1) distinguish between salient and no salient pole rotor
TRANSCRIPT
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AALIM MUHAMMED SALEGH COLLEGE OF ENGINEERING DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
MODEL EXAM
EE6504 ELECTRICAL MACHINES II
PART-A
1) Distinguish between salient and no salient pole rotor construction and mention their
application in Different power plants.
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2) Explain the concept of armature reaction in alternator?
Effect of the armature flux on the main flux affecting its value and the distribution is
called armature reaction.
The effect of the armature flux not only depends on the magnitude of the current flowing
through the armature winding but also depends on the nature of the power factor of the load
Connected to the alternator.
Armature reaction for unity power factor (cross magnetizing)
Armature reaction for lagging power factor(demagnetizing)
Armature reaction for leading power factor (magnetizing)
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3) What is an infinite bus? (Infinite means many)
A system having a constant voltage and constant frequency regardless of the load is called
an Infinite Busbar system. Thus, an infinite bus has a large power system. The amount of
real and reactive power is drawn or supplied, does not affect its voltage and frequency. They
both remain constant.
4) What are need, condition and methods for parallel operation generators?
The conditions to be satisfied for parallel operation of alternators are:
i. The terminal voltage of the incoming alternator must be same as that of the existing
alternator (bus bar voltage).
ii. The speed of the incoming alternator must be such that its frequency must be same as that
of existing alternator (bus bar frequency).
The phase sequence of the incoming alternator should be identical with the phase sequence of
existing alternator (bus bar phase sequence).
The advantages of parallel operation are:
i. It ensures continuity of power supply to consumers in case of breakdown or shut down for
maintenance of an alternator in a generation station.
ii. The load on any system, changes from time to time. Alternators can be shut down during
light load periods any they can be connected during peak load hours.
iii. When demand on power increases, new alternator can be installed to operation in parallel.
iv. Repair and maintenance of individual machines can be carried one after the other without
effecting normal work.
The three methods for synchronizing alternators are:
i. Bright lamp method.
ii. Dark lamp method.
iii. Dark bright method.
iv. Synchroscope method
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5) What is importance of damper winding in synchronous generator and motor?
Synchronous motors. This additional rotor winding is usually referred to as a damper
winding because of its additional property of damping out any oscillation that might be
caused by sudden changes in the load on the rotor when in synchronism.
6) What is synchronous condenser? Mention its application.
A synchronous motor under over excited condition operates at leading power factor. A synchronous
motor at no-load behaves as a condenser by simply varying its excitation. The machine operated under
such a condition (motor on no-load or light load) is known as synchronous condenser.
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7) What is significance of capability curve of synchronous generator?
The capability curve of the synchronous generator defines the bounds within which it can operate
safely. The various bounds imposed are.
i. MVA – loading cannot exceed the generator rating.
ii. MW- loading cannot exceed turbine rating.
iii. The maximum fI cannot exceed a specified value imposed by rotor.
iv. The generator must operate at safe margin away from steady state stability limit 90
8) What is hunting in synchronous machine? How to eliminate it?
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9) Draw the power flow diagram of an induction motor.
10) What is cogging and crawling, skewing?
Cogging
Stator and rotor poles gets locked with each other.
The motor refuses to start.
Crawling
Due to 7 th harmonics, machine run 1/7thof the synchronous speed.
Skewing
The motor slots are given a slight skew so that:
1) It helps to make the motor run quietly by reducing magnetic hum.
2) It helps in reducing locking tendency of the rotor.
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11) Define- Slip. Derive an expression for the frequency of rotor circuit.
The difference between the synchronous speed sN and the actual speed of the motor is
known as slip. It is always expressed as percentage of synchronous speed.
% 100sNNslips
Ns
When the rotor starts revolving, the frequency depends upon the relative speed or slip speed.
Let at any slip speed, the frequency of rotor current be f , then
120
s
fN N
P Also
120s
fN
P
sr
s
s
N NfS
f N
f Sf
12) A 20 pole, 50Hz, 3phase induction motor runs at 285 rpm. Calculate the frequency of
rotor current.
120 12050300
20
300283% 1005%
300
0.0550 25
s
s
s
f
fN rpm
p
N Nslip
N
rotorfrequencyf S Hz
13) What is plugging and regenerative braking?
