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EE 8401 — ELECTRICAL MACHINES – II
UNIT I SYNCHRONOUS GENERATOR 9Constructional details – Types of rotors –winding factors- emf equation – Synchronous reactance –Armature reaction – Phasor diagrams of non salient pole synchronous generator connected to infinite bus--Synchronizing and parallel operation – Synchronizing torque -Change of excitation and mechanical input- Voltage regulation – EMF, MMF, ZPF and A.S.A methods – steady state power angle characteristics– Two reaction theory –slip test -short circuit transients - Capability Curves
PART A — (10 2 = 20 marks)
1. Why almost all large size Synchronous machines are constructed with rotating field
system type? (Nov/Dec 2008) & (May/June 2012)
The following are the principal advantages of the rotating field system type
construction of Synchronous machines:
The relatively small amount of power, about 2%, required for field system via slip-
rings and brushes.
For the same air gap dimensions, which is normally decided by the kVA rating, more
space is available in the stator part of the machine for providing more
Insulation to the system of conductors, especially for machines rated for 11kV or above.
Insulation to stationary system of conductors is not subjected to mechanical stresses due
to centrifugal action.
Stationary system of conductors can easily be braced to prevent deformation.
It is easy to provide cooling arrangement for a stationary system of conductors.
Firm stationary connection between external circuit and system of conductors
Enable he machine to handle large amount of volt-ampere as high as 500MVA.
2. Write down the equation for frequency of emf induced in an Alternator.
Frequency of emf induced in an Alternator,f ,expressed in cycles per second or Hz,
is given by the following equation F = (PN)/120 Hz, Where P- Number of poles N-Speed in
rpm.
3. How are alternators classified? (Apr/May 2005)
According to type of field system
(i) Stationary field system type
(ii) Rotating field system type
According to shape of field system
(i) Salient pole type
(ii) Smooth cylindrical type
4. Name the types of Alternator based on their rotor construction.(Apr/May 2013)
Alternators can be classified into the following two types according to its rotor
Construction.
a. Smooth cylindrical type alternator
b. Salient pole alternator
5. Why do cylindrical Alternators operate with steam turbines?(Apr/May 2005)
Steam turbines are found to operate at fairly good efficiency only at high speeds.
The high speed operation of rotors tends to increase mechanical losses and so the rotors
should have a smooth external surface. Hence, smooth cylindrical type rotors with less
diameter and large axial length are used for Synchronous generators driven by steam
turbines with either 2 or 4 poles.
6. Which type of Synchronous generators are used in Hydro-electric plants and why?
As the speed of operation is low for hydro turbines use din Hydro-electric plants,
salient pole type Synchronous generators are used. These allow better ventilation and also
have other advantages over smooth cylindrical type rotor.
7. What are the advantages of salient pole type construction used for Synchronous
machines?
Advantages of salient-pole type construction are :
(i) They allow better ventilation.
(ii) The pole faces are so shaped that the radial air gap length increases
from the pole center to the pole tips so that the flux distribution in the air-
gap is sinusoidal in shape which will help the machine to generate
sinusoidal emf.
(iii) Due to the variable reluctance the machine develops additional reluctance
power which is independent of excitation.
8. Why is the stator core of Alternator laminated?(Apr/May 2011)
The stator core of Alternator is laminated to reduce eddy current loss.
9. How does electrical degree differ from mechanical degree?
Mechanical degree is the unit for accounting the angle between two points based on
their mechanical or physical placement. Electrical degree is used to account the angle
between two points in rotating electrical machines. Since all electrical machines operate
with the help of magnetic fields, the electrical degree is accounted with reference to the
magnetic field. 180 electrical degree is accounted as the angle between adjacent North and
South poles.
10. What is distributed winding? (Apr/May 2000)
When coil-sides belonging to each phase are housed or distributed in more than one
slot under each pole region then the winding is called distributed winding. A full pitch coil
has width of coil otherwise called coil-span as 180º - angle between adjacent slots in
electrical degree and x= 1,2,3…
11. Why is short pitch winding preferred over full-pitch winding?(April/May 2010)
(i) Waveform of the emf can be approximately made to a sine wave
and distorting harmonics can be reduced or totally eliminated.
(ii) Conductor material , copper , is saved in the back and front end
connections due to less coil-span.
(iii)Fractional slot winding with fractional number of slots/phase can be
used which in turn reduces the tooth ripples.
(iv)Mechanical strength of the coil is increased.
12. Define winding factor. (Apr/May 2001)
The winding factor Kd is defined as the ratio of phasor addition of emf induced in
all the coils belonging to each phase winding to their arithmetic addition.
13. Why are Alternators rated in kVA and not in kW? (Nov/Dec 2008)
The continuous power rating of any machine is generally defined as the power the
machine or apparatus can deliver for a continuous period so that the losses incurred in the
machine gives rise to a steady temperature rise not exceeding the limit prescribed by the
insulation class. Apart from the constant loss incurred in Alternators is the copper loss,
occurring in the 3 –phase winding which depends on I2 R, the square of the current
delivered by the generator. As the current is directly related to apparent – power delivered by
the generator , the Alternators have only their apparent power in VA/kVA/MVA as their
power rating.
14. What are the causes of changes in voltage in Alternators when loaded? (Nov/Dec
2012)
Variations in terminal voltage in Alternators on load condition are due to the
following three causes:
a. Voltage variation due to the resistance of the winding, R
b. Voltage variation due to the leakage reactance of the winding, Xt
15. What is meant by armature reaction in Alternators? (Nov/Dec 2012) (Nov/Dec
2013)
The interaction between flux set up by the current carrying armature and the main
is defined as the armature reaction.
