Q1. What do you mean by harmonics?
Definition for a harmonic is-a sinusoidal component of a periodic wave or quantity having a
frequency that is an integral multiple of the fundamental frequency.
The main sources of harmonic component are the phase angle controlled rectifiers and inverters
due to the fast switching action and firing scheme.
Filters are used to eliminate these harmonics as they may cause overheating of capacitors and
nearby generators and interfere with telecommunication systems.
Q2.Hunting of a synchronous motor:
When the mechanical load is constant, the rotor settles down to an absolutely constant speed with
the torque angle fixed by the particular delivered power.
When there is a change in load the rotor speed changes momentarily until the torque angle
adjusts itself to the new power.
If the load increases, the rotor slips backwards to an increased torque angle, while reduction
causes the rotor to advance a smaller torque angle.
Due to the M.I of the rotor either it overshoots or undershoots the mean position.
During this overshooting and undershooting some K.E. is stored in the rotor which causes
oscillation of the rotor periodically.
This rapid forward and backward motion of the rotor as it resolves at the average constant speed
is called HUNTING.
DISADVANTAGES:
Hunting is an objectionable characteristic of a synch motor as it produces severe mechanical
stress as well as great variation in current and power taken by the motor.
In order to prevent hunting, dampers (or) damping grids are employed. Damper windings are
short circuited; copper bars are embedded in the faces of the field poles of the motor.
Question: what is "p.u." in electrical engg and what is the advantage of p.u. quantities?
Answer: p.u. stands for per unit (p.u=actual value/base value).
Advantages:
•
•
wide variation of electrical parameters gets eliminated by p.u. representation
Since the p.u. impedances of any component remains the same on either side of a
transformer, a power system can be analyzed on a single voltage level as the different
voltage level vanishes.
Question: Give two basic speed control scheme of DC shunt motor?
Answer: 1. By using flux control method: in this method a rheostat is connected across the field
wdg to control the field current. So by changing the current the flux produced by the field wdg
can be changed, and since speed is inversely proportional to flux speed can be controlled.
2. Armature control method: in this method a rheostat is connected across armature wdg and by
varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly
proportional to Eb-IaRa the speed can be controlled.
Question: what is meant by armature reaction?
Answer: The action of armature m.m.f. on the main m.m.f. is known as armature reaction.
Question: Enlist the types of dc machines?
Answer: D.C. machines are classified into two types 1) separately excited 2) self excited which
is further classified into 1)series 2)shunt and 3)compound(which is further classified into
cumulative and differential).
Question: why, when birds sit on transmission lines doesn't get shock?
Answer: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical
shock... if birds touch 2 lines than the circuit is closed and they get electrical shock... so if a
human being touches single one line (phase) then he doesn't get shock .if he is standing on the
ground then touching the line (phase) he will get a shock because the ground on what we are
standing is like line (ground bed - like neutral) that means that human who touches the line
closes the circuit between phase and neutral.
Question: what is ACSR cable?
Answer: ACSR means Aluminum conductor, steel reinforced, e.g. 7/24 ACSR conductor has 7
steel strands forming a central core, around which there are two layers of aluminum strands(24).
Question: what is electric traction?
Answer: Traction means transport system i.e. transport of men and material from one place to
other. Electric traction means use of the electricity for all these .electric traction services are
broadly classified to railways, trams, trolleys, battery driven and solar powered vehicles.
Question: Why is the starting current high in a DC motor?
Answer: In DC motors, Voltage equation is V=Eb-IaRa
(V = Terminal voltage, Eb = Back emf in Motor, Ia = Armature current, Ra = Armature
resistance).At starting, Eb is zero. Therefore, V=IaRa, Ia = V/Ra, where Ra is very less value and
Ia becomes very high.
Question: What are the transformer losses?
Answer: TRANSFORMER LOSSES –
There are mainly two kinds of losses in a transformer namely
1. Copper loss (ohmic loss) and 2.magnetic loss (core loss).
Copper losses are caused by the resistance of the wire (I2R).
Magnetic losses are caused by eddy currents and hysteresis in the core.
In addition to the core loss and ohmic loss the following two losses are also present in the
transformer stray load loss and dielectric loss.
Question: what is Ferranti effect?
Answer: when receiving end voltage is greater than the sending end voltage due to no load or
light loading, this effect is known as Ferranti effect.
Question: types of cooling system in transformers?
Answer: 1. ONAN (oil natural, air natural)
2. ONAF (oil natural, air forced)
3. OFAF (oil forced, air forced)
4. ODWF (oil direct, water forced)
5. OFAN (oil forced, air forced)
Short questions and answers
EE1251 Electrical Machines II
1. Why almost all large size Synchronous machines are constructed with rotating
field system type?
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 Altenator.
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?
According to type of field system
• Stationary field system type
• Rotating field system type
According to shape of field system
• Salient pole type
• Smooth cylindrical type
4. Name the types of Alternator based on their rotor construction.
Alternators can be classified into the following two types according to its rotor
construction
• Smooth cylindrical type alternator
• Salient pole alternator
5. Why do cylindrical Alternators operate with steam turbines?
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 :
• They allow better ventilation
• 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
• 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?
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 the relation between electrical degree and mechanical degree?
(OHFWULFDOGHJUHH HDQGPHFKDQLFDOGHJUHH PDUHUHODWHGWRRQHDQRWKHUE\
the number of poles P, the electrical machine has, as given by the following
equation
H 3 P
11. What is distributed winding?
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
:KHUH - angle between adjacent slots in electrical degree and x=
1,2,3…
12. Why is short pitch winding preferred over full-pitch winding ?
Advantages
•
Waveform of the emf can be approximately made to a sine
wave and distorting harmonics can be reduced or totally
eliminated.
• Conductor material , copper , is saved in the back and front
end connections due to less coil-span.
• Fractional slot winding with fractional number of
slots/phase can be used which in turn reduces the tooth
ripples.
• Mechanical strength of the coil is increased.
13. Write down the formula for distribution factor.
Kd = sin(m PVLQ RU.dn = sin(mn Psin(n m= number of slots/ pole/ phase
DQJOHEHWZHHQDGMDFHQWVORWVLQHOHFWULFDOGHJUHH
n = order of harmonic
14. Define winding factor.
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.
15. Why are Alternators rated in kVA and not in kW?
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.
16. What are the causes of changes in voltage in Alternators when loaded?
Variations in terminal voltage in Alternators on load condition are due to the
following three causes:
• Voltage variation due to the resistance of the winding, R
•
Voltage variation due to the leakage reactance of the
winding, Xt
• Voltage variation due to the armature reaction effect, Xa
17. What is meant by armature reaction in Alternators?
The interaction between flux set up by the current carrying armature
FRQGXFWRUV a and the main fieOGIOX[ m is defined as the armature reaction.
18. 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 syncheornous reactance.
19. 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.
In complex form
Zs = (R + jXs )
In polar form
Zs = | Zs | <
Where
| Zs | = ¥ 52 + X2s )
And
WDQ-1 (Xs /R)
20. 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.
21. Upon what factors does the load angle depend?
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angle is positive during generator operation and negative during motor
operation.
22. 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.
23. Define the term voltage regulation of Alternator.
The voltage regulation of an Alternator is defined as the change in terminal
voltage from no-load to load condition expressed as a fraction or percentage of
terminal voltage at load condition ; the speed and excitation conditions
remaining same.
