98.77% when delivering full – load at 0.8 p.f. lagging and 99.13% at half full – load at unity p.f. Calculate (i) Iron loss and (ii) Full – load copper loss.

UNIT-III INDUCTION MOTORS

PART-A

1. Write two extra features of slip ring induction motor

• Rotor has 3 phase winding

• Extra resistance can be added into the rotor circuit for speed control and also improving PF with the help of slip rings.

2. Why an induction motor is called a rotating transformer?

The rotor receives same electrical power in exactly the same way as the secondary of a two winding transformer receiving power from primary. That is why induction motor is called a rotating transformer.

3. Why an induction motor never runs at synchronous speed?

If it runs at synchronous speed then there would be no relative speed between the two, hence no rotor emf, so no rotor current, then no rotor torque to maintain rotation.

4. What are slip rings?

The slip rings are made of copper alloys and are fixed around the shaft insulating it. Through these slip rings and brushes, rotor winding can be connected to external circuit.

5. State the types of 3-phase induction motor based on construction.

i. Squirrel cage Induction Motor

ii. Phase Wound or slip ring Induction Motor

6. State the expression for synchronous speed.

Ns = 120 f

P

f= frequency in Hertz

P= No. of poles

7. What factors determine the direction of induction motor and how can it be reversed?

By reversing any 2 phase of the supply, direction of Induction Motor can be reversed.

8. Why an induction motor cannot develop torque when running at synchronous speed?

Induction motor cannot develop torque since the slip is zero when the motor runs at synchronous speed .

9. State the expression for torque developed in an induction motor, and state the factors affecting the running torque.

T = Ks E22R22 N-m

(R22 +(sX2)2)1/2

Slip affects the running torque

E2 = Rotor induced EMF

R2 = Rotor resistance

X2 = Rotor reactance

S= Slip

K = 3/2(Ns

10. State the expression for starting torque of a 3-phase induction motor and state the factors affecting starting torque.

Tstart = K E22R22 N-m

(R22 +X22)1/2

E2 = Rotor induced EMF

R2 = Rotor resistance

X2 = Rotor reactance

K = 3/2(ns

11. State the condition for achieving maximum torque and state the expression for maximum running torque.

Sm = R2/X2

Tm= k E22 N-m

2X2

12. What is the need for starters in 3-phase induction motors?

To limit the high starting current by supplying reduced voltage to the motor at the time of starting. Such a reduced voltage is applied only for a short period and once rotor gets accelerated, full normal rated voltage can be applied.

13. State the different types of starters used in induction motor.

1. Stator resistance starter

2. Autotransformer starter

3. Star-Delta starter

4. Rotor resistance starter

5. Direct on line starter

14. State the methods of speed control of 3-phase induction motors.

Basically by 2 Methods

1. From Stator side

1.1 Supply frequency control called v/f control

1.2 Supply voltage control

1.3 Controlling number of stator poles

1.4 Adding rheostats in stator circuit.

2. From Rotor side

2.1 Adding external resistance in rotor circuit

2.2 Cascade control

15. Briefly explain the phenomenon of crawling in 3-phase induction motors.

Crawling occurs particularly in Squirrel Cage Induction motor. Motor runs at a speed as low as one-seventh of their synchronous speed Ns. This phenomenon is known as Crawling. This is due to the fact that the ac winding of the stator produces a flux wave which is not a pure sine wave. It consist of a fundamental wave , which revolves at synchronous speed and odd harmonics. Due to this harmonic content the motor will not accelerate up to its normal speed but will remain running at a speed, which is nearly one seventh of its full speed.

16. Briefly explain the phenomenon of Cogging in 3-phase induction motors.

The rotor of squirrel cage Induction motor sometimes refuses to start at all, particularly when the voltage is low. This happens when the number of stator teeth S1 is equal to the number of rotor teeth S2 and due to the magnetic locking between the stator and rotor teeth. This is also referred as teeth-locking.