16. What do you mean by synchronous reactance?
Synchronous reactance X s= (Xl + Xa) The value of leakage reactance Xl is
constant for a machine based on its construction. Xa depends on saturating condition of the
machine. It is the addition of Xa , which represent the armature reaction effect between two
synchronously acting magnetic fields that makes the total reactance Xa to be called
synchronous reactance.
17. What is meant by synchronous impedance of an Alternator?
The complex addition of resistance, R and synchronous reactance , jXs can be
represented together by a single complex impedance Zs called synchronous impedance.
18. What is meant by load angle of an Alternator?
The phase angle introduced between the induced emf phasor, E and terminal voltage
phasor , U during the load condition of an Alternator is called load angle.
19. Name the various methods for predetermining the voltage regulation of 3-phase
Alternator.
The following are the four methods which are used to predetermine the voltage
regulation of smooth cylindrical type Alternators
(i) Synchronous impedance / EMF method
(ii) Ampere-turn / MMF method
(iii) Potier / ZPF method
(iv)American Standards Association method
20. How synchronous impedance is calculated from OCC and SCC?
Synchronous impedance is calculated from OCC and SCC as |Zs| = E0/Isc(for same If)
A compromised value of Zs is normally estimated by taking the ratio of (E0/Isc) at normal
field current Ifn. A normal field current Ifn is one which gives rated voltage Vr on open
circuit. |Zs| = Vr/Iscn
21. What are the advantages and disadvantages of estimating the voltage regulation of
an Alternator by EMF method?
Advantages:
(i) Simple no load tests (for obtaining OCC and SCC) are to be conducted.
(ii) Calculation procedure is much simpler.
Disadvantages:
(i) The value of voltage regulation obtained by this method is always higher than the
actual value.
22. Why is the synchronous impedance method of estimating voltage regulation
considered as pessimistic method?
Compared to other methods, the value of voltage regulation obtained by the synchronous
impedance method is always higher than the actual value and therefore this method is
called the pessimistic method.
23. In what way does the ampere-turn method differ from synchronous impedance
method?
The ampere-turn /MMF method is the converse of the EMF method in the sense that
instead of having the phasor addition of various voltage drops/EMFs, here the phasor
addition of MMF required for the voltage drops are carried out. Further the effect of
saturation is also taken care of.
24. What are the test data required for predetermining the voltage regulation of
an Alternator by MMF method?
Data required for MMF method are :
(i) Effective resistance per phase of the 3-phase winding R
(ii) Open circuit characteristic (OCC) at rated speed/frequency
(iii)Short circuit characteristic (SCC) at rated speed/frequency
25. Why is the MMF method of estimating the voltage regulation considered as the
optimistic method?
Compared to the 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 care
of saturation effect. As this method require more effort, the final result is very close to the
actual value. Hence this method is called optimistic method.
26. State the condition to be satisfied before connecting two alternators in
parallel.(Nov/Dec 2009) (Apr/May 2013)
The following are the three conditions to be satisfied by synchronizing the additional
Alternator with the existing one or the common bus-bars:
(i) The terminal voltage magnitude of the incoming Alternator must be made equal
to the existing Alternator or the bus-bar voltage magnitude.
(ii) The phase sequence of the incoming Alternator voltage must be similar to the bus-
bar voltage.
(iii)The frequency of the incoming Alternator voltage must be the same as the bus-bar
voltage.
27. How do the synchronizing lamps indicate the correctness of phase sequence
between existing and incoming Alternators?
The correctness of the phase sequence can be checked by looking at the three sets of
lamps connected across the 3-pole of the synchronizing switch. If the lamps grow bright
and dark in unison it is an indication of the correctness of the phase sequence. If on the
other hand, they become bright and dark one after the other, connections to any two
machine terminals have to be interchanged after shutting down the machine.
28. What are the advantages and disadvantages of three dark lamps method of
synchronizing?
Advantages:
(i) The synchronous switch using lamps is inexpensive
(ii) Checking for correctness of the phase sequence can be obtained in a simple
manner which is essential especially when the Alternator is connected for the first
time or for fresh operation after disconnection .
Disadvantages:
(i) The rate of flickering of the lamps only indicates the frequency difference
between the bus-bar and the incoming Alternator. The frequency of the incoming
Alternator in relation to the bus-bar frequency is not available.
29. How synchronoscope is used for synchronizing Alternators?
Synchronoscope can be used for permanently connected Alternators where the
correctness of phase sequence is already checked by other means. Synchronoscope is
capable of rotating in both directions. The rate of rotation of the pointer indicates the
amount of frequency difference between the Alternators. The direction of rotation indicates
whether incoming Alternator frequency is higher or lower than the existing Alternator. The
TPST switch is closed to synchronize the incoming Alternator when the pointer faces the top
thick line marking.
30. Why synchronous generators are to be constructed with more synchronous
reactance and negligible resistance?
The presence of more resistance in the Synchronous generators will resist or oppose their
synchronous operation. More reactance in the generators can cause good reaction between
the two and help the generators to remain in synchronism in spite of any disturbance
occurring in any one of the generators.
31. List the factors that affect the load sharing in parallel operating generators?
The total active and reactive power delivered to the load, connected across the common
bus-bars, are shared among Synchronous generators, operating in parallel, based on the
following three factors.
(i) Prime-mover characteristic/input
(ii) Excitation level and
(iii)Percentage synchronous impedance and its R/X ratio
32. How does the change in prime mover input affect the load sharing?
An increase in prime-mover input to a particular generator causes the active power shared
by it to increase and a corresponding decrease in active-power shared by other generators.