Voltage regulation in percentage , URP = [(|E|-|U|)/|U| ]x 100
24. What is the necessity for predetermination of voltage regulation?
Most of the Alternators are manufactured with large power rating , hundreds
of kW or MW, and also with large voltage rating upto 33kV. For Alternators
of such power and voltage ratings conducting load test is not possible. Hence
other indirect methods of testing are used and the performance like voltage
regulation then can be predetermined at any desired load currents and power
factors.
25. Name the various methods for predetermining the voltage regulation of 3-phase
Alternator.
The following are the three methods which are used to predetermine the
voltage regulation of smooth cylindrical type Alternators
• Synchronous impedance / EMF method
• Ampere-turn / MMF method
• Potier / ZPF method
26. 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
Ur on open circuit.
|Zs| = Ur/Iscn
27. What are the advantages and disadvantages of estimating the voltage regulation of
an Alternator by EMF method?
Advantages:
•
•
Disadvantages:
•
Simple no load tests (for obtaining OCC and SCC) are to be
conducted
Calculation procedure is much simpler
The value of voltage regulation obtained by this method is
always higher than the actual value
28. 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.
29. 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.
30. What are the test data required for predetermining the voltage regulation of an
Alternator by MMF method?
Data required for MMF method are :
• Effective resistance per phase of the 3-phase winding R
• Open circuit characteristic (OCC) at rated speed/frequency
• Short circuit characteristic (SCC) at rated speed/frequency
31. 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.
32. State the condition to be satisfied before connecting two alternators in parallel
The following are the three conditions to be satisfied by synchronizing the
additional Alternator with the existing one or the common bus-bars.
• The terminal voltage magnitude of the incoming Alternator
must be made equal to the existing Alternator or the bus-bar
voltage magnitude.
• The phase sequence of the incoming Alternator voltage
must be similar to the bus-bar voltage.
• The frequency of the incoming Alternator voltage must be
the same as the bus-bar voltage.
33. 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.
34. What are the advantages and disadvantages of three dark lamps method of
synchronizing?
Advantages:
•
The synchronous switch using lamps is inexpensive
•
Disadvantages:
•
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
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.
35. 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 synchronise the incoming Alternator when the pointer faces the top
thick line marking.
36. 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.
37. 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
• Prime-mover characteristic/input
• Excitation level and
• Percentage synchronous impedance and its R/X ratio
38. How does the change in prime mover input affect the load sharing?
An increase in prime-mover input to a particular generator causes the activepower 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.
39. 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.
40. 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
• 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.
• The excitation of the outgoing generator has to be decreased
and that of other generators have to be increased and by this
the entire reactive-power delivered by the outgoing
generator is transferred to other generators.
• After ensuring the current delivered by the outgoing
generator is zero, it has to be disconnected from parallel
operation.
41. 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.
42. How does increase in excitation of the Alternator connected to infinite bus-bars
affect this operation?
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.
43. 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.
44. What are the inherent characteristics of plain 1-phase Induction motor ?
A plain 1-phase Induction motor is not used in practice due to the following
inherent characteristics
• A plain 1-phase Induction motor does not have any starting torque
• However, if the rotor is initially given a starting torque, by some means,
the motor can pick up its speed in a direction at which the initial torque is
given and deliver the required output.
45. Name the two different theories with which principle of 1-phase induction motors
are explained.
The two different theories are
• Double revolving field theory
• Cross field theory
46. State double revolving field theory.
Double revolving theory, formulated by Ferrari, states that a single pulsating
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synchronous speed proportional to the frequency of the pulsating field.
47. Name any four types of 1-phase induction motors.
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
48. Why are centrifugal switches provided on many 1-phase Induction motors?
Centrifugal switches are provided on many 1-phase Induction motors to
disconnect the starting / auxiliary winding from the supply when the motor
reaches about 70% of its synchronous speed.
49. How is the direction of a capacitor start Induction motor be reversed?
The direction of rotation can be reversed by interchanging the terminals of
either the main winding or the starting winding.
50. State the principle of 3 phase IM?
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.
51. 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.
52. An induction motor is generally analogous to ?
It is analogous to a winding rotating transformer with its secondary circuit closed’
.
53. Can the starting torque of of a slip ring induction motor being increased?
Yes. It can be increased by adding resistances to the rotor.
54. What would happen if a 3 phase induction motor is switched on with one phase
disconnected?
The motor is likely to burn .
55. 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.
56. State the advantages of skewing?
It reduces humming and hence quite running of motor is achieved.
It reduces magnetic locking of the stator and rotor.
67 State the condition at which the starting torque developed in a slip-ring induction
motor is maximum.
When R2=X2
68 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.
68 What is slip of an induction motor?
The slip speed expressed as the ratio of synchronous speed is defined as slip.
Percentage slip
S=Ns-N/Ns*100
69 How the magnitude of rotor emf is related to the slip in an I M?
Rotor circuit emf per phase E2r=SE2
70 How the frequency of rotor emf is related to the slip in an I M?
Frequency of rotor emf/current fr=Sfs
71 What is the normal value of slip of an I M operating at full load?
3 - 5%
72 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.
73 State the condition at which the torque developed in a 3 phase induction motor is
maximum.
When R2=SX2
74. What are the advantages of slip-ring I M over cage I M?
(i) Rotor circuit is accessible for external connection.
(ii) By adding external resistance to the rotor circuit the starting current is
reduced with the added advantage of improving starting torque.
(iii) Additional speed control methods can be employed with the
accessibility in the rotor circuit.
75. What are the losses occurring in an I M and on what factors do they depend?
Magnetic losses Wi
Electrical losses Wcu
Mechanical losses Wm
For I M operating in normal condition (with constant voltage and frequency)
magnetic and mechanical losses remain constant whereas electrical losses
vary in square proportion to the current.
76. What care should be taken at the time of construction to reduce eddy current
losses in I M?
Make the resistance of the core body as large as possible.This is
achieved by laminating the stator core,stacked and revetted at right angles to
the path of eddy current.The laminations are insulated from each other by thin
coat of varnish.
77. Why is there not appreciable magnetic losses in the rotor core of Induction
motors?
Although the rotor core is also subjected to magnetic flux reversals and since
the frequency of flux reversals in the rotor, fr = Sfs, is very small, the iron loss
incurred in the rotor core is negligibly small.
78. What is meant by synchronous watt?
With the power input to the motor Pi, after the losses in the stator winding.
Wcu1 and stator core, Wi, are met with, the remaining power is transferred to
the rotor by the rotating magnetic field as power input to the rotor Pir
Pir = Pi –wau1 - Wi
The power input to the rotor Pir is transferred from the stator
to the rotor by rotating magnetic field which rotates at
synchronous speed Ns. Torque Td is developed in the rotor as
Or
a result of Pir and the equation for Pir can alternatively be
expressed as
Pir 1s/60). Td W
Td = Pir Syn. W
79. 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
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
80. Name the tests to be conducted for predetermining the performance of 3-phase
induction machine.
(a) No load test
(b) Blocked rotor test
81. What are the informations obtained from no-load test in a 3-phase I M?
(i) 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
82What are the informations obtained from blocked rotor test in a 3-phase I M?
(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
83 What is circle diagram of an I M?
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.
84 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.
85 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
86 State the characteristic features of synchronous motor.
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.
87 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.
88 Name any two methods of starting a synchronous motors
• By an extra 3 phase cage induction motor
• By providing damper winding in pole phases
• By operating the pilot excitor as a dc motor
89 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.