17. List the various losses in a 3-phase induction motor.

i. Stator Copper loss

ii. Rotor Copper loss

iii. Iron loss

iv. Friction and windage loss

18. List the few applications of 3-phase induction motor.

Squirrel cage motor has moderate starting torque and constant speed

Uses: Driving fans, Blowers, Water pumps, lathe machines, drilling machine

Slip ring motor has high starting torque and maximum torque

Uses: lifts, hoists, Elevators, cranes, compressors

19. Why skewing is used in rotor core assembly?

Skewing is used in rotor core assembly to avoid magnetic hum and to avoid interlocking.

20. What is the use of no voltage coil in a DOL starter?

No voltage coil gets de-magnetized when the supply fails and motor gets switched off from the supply.

21. Draw the speed-Torque characteristics of 3-phase Induction motor.

[pic]

22. Why-squirrel cage induction motors are commonly used in domestic pump sets?

Squirrel cage motor has moderate starting torque and constant speed characteristic hence commonly used in domestic pump sets.

23. What is the frequency of the induced emf of an induction motor

fr = S f

where fr - frequency of rotor induced emf

S - slip

f - frequency of the supply voltage

24. What are the advantages of primary resistance starter of induction motor?

• Starting torque to full load torque is x2 of that obtained with direct switching or across the line starting

• This method is useful for smooth starting of small machines only.

PART-B

1. Explain how a rotating magnetic field is produced when a 3-phase supply is connected across the 3-phase stator windings of an induction motor.

2. With the help of neat diagrams explain the construction of a 3-phase induction motor.

3. Explain with diagram any one method of starting a slip ring induction motor.

4. Discuss how the speeds of three phase induction motors can be controlled.

5. Draw a neat diagram for a star-delta starter and explain.

6. Explain the principle of working of 3-phase Induction motor. Also name the methods of starting and prove that the rotor cannot run at synchronous speed.

7. Explain the method of speed control of a slip ring Induction motor with a neat diagram.

8. Discuss briefly the torque developed by a 3- phase Induction motor and derive the expression for the torque developed by a 3-phase Induction motor.

9. Compare a cage Induction motor with a slip ring motor with reference to construction, performance, and applications.

10. a) Explain a method of determining equivalent circuit of 3- phase Induction motor.

b) Prove mathematically that a rotating magnetic field is produced by the application of 3-phase supply to the stator of a 3- phase Induction motor.

11. How does a rotor rotate in an Induction Motor?

12. a) Draw the torque /slip curves for several values of rotor circuit resistance of a induction motor and explain torque/slip curve.

b) Draw and explain the equivalent circuit of 3 phase induction motor.

13. Explain the working of single phase induction motor.

TUTORIALS

1. A 50 Hz, 4 pole, 3 phase induction motor has a rotor current of frequency 2 Hz . Determine (i) The Slip, (ii) Speed of the Motor.

2. A 6 pole, 3 phase, 50 Hz induction motor is running at full – load with a slip of 4%. The rotor is star-connected and its resistance and stand still reactance are 0.25 Ω and 1.5 Ω per phase. The emf. between slip rings is 100V. Find the rotor current per phase and p.f. assuming the slip rings are short – circuited.

3. A 8 pole, 3 phase, 50 Hz induction motor running with a slip of 4% is taking 20KW. Stator losses amount to 0.5 kW. If the mechanical torque lost in friction is 16.25 N-M, find (i) B.H.P, (ii) Efficiency

4. A 4 pole, 3 phase, 50Hz induction motor has a star- connected rotor. The rotor has a resistance of 0.1 Ω per phase and standstill reactance of 2 Ω/phase. The induced emf between the slip rings is 100V. If the full load speed is 1460 rpm, calculate (i) the Slip

(ii) The E.m.f induced in the rotor in each phase, (iii) the rotor reactance per phase, (iv) The rotor current and (v) Rotor Power factor. Assume slip – rings are short circuited.

5. The rotor resistance and standstill reactance per phase of a 3- Phase induction motor are 0.02 Ω and 0.1 Ω respectively. What should be the value of the external resistance per phase to be inserted in the rotor circuit to give maximum torque at starting?