The change in reactive power sharing is less appreciable. The frequency of the bus-bar
voltage will also subjected to slight increase in value.
33. How does change in excitation affects the load sharing?
The decrease in excitation in one generator causes the reactive power shared by it to
decrease and a corresponding increase in reactive-power shared by other generators. The
change in active-power sharing is less appreciable. There will be a slight decrease in
terminal voltage magnitude also.
34. What steps are to be taken before disconnecting one Alternator from
parallel operation?
The following steps are to be taken before disconnecting one Alternator from parallel
Operation:
(i) The prime-mover input of the outgoing generator has to be decreased and that of
other generators has to be increased and by this the entire active-power delivered by
the outgoing generator is transferred to other generators.
(ii) The excitation of the outgo generator has to be decreased and that of othering
generators have to be increased and by this the entire reactive-power delivered by
the outgoing generator is transferred to other generators.
(iii)After ensuring the current delivered by the outgoing generat r is zero, it has to beo
disconnected from parallel operation.
35. What is meant by infinite bus-bars?
The source or supply lines with non-variable voltage and frequency are called infinite
bus-bars. The source lines are said to have zero source impedance and infinite rotational
inertia.
36. How does increase in excitation of the Alternator connected to infinite bus-
bars affect this operation? (Apr/May 2014)
Increase in excitation level of the synchronous generator will effectively increase the
reactive component of the current supplied by the generator and hence the active power
delivered.
37. Upon what factors does the load angle depend?
Angle is positive during generator operation and negative during motor operation.
38. An Alternator is found to have its terminal voltage on load condition more
than that on no load. What is the nature of the load connected?
The nature of the load is of leading power factor , load consisting of resistance and
capacitive reactance.
39. What are the principal advantages of rotating field type construction?
Relatively small amount of power required for field system can easily supplied to
rotating system 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 easily be braced to prevent deformation.
40. 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.
41. Which type of synchronous generators are used in hydroelectric plants and why?
As the speed of operation is low, for hydro turbines used in hydroelectric plants,
salient pole type sync. generator is used because it allows better ventilation also
better than smooth cylindrical type rotor.
42. 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.
43. Define voltage regulation of the alternator? (April/May 2011) (Nov/Dec 2013)
It is defined as the increase in terminal voltage when full load is thrown off, assuming
field current and speed remaining the same.
% Reg = [(E0 – V)/V]x100 Where E0 = no terminal voltage V = full load rated terminal
voltage
44. How is armature winding in alternators is different from those used in dc machines?
The arm. winding of the alternator is placed in the stator, but in the case of dc machines
the arm winding is placed in the rotor.
45. Distinguish the use of salient pole and round rotor synchronous machines.
(April/May 2015)
Salient Pole synchronous machine is used in Hydro Electric Power Station. Round rotor
synchronous machine is used in Thermal Power station.
46. What are the essential elements for generating emf in alternators? (Nov/Dec 2014)
The three essential elements for generating emf in alternators are:
(i) The conductor or coil
(ii) The magnetic field (or) flux
(iii) Relative motion between conductor (or) coil and magnetic field or flux.
47. How will you distinguish between the two types of large synchronous generator
from their appearance? (May/June 2014)
The two types of large synchronous generator are:
(i) Salient Pole Synchronous generator &
(ii) Smooth Cylindrical Rotor Synchronous generator.
The difference between these two from their appearance are: Salient Pole Synchronous
generator has large diameter and small axial length. But Smooth Cylindrical Synchronous
generator has small diameter and large axial length.
PART B — (5 16 = 80 marks)
1. Find the no load phase and line voltages of a star connected 3 phase, 6 pole alternator
which runs at 1200 rpm, having flux per pole of 0.1wb sinusoidally distributed. It’s
stator has 54 slots having double layer winding. Each coil has 8 turns and the coil is
chorded by 1 slot. (Nov/Dec 2005)
2. The open circuit and short circuit readings for a 3 phase, star connected 1000kVA,
2000V, 50Hz, synchronous generator are :
Field current, A 10 20 25 30 40 50O.C. terminal voltage, V 800 1500 1760 2000 2350 2600S.C. armature current, A - 200 250 300 - -
The armature effective temperature is 0.2Ω per phase. Draw the characteristics curves
and estimate the full-load percentage regulation at 0.8pf lagging and 0.8pf leading.
3. A 3300V, 3phase star connected alternator has a full load current of 100A. On short
circuit a field current of 5A was necessary to produce full load current. The emf on
open circuit for the same excitation was 900V. The armature resistance was 0.8
Ω/phase. Determine the full load voltage regulation for (1)0.8pf lagging (2)0.8pf leading.
(Apr/May 2005)
4. A 3 phase, 50Hz star connected 2000kA for a certain field excitation. With the same
excitation, the open circuit voltage was 900V. The resistance between a pair of
terminals was 0.12 Ω. Find the full load regulation at UPF and 0.8pf lagging. Draw
the phasor diagrams. (Nov/Dec 2009)
5. A 3 phase 16 pole alternator has a star connected winding with 144 slots and 10
conductors per slot. The flux per pole 0.04wb and is sinusoidally distributed. The speed
is 375 rpm. Find the frequency, phase e.m.f. and line e.m.f. The coil span is 160ο
electrical.(Nov/Dec 2008)
6. (a) Describe the principle and construction of slow speed operation generator with
neat diagram. (Apr/May 2005)
(b) Derive the e.m.f. equation of alternator. (Nov/Dec 2012)
7. What are the methods of determining regulation of alternator? Discuss each briefly
(Apr/May 2005) & (Nov/Dec 2012)
8. Explain the procedure for POTIER method to calculate voltage regulation of alternator.
(Apr/May 2005)
9. For a salient pole synchronous machine, prove the d-axis synchronous reactance
Xd, can be obtained from its OCC and SCC. Neglect armature resistance.