90 Why a synchronous motor is a constant speed motor?
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.
91 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
DQJOH 92 At what load angle is power developed in a synchronous motor becomes its
maximum value ?
:KHQLWVORDGDQJOH LVHTXDOWRWKHLPSHGDQFHDQJOH 93 What are V and inverted V curves of synchronous motor ?
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 V curve.
94 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.
95 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.
96 How the synchronous motor can be used as s synchronous condenser?
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.
97 What type of single phase induction motor would you use for the following
applications?
(i) Ceiling fan (ii) Wet grinder
Ceiling fan – capacitor start and run motor
Wet grinder – capacitor start motor
98 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.
99 What will be the direction of rotation of a shaded pole single phase induction
motor?
The motor rotates in the direction specified by the unshaded to shaded
region in the pole phase
100What is the property of a single phase single winding induction motor?
It has zero starting torque
101Which winding in a double cage induction motor has high resistance and low
inductance?
Outer cage winding
Long answer Questions
1. State the requirements for paralleling of alternators.
2. A two pole,50 Hz, 3-phase, turbo alternator is excited to generate the bus-bar
voltage of 11 kV on no load.The machine is star connected and the short
circuit current for this excitation is 1000A. Calculate the synchronizing power
per degree of mechanical displacement of the rotor and the corresponding
synchronizing torque.
3. A 3300V, 3 phase 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 e m f on open circuit for the same excitation was 900V.The
armature resistance was 0.8 ohm/phase. Determine the full load voltage
regulation for (i) 0.8 p f lagging (ii) 0.8 p f leading.
4. Explain the construction and principle of operation of 3-phase alternator
5. Develop the equivalent circuit of three phase I M
6. Explain the various techniques of speed control of 3-phase I M
7. A 3-phase ,star connected, 16 pole alternaror has 192 slots with 8 conductors
per slot, coil span=160 electrical degrees, speed of alternator=375 rpm, flux
per pole=55mWb.Calculate the line and phase values of EMF generated.
8. The following data refers to a 20pole,460V,60Hz, 3-phase I M :
R1=2 ohm,X1=1 ohm, R2’ =3 ohm, X2’ =7 ohm. When the motor is tested on
no load, it is observed that it takes 3.9A and the total core loss is 450W.By
using an approximate equivalent circuit at 5% slip, calculate
(i) Rotor current (ii) Supply current and pf (iii) Gross load torque
(iv) draw the equivalent circuit.
9. Explain about crawling and cogging
10. Describe any two methods of determining the voltage regulation of 3-phase
alternator
11. Explain the operation of single phase induction motor on the basis of double
field revolving theory.
12. Explain the operation of the types of stepper motors.
13. A 3 MVA , 50Hz, 11 kV, 3-phase star connected alternator supplies 100A at
zero p f leading. The line voltage is 12370V.When the load is removed, the
line voltage is 1100V.Find the regulation at full load,0.8 pf lagging.
Ra=0,4ohm/phase.
14. Bring out the characteristics of two alternators working in parallel. What is the
effect of change in excitation on load sharing.
15. Derive the equation for torque developed by an I M. Draw the tipical torqueslip curve and deduce the condition for maximum torque.
16. Write a note on Hysterisis motor
17. Write short notes on (i)A C series motor (ii) Reluctance motor
18. How do you determine the direct axis and quadrature axis reactances of a
salient pole alternator?
19. A three phase star connected alternator has direct axis synchronous reactance
of 0,7p u and quadrature axis reactance of 0,5 p u,If the generator delivers
kVA at rated voltage at full load and lagging, find the percentage
regulation.Resistance drop at full load is 0,017 p u.
20. Two alternators are connected in parallel, what happens when we (i) increase
the excitation of one machine (ii) increase the steam supply of one machine.
21. Two similar 3000kVA synchronous generators work in parallel.The governor
of first machine is such that frequency drops uniformly from 50Hz on no load
to 48 Hz on full load.The corresponding speed drop of second machine is from
50Hz to 47.5Hz.determine the following
(i) How will the two machines share a load 5000kW at full load
(ii) What is the maximum load at u p f that can be delivered without
overloading either of the two machines.
22. Explain why synchronous motor is not self starting.
23. Explain the various schemes of starting of 3 phase I M
24. Describe how 3-phase supply produces a rotating magnetic field of constant
value at constant speed with vector diagrams.
25. Derive the EMF Equation of 3 phase alternator .Define distribution factor and
coil span factor?
ELECTRICAL MACHINES:
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35.
What is the basic principle of operation of a DC Generator?
What are the different types of D.C.Generators.
What is long compound and short compound?
What are the factors affecting EMF generated in a DC Generator?
What is armature reaction?
What is a compensating winding in DC Machine?
What are interpoles?
What is commutation?
What are the causes of self-excitation and what are the remedies?
Name applications for different types of generators.
What is back emf?
Why a starter is required to start a dc motor?
Explain the basic principle of operation of a starter?
Explain the characteristics of different types of motors and their applications.
How is the dc motor speed is controlled?
What is the advantage of Swinburne’s Test?
What are the different losses in a DC Motor?
What is the condition for maximum efficiency?
What is the purpose of Hopkinson’s Test?
What is a transformer and its basic principle of operation?
How many types of transformer are there based on construction?
What is the purpose of laminations in the core of a transformer?
What are the different losses in a transformer?
What is the condition for maximum efficiency?
What is the effect of variation of supply frequency on iron losses?
What is all day efficiency? Is it for all types of transformers, explain.
What are the different tests carried on the transformer
How the equivalent circuit parameters of a transformer are obtained?
What is regulation? And how it is obtained /calculated?
What is Sumpner’s test and its purpose?
What is an autotransformer? Is it advantageous when compared to two winding transformer.
What is Scott connection?
What are the conditions required for parallel operation of transformers?
Explain the principle of operation of three phase induction motor?
How is a rotating magnetic field is produced? And what is its speed of rotation?
36. Is the frequency of voltage/current in the rotor of induction motor different from supply
frequency, explain.
37. What happens if the rotor of an induction motor is open circuited and three phase supply is
given to stator?
38. What is slip?
39. What is the slip at stand-still?
40. What are the different types of induction motors based rotor construction?
41. What are the advantages of slip ring induction motor?
42. What is the condition for obtaining maximum torque from a 3-phase induction motor?
43. What is the relationship between rotor input and copper losses?
44. What is a circle diagram and how it is conducted?
45. What are different types of starting an induction motor?
46. Can the speed of an induction motor be controlled? How?
47. Applications of different types of induction motors?
48. Explain the principle of operation of an alternator.
49. What is pitch factor and distribution factor?
50. How harmonics can be suppressed in an alternator?
51. What is regulation of an alternator and how it is obtained?
52. What is synchronizing power torque?
53. What are sub-transient, transient and steady-state reactance?
54. Explain the principle of operation of a single phase induction motor.
Power Electronics(2 marks)
ELECTRICAL AND ELECTRONICS DEPARTMENT
SHORT QUESTIONS AND ANSWERS
EE1301-Power Electronics
1. Why IGBT is very popular nowadays?
a. Lower hate requirements
b. Lower switching losses
c. Smaller snubber circuit requirements
2. What are the different methods to turn on the thyristor?
a. Forward voltage triggering
b. Gate triggering
c. dv/dt triggering
d. Temperature triggering
e. Light triggering
3. What is the difference between power diode and signal diode?
S.No.