6. A 3 phase Induction motor is running at 1740 rpm on a 60Hz supply. Calculate the no. of poles, the slip and the rotor frequency.

7. The power supplied to a 3- phase Induction motor is 40 kw and the corresponding stator losses are 1.5KW. Calculate the total mechanical power developed and rotor copper loss if slip is 0.04 per unit.

8. A 3- phase, 6- pole, 50 Hz, Induction motor has a slip of 1% at no load and 3% at full Load. Find i) No load speed ii) Full load speed iii) Frequency of torque current at full load.

UNIT-IV SYNCHRONOUS AND SPECIAL MACHINES

PART-A

1. What are the principle advantages of rotating field type construction?

• Relatively small amount of power required for field system can easily be supplied to the rotating field using slip rings and brushes

• More space is available in the stator part of the machine to provide more insulation.

• It is easy to provide cooling system.

• Stationary system of conductors can be easily braced to prevent deformation.

2. Give the emf equation of a 3-phase alternator.

Eph = 4.44 f(Tph volts

Where f = frequency of induced emf in Hz

( = flux per pole in wb

Tph= Number of turns per phase

3. Name two types of alternators.

i. Salient pole type

ii. Smooth cylindrical type

4. A 2-pole synchronous alternator is driven at 3600 rpm. What will be the frequency of the generated emf?

f = PN Hertz

120

f= 2* 3600 = 60 Hertz

120

5. Name the various methods of starting a Synchronous Motor and application of synchronous Motor.

i. Using pony motors

ii. Using damper winding

iii. As a slip ring Induction motor

iv. Using a small dc machine coupled to it.

6. Salient pole arrangement is adopted for low sped alternator. State whether it true or false with proof.

True. Mechanical strength of Salient pole arrangement alternator is less , hence this is used as low speed alternators from 125 rpm to 500 rpm.

7. Define the term voltage regulation of an alternator.

% regulation = Eph – Vph x 100

Vph

Where Eph = No load induced emf per phase

Vph = Phase value of rated voltage

8. What is the principle of stepper motor?

Stepper works by the principle to convert a train of input pulses i.e., a square wave pulses into a precisely defined increment in the shaft position or Stepper motor is a electromechanical device which actuates a train of step movements of shafts in response to train of input pulses. The step movement may be angular or linear.

9. Name the types of stepping motors.

i. Variable reluctance Stepper motor

ii Permanent Magnet Stepper Motor

iii Hybrid stepper Motor

10. What is the principle of working of a hysteresis motor?

This is the synchronous motor ( magnetic locking) which does not require any dc excitation to the rotor and it uses non projected poles.

11. Give two applications of stepper motor.

i. Widely used in computer peripherals such as serial printers , linear stepper motors to printers , tape drivers , floppy disc drives etc.,

ii. Stepper motors are also used in serial printers in typewriters or word process system , numerical control of machine tools, robotic control systems, space crafts etc.,

12. What are the methods to determine the regulation of an alternator?

i. Direct loading method

ii. Synchronous impedance method or EMF method

iii. Ampere- turns Method or MMF Method

iv. Zero power factor Method

v. ASA modified form of MMF Method

vi. Two reaction Theory.

13. Which method of finding regulation is called as (a) Pessimistic method (b) Optimistic method? Give reasons for your answer.

i. Pessimistic method or Synchronous impedance method or EMF method

This method leads to a high value of percentage regulation than the actual results.

ii. Optimistic method or Ampere- turns Method or MMF Method

This method leads to a low value of percentage regulation than the actual results.

14. Why are alternators rated in KVA and not in KW?

As load increases I2R loss also increases, as the current is directly related to apparent power delivered by generator, the alternator has only their apparent power in VA/KVA/MVA as their power rating.

15. Why the synchronous impedance method of estimating voltage regulation is considered as pessimistic method?

Compared to other method, the value of voltage regulation obtained by this method is always higher than the actual value so it is called as pessimistic method.