10. Explain the condition for parallel operation of 3 phase alternator with neat diagram.
(Apr/May 2008) & (Nov/Dec 2012)
11. Explain the phenomenon of armature reaction in alternator for different load power
factors. (May/June 2012)
12. Explain clearly the ZPF method of determining the regulation of an alternator?
(May/June 2012)
13. With neat sketch describe the construction and principle of operation of salient pole
alternator. (Nov/Dec 2012)
14. Describe the POTIER method of determining the regulation of an alternator. (May/June
2013)
15. A 3.3 kV alternator gave the following results:
Field current (A): 16 25 37.5 50 70
OC Voltage (kV): 1.55 2.45 3.3 3.75 4.15
A field current of 18A is found to cause the full load current to flow through the
winding during short circuit test. Predetermine the full load voltage regulation at (1) 0.8
pf lag and (2) 0.8 pf lead by MMF method. (May/June 2013)
16. Describe the slip test for finding Xd and Xq. (May/June 2013)
17. Two similar, 3 phase alternators work in parallel and deliver a total real power of 1800
kW at 11 kV and at 0.85 pf lagging to the load. Each alternator initially supplied half
the load power. The excitation of the first alternator is then increased such that its line
current becomes 60A lagging. Find the line current delivered by the second alternator.
(May/June 2013)
18. Derive an expression for the emf induced in an alternator. (Nov/Dec 2013)
19. A 3-phase 16 pole alternator has star connected winding with 144 slots and 10
conductors per slot. The flux per pole is 0.04 Wb and is distributed sinusoidally. The
speed is 375 rpm. Find the frequency, phase emf, and line emf. The coil span is 120º
electrical. (Nov/Dec 2013)
20. List the methods used to pre determine the voltage regulation of sysnchronous machine
and explain the MMF method. (Nov/Dec 2013)
21. A 3-phase, star-connected, 1000 kVA, 11,000V alternator has rated current if 52.5A.
The ac resistance of the winding per phase is 0.45Ω. The test results are given below:
OC Test: field current = 12.5A, voltage between lines = 422V.
SC Test: field current = 12.5A, line current = 52.5A
Determine the full load voltage regulation of the alternator (i) 0.8 pf lagging and (ii)
0.8 pf leading. (May/June 2014)
Describe a method of determining direct and quadrature axis reactance of salient pole
alternator. (May/June 2014)
22. Develop the formula for the induced emf in an alternator. (Nov/Dec 2014)
23. Describe the method of determining the voltage regulation of an alternator by
synchronous impedance method. (Nov/Dec 2014)
24. Describe the salient constructional features of ac generators driven by (1) diesel
engines (2) steam engines. (Nov/Dec 2014)
25. A 3-phase star-connected salient pole synchronous generator is driven at a speed near
synchronous with the field circuit open, and the stator is supplied from a balanced 3-
phase supply. Voltmeter connected across the line gave minimum and maximum
readings of 2800 volts and 2820 volts. The line current fluctuated between 360A and
275A. Find the direct and quadrature – axis reactance per phase. Neglect armature
resistance. (Nov/Dec 2014)
26. Explain the EMF and MMF method of evaluating the synchronous reactance.
(April/May 2015)
27. Derive an expression for real and reactive power outputs of asynchronous generator.
(April/May 2015)
28. Illustrate a method for determining the direct and quadrature axis reactances of a
salient pole synchronous generator. (April/May 2015)
UNIT II SYNCHRONOUS MOTOR 9Principle of operation – Torque equation – Operation on infinite bus bars - V and Inverted V curves –Power input and power developed equations – Starting methods – Current loci for constant power input, constant excitation and constant power developed-Hunting – natural frequency of oscillations – damper windings- synchronous condenser
PART A1. State the characteristic features of synchronous motor.(MAY 2014)
a. the motor is not inherently self starting
b. The speed of operation is always in synchronous with the supply
frequency irrespective of load conditions
c. The motor is capable of operating at any power factor.
2. In what way synchronous motor is different from other motors?
All dc and ac motors work on the same principle. Synchronous motor operates due
to magnetic locking taking place between stator and rotor magnetic fields.
3. Name any two methods of starting a synchronous motors (April/May 2010),(MAY
2013),(MAY 2014)
(i) By an extra 3 phase cage induction motor
(ii) By providing damper winding in pole phases
(iii) By operating the pilot exciter as a dc motor
4. What is the effect on speed if the load is increased on a 3 phase synchronous motor?
The speed of operation remains constant from no load to maximum load in the
motor operating at constant frequency bus bars.
5. Why a synchronous motor is a constant speed motor? (Nov/Dec 2012)
Synchronous motor work on the principle of force developed due to the magnetic
attraction established between the rotating magnetic field and the main pole feed. Since the
speed of rotating magnetic field is directly proportional to frequency the motor operates at
constant speed.
6. What is the phasor relation between induced emf and terminal voltage of a 3 phase
Synchronous motor?
The rotating magnetic field is initially established by the prime source of supply V.
The main field then causes an emf e to get induced in the 3 phase winding. Hence when the
machine operates as a synchronous motor the emf phasor always lags the terminal voltage
phasor by the load/torque.