Power diode Signal diode
1.Constructed with n-layer, called
drift region between p+ layer and
n+ layer.
Drift region is not present.
2.
The voltage, current and power
ratings are higher.
Lower
3.
Power diodes operate at high
speeds.
Operates at higher switching speed.
4. IGBT is a voltage controlled device. Why?
Because the controlling parameter is gate-emitter voltage.
5. Power MOSFET is a voltage controlled device. Why?
Because the output (drain) current can be controlled by gate-source voltage.
6. Power BJT is a current controlled device. Why?
Because the output (collector) current can be controlled by base current.
7. What is the relation between and ?
=
1=
1–
8. What are the different types of power MOSFET?
a. N-channel MOSFET
b. P-channel MOSFET
9. How can a thyristor turned off?
A thyristor can be turned off by making the current flowing through it to zero.
10. Define latching current.
The latching current is defined as the minimum value of anode current which it
must attain during turn on process to maintain conduction when gate signal is removed.
11. Define holding current.
The holding current is defined as the minimum value of anode current below
which it must fall to for turning off the thyristor.
12. What is a snubber circuit?
It consists of a series combination of a resistor and a capacitor in parallel with the
thyristors. It is mainly used for dv / dt protection.
13. What losses occur in a thyristor during working conditions?
a. Forward conduction losses
b. Loss due to leakage current during forward and reverse blocking.
c. Switching losses at turn-on and turn-off.
d. Gate triggering loss.
14. Define hard-driving or over-driving.
When gate current is several times higher than the minimum gate current
required, a thyristor is said to be hard-fired or over-driven. Hard-firing of a thyristor
reduces its turn-on time and enhances its di/dt capability.
15. Define circuit turn off time.
It is defined as the time during which a reverse voltage is applied across the
thyristor during its commutation process.
16. Why circuit turn off time should be greater than the thyristor turn-off time?
Circuit turn off time should be greater than the thyristor turn-off time for reliable
turn-off, otherwise the device may turn-on at an undesired instant, a process called
commutation failure.
17. What is the turn-off time for converter grade SCRs and inverter grade SCRs?
Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter
grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.
18. What are the advantages of GTO over SCR?
a. Elimination of commutation of commutating components in forced commutation,
resulting in reduction in cost, weight and volume.
b. Reduction in acoustic noise and electromagnetic noise due to elimination of
commutation chokes.
c. Faster turn-off, permitting high switching frequencies.
d. Improved efficiency of the converters.
19. What is meant by phase controlled rectifier?
It converts fixed ac voltage into variable dc voltage.
20. Mention some of the applications of controlled rectifier.
a. Steel rolling mills, printing press, textile mills and paper mills employing dc
motor drives.
b. DC traction
c. Electro chemical and electro-metallurgical process
d. Portable hand tool drives
e. Magnet power supplies
f. HVDC transmission system
21. What is the function of freewheeling diodes in controlled rectifier?
It serves two process.
a. It prevents the output voltage from becoming negative.
b. The load current is transferred from the main thyristors to the freewheeling diode,
thereby allowing all of its thyristors to regain their blocking states.
22. What are the advantages of freewheeling diodes in a controlled in a controlled rectifier?
a. Input power factor is improved.
b. Load current waveform is improved and thus the load performance is better.
23. What is meant by delay angle?
The delay angle is defined as the angle between the zero crossing of the input
voltage and the instant the thyristor is fired.
24. What are the advantages of single phase bridge converter over single phase mid-point
converter?
a. SCRs are subjected to a peak-inverse voltage of 2Vm in a fully controlled bridge
rectifier. Hence for same voltage and currnt ratings of SCrs, power handled by
mid-point configuration is about
b. In mid-point converter, each secondary winding should be able to supply the load
power. As such, the transformer rating in mid-point converter is double the load
rating.
25. What is commutation angle or overlap angle?
The commutation period when outgoing and incoming thyristors are conducting is
known as overlap period. The angular period, when both devices share conduction is
known as the commutation angle or overlap angle.
26. What are the different methods of firing circuits for line commutated converter?
a. UJT firing circuit.
b. The cosine wave crossing pulse timing control.
c. Digital firing schemes.
27. Give an expression for average voltage of single phase semiconverters.
Average output voltage Vdc = (Vm / ) (1 + cos ).
28. What is meant by input power factor in controlled rectifier?
The input power factor is defined as the ratio of the total mean input power to the
total RMS input volt-amperes.
PF = ( V1 I1 cos 1 ) / ( Vrms Irms) where V1 = phase voltage, I1 = fundamental
component of the supply current, 1 = input displacement angle, Irms = supply rms current.
29. What are the advantages of six pulse converter?
a. Commutation is made simple.
b. Distortion on the ac side is reduced due to the reduction in lower order harmonics.
c. Inductance reduced in series is considerably reduced.
30. What is meant by commutation?
It is the process of changing the direction of current flow in a particular path of
the circuit. This process is used in thyristors for turning it off.
31. What are the types of commutation?
a. Natural commutation
b. Forced commutation
32. What is meant by natural commutation?
Here the current flowing through the thyristor goes through a natural zero and
enable the thyristor to turn off.
33. What is meant by forced commutation? In this commutation, the current flowing through
the thyristor is forced to become zero by external circuitry.
34. What is meant by dc chopper?
A dc chopper is a high speed static switch used to obtain variable dc voltage from
a constant dc voltage.
35. What are the applications of dc chopper?
a. Battery operated vehicles
b. Traction motor control in electric traction
c. Trolly cars
d. Marine hoists
e. Mine haulers
f. Electric braking.
36. What are the applications of dc chopper?
Chopper provides
a. High efficiency
b. Smooth acceleration
c. Fast dynamic response
d. Regeneration
37. What is meant by step-up and step-down chopper?
In a step- down chopper or Buck converter, the average output voltage is less than
the input voltage. In a step- up chopper or Boost converter, the average output voltage is
more than the input voltage.
38. Write down the expression for average output voltage for step down chopper.
Average output voltage for step down chopper V0 = Vs, is the duty cycle
39. Write down the expression for average output voltage for step up chopper.
Average output voltage for step down chopper V0 = Vs, is the duty cycle
140. What is meant by duty-cycle?
Duty cycle is defined as the ratio of the on time of the chopper to the total time
period of the chopper. It is denoted by a.
41. What are the two types of control strategies?
a. Time Ratio Control (TRC)
b. Current Limit Control method (CLC)
42. What is meant by TRC?
In TRC, the value of Ton / T is varied in order to change the average output
voltage.
43. What are the two types of TRC?
a. Constant frequency control
b. Variable frequency control
44. What is meant by FM control in a dc chopper?
In frequency modulation control, the chopping frequency f (or the chopping
period T) is varied. Here two controls are possible.
a. On-time Ton is kept constant
b. Off period Toff is kept constant.
45. What is meant by PWM control in dc chopper?
In this control method, the on time Ton is varied but chopping frequency is kept
constant. The width of the pulse is varied and hence this type of control is known as Pulse
Width Modulation (PWM).
46. Write down the expression for the average output voltage for step down and step up
chopper.
Average output voltage for step down chopper is VO = a VS. Average output
voltage for step up chopper is VO = a VS x [1/ ( 1- a )].
47. What are the different types of chopper with respect to commutation process?
a. Voltage commutated chopper.
b. Current commutated chopper.
c. Load commutated chopper.