16. Why MMF method of estimating voltage regulation is considered as optimistic method?

Compared to EMF method, MMF method involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes core saturation effect. As this method require more effort, final result is very close to actual value, hence this method is called as optimistic method.

17. What are the advantages of salient type pole construction used in synchronous machines?

They allow better ventilation, the pole faces are so shaped radial air gap length increases from pole center to pole tips so flux distortion in air gap is sinusoidal so emf is also sinusoidal.

18. What do you mean by reluctance motor?

A reluctance motor is a type of synchronous electric motor that induces non-permanent magnetic poles on the ferromagnetic rotor. Torque is generated through the phenomenon of magnetic reluctance.

19. What are the advantage and disadvantage of reluctance motor?

Reluctance motors can have very high power density at low-cost, making them ideal for many applications.

Disadvantages are high torque ripple when operated at low speed, and noise caused by torque ripple. Until recently, their use has been limited by the complexity inherent in both designing the motors and controlling them.

20. What are the applications of synchronous reluctance motor?

• SRM's are used in some washing machine designs.

• SRM's are commonly used in the control rod drive mechanisms of nuclear reactors.

PART-B

1. a) Why a synchronous motor is not self-starting? Which method is used to make a synchronous motor self-starting

b) Write a brief note on hunting of synchronous motor.

2. A 3-phase, 16-pole alternator has a star connected winding with 144 slots and 100 conductors per slot. The chording is by one slot pitch. The flux per pole is 0.03 Wb sinusoidally distributed and the speed is 375 rpm. Find the frequency, phase and line voltage.

3. Derive from fundamentals principles, the equation for emf generated per phase of a 3-phase alternator.

4. a) Draw the phasor diagrams of a synchronous motor working under (i) lagging p.f (ii) UPF and (iii) leading p.f.

b) Discuss the various methods of starting of synchronous motor.

5. a)Draw and explain the phasor diagram of a synchronous motor operating at leading power factor.

b) Explain the methods of staring synchronous motors.

6. a) Discuss the basic principle of synchronous motor.

b) Explain any one method of starting a synchronous motor.

7. Explain the construction and working of Brushless alternator with neat sketch.

8. Explain the construction and working of Permanent magnet synchronous machine with neat sketch.

9. Explain sequential circuit reluctance motor and Hysteresis motor with neat sketches the principle of operation and the application.

10. Write short notes on Stepper motors.

11. Explain the principle of operation of a reluctance motor.

TUTORIALS

1. A 2 MVA 3 – phase, 8-pole alternator is connected to 6000V, 50Hz bus bars and has a synchronous reactance of 4 Ω per phase. Calculate the synchronizing power and synchronizing torque per mechanical degree of rotor displacement at no load. Assume normal excitation.

2. A 4 pole, 50Hz, 3 phase mesh connected alternator has a single layer stator winding distributed in 36 slots, each slot containing 16 conductors. The flux per pole is 0.04 wb. Calculate the terminal emf on open circuit.

3. A 6 - pole, 3 – phase, 50 Hz I.M has a slip of 1% at No load & 3% at F.L. Find the No load speed & frequency of rotor current at F.L.

4. a) Derive the E.M.F equation of an alternator.

b) A 3 phase star connected alternator is excited to give 6599 V between lines on open circuit. It has a resistance of 0.5 ohm and a synchronous reactance of 5 ohm per phase. Determine the terminal voltage and regulation at full load current of 130A when the load p.f is 0.8 lagging.

5. a) Discuss the effect of excitation in synchronous motor.

b) A 500 V, synchronous motor develops 7.5 h.p. The power factor is 0.8 lagging, effective resistance 0.75 ohm and iron and friction losses are 400 watts. Find the armature current and power supplied to the motor.

6. A 3-Phase, 10-pole, star-connected alternator runs at 600 rpm. It has 120 stators with 8 conductors per slot, and the conductors of each phase are connected in series. Determine the phase electro-motive force if the flux/pole is 56 mWb.

7. An alternator on open circuit generates at 60Hz, when the field current is 3.6 Amps. Neglecting saturation, determine the open circuit emf when frequency in 40Hz and the field current is 2.4 amperes.