7. What are V and inverted V curves of synchronous motor? (Nov/Dec 2009),(MAY
2013)
The variation of magnitude of line current with respect to the field current is called
V curve . The variation of power factor with respect to the field current is called inverted
Vcurve.
8. What happens when the field current of a synchronous motor is increased
beyond the normal value at constant input?
Increase in emf causes the motor to have reactive current in the leading direction.
The additional leading reactive current causes the magnitude of line current, accompanied
by the decrease in power factor.
9. Distinguish between synchronous phase modifier and synchronous condenser
A synchronous motor used to change the power factor or power factor in the supply
lines is called synchronous phase modifier. A synchronous motor operated at no load with
over excitation condition to draw large leading reactive current and power is called a
synchronous condenser.
10. How the synchronous motor can be used as s synchronous condenser? (Nov/Dec
2012),(NOV 2014)
Synchronous motor is operated on over excitation so as to draw leading reactive
current and power from the supply lines. This compensates the lagging current and power
requirement of the load making the system power factor to become unity. The motor does
the job of capacitors and hence called as synchronous condenser.
11. What is hunting how can it be prevented? (Nov/Dec 2008) , (May /June 2012) ,
(Nov/Dec 2012),(NOV 2013)
When a sync motor is used for driving a fluctuating load, the rotor starts oscillating about
its new position of equilibrium corresponding to the new load. This is called hunting or
phase swinging. To prevent hunting dampers are damping grids are employed.
12. What are different torques of a synchronous motor?
1. Starting torque
2. Running torque
3. Pull-in torque
4. Pull-out torque
13.What could be the reasons if a 3-phase synchronous motor fails to start?(NOV 2014)
(i) Voltage may be too low.
(ii) Too much starting load.
(iii) Open circuit in one phase or short circuit.
(iv) Field excitation may be excessive.
PART – B
1. (a) Draw the equivalent circuit and phasor diagram of a synchronous motor.
(b) Explain the significance of V and inverted V curves.(Nov2012 ).,(NOV2013),(NOV2014)
2. Discuss
a. Constant excitation circle and
b. Constant power circle.(Nov/Dec2012),(NOV2104)
3. Derive the mechanical power developed per phase of a synchronous motor.(OCT-2007)
4. A 3300V, 3 phase synchronous motor running at 1500 rpm has its excitation kept
constant corresponding to no-load terminal voltage of 3000V. Determine the power
input, power factor and torque developed for an armature current of 250A if the
synchronous reactance is 5 Ω per phase and armature resistance is neglected.
5.A synchronous motor having 40% reactance and negligible resistance is to be operated at
rated voltage at UPF, 0.8pf lag, 0.6pf lag, 0.8pf lead and 0.6pf lead. What are the values of
induced e.m.f. (OCT-2008)
6..A 75 kW,400 V,4 pole , 3-Ф, star connected synchronous motor has a resistance and
synchronous reactance per phase of 0.04 Ω and 0.4 Ω respectively. Compute for full
load 0.8pf lead the open circuit e.m.f. per phase and gross mechanical power developed.
Assume an efficiency of 92.5%. (OCT-2008),(MAY2014)
7. A 6600 V, 3-Ф, star connected synchronous motor draws a full load current of 80 A at
0.8pf leading. The armature resistance is 2.2 Ω and reactance of 22 Ω per phase. If the stray
losses of the machine are 3200w. Find (i) e.m.f. induced (ii) Output power (iii) Efficiency of
the machine. (OCT-2009)
8.A 2000 V, 3 phase, 4 pole, star connected synchronous motor runs at 1500 rpm.
The excitation is constant and corresponding to an open circuit voltage of 2000 V. The
resistance is negligible in comparison with synchronous reactance of 3.5 Ω/ph. For an
armature current of 200 A. Determine (i) power factor (ii) power input (iii) torque developed.
9. Why is a synchronous motor not self staring? Explain. (May/June 2012)
10. Explain the effect of variable excitation on the behaviour of the synchronous motor under
constant load conditions. (May/June 2012).
11. Derive an expression for the maximum torque developed per phase of a synchronous
motor.(May/June 2012)
12. Draw and explain the current loci of synchronous motor for (i) constant power input and
(ii) constant excitation. Also obtain the minimum and maximum excitation for given
mechanical power. (May/June 2013),(MAY 2014),(MAY 2015)
13. Derive the expression for power developed in a synchronous motor. Also find the
condition for maximum power developed. (May/June 2013)
14. Draw and explain the equivalent circuit and phasor diagram of a cylindrical rotor
synchronous motor operating at different power factors.(NOV 2013)
15. What are the different methods of starting synchronous motor?(NOV 2014)
16. Illustrate the phenomenon of hunting and explain the use of damper winding with the help
of dynamic equations.(MAY 2015)
UNIT III THREE PHASE INDUCTION MOTOR 9Constructional details – Types of rotors –- Principle of operation – Slip –cogging and crawling-Equivalent circuit – Torque-Slip characteristics - Condition for maximum torque – Losses and efficiency – Load test - No load and blocked rotor tests - Circle diagram – Separation of losses –Double cage induction motors – Induction generators – Synchronous induction motor.
PART A — (10 2 = 20 marks)1. How does the shaft torque differ from the torque developed in 3-phase Induction
motor?
The mechanical power developed Pd causes the rotor to rotate at a speed Nr due to
the torque Td developed in the rotor . Therefore, equation for Pr can be written as Pd
1rTd / 60 The remaining power, after the mechanical losses Wm are met with, available
in the shaft as mechanical power output Po = Pd – Wm .The mechanical power output Po,
which is less than Pd is available in the shaft running at a speed of Nr and with a shaft
torque T. Therefore the shaft torque (T) is slightly less than the torque developed Td, Pd_
1rT / 60 Wm = Pd – Po_ _>_ 1r(Td – T)] / 60
2. Name the tests to be conducted for predetermining the performance of 3-phase
induction machine.