48. What is meant by voltage commutation?
In this process, a charged capacitor momentarily reverse biases the conducting
thyristor and turn it off.
49. What is meant by current commutation?
In this process, a current pulse is made to flow in the reverse direction through
the conducting thyristor and when the net thyristor current becomes zero, it is turned off.
50. What is meant by load commutation?
In this process, the load current flowing through the thyristor either becomes zero
or is transferred to another device from the conducting thyristor.
51. What are the advantages of current commutated chopper?
a. The capacitor always remains charged with the correct polarity.
b. Commutation is reliable as load current is less than the peak commutation current
ICP.
c. The auxiliary thyristor TA is naturally commutated as its current passes through
zero value.
52. What are the advantages of load commutated chopper?
a. Commutating inductor is not required.
b. It is capable of commutating any amount of load current.
c. It can work at high frequencies in the order of kHz.
d. Filtering requirements are minimal.
53. What are the disadvantages of load commutated chopper?
a. For high power applications, efficiency becomes very low because of high
switching losses at high operating frequencies.
b. Freewheeling diode is subjected to twice the supply voltage.
c. Peak load voltage is equal to twice the supply voltage.
d. The commutating capacitor has to carry full load current at a frequency of half
chopping frequency.
e. One thyristor pair should be turned-on only when the other pair is commutated.
This can be realized by sensing the capacitor current that is alternating.
54. What is meant by inverter?
A device that converts dc power into ac power at desired output voltage and
frequency is called an inverter.
55. What are the applications of an inverter?
a. Adjustable speed drives
b. Induction heating
c. Stand-by aircraft power supplies
d. UPS
e. HVDC transmission
56. What are the main classification of inverter?
a. Voltage Source Inverter
b. Current Source Inverter
57. Why thyristors are not preferred for inverters?
Thyristors require extra commutation circuits for turn off which results in
uncreased complexity of the circuit. For these reasons thyristors are not preferred for
inverters.
58. How output frequency is varied in case of a thyristor?
The output frequency is varied by varying the turn off time of the thyristors in the
inverter circuit, i.e. the delay angle of the thyristors is varied.
59. Give two advantages of CSI.
a. CSI does not require any feedback diodes.
b. Commutation circuit is simple as it involves only thyristors.
60. What is the main drawback of a single phase half bridge inverter?
It require a 3-wire dc supply.
61. Why diodes should be connected in antiparallel with thethyristors in inverter circuits?
For RL loads, load current will not be in phase with load voltage and the diodes
connected in antiparallel will allow the current to flow when the main thyristors are
turned off. These diodes are called feedback diodes.
62. What types of inverters require feedback diodes?
VSI with RL load.
63. What is meant a series inverter?
An inverter in which the commutating elements are connected in series with the
load is called a series inverter.
64. What is the condition to be satisfied in the selection of L and C in a series inverter?
R2 < 4L
C
65. What is meant a parallel inverter?
An inverter in which the commutating elements are connected in parallel with the
load is called a parallel inverter.
66. What are the applications of a series inverter?
The thyristorised series inverter produces an approximately sinusoidal waveform
at a high output frequency, ranging from 200 Hz to 100kHz. It is commonly used for
fixed output applications such as
a. Ultrasonic generator.
b. Induction heating.
c. Sonar Transmitter
d. Fluorescent lighting.
67. How is the inverter circuit classified based on commutation circuitry?
a. Line commutated inverters.
b. Load commutated inverters.
c. Self commutated inverters.
d. Forced commutated inverters.
68. What is meant by McMurray inverter?
It is an impulse commutated inverter which relies on LC circuit and an auxiliary
thyristor for commutation in the load circuit.
69. What are the applications of a CSI?
a. Induction heating
b. Lagging VAR compensation
c. Speed control of ac motors
d. Synchronous motor starting.
70. What is meant by PWM control?
In this method, a fixed dc input voltage is given to the inverter and a controlled
ac output voltage is obtained by adjusting the on and off periods of the inverter
components. This is the most popular method of controlling the output voltage and this
method is termed as PWM control.
71. What are the advantages of PWM control?
a. The output voltage can be obtained without any additional components.
b. Lower order harmonics can be eliminated or minimized along with its output
voltage control. As the higher order harmonics can be filtered easily, the filtering
requirements are minimized.
72. What are the disadvantages of the harmonics present in the inverter system?
a. Harmonic currents will lead to excessive heating in the induction motors. This
will reduce the load carrying capacity of the motor.
b. If the control and the regulating circuits are not properly shielded, harmonics from
power ride can affect their operation and malfunctioning can result.
c. Harmonic currents cause losses in the ac system and can even some time produce
resonance in the system. Under resonant conditions, the instrumentation and
metering can be affected.
d. On critical loads, torque pulsation produced by the harmonic current can be
useful.
73. What are the methods of reduction of harmonic content?
a. Transformer connections
b. Sinusoidal PWM
c. Multiple commutation in each cycle
d. Stepped wave inverters
74. Compare CSI and VSI.
S. No. VSI CSI
1.
Input voltage is maintained
constant
Input current is constant but
adjustable
2.
The output voltage does not
depend on the load
The output current does not depend
on the load
3.
The magnitude of the output
current and its waveform
depends on the nature of the
load impedance
The magnitude of the output voltage
and its waveform depends on the
nature of the load impedance
4.
It requires feedback diodes It does not requires feedback diodes
5. Commutation circuit is
complicated i.e. it contains
capacitors and inductors.
Commutation circuit is simple i.e. it
contains only capacitors.
75. What are the disadvantages of PWM control?
SCRs are expensive as they must possess low turn-on and turn-off times.
76. What does ac voltage controller mean?
It is device which converts fixed alternating voltage into a variable voltage
without change in frequency.
77. What are the applications of ac voltage controllers?
a. Domestic and industrial heating
b. Lighting control
c. Speed control of single phase and three phase ac motors
d. Transformer tap changing
78. What are the advantages of ac voltage controllers?
a. High efficiency
b. Flexibility in control
c. Less maintenance
79. What are the disadvantages of ac voltage controllers?
The main draw back is the introduction of harmonics in the supply current and the
load voltage waveforms particularly at low output voltages.
80. What are the two methods of control in ac voltage controllers?
a. ON-OFF control
b. Phase control
81. What is the difference between ON-OFF control and phase control?
ON-OFF control: In this method, the thyristors are employed as
switches to connect the load circuit to the source for a few cycles of the load voltage and
disconnect it for another few cycles. Phase control: In this method, thyristor switches
connect the load to the ac source for a portion of each half cycle of input voltage.
82. What is the advantage of ON-OFF control?
Due to zero-voltage and zero current switching of thyristors, the harmonics
generated by the switching action are reduced.
83. What is the disadvantage of ON-OFF control?
This type of control is applicable in systems that have high mechanical inertia and
high thermal time constant.
84. What is the duty cycle in ON-OFF control method?
Duty cycle K = n/ (n + m), where n = no. of ON cycles, m = no. of OFF cycles.
85. What is meant by unidirectional or half-wave ac voltage controller?
Here the power flow is controlled only during the positive half-cycle of the input
voltage.
86. What are the disadvantages of unidirectional or half-wave ac voltage controller?
a. Due to the presence of diode on the circuit, the control range is limited and the
effective RMS output voltage can be varied between 70.7% and 100%.
b. The input current and output voltage are asymmetrical and contain a dc
component.If there is an input transformer, sdaturation problem will occur
c. It is only used for low power resistive load.