UNIT-V

1. What are the different parts of power system?

i. Generation

ii. Transmission

iii. Distribution

2. Mention three power generation system

i. Thermal

ii. Hydro Electric

iii Nuclear power system

3. What is the important disadvantage of Nuclear power station

i. Pollution

ii. Control is difficult

4. Why power is transmitted at a high voltage.

i. to reduce losses and

ii. to reduce the volume of conductors.

5. What are the two types of distribution system

i.Primary ii.Secondary

6. Mention the advantage of AC transmission

AC voltages can be stepped up or down using transformers.

7. Mention the advantage of underground cables.

Public safety

8. Mention the materials used as overhead conductors

Copper and Aluminum

9. Mention the materials used for line supply

Wooden poles and Steel poles

10. Name the different types of transmission system

Overhead System and Underground System

11. What are the different types of EHVDC System

i. Monopolar EHVDC

ii Bipolar EHVDC

iii Homopolar EHVDC

iv Back to Back EHVDC

v. Multi terminal EHVDC

12. Name the types of Insulators

i. Pin type insulators

ii. Suspension type insulators

iii. Strain insulators

iv. Shackle insulators

v. Stay insulators

13. Name some commonly used insulating materials

i. Porcelian ii. Glass iii. Synthetic resin

14. Write the different methods of testing the cables

i. Continuity test

ii Insulation test

iii Murray loop test

iv. Fall of potential test

v. DC charge and Discharge test

15. Write the components of Distribution.

i. Substation

ii. Local Distribution station

iii. Feeders

iv. Distributors

v. Service mains

16. Define String Efficiency

String Efficiency = Voltage across the string

n x voltage across disc nearest to conductor

n = number of disc in the string

17. Why transmission lines are 3 phase 3 wire circuits while distribution lines are 3 phase 4 wire circuits?

A Balanced 3 phase circuit does not require the neutral conductor, as the instantaneous sum of the 3 line currents are zero. Therefore the transmission lines and feeders are 3 phase 3 wire circuits. The distributors are 3 phase 4 wire circuits because a neutral wire is necessary to supply the 1 phase loads of domestic and commercial consumers.

18. Mention the disadvantages of high voltage ac transmission.

• An ac line requires more copper than a dc line.

• The construction of an ac line is more complicated than a dc transmission line.

• Due to skin effect in the ac system the effective resistance of the line is increased.

19. Mention the range of surge impedance in underground cables .

40-60 ohms

20. Mention the range of surge impedance in overhead transmission lines .

400-600 ohms

21. Why cables are not used for long distance transmission?

Cables are not used for long distance transmissions due to their large charging currents.

22. Mention the 3 main parts of the cable?

Conductor, dielectric, sheath

23. Mention the differences between 3 wire and 3 phase 4 wire distribution system?

i) 3 phase 3 wires are employed for balanced loads, but 3 phase 4 wire is employed for unbalanced loads.

ii) 3 phase 3 wires are used for transmission but 3 phase 4 wire is used for distribution of power to consumers.

24. Mention the demerits of HVDC transmission.

i. Electric power cannot be generated at high dc voltages.

ii. The dc voltages cannot be stepped up for transmission of power at high voltages.

iii. The dc switches and circuit breakers have their own limitations.

25. What are the advantages of high voltage ac transmission?

• The power can be generated at high voltages.

• The maintenance of ac substation is easy and cheaper.

26. Mention the limitations of using very high transmission voltage.

• The increased cost of insulating the conductor.

• The increased cost of transformers, switch gears and other terminal apparatus.

PART - B

1. The DC distribution cable is 1000 m long and is loaded as shown in Fig. The resistance of each conductor is 0.05 ohms per 1000 m. calculate the voltage at load point.