No load test
Blocked rotor test
3. . What are the informations obtained from no-load test in a 3-phase I M?
No –load input current per phase,Io
(ii) No load powerfactor and hence no load phase angle
(iii) Iron and mechanical losses together
(iv) elements of equivalent circuit shunt branch
4. What are the informations obtained from blocked rotor test in a 3-phase I M?(Oct
2005)
(i)Blocked rotor input current per phase at normal voltage
(ii) Blocked rotor power factor and hence phase angle
(iii) Total resistance and leakage reactance per phase of the motor
as refered to the stator
5. What is circle diagram of an I M?(Apr 2001)
When an I M operates on constant voltage and constant frequency source,the loci of
stator current phasor is found to fall on a circle.This circle diagram is used to predict the
performance of the machine at different loading conditions as well as mode of operation.
6. . What are the advantages and disadvantages of circle diagram method of
predetermining the performance of 3 –phase I M?
The prediction can be carried out when any of the following information is available
The input line current., the input power factor, The active power input, The reactive power
input, The apparent power input, The output power , The slip of operation, The torque
developed, The equivalent rotor current per phase, Maximum output power, Maximum
torque developed. The only disadvantage is, being a geometrical solution, errors made
during measurements will affect the accuracy of the result.
7. What are the advantages and disadvantages of direct load test for 3 –phase I M?
Advantages Direct measurement of input and output parameters yield accurate Results
Aside from the usual performance other performances like mechanical Vibration, noise
etc can be studied. By operating the motor at full load for a continuous period, the final
steady
temperature can be measured. Disadvantages Testing involves large amount of power and the
input energy and the entire energy delivered is wasted Loading arrangement cannot be
provided for motors of large power rating
8. State the principle of 3 phase IM?(Apr 2008)
While starting, rotor conductors are stationary and they cut the revolving magnetic field
and so an emf is induced in them by electromagnetic induction. This induced emf produces a
current if the circuit is closed. This current opposes the cause by Lenz’s law and hence the
rotor starts revolving in the same direction as that of the magnetic field.
9. Induction motor can run at synchronous speed ? True or false? Explain .
No, if the speed of induction motor is Ns then the relative speed between the rotating flux
and the rotor will be zero and so no torque is produced.
10. An induction motor is generally analogous to ?
It is analogous to a winding rotating transformer with its secondary circuit closed’ .
11. Can the starting torque of of a slip ring induction motor being increased?(Apr
2007),(MAY 2015)
Yes. It can be increased by adding resistances to the rotor
12. What would happen if a 3 phase induction motor is switched on with one phase
disconnected?
The motor continues to run with reduced voltage.
13. What happens if the air gap flux density in an induction motor increases?
The increase in air gap flux increases iron loss and hence efficiency decreases
14. State the advantages of skewing?(April 2008),(NOV 2013)
It reduces humming and hence quite running of motor is achieved. It reduces magnetic
locking of the stator and rotor.
15. State the condition at which the torque developed in a slip-ring induction motor is
maximum under running condition.(May 2009),(MAY 2013)
R2=SX2
16. What are the effects of increasing rotor resistance on starting current and
starting torque?
The additional external resistance reduces the rotor current and hence the current drawn
from the supply. It improves the starting torque developed by improving the power factor in
high proportion to the decrease in rotor current.
17. What is slip of an induction motor?(Nov/Dec 2012) &(Nov/Dec 2011),(MAY
2013),(NOV 2013),(NOV 2014)
The slip speed expressed as the ratio of synchronous speed is defined as slip.
Percentage slip S=Ns-N/Ns*100
18. How the magnitude of rotor emf is related to the slip in an I M? (Apr 2001)
Rotor circuit emf per phase E2r=SE2
19. How the frequency of rotor emf is related to the slip in an I M?
Frequency of rotor emf/current fr=Sfs
20. What is the normal value of slip of an I M operating at full load?(Apr 2010)
3 - 5%
21. Why is not possible for the rotor speed of an I M to be equal to the speed of its
rotating magnetic field?
The machine will not be able to develop any mechanical torque to run as a motor.
22. How the direction of rotation of a 3-phase induction motor can be
reversed? (May/June 2012)
If the phase sequence of the motor is changed, direction of rotation will be
changed.
PART B — (5 16 = 80 marks)
1. Explain the construction and working of both the types of three phase induction motor.
(M a y/ Ju n e 2 0 12 ) & ( N o v / D e c 2 0 12 ),(NOV2013),(NOV2014)
2. Explain the power flow diagram and torque slip characteristics of induction motor.
(APR-2005)
3. Derive the torque equation of a three phase induction motor. (APR-2006)
4. Develop an equivalent circuit for three phase induction motor. State the difference
between exact and approximate equivalent circuit.( M a y/ J u ne 2012 ) & ( N o v /
D e c 2012 ),(MAY2013)
5. The power input to the rotor of a 3-Ф, 50 HZ, 6 pole induction motor is 80KW .The
rotor e.m.f. makes 100 complete alternations per minute. Find :
1) Slip
2) Motor Speed
3) Mechanical power developed
4) Rotor copper loss per phase
5) Rotor resistance per phase if rotor current is 65 A
6) Torque developed
6. Derive the equation for torque developed by an induction motor .Draw a typical
torque – slip curve and deduce the condition for maximum torque.(APR-2007),(MAY
2015)
7. Draw the torque slip characteristics of three phase induction motors in detail(MAY
2013),(MAY 2014)