87. What is meant by bidirectional or half-wave ac voltage controller?
Here the power flow is controlled during both cycles of the input voltage.
88. What is the control range of firing angle inac voltage controller with RL load?
The control range is F < a <180°, where F = load power factor angle.
89. What type of gating signal is used in single phase ac voltage controller with RL load?
High frequency carrier gating signal is used for single phase ac voltage controller
with RL load.
90. What are the disadvantages of continuous gating signal?
a. More heating of the SCR gate.
b. Increases the size of pulse transformer.
91. What is meant by high frequency carrier gating?
Thyristor is turned on by using a train of pulses from a to p. This type of signal
is called as high frequency carrier gating.
92. What is meant by sequence control of ac voltage regulators?
It means that the stages of voltage controllers in parallel triggered in a proper
sequence one after the other so as to obtain a variable output with low harmonic content.
93. What are the advantages of sequence control of ac voltage regulators?
a. System power factor is improved.
b. Harmonics are reduced in the source current and the load voltage.
94. What is meant by cyclo-converter?
It converts input power at one frequency to output power at another frequency
with one-stage conversion. Cycloconverter is also known as frequency changer.
95. What are the two types of cyclo-converters?
a. Step-up cyclo-converters
b. Step-down cyclo-converters
96. What is meant by step-up cyclo-converters?
In these converters, the output frequency is less than the supply frequency.
97. What is meant by step-down cyclo-converters?
In these converters, the output frequency is more than the supply frequency.
98. What are the applications of cyclo-converter?
a. Induction heating
b. Speed control of high power ac drives
c. Static VAR generation
d. Power supply in aircraft or ship boards
99. What is meant by positive converter group in a cycloconverter?
The part of the cycloconverter circuit that permits the flow of current during
positive half cycle of output current is called positive converter group.
100.What is meant by negative converter group in a cycloconverter?
The part of the cycloconverter circuit that permits the flow of current during negative half
cycle of output current is called negative converter group.
BIG QUESTIONS AND ANSWER KEY
EE1301- Power Electronics
]
1.Draw the two transistor model of SCR and derive an expression for anode current.
Ans:
• Schematic diagram (2M)
• Two transistor model diagram (2M)
• Operation (4M)
2.Explain the characteristics of SCR
Ans:
• Equivalent circuit
• V-I characteristics
• Switching characteristics
3.Describe the various methods of thyristor turn on.
Ans:
• Forward voltage triggering
• The dv/dt triggering
• Gate triggering
• Temperature triggering
• Light triggering
4.Explain the operation of MOSFET
Ans:
• Basic structure, symbol
• Operation
• V-I characteristics, Switching characteristics
5.Explain the operation of IGBT
Ans:
• Basic structure, symbol
• Operation
• V-I characteristics, Switching characteristics
6.Derive the expressions for average output voltage and rms output voltage of 1
semiconverter.
Ans:
• 1 semiconverter bridge circuit
• waveforms of vo , io , ifd, is, VT1, VT2
• Operation
• Average output voltage, rms value of output voltage expression
7.Describe the working of 1 fully controlled bridge converter in the Rectifying mode
and inversion mode. And derive the expressions for average output voltage and rms
output voltage.
Ans:
• 1 full converter bridge circuit
• waveforms of vo , io , ifd, is, VT1, VT2
• Operation for < 900, > 900
• Average output voltage, rms value of output voltage expression
8. Describe the working of 3 fully controlled bridge converter in the Rectifying mode
and inversion mode. And derive the expressions for average output voltage and rms
output voltage.
Ans:
• 3 full converter bridge circuit
• waveforms of vo , io , iA, vs
• Operation
• Average output voltage expression
9.Describe the working of 3 semi converter. And derive the expressions for average
output voltage and rms output voltage.
Ans:
• 3 semi converter bridge circuit
• waveforms of vo , io , iA, vs
• Operation
• Average output voltage expression
10. Describe the working of Dual converter.
Ans:
• 3 dual converter bridge circuit
• waveforms of vo , io , vo1 , vo2 , i1, i2 , vs , ic
• Operation for with circulating current and without circulating current
• load voltage expression, peak value icp
11. FOR A Type A chopper (first quadrant), express the following variables as a function
of Vs, R and duty cycle in case the load is resistive average output voltage and
current.
Ans:
• chopper circuit
• output voltage & current waveforms
• Average load voltage expression
12.Describe the principle of step-up chopper. Derive an expression for the average output
voltage in terms of input dc voltage & duty cycle.
Ans:
• chopper circuit
• output voltage & current waveforms
• Average load voltage expression
13.Describe the working of four quadrant chopper. .
Ans:
• chopper circuit
• operation
14.Explain the working of current commutated chopper with aid of circuit diagram and
necessary waveforms. Derive an expression for its output voltage.
Ans:
• chopper circuit
• Modes of operation- equivalent circuit diagrams
• Current & voltage waveforms
• Design
15. Explain the working of voltage commutated chopper with aid of circuit diagram and
necessary waveforms. Derive an expression for its output voltage.
Ans:
• chopper circuit
• Modes of operation- equivalent circuit diagrams
• Current & voltage waveforms
• Design of C & L
16.Describe the operation of series inverter with aid of diagrams. Describe an
expression for output frequency, current and voltages. What are the disadvantages of
basic series inverter?
Ans:
• series inverter circuit
• Current & voltage waveforms
• Operation
• Expression for output frequency, VL, VC
18. State different methods of voltage control inverters. Describe about PWM control in
inverter.
Ans:
• External control of ac output voltage
• External control of dc input voltage
• Internal control of Inverter
PWM inverter
• Single pulse modulation
• Multiple pulse modulation
• Sinusoidal pulse modulation
19. Explain the operation of 3 bridge inverter for 1800 degree mode of operation with aid
of relevant phase and line voltage waveforms.
Ans:
• Inverter circuit
• operation- equivalent circuits
• Waveforms of phase and line voltage
20. Explain the operation of 3 bridge inverter for 1200 degree mode of operation with aid
of relevant phase and line voltage waveforms.
Ans:
• Inverter circuit
• operation- equivalent circuits
• Waveforms of phase and line voltage
21. Draw the circuit diagram of 1 auto sequential commutated current source inverter
and explain its operation with equivalent circuits for different modes and necessary
waveforms.
Ans:
• Inverter circuit
• Modes of operation- equivalent circuits
• Waveforms of ic, vc , io
• Expression for tc, vc, vL
22.Draw the circuit diagram of 1 capacitor commutated current source inverter
and explain its operation with equivalent circuits for different modes and necessary
waveforms.
Ans:
• Inverter circuit
• Modes of operation- equivalent circuits
• Waveforms of ic, vo , io, iT1, iT2, vT1, vT2
• Expression for vc, vL , vo , io, tc
23.Explain the operation of multistage control of AC voltage controllers with neat
diagram.
Ans:
• Circuit diagram
• Operation
24.Explain the operation of 1 AC voltage controller with RL load.
Ans:
• Circuit diagram
• Operation
• Waveforms
•
25.Explain the operation of sequence control of AC voltage controller..
Ans:
• Circuit diagram
• Operation
• Waveforms
26. Explain the operation of 1 sinusoidal AC voltage controller..
Ans:
• Circuit diagram
• Operation
• Waveforms
27. For a 1 voltage controller, feeding a resistive load, draw the waveforms of source
voltage, gating signals, output voltage and voltage across the SCR. Describe the
working with reference to waveforms drawn.
