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EE2302 ELECTRICAL MACHINES – II
UNIT I
SYNCHRONOUS GENERATOR
1. Why are almost all large size synchronous machines constructed with rotating field
system type?
The following are the principal advantages of the rotating field system type
construction of synchronous machines:
1. The relatively small amount of power, about 2%, required for field system via sliprings and brushes.
2. For the same air gap dimensions, which are 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.
3. Insulation to stationary system of conductors is not subjected to mechanical stresses
due to centrifugal action.
4. Stationary system of conductors can easily be braced to prevent deformation.
5. It is easy to provide cooling arrangement for a stationary system of conductors.
6. Firm stationary connection between external circuit and system of conductors enable
the machine to handle large amount of volt-ampere as high as 500MVA.
2. Write 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
1. Stationary field system type
2. Rotating field system type
3. According to shape of field system
4. Salient pole type
5. Smooth cylindrical type
4. Name the types of alternators based on their rotor construction.
Alternators can be classified into the following two types according to its rotor construction.
They are
mooth cylindrical type alternator
1.
2. 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
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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 is used in Hydro-electric plants?
As the speed of operation is low for hydro turbines used in 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 i n s ynchronous
machines?
The advantages of salient-pole type construction are as follows:
1. They allow better ventilation
2.
pole faces are so shaped that the radial air gap length increases from the pole
centre 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
to variable reluctance the machine develops additional reluctance power
3.
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 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 - angle between adjacent slots in
electrical degree and x= 1,2,3…
11. Why is short-pitch winding preferred over full-pitch winding?
Short-pitch winding is preferred over full-pitch winding because,
1. Waveform of the emf can be approximately made to a sine wave and distorting
harmonics can be reduced or totally eliminated.
2. Conductor material copper, is saved in the back and front end connections due to
less coil-span.
3. Fractional slot winding with fractional number of slots/phase can be used which in
turn reduces the tooth ripples.
4. Mechanical strength of the coil is increased
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12. 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.
13. 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 I2R, the square of the current
delivered by the generator. As the current is directly related to the 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?
Variations in terminal voltage in Alternators on load condition are due to the
following causes:
1. Voltage variation due to the resistance of the winding, R
2.
tage variation due to the leakage reactance of the winding, Xt
15. What is meant by armature reaction in Alternators?
The interaction between flux set up by the current carrying armature and the main is
defined as the armature reaction.
16. 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 ).
17. 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.
UNIT II SYNCHRONOUS MOTOR
1. Write 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:
1. Synchronous impedance / EMF method
2. Ampere-turn / MMF method
3. Potier / ZPF method
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2. How is synchronous impedance 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
3. What are the advantages and disadvantages of estimating the voltage regulation of an
Alternator by EMF method?
Advantages:
1. Simple no load tests (for obtaining OCC and SCC) are to b
2. Calculation procedure is much simpler
Disadvantage:
The value of voltage regulation obtained by this method is always higher than
the actual value
4. Why i s t h e s yn ch ron ou s i mp ed an c e meth od o f es ti m ati n g v ol tag e
r egu l at i on considered as a 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.
5. 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.
6. What are the test data required for predetermining the voltage regulation of an
alternator by MMF method?
Data required for MMF method are:
1. Effective resistance per phase of the 3-phase winding R
2. Open circuit characteristic (OCC) at rated speed/frequency
3. Short circuit characteristic (SCC) at rated speed/frequency
7. Why is the MMF method of estimating the voltage regulation considered an
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 requires more effort, the final result is very close to the
actual value. Hence this method is called optimistic method.
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8. Write the conditions 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.
1. The terminal voltage magnitude of the incoming alternator must be made equal to the
existing alternator or the bus-bar voltage magnitude.
2. The phase sequence of the incoming alternator voltage must be similar to the bus- bar
voltage.
3. The frequency of the incoming alternator voltage must be the same as the bus-bar
voltage.
9. 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.
10. What are the advantages and disadvantages of three dark lamps method of
synchronizing?
Advantages:
1. The synchronous switch using lamps is inexpensive
2. 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.
Disadvantage:
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.
11. How is synchronoscope 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.
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12. Why a r e synchronous generators 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.
13. List out 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
1. Prime-mover characteristic/input
2. Excitation level
3. Percentage synchronous impedance and its R/X ratio
14. 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 subject to slight increase in value.
15. How does change in excitation affect 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.
16. 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:
1. 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
2. The excitation of the outgoing generator has to be decreased and that of other
generators has to be increased and by this the entire reactive-power delivered by
the outgoing generator is transferred to other generators
3. After ensuring that the current delivered by the outgoing generator is zero, it has to
be disconnected from parallel operation.
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17. 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.
18. 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.
19. An a lternator 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.
UNIT III – THREE PHASE INDUCTION MOTOR
1. State the principle of 3 phase induction motor.
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.