Braking is an equipment to reduce the speed of any moving or rotating equipment,
Plugging In this method the terminals of supply are reversed, as a result the generator torque also
reverses which resists the normal rotation of the motor and as a result the speed decreases
Regenerative braking takes place whenever the speed of the motor exceeds the synchronous speed
This braking method is called regenerative braking because here the motor works as generator
and supply itself is given power from the load motor will act as a generator and the direction of
current flow through the circuit and direction of the torque reverses and braking takes place.
14) Two induction motor are used for the cascade control. The main motor has 4 poles while the
auxiliary motor has 6 poles. Determine the various synchronous speeds possible for the set.
Assume supply frequency 50Hz.
Solution:
PA = 4, PB = 6 and f = 50 Hz.
1) 12012050
1500.. ....'A' running alone4
S
A
fN rpmP
2) 12012050
1000.. ....'B' running alone6
S
B
fN rpmP
3) 12012050
600.. ....Cumulative cascade10
S
AB
fN rpmPP
4) 12012050
3000.. ....Differential cascade2
S
AB
fN rpmPP
Though PA – PB is negative, in practice the effective poles are 2 only. The negative sign indicates
the direction of torque.
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15) What is the need for AC starters? Give the relation between the starting torque and the full
load current in an induction motor.
Induction motor can be thought as a polyphase transformer with a short circuited rotating
secondary So, if normal supply voltage is supplied to the stationary rotor a very large initial
current would be taken up by the stator for about seven times of the full load current.
So to prevent
i) winding damage of the motor itself
ii) High capacity motor creates voltage fluctuation (voltage sag) in the nearby circuits
and damages the appliance connected with it.
2
st scf
f f
T IS
T I
16) Mention the methods of speed control of 3 phase induction motors.
1. Control from stator side
a. by changing the applied voltage
b. by changing the applied frequency
c. by changing the number of stator pole,
2. Control from rotor side a .rotor rheostat control,
b. by operating two rotor in concatenation or cascade,
c. by injection an emf in the rotor circuit.
17) Draw the equivalent circuit of single phase induction motor without core loss.
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18) What are the applications and characteristics of Capacitor start motors?
Applications:
a) fans
b) Blowers
c) Centrifugal pump
Characteristics:
a) The percentage of rated starting torque is 100% - 200%
b) The breakdown torque is up to 250%
c) The power factor of the motor is in the range of 0.75 to 0.9
d) The efficiency of the motor is 60% to 70%
e) The power rating of the motor is 1/8 to 1 hp
19) What is magnetic levitation? List the applications of magnetic levitation
Magnetic levitation is a method by which an object is suspended with no support other than
magnetic fields. Magnetic force is used to counteract the effects of the gravitational and any other
accelerations.
Applications
Electromagnetic Suspension (EMS)
Electrodynamic Suspension (EDS)
Magnetically Levitated Trains
Launching Rockets
Maglev Fan
Maglev Heart Pump
Analyzing Food and Beverages
20) What is a Stepper motor? Give the application of Stepper motor.
The Stepper motor is a special type of Synchronous motor which is designed to rotate through a
specific angle for each electrical pulse received by its control unit.
The applications of stepper motor are:
d) Paper feed motors in type writers and printers.
e) Positioning of print heads, pins in X-Y plotters.
f) Recording heads in computer disk drives.
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PART B
11a) i) Derive the EMF equation of an alternator also define pitch factor and distribution factor.(8)
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11aii)
bii) Explain about the blondel two reaction theory the determination of direct and quadrature axis Synchronous reactance using slip test.
Determination of Xd and Xq by slip test:
The direct and quadrature axis reactances Xd and Xq can be of a synchronous machine can be
experimentally determined by a simple test known as slip test. Basic circuit diagram for conducting
this test is shown in figure 49. Here the armature terminals are supplied with a subnormal voltage of
rated frequency with field circuit left open. The generator is driven by a prime mover at a slip speed
which is slightly more or less than the synchronous speed. This is equivalent to the condition in which
the armature mmf remains stationary and rotor rotates at a slip speed with respect to the armature
mmf. As the rotor poles slip through the armature mmf the armature mmf will be in line with direct
axis and quadrature axis alternately. When it is in line with the direct axis the armature mmf directly
acts on the magnetic circuit and at this instant the voltage applied divided by armature current gives
the direct axis synchronous reactance. When the armature mmf coincides with the quadrature axis then
the voltage impressed divided by armature current gives the quadrature axis synchronous reactance.
Since Xd > Xq the pointers of the ammeter reading the armature current will oscillate from a minimum
to a maximum. Similarly the terminal voltage will also oscillate between the minimum and maximum.
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12a) Explain the principle of working methods of starting methods of synchronous motor.(8)
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