8. Explain the tests required to be performed to obtain the data for the circle diagram.
( May/June 2012)
9. Explain with necessary diagrams the principle and operation of double cage induction
motor.(MAY 2013)
10. Explain the principle of operation of synchronous induction motor.(NOV 2014)
11. An induction motor has an efficiency of 0.9 when the shaft load is 45 KW.At this
load,stator ohmic losses and rotor ohmic losses each equal to iron losses.The
mechanical losses is one-third of no load losses.Neglect the ohmic losses at no-
load.calculate the slip.(NOV 2014)
12. A 3300 V,10-pole ,50 Hz three phase star connected induction motor has slip ring
rotor resistance per phase = 0.015 ohm and standstill reactance per phase =
0 .25ohm. If the motor runs at 2.5% slip on full load, find:
1) Speed of the motor
2) Speed at which the torque will be maximum
3) The ratio of maximum torque to full load torque.
13. A 3-Ф, 4-pole, 50 Hz induction motor is running at 1440 rpm. Determine the slip
speed and slip.
UNIT IV STARTING AND SPEED CONTROL OF THREE PHASE INDUCTIONMOTOR 9Need for starting – Types of starters – DOL, Rotor resistance, Autotransformer and Star-delta starters – Speed control – Voltage control, Frequency control and pole changing – Cascaded connection-V/f control – Slip power recovery scheme- Braking of three phase induction motor: Plugging, dynamic braking and regenerative braking.
PART A — (10 2 = 20 marks)
1. State two advantage of speed control of IM by injecting an emf in the rotor
circuit. (Apr 2000)
1. speed within the working range can be
obtained. 2.Improves the power factor of the
system.
2. Point out the disadvantages of rotor rheostat control to obtain variable speed of
induction motor. (APR-2006)
Reduced efficiency, speed changes vary widely with load variation, unbalance in
voltage and current.
3. Give the functions performed by induction motor starter.(May/June 2012)
To limit the starting current, to start the motor, to protect from over load condition and
low voltage condition.
4. List out the methods of speed control in 3 phase induction motor.(MAY 2013),
(NOV 2013)
Stator voltage control, Stator frequency control, V/F control , pole changing method,
rotor resistance control cascade control, Injection of emf in the rotor circuit, slip power
recovery scheme
5. List out the methods of speed control in 3 phase Squirrel cage induction motor.
(Nov/Dev 2012)
Stator voltage control, Stator frequency control, V/F control , pole changing method,
6. Why stator resistance rarely used?(Apr 2005)Due to addition of resistance in the stator side cause the voltage available to the motor
X times the normal voltage i.e. The starting current drawn by the motor as well as the
current drawn from the supply get reduced by X times where as the starting torque
developed gets reduced by X2 times.
7. What are the effects of increasing rotor resistance in the rotor circuit of a 3-phase induction motor as starting?(Nov/Dec 2011)
Due to addition of resistance in rotor circuit by the stator not only reduces the staring
current, in addition to that the starting torque developed than those given by DOL starting.
8. What are the advantages of Electronic starter?(Apr 2001)The moving parts and contacts get completely eliminated. The arcing problem gets eliminated.Minimum maintenance is required as there are no moving parts. The operation is reliableStarting time also gets reduced.
9. What are the various types of starters in a 3-phase induction motor? (Nov/Dec 2007)&(Nov/Dec 2009)Stator resistance starter
Autotransformer starter
Star-delta starter
Rotor resistance starter
Direct on line starter
10. What is autotransformer starter?(MAY 2015)A three phase star connected autotransformer can be used to reduce the voltage
applied to the stator. Such a starter is called as autotransformer starter.
11. What is star-delta starter?(Nov/Dec 2009)This is the cheapest starter of all and hence used very commonly for the induction motors.
It uses triple pole double throw (TPDT) switch. The switch connects the stator winding in
star at start. Hence per phase voltage get reduced by the factor 1/.3.Due to this reduced
voltage, the staring current is limited.
12. What is the function of starters? (May/June 2012)For large capacity induction motors is to reduce the starting current
Having necessary control devices to limit the starting current
All starters are provided devices to protect the motor against overload and loss of
supply voltage.
13. What is Soft Starter?(AU-MECH Apr 2011)Soft starting an ac motor refers to any one of several starting methods that limit the starting
current and torque of the motor. The reduced voltage starters, and will be referred to as soft
starting. The soft starter eliminates unnecessary jerks during the start. Gradually, the voltage
and the torque increase so that the machinery starts to accelerate the motor by means of
thyrister (SCRs).
14. 20. Draw the Schematic circuit diagram of Soft Starter
15. What are the advantages of soft starter?Reduced wear on mechanical gears, chains and sprockets, and unexpected repair of broken
belts and jammed gearboxes.
Lower inventory of spare mechanical parts and operating costs.
Increased production rates by reducing machine maintenance downtime.
Prolonged life of electrical switchgear with lower inrush currents.
