1. What does hunting of synchronous motor mean?
load applied to the synchronous motor is suddenly increased or decreased, the
rotor oscillates about its synchronous position with respect to the stator
field. This action is called hunting
2. What could be the reasons if a 3-phase
synchronous motor fails to start?
usually due to the following reasons
Voltage may be too low. ,
much starting load.
excitation may be excessive
3. What is synchronous condenser?
over-excited synchronous motor under no load ,used for the improvement of power
factor is called as synchronous condenser because, like a capacitor it takes a
4. Write the applications of synchronous motor.
for power factor improvement in sub-stations and in industries. b. Used in
industries for power applications.
for constant speed drives such as motor-generator set, pumps and compressors.
5. What is an inverted 'V' curve?
constant load, if the power factor is plotted against various values of field
exciting current, the curve formed is inverted V Shape and called as inverted
'V' curve. Also draw draws the graph.
6. A synchronous motor starts as usual but fails to
develop its full torque. What could it be due to?
voltage may be too low.
spool may be reversed.
c. There may
be either open-circuit or short-circuit in the field.
7. What are the two types of 3-phase induction
Squirrel cage induction motor. b. Slip ring induction motor.
8. Write the two extra features of slip ring
is having 3-phase winding.
resistance can be added in the rotor circuit by connecting through the help of
three slip rings for improving the power factor, increasing Starting Torque,
limiting the starting current.
9. Can we add extra resistance in series with
squirrel cage rotor? State the reason?
add extra resistance in series with the rotor because all the copper bars of
the rotor are short circuited in both the sides by copper end rings to have a
10. Why an induction motor is called rotating
receives electrical power in exactly the same way as the secondary of a two
winding transformer receiving its power from primary. That is why an induction
motor can be called as a rotating transformer
which primary winding is stationary but the secondary is free to rotate.
11. Why an induction motor will never run at its
runs at synchronous speed then there would be no related speed between the two,
hence no rotor emf, no rotor current so no rotor torques to maintain rotation.
That is why the rotor runs at its synchronous speed.
12. Define SCR?
circuit ratio (SCR) is defined as the ratio of field current required to
produce rated voltage on open-circuit to field current required to produce
rated armature current with the terminals shorted, while the machine runs at
13. When does a synchronous motor get over
field excitation of the motor is increased, the field flux will become strong
and Eb will increase. As a result Eb will exceed V and the motor will be called
an over excited motor.
14. Define pullout torque?
pullout torque is the torque, beyond which the synchronous link between field
poles and resultant flux wave is severed and the machine falls out-of-slip.
15.State the characteristic features of synchronous
the motor is not inherently self starting
The speed of operation is always in synchronous
with the supply frequency irrespective of load conditions
is capable of operating at any power factor.
16.In what way synchronous motor is different from
work on the same principle. Synchronous motor operates due to magnetic locking
taking place between stator and rotor magnetic fields.
17.Name any two methods of starting a synchronous
By an extra
3 phase cage induction motor
damper winding in pole phases
the pilot excitor as a dc motor
18.What is the effect on speed if the load is increased
on a 3 phase synchronous motor?
of operation remains constant from no load to maximum load in the motor
operating at constant frequency bus bars.
19.Why a synchronous motor is a constant speed
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
20.What is the phasor relation between induced emf and
terminal voltage of a 3 phase Synchronous motor?
he main field then causes an emf eto get induced in the 3phase winding. Hence
when the machine operates as asynchronous motor the emf phasor always lags the
terminal voltage phasor by the load/torque
21.What are V and inverted V curves of synchronous motor?
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 in verted
22.What happens when the field current of asynchronous
motor is increased beyond the normal value at constant input?
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.
23.Distinguish between synchronous phase modifier
and synchronous condenser.
motor used to change the power factor or power factor in the supply lines is called
synchronous phase modifier. Asynchronous motor operated at no load with over excitation
condition to draw large leading reactive current and power is called asynchronous
24.Howthe synchronous motor can be used as s
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 be come unity. The motor does the
job of capacitors and hence called as