O 300 m A 600 m B 100 m C

250 V 100 A 80 A 50 A

2. A 3 core 400 V, 250 m long cable supplies a load of 80 kW at 0.8 power factor lag for 4000 hours per annum. The capital cost per meter length is Rs. (15 a+ 25) where 'a' is the cross sectional area of each conductor in sq.cm. The resistance per km of a cable of cross sectional area 1 cm2 0.017hm. If the cost of the energy wasted is 10 paise per unit and the rate of interest and depreciation is 10 % calculate the most economical cross section of the conductor.

3. A 3 Wire, DC system is to be converted to a 3 phase, 4 wire AC system, by the addition of another conductor equal in section to one of the outers. If the percentage copper loss and voltage at the consumer terminals are to remain the same in the two cases, calculate the additional percentage load that can be supplied by the AC system. Assume balanced load in the two cases and unity power factor in the case of AC.

4. A 3 phase ring main system has 4 busbars with 11 kV fed at point A. It supplies balanced loads of 50 A at 0.8 p.f lag at B, 120 A at u.p.f. at C and 70 A at 0.866 p.f lag at D, all power factors with respect to the voltage at A. The impedances between various sections are as follows: ZAB=(1+j0.6), ZBC=(1.2+j0.4), ZCD=(0.8+j0.5), ZDA=(3+j2) ohms. Calculate the currents in various sections and the busbar voltages at B,C and D.

5. An electric train taking a constant current of 600 A moves on a section of line between two substations 8 km apart and kept at 575 V and 590 V respectively. The track resistance, including go and return, is 0.04 ohms per km. Find the point of minimum potential and the currents supplied by the two substations at the instant.

6. State and explain Kelvin's law. Also state the limitations of Kelvin's law.

7. An insulator string consists of 3 units each having a safe working voltage of 154 kV. The ratio of self capacitance of each unit is 8:1. Find the maximum safe working voltage of the string. Also find the string efficiency.

8. A single core, lead covered cable is to be designed for 66 kV to earth. Its conductor radius is 0.5 cm and its three insulating materials A, B and C have relative permittivity’s of 4,4 and 2.5 with maximum permissible stresses of 50, 40 and 30 kV/cm respectively. Find the minimum internal diameter of the lead sheath.

9. Calculate the capacitance and charging current of a single core cable used on a 3 phase, 66 kV system. The cable is 1 km lone having a core diameter 15 cm and sheath diameter 60 cm. The relative permittivity of insulation is 3.6 and the supply frequency is 50 Hz.

10. A single core cable has a conductor diameter of 1 cm and insulation thickness of 0.4 cm. If the specific resistance of insulation is 5 *10 14 ohm-cm, calculate the insulation resistance for a 2 km length of the cable.

11. If V1 , V2 and V3 are the potentials to earth of any three successive link pins of a suspension insulator, show that m V1 –(2m+1) V2 +m V3 =0 where m is the ratio of the capacitance of each unit to the capacitance of link pin to earth.

12. A 250m, 2 wire d.c distributor fed from one end is loaded uniformly at the rate of 1.6A/meter. The resistance of each conductor is 0.0002 ohm per meter. Find the voltage necessary at feed point to maintain 250V, (i) at the far end, (ii) at the mid – point of the distributor.

13. Calculate the voltage at a distance of 200m of a 300m long distributor uniformly loaded at the rate of 0.75 A per meter. The distributor is fed at one end at 250V. The resistance of the distributor (go and return) per meter is 0.00018 ohm. Also find the power loss in the distributor.

14. A 2 wire D.C. distributor AB is fed from both ends. At feeding point A, the voltage is maintained as at 230V and at 235V. The total length of the distributor is 200 meters and loads are tapped off as under,

25A at 50 meters from A; 50A at 75 meters from A

30A at 100 meters from A; 40A at 150 meters from A

The resistance per kilometer of one conductor is 0.3 ohm, Calculate

i) Currents in various sections of the distributor,

ii) Minimum voltage and the point at which it occurs.

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θ

E1

E2

IW

i0

I

V1

Load

R1

R2

X2

X1

R0

X0

V1

I1

I2

ZL

VL

E2=E1

I1

I2

Load

R1

X2

X1

R0

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V1

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I2

I0

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R01

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