Soft stops on pumping applications reduce piping system stresses and “hammer” effect.PART B — (5 16 = 80 marks)
1. With neat diagrams explains the working of any two types of starters used for squirrel
cage type 3-Ф induction motor.(APR-2000),(NOV 2013)
2. Discuss the various starting methods of induction motors.( May/June 2012)
3. Explain the different speed control methods of phase wound induction motor. (APR-
2000)
4. Explain the various schemes of starting squirrel cage induction motor. (APR-2008)
5. Explain the speed control of 3-Ф squirrel cage induction motor by pole changing.
6. Discuss the theory of star – delta starter. (APR-2007) &( May/June 2012) &(Nov/Dec
2012),(MAY 2013),(NOV 2014)
7. Explain briefly the various speed control schemes of induction motors.
8. Explain in detail the slip power recovery scheme.( Nov / Dec 2 0 12 ),(MAY2013) ,
(NOV 2013)
9. Explain the various techniques of speed control of induction motor from rotor side
control. (APR-2001),(MAY 2014),(MAY 2015)
10. Explain the cascade operation and variable frequency of induction motors to obtain
variable speed.( May /Jun e 2012),(MAY 2013)
3 IM3
Sup
ply
UNIT V SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES 9Constructional details of single phase induction motor – Double field revolving theory and operation –Equivalent circuit – No load and blocked rotor test – Performance analysis – Starting methods of single-phase induction motors – Capacitor-start capacitor run Induction motor- Shaded pole induction motor - Linear induction motor – Repulsion motor - Hysteresis motor - AC series motor- Servo motors- Stepper motors - introduction to magnetic levitation systems.
PART A — (10 2 = 20 marks)1. After servicing a single phase fan it was found to run in reverse direction.
What could be the reason?
The connection to the starting/ auxiliary winding would have reversed.
2. What will be the direction of rotation of a shaded pole single phase induction
motor? (Nov 2002)(MAY 2013)
The motor rotates in the direction specified by the unshaded to shaded region in
the pole phase
3. In what respect does a 1-phase Induction motor differ from a 3-phase Induction
motor?
Construction wise a plain 1-phase Induction motor is more or less similar to a 3-phase
squirrel-cage Induction motor except that its stator is provided with only 1-phase winding.
4. What are the inherent characteristics of plain 1-phase Induction motor(NOV 2013)
A plain 1-phase Induction motor is not used in practice due to the following inherent
characteristics
-phase Induction motor does not have any starting torque
pick up its speed
in a direction at which the initial torque is given and deliver the required output.
5. Name the two different theories with which principle of 1-phase induction
motors are explained.
The two different theories are
6. State double revolving field theory.(May/June 2009),(NOV 2013)
Double revolving theory, formulated by Ferrari, states that a single pulsating
synchronous speed proportional to the frequency of the pulsating field.
7. Name any four types of 1-phase induction motors(Nov/Dec 2012)
Based on the method of starting arrangement provided, the 1-phase Induction motors
are classified as follows
(i) Split-phase motor
(ii) Capacitor start motor
(iii) Capacitor start and run motor
(iv) Shaded pole motor
(v) Repulsion start Induction run motor
8. How is the direction of a capacitor start Induction motor be reversed?(NOV 2014)
The direction of rotation can be reversed by interchanging the terminals of either the
main winding or the starting winding
9. . Define step angle?(Apr-2005)
It is defined as angle through which the stepper motor shaft rotates for each command
pulse. It is denoted as β, i)β=[(Ns-Nr)/ Ns.Nr]x360o
Where Ns = no. of stator poles or stator teeth
Nr = no. of rotor poles or rotor teeth
ii) β = 3600/mNr
Where m= no. of stator poles
10. What are different types of stepper motor?(Apr-2006)
1.Variable reluctance (VR) motor
2. Permanent magnet (PM) stepper motor
3. Hybrid stepper motor
11. What is the advantage in using stepper motor?(April 2008)
1. it can drive open loop without feedback
2. it requires little or no maintenance.
12. Give the applications of stepper motor?(Nov/Dec 2007)
1. Robotics
2. Computer peripherals
3. Facsimile machine
4. Aerospace
13. What is the adv. of reluctance m/c?(APR 2001)
1. Motor speed is constant
2. Simple construction14. Why a single phase induction motor does not Self start? (April/May
2010) &(April/May 2011)
When a single phase supply is fed to the single phase induction motor, Its stator winding
produces a flux which only alternates along one space axis. It is not a synchronously revolving
field, as in the case of a 2 or 3phase stator winding, fed from 2 or 3 phase supply.
15. What type of single phase induction motor would you use for the following
applications?
(i) Ceiling fan (ii) Wet grinder (Nov/Dec 2011),(MAY2014)
Ceiling fan – capacitor start and run motor, Wet grinder – capacitor start motor
PART B — (5 16 = 80 marks)
1. Give the classification of 1-Ф motors. Explain the types of 1-Ф induction motors.
(APR-2000)
2. Explain the double field revolving theory for operation of 1-Ф induction motor.
(APR-2004) & ( May/June 2012) &( Nov/Dec 2012),(MAY 2013),(MAY 2015)
3. What are the types of 1-Ф induction motor? Explain any two in detail. (APR-2006)
4. Explain the shaded pole induction motor with diagram.(May/ June 2012) &( Nov/ Dec 2 012)
5. Develop equivalent circuit of a 1-Ф induction motor ignoring core losses. (APR-2009)
6. Explain the working principle of single phase induction motor .Mention its four
applications.(APR-2005)
7. What is the principle and working of hysteresis motor? Explain briefly.
8. Explain the construction and working of stepper motor.(APR-2000),(MAY 2015)
9. Explain the principle of operation and applications of reluctance motor. (NOV 2013)
10. Explain the principle of operation and applications of reluctance motor and hysteresis
motor. (APR-2002) &(May/June 2012) &( Nov / Dec 2012) ,(MAY 2014),(NOV
2014)
11. Explain the principle of operation of capacitor start single phase induction motor and
AC series motor.(MAY 2013),(NOV 2013),(MAY 2014)
12. Explain the theory of brushless dc machine.(MAY 2015)
