When the rotor of the synchronous motor rotates an EMF is induced in its stator winding due to flux produced by the rotating rotor. This induced EMF is alternating and is given by,
Eb/ph = 4.44 KcKdφfTph
This EMF is produced due to the stator (armature conductors) cutting the rotor flux. This induced EMF always opposes the supply voltage according to the Lenz’s law hence called back EMF. In above expression, all other quantities are constant except flux φ.
Therefore, Eb α φ
But the rotor flux φ depends on the rotor excitation. Hence the back EMF Eb is proportional to or dependent on the rotor (field excitation).
Effect of Excitation on Synchronous Motor
If the excitation of a synchronous motor is changed, keeping the load constant, the motor power factor will get automatically adjusted to keep its active component constant under all excitation conditions.
The active component of the armature current drawn by the motor is constant because load, losses and applied voltage to the motor are constant.
Depending on the level of excitation, the synchronous motor is capable of operating in one of the following four conditions:
- Normally Excited
- Under Excited
- Over Excited
- Critically Excited
In this case, induced back EMF Eb is equal to the applied voltage V.
Under Excited Synchronous Motor
If the field excitation is such that back EMF Eb is less than the applied voltage V, then the motor is said to be under-excited. An under excited synchronous motor has a lagging power factor.
Over Excited Synchronous Motor
If DC field excitation of a synchronous motor is such that back EMF Eb is greater than applied voltage V, then the motor is said to be over excited. An over excited synchronous motor draws leading current.
An over excited synchronous motor running at no load is known as the synchronous capacitor or synchronous condenser. Synchronous capacitors are always totally enclosed. The shaft does not extend beyond the case of the motor. They are used for correcting the power factor of a lagging load such as transformers and induction motors in an installation.
Power factor correction means raising the power factor of the load from its low value to higher value. Some industrial loads run at very low power factors. It is advantageous to raise this power factor to unity or near about.
Any increase in power factor, increases supply capacity, efficiency and improves the operating characteristics of the system. Synchronous capacitors cancel the lagging kVARs of the installation with their leading kVARs. They are usually connected in parallel with the incoming power lines to the plant.
Critically excitation is defined as the excitation for which the power factor of the motor is unity. In this condition, with the change in excitation the power factor changes.
V Curves of Synchronous Motor
When the excitation of a three-phase synchronous motor taking a constant power, is varied, it changes the operating power factor of the motor.
For a constant input power and terminal voltage only increase in power factor, causes a decrease in armature current and vice versa.
The armature current will be minimum at unity power factor and increases when the power factor decreases on either side (lagging or leading).
Hence the variation in excitation (field current) causes the variation in armature current.
If we plot a curve between field current on X-axis and armature current on Y-axis, the curve so obtained is called V curves of synchronous motor.
The V curves at different power inputs are shown in Figure. With the increase in load, the V curves get shifted upwards as shown in Figure.
Thanks for reading about over excited synchronous motor and under excited synchronous motor.
Alternators – 5 | Objective Type Question Answers
#1 If the driving force of both the alternators running in parallel is changed, this will result in change in
#2 In an alternator, when the load power factor is unity
the armature flux will be cross-magnetising
#3 The Poiter’s triangle separates the
armature leakage reactance and armature reaction m.m.f.
#4 In an alternator zero power factor method is used to find the
#5 If the driving power from the prime-mover driving an alternator is lost but the alternator remains connected to the supply network and field supply is on, then the alternator will
behave as a synchronous motor and will rotate in the same direction
#6 In turbo-alternators, smooth cylindrical type rotors used have long axial length because
number of armature conductors being less they have to be necessarily long for generating the required voltage
#7 Large diameter salient-pole rotors have short axial length mainly because
number of armature conductors held in the large circumference, rotor being very large, they need not be long
#8 At leading power factor, the armature flux in an alternator
aids the rotor flux
#9 Three-phase alternators are invariably star-connected because
higher terminal voltage is obtained
#10 Which of the following conditions does not have to be met by akernators working in parallel ?
The machines must have equal kVA ratings.
#11 The fictitious part of synchronous reactance takes care of
#12 In an alternator, the voltage of field system is usually
less than 200 V
#13 In an alternator, pitch factor is the ratio of the e.m.fs. of
short pitch coil to full pitch coil
#14 When two alternators are running in exact synchronism, the synchronizing power will be
#15 In an alternator if the armature reaction produces demagnetization of the main field, the power factor should be
zero, lagging load
#16 In an alternator, the armature reaction influences
generated voltage per phase
#17 An alternator operating at lower voltage for the same power rating will be
larger in size
#18 When two alternators are running in parallel their kVAR load share and kW load share are changed by changing their
excitation, driving torque respectively
#19 In an alternator one of the advantages of distributing the winding is to
improve voltage waveform
#20 . ………. plays an important role in over speed protection of a generator ?
#21 Which of the following is the common synchronous speed in R.P.M. between 50 Hz and be 60 Hz alternators ?
#22 Salient pole type rotors as compared to cylindrical pole type are
larger in diameter and smaller in axial length
#23 Which of the following relays come into operation in the event of the failure of prime-mover connected to the generator ?
Reverse power relay
#24 In order to reduce the harmonics in the e.m.f. generated in an alternator
all of the above
#25 The permissible duration for which a generator of rated frequency 50 Hz can nut at 46 Hz is
#26 Due to which of the following reasons, for aircraft alternators high frequency is used ?
To reduce the bulk
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- Parts of Synchronous Generator
- Synchronization of Alternators
- Synchronous Motor Working and Construction
- Methods of Starting of Synchronous Motor
- Damper Winding in Synchronous Motor
- Effect of Excitation on Synchronous Motor
- Advantages of Synchronous Motor
- Compare Synchronous Motor and Induction Motor