2. Why is an induction motor called a rotating transformer?
The rotor receives electric power in exactly the same way as the secondary of
a two-winding transformer receiving its power from the primary. Therefore, an induction
motor can be called as a rotating transformer i.e. one in which primary winding is stationary
but the secondary is tree to rotate.
3. List out the advantages of skewing.
The advantages of skewing are,
1. It reduces humming effect.
2. It reduces magnetic locking of the stator and rotor.
4. What are the effects of increasing rotor resistance on starting current and starting
torque?
The effects of increasing rotor resistance on starting current and starting torque are:
1. The additional external resistance reduces the rotor current and hence the current
drawn from the supply.
2. It improves the starting torque developed by improving the power factor in high
proportion to the decrease in rotor current.
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5. What is slip of an induction motor?
The slip speed is defined as the percentage difference of relative speed to
synchronous speed. It is expressed as,
S = (Ns - N)/Ns*100
6. What are the advantages of slip-ring induction motor?
The advantages of slip-ring induction motor are:
1. Rotor circuit is accessible for external connection.
2. By adding external resistance to the rotor circuit the starting current is reduced with
the added advantage of improving starting torque.
3. Additional speed control methods can be employed with the accessibility in the rotor
circuit.
7. What are the various losses that occur in an induction motor?
The various losses that occur in an induction motor are:
1. Magnetic losses Wi,
2. Electrical losses Wcu
3. Mechanical losses Wm
8. What are the tests to be conducted for predetermining the performance of a 3 phase
induction Motor?
The tests to be conducted for predetermining the performance of a 3 phase induction motor
are:
1. No load test
2. Blocked rotor test
9. What is meant by circle diagram of an induction motor?
This circle diagram is used to predict the performance of the machine at different
loading conditions as well as mode of operation. 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.
10. What are the advantages of direct load test for a 3 phase induction motor?
The advantages of direct load test for a 3 phase induction motor are,
1. Direct measurement of input and output parameters yield accurate results
2. Aside from the usual performance other performances like mechanical vibration,
noise etc can be studied.
3. By operating the motor at full load for a continuous period, the final steady
temperature can be measured.
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11. What is cogging of an induction motor?
When the number of teeth in stator and rotor are equal, the stator and rotor teeth have
a tendency to align themselves exactly to minimum reluctance position. In such case the
rotor may refuse to accelerate. This phenomenon is called magnetic locking or cogging.
UNIT IV – STARTING AND SPEED CONTROL OF
THREE PHASE INDUCTION MOTOR
1. Write the different types of starters used for induction motor.
The different types of starters used for induction motor are,
1. Primary resistor
2. Autotransformer starter
3. Star-delta starter
4. Rotor rheostat
2. What are the advantages of a primary resistance starter of induction motor?
The advantages of a primary resistance starter of induction motor are,
1. Starting torque to full load torque is x2 of that obtained with direct switching or
across the line starting.
2. This method is useful for smooth starting of small machines only.
3. What are the advantages of an autotransformer starter?
The advantages of an autotransformer starter are,
1. Reduced voltage is applied across the motor terminal.
2. There is a provision for no-voltage and over-load protection.
4. Write the relationship between starting current and full load current of an
autotransformer starter.
The relationship between starting current and full load current of an autotransformer
starter is given by, I2 = K. Isc
where,
K = Transformation ratio
I2 = Full load current
Isc = Starting current
5. Write the relationship between starting torque of an induction motor with an
autotransformer starter and star delta starter
Star delta starter is equivalent to an autotransformer starter in the ratio of 58% in
approximation.
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6. How is the starting current reduced using rotor resistance starter?
The controlling resistance is in the form of a rheostat, connected in star. The
resistance being gradually cut-out of the rotor circuit as the motor gathers speed. Increasing
the rotor resistance, not only in the rotor current reduced at starting, but at the same time
starting torque is also increased due to improvement in power factor.
7. Write the methods of speed control on stator side of an induction motor.
The methods of speed control on stator side of an induction motor are,
1. By changing the applied voltage
2. By changing the applied frequency
3. By changing the number of stator poles.
8. Write the methods of speed control from rotor side of an induction motor.
The methods of speed control from rotor side of an induction motor are,
1. Rotor rheostat control
2. By operating two motors in concatenation or cascade
3. By injecting an emf in the rotor circuit
9. What are the applications of speed control of an induction motor by pole changing
method?
The applications of speed control of an induction motor by pole changing method are,
1. Elevator motors
2. Traction motors
3. Small motors driving machine tools
10. How is the speed control achieved by changing the number of poles?
Synchronous speed of induction motor can also be changed by changing the number
of stator poles. This change of number of poles is achieved by having two or more entirely
independent stator windings in the same slots.
11. What are the limitations of rotor rheostat speed control of an induction motor
The limitations of rotor rheostat speed control of an induction motor are,
1. With increase in rotor resistance, I2R losses also increase which decrease the
operating efficiency of the motor. In fact, the loss is directly proportional to the
reduction in the speed.