Ans:
• Circuit diagram
• Operation
• Waveforms
1. The variation of rotor torque with power output of 3-Ø squirrel cage induction motor has the
shape:
2.
In case the field of a synchronous motor is under excited, the power factor will be
(A) leading
(B) lagging
(C) zero
(D) unity.
Ans: Lagging
3.In a three-phase induction motor
power factor at starting is high as compared to that while running
power factor at starting is low as compared to that while running
power factor at starting in the same as that while running
power factor at starting is low as compared to that while running
Ans: D
5.The torque of a rotor in an induction motor under running condition is maximum
at the unit value of slip
at the zero value of slip
at the value of the slip which makes rotor reactance per phase equal to the
resistance per phase
at the value of the slip which makes the rotor reactance half of the rotor
Ans: C
6.5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest ?
730 r.p.m.
960 r.p.m.
1440 r.p.m.
2880 r.p.m.
12.In a three-phase induction motor
iron losses in stator will be negligible as compared to that in rotor
iron losses in motor will be neg�ligible as compared to that in rotor
iron losses in stator will be less than that in rotor
iron losses in stator will be more than that in rotor
4.The number of slip rings on a squirrel cage induction motor is usually
two
three
four
none
5.In case the air gap in an induction motor is increased
the magnetising current of the rotor will decrease
the power factor will decrease
speed of motor will increase
the windage losses will increase
Ans b
7.In case of the induction motors the torque is
inversely proportional to (Vslip)
directly proportional to (slip)2
inversely proportional to slip
directly proportional to slip
18.The good power factor of an induction motor can be achieved if the average flux
density in the air gap is
absent
small
large
infinity
3.An induction motor is
self-starting with zero torque
self-starting with high torque
self-starting with low torque
non-self starting
6.In a three-phase induction motor, the number of poles in the rotor winding is always
zero
more than the number of poles in stator
less than number of poles in stator
equal to number of poles in stator
8.The maximum torque in an induction motor depends on
frequency
rotor inductive reactance
square of supply voltage
all of the above
11.The power factor of an induction motor under no-load conditions will be closer to
0.2 lagging
0.2 leading
0.5 leading
unity
21.Slip of an induction motor is negative when
magnetic field and rotor rotate in opposite direction
rotor speed is less than the syn-chronous speed of the field and are in the same
direction
rotor speed is more than the syn-chronous speed of the field and are in the same
direction
none of the above
22.A change of 4% of supply voltage to an induction motor will produce a change of appromimately
4% in the rotor torque
8% in the rotor torque
12% in the rotor torque
16% in the rotor torque
5.The low power factor of induction motor is due to
rotor leakage reactance
stator reactance
the reactive lagging magnetizing current necessary to generate the magnetic flux
all of the above
11.In an induction motor if air-gap is increased
the power factor will be low
windage losses will be more
copper loss will reduce In an induction motor
18.In induction motor, percentage slip depends on
supply frequency
supply voltage
copper losses in motor
none of the above
Ans . C
Ans C
bearing friction will reduce
Ans. A
Ans D
3.
The power factor of the 3 phase induction motor will be maximum when it operates at:
a) Full load
b) No-load
c) Maximum slip
d) Maximum torque
13. A 440 V, 20 HP 3-ph motor operates at full load, 88% efficiency and 0.65 power factor lagging:
A. Find the current drawn by the motor
B. Find the real and reactive power absorbed by the motor
A. Pin (Input power) = 20 x 746 / 0.88 = 16955 W
IL (Input current) = 16955 / (√3 x 440 x 0.65) = 34.2 A
B. PF = 0.65 ∴ = cos-1(0.65) = 49.5o (also sin 49.5 °C = 0.76)
P (kW) = √3 x VL x IL x cos 49.5o = √3 x 440 x 34.2 x 0.65 = 16.95 kW
Q (kVAr) = √3 x VL x IL x sin 49.5o = √3 x 440 x 34.2 x 0.76 = 19.8 kVAr
S (kVA) = √3 x VL x IL = √3 x 440 x 34.2 = 26.1 kVA
{Note also: S2 = √(P2+Q2)}
[10] A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source
drives a mechanical load. The torque-speed characteristics of the motor (solid curve) and of
the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the
following options correctly describes the stability of A and B? [GATE 2009]
(a) A is stable B is unstable
(b) A is unstable B is stable
(c) Both are stable
(d) Both are unstable
Ans: B
1.
The torque developed in an induction motor is nearly proportional to
o
A) 1 / V
o
B) V
o
C) V2
o
D) none of these.
Ans ©
1.
The torque developed by a 3-phase induction motor is approximately proportional to
o
A) square root of s
o
B) s2
o
C) s
o
D) 1 / s.
Ans. C
Ques2: A 12 pole 3 φ alternator driver at speed of 500 r.p.m. supplies power to an 8 pole 3 φ induction motor. If the slip
of motor is 0.03p.u, calculate the speed.
Solution
Frequency of supply from alternator, f=PN/120
=12*500/120 = 50hz
where P= no of poles on alternatev
N=alternator speed is r.p.m.
Synchronous speed of 3 φ induction motor
N=120f/Pm
=120*50/8 = 750 r.p.m.
Speed of 3 φ induction motor N=Ns (1-s)
=750(1-0.03) = 727.5 r.p.m.
Ques8: A 3 φ, 400V wound rotor has delta connected stator winding and star connected rotor winding. The stator has 48
turns/phase while rotor has 24 turns per phase. Find the stand still or open circuited voltage across the slip rings
Solution
Stator e.m.f/phase E1 = 400V
Statur turns/phase N1 = 48
Rotor turns/phase N2 = 24
K= N2/N1 = 24/48 = 1/2
Rotor e.m.f/phase = KE1 = 1/2 * 400 = 200V
Voltage between slip rings = Rotor line voltage = √ 3 x 200 = 346 volt
1.
If 90 per cent of normal voltage and 90 per cent of normal frequency are applied to a transformer, the per cent charge in
hysteresis losses will be
(a) 20% (b) 4.7% (c) 19% (d) 21%
(B)
2.
If 110 per cent of normal voltage and 110 per cent of normal frequency is applied to a transformer, the percentage
change of eddy current losses will be
(a) 10% (b) 20% (c) 25% (d) 21%
(D)
1.
The no-load current in a transformer lags the applied voltage by
(a) 90° – 95° (b) About 80° – 85° (c) 0° – 15° (d) About 110°
Ans B.
Transformer is laminated to reduce
(a) Hystersis loss
(b) Eddy current loss
(c) Partly (a) and partly (b)
(d) Increases exciting current
Ans. B
The value of flux in the emf equation of a transformer is
(a) rms
(b) Average
(c) Maximum
(d) Integral wave cycle
Ans. C
1.
The leakage flux in a transformer depends upon the value of
(a) Load current (b) Supply frequency
(c) Mutual flux
(d) None of these
Ans. A
1.
The voltage regulation of a transformer at full-load 0.8 power factor lag is 6 per cent. Its voltage regulation at full-load
0.8 power factor lead will be
(a) Negative (b) 54% (c) Positive (d) Zero
(a)
(b)
(a) 99%
The full-load efficiency of a transformer at 0.85 p.f. lag is 97 per cent. Its efficiency at full load 0.85 bpower factor lead
will be
(b) 96%
(c) 97%
(d) 98%
Ans ©
1.