2. Double dependence of speed, not only on R2 but also on load as well.
12. Write the three possible methods of speed control of cascaded connection of an
induction motor.
The three possible methods of speed control of cascaded connection of an induction motor
are,
1. Main motor may be run separately from the supply
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2. Auxiliary motor may be run separately from the mains.
3. The combination may be connected in cumulative cascade.
13. How is the tandem operation of induction motor started?
When the cascaded set is started, the voltage at frequency, f is applied to the stator
winding of main motor. An induced emf of the same frequency is produced in main motor
(rotor) which is supplied to the auxiliary motor. Both the motors develop a forward torque.
As the shaft speed rises, the rotor frequency of main motor falls and so does the synchronous
speed of auxiliary motor. The set settles down to a stable sped when the shaft speed become
equal to the speed of rotating field of Auxiliary motor
14. Write the method of speed control by injecting emf in the rotor circuit.
The speed of an induction motor is controlled by injecting a voltage in the rotor
circuit. It is necessary for the injected voltage to have the same frequency as the slips
frequency
15. What are the advantages and disadvantages of slip power scheme?
The advantages of slip power scheme are,
1. Easier power control
2. Higher efficiency
The disadvantages of slip power scheme are,
1. Reactive power consumption.
2. Low power factor at reduced speed.
16. Write the types of slip power recovery schemes.
The types of slip power recovery schemes are,
1. Scherbius system
2. Kramer drive
UNIT V
SINGLE PHASE INDUCTION MOTORS AND SPECIAL MACHINES
1. Write the two windings of a single-phase induction motor.
The two windings of a single-phase induction motor are,
1. Running winding (main winding)
2. Starting winding (auxiliary winding)
2. What are the various methods available for making a single-phase motor self-starting?
The various methods available for making a single-phase motor self-starting are,
1. By splitting the single phase
2. By providing shading coil in the poles
3. Repulsion start method.
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3. What is the main difference between the principle of operation of a 3 phase and single
phase induction motors?
When three-phase supply is given to 3-phase induction motor, a rotating magnetic
field is produced and the rotor-starts rotating. But when single-phase supply is given to
single-phase motor only a pulsating flux is produced. So motor is not self-starting. Therefore
to make it self-starting split-phase arrangement is made by providing an auxiliary winding.
4. Write the main difference in construction of an AC series motor and a DC series
motor.
The main difference in construction of an AC series motor and a DC series motor are,
1. The entire iron structure of the field cores and yoke are laminated to reduce the eddy
current loss
2. Number of turns in the field winding is reduced to have large reactance and higher
power factor.
3. AC series motors are provided with commutating poles
5. Write the advantages of a capacitor run type motor.
The advantages of a capacitor run type motor are,
1. It has high starting and running torques
2. Current drawn is less because of higher power factor
3. It can be started with some load.
6. How can the rotation of a universal motor be reversed?
1. The direction of rotation of the concentrated-pole (or salient-pole) type universal
motor may be reversed by reversing the flow of current through either the armature
or field windings.
2. The direction of rotation of the distributed field compensating type universal motor
may be reversed by interchanging either the armature or field leads and shifting the
brushes against the direction in which the motor win rotate.
7. Why is a single-phase induction motor not self-starting?
When the motor is fed from a single-phase supply, its stator winding produces an
alternating or pulsating flux, which develops no torque. Therefore, a single-phase motor is
not self-starting.
8. List out the applications of a universal motor.
The applications of a universal motor are,
1. Used for sewing machines
2. Table fans
3. Vacuum cleaners
4. Hair driers
5. Blowers
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9. Write the advantages of capacitor run induction motor.
The advantages of capacitor run induction motor are,
1. Running torque is more.
2. Power factor during running is more, thereby line current is reduced.
10. What is a universal motor?
A universal motor is defined as a motor, which may be operated either on direct
current or single phase A.C supply, at approximately, the same speed and output.
11. What is the use of shading ring in a shaded pole motor?
The shading coil causes the flux in the shaded portion to lag behind the flux in
unshaded portion of pole. This gives in effect a rotation of flux across the pole face and
under the influence of this moving flux a starting torque is developed.
12. Write the advantages of a capacitor start motor.
The advantages of a capacitor start motor are,
1. The starting current of capacitor start motor is less than resistance split phase motor
2. Starting torque of the capacitor motor is twice that of resistance start motor.
13. What are the different types of single phase induction motor?
The different types of single phase induction motor are,
1. Split-phase motor
2. Shaded pole motor
3. Single phase series motor
4. Repulsion motor
5. Reluctance motor
14. Write the two different theories with which principle of 1 phase induction motors are
explained.
The two different theories employed for explaining the principle of single phase induction
motors are
1. Double revolving field theory
2. Cross field theory