A 220V, 150V bulb is connected in series with the primary of a 220/1,100 V, 50 Hz transformer. If the load on the
secondary side is disconnected, the brightness of the bulb will
(a) Decrease
(b) Increase
(c) Be unaffected (d) Decrease to a very low value
Ans d
1.
The open circuit test of a transformer gives information about
(a) Core losses of the transformer (b) Cu losses of the transformer
(c) Exciting current
(d) None of these
Ans a and c
2.
The short circuit test of a transformer gives the information of
(a) Cu losses of the transformer (b) Core losses of the transformer
(c) Winding circuit impedance
(d) None of these
Ans a and c
A transformer is connected to a constant voltage source. If the supply frequency decreases, the magnetic flux in the core will
(a) Increase towards saturation
(b) Decrease
(c) Remain unchanged
(d) None of these
Ans (a)
In operating a 400 Hz transformer at 50 Hz
(a) Only voltage is reduced in the same proportion as the frequency
(b) Only kVA is reduced in the same proportion as the frequency
(c) Both voltage and kVA rating are reduced in the same proportion as the frequency
(d) None of the above
Ans c
1.
While performing short circuit test on a transformer, the impressed voltage magnitude is kept constant but the frequency
is increased. The short circuit current will
(a) Increase1
(b) Decrease
(c) Remain the same (d) None of the above
(b)
1.
A transformer has a hysteresis loss of 30 W at 240 V, 60 Hz. The hysteresis loss at 200 V, 50 Hz will be
(a) 20.8 W (b) 25 W (c) 30 W (d) 36 W
Ans d
In a transformer, the exciting current will be in phase quadrature with the impressed voltage provided
(a) Only the leakage impedance drop is ignored
(b) Only the core loss is ignored
(c) Both the leakage and impedance drop and the core loss are ignored
(d) Only no-load copper loss is ignored
Ans b
1.
In a transformer operating at constant voltage if the input frequency increases the core loss will
(a) Increase
(b) Decrease
(c) Remain constant (d) Increase as the square of the frequency
Ans a
Choose the correct statement:
(a) emf per turn in high-voltage winding is more than the emf per turn in low-voltage winding
(b) emf per turn in both the finding are equal
(c) emf per turn in both the windings are not equal
(d) None of these
Ans b
One transformer has leakage impedance of 1 + j 4 Ω and 3 + j 4 Ω for its primary and secondary windings, respectively. The
transformer has
(a) Low voltage primary
(b) High voltage primary
(c) Medium voltage primary
(d) Insufficient data
Ans a.
A single-phase transformer has its maximum efficiency at 60 per cent of full load. At full load copper loss will be
(a) Equal to core loss
(b) Less than core loss
(c) More than core loss
(d) Zero
Ans c
428) One 200V, 100W bulb is connected in series with the primary of a 200V, 10 kVA transformer. If its
secondary is open circuited, then the bulb will have
A.
full
brightness.
B.
poor
brightness.
C.
little
less
than
D. more than full brightness.
Ans b
The turns ratio required to match an 80
source to a 320
load is
A.
80
B.
20
C.
4
D.
2
Ans D.
full
brightness.
1.
2.In a three-phase induction motor
iron losses in stator will be negligible as compared to that in rotor
iron losses in motor will be negligible as compared to that in rotor
iron losses in stator will be less than that in rotor
5.In case the air gap in an induction motor is increased
the magnetising current of the rotor will decrease
the power factor will decrease
speed of motor will increase
B
8.The maximum torque in an induction motor depends on
frequency
rotor inductive reactance
square of supply voltage
all of the above
B and c
21.Slip of an induction motor is negative when
magnetic field and rotor rotate in opposite direction
rotor speed is less than the syn-chronous speed of the field and are in the same
direction
rotor speed is more than the syn-chronous speed of the field and are in the same
direction
none of the above
22.A change of 4% of supply voltage to an induction motor will produce a change of appromimately
4% in the rotor torque
8% in the rotor torque
12% in the rotor torque
16% in the rotor torque
5.The low power factor of induction motor is due to
rotor leakage reactance
stator reactance
the reactive lagging magnetizing current necessary to generate the magnetic flux
all of the above
11.In an induction motor if air-gap is increased
the power factor will be low
windage losses will be more
copper loss will reduce In an induction motor
Ans . C
bearing friction will reduce
Ans D
13. A 440 V, 20 HP 3-ph motor operates at full load, 88% efficiency and 0.65 power factor lagging:
A. Find the current drawn by the motor
B. Find the real and reactive power absorbed by the motor
A. Pin (Input power) = 20 x 746 / 0.88 = 16955 W
IL (Input current) = 16955 / (√3 x 440 x 0.65) = 34.2 A
B. PF = 0.65 ∴ = cos-1(0.65) = 49.5o (also sin 49.5 °C = 0.76)
P (kW) = √3 x VL x IL x cos 49.5o = √3 x 440 x 34.2 x 0.65 = 16.95 kW
Q (kVAr) = √3 x VL x IL x sin 49.5o = √3 x 440 x 34.2 x 0.76 = 19.8 kVAr
S (kVA) = √3 x VL x IL = √3 x 440 x 34.2 = 26.1 kVA
{Note also: S2 = √(P2+Q2)}
[10] A 3-phase squirrel cage induction motor supplied from a balanced 3-phase source
drives a mechanical load. The torque-speed characteristics of the motor (solid curve) and of
the load (dotted curve) are shown. Of the two equilibrium points A and B, which of the
following options correctly describes the stability of A and B? [GATE 2009]
(a) A is stable B is unstable
(b) A is unstable B is stable
(c) Both are stable
(d) Both are unstable
Ans:
1.
The no-load current in a transformer lags the applied voltage by
(a) 90° – 95° (b) About 80° – 85° (c) 0° – 15° (d) About 110°
Transformer is laminated to reduce
(a) Hystersis loss
(b) Eddy current loss
(c) Partly (a) and partly (b)
(d) Increases exciting current
Ans. B
1.
The voltage regulation of a transformer at full-load 0.8 power factor lag is 6 per cent. Its voltage regulation at full-load
0.8 power factor lead will be
(a) Negative (b) 54% (c) Positive (d) Zero
(a)
1.
A 220V, 150W bulb is connected in series with the primary of a 220/1,100 V, 50 Hz transformer. If the load on the
secondary side is disconnected, the brightness of the bulb will
(a) Decrease
(b) Increase
(c) Be unaffected (d) Decrease to a very low value
Ans d
In operating a 400 Hz transformer at 50 Hz
(a) Only voltage is reduced in the same proportion as the frequency
(b) Only kVA is reduced in the same proportion as the frequency
(c) Both voltage and kVA rating are reduced in the same proportion as the frequency
(d) None of the above
Ans c
1.
While performing short circuit test on a transformer, the impressed voltage magnitude is kept constant but the frequency
is increased. The short circuit current will
(a) Increase1
(b) Decrease
(c) Remain the same (d) None of the above
(b)
Choose the correct statement:
(a) emf per turn in high-voltage winding is more than the emf per turn in low-voltage winding
(b) emf per turn in both the windings are equal
(c) emf per turn in both the windings are not equal
(d) None of these
Ans b
One transformer has leakage impedance of 1 + j 4 Ω and 3 + j 4 Ω for its primary and secondary windings, respectively. The
transformer has
(a) Low voltage primary
(b) High voltage primary
(c) Medium voltage primary
(d) Insufficient data
Ans a.