Q.1. Why electric motor isn't self starting?
Ans. electric motor may be a doubly excited machine
i.e., two electrical inputs are provided thereto. It’s stator winding which
consists of a 3 phase winding is given 3 phase supply and rotor is given DC
supply. The three phase stator winding carrying 3 phase currents produces 3
phase rotating magnetic flux. The rotor carrying DC supply also produces a
continuing flux. At a specific instant rotor and stator poles could be of same
polarity (N-N or S-S) causing repulsion on rotor and therefore the very next
second it'll be N-S causing attraction. But thanks to inertia of the rotor,
it's unable to rotate in any direction thanks to attractive or repulsion and
remain in standstill condition. Hence it's not self starting.
Q.2. What are the methods of starting synchronous
motor?
Ans. 1. Pony motor starting (using AC or DC motor)
2. Starting as cage induction motor or
using damper winding
3. Starting as connection induction moor
or electric motor
4. Using DC machine coupled thereto
Q.3. Why a electric motor may be a constant speed
motor?
Ans. When load increase in electric motor the load
angle δ also increases but speed remains constant. The further increase in load
cause further increase in load angle. When load angle reaches 90 degree
electrical then the motor comes out of synchronism. Hence the motor rotates at
synchronous speed otherwise it comes out of synchronism.
Q.4. what's hunting?
Ans. The sudden changes of load on electric motor
found out oscillations within the rotor. Such oscillation of rotor about its
new equilibrium position is named hunting.
Q.5. What are the consequences of hunting?
Ans. 1. Loses synchronism
2. Develops mechanical stress on rotor
head
3. Increase machine losses
4. Increase temperature of machine
Q.6. Mention some specific application of
synchronous motor?
Ans. 1. Constant speed load service
2. Reciprocating compressor drives
3. Power factor correction
4. Voltage regulation of transmission
lines
Q.7. what's a synchronous capacitor?
Ans. An overexcited electric motor is named
synchronous capacitor, because, sort of a capacitor, it takes a number one
current.
Q.8. What might be the explanations if a electric
motor fails to start?
Ans. It's usually thanks to the subsequent causes:
1.
Voltage could also be too low
2.
Some faulty connection in auxiliary apparatus
3. An
excessive amount of starting load
4. Open-circuit
in one phase or short-circuit
5.
Field excitation could also be excessive
Q.9. What are the causes of faulty starting of a
synchronous motor?
Ans. It might be thanks to the subsequent causes:
1. Voltage could also
be too low- a minimum of half voltage is required for starting
2. There could also be
open- circuit in one phase- thanks to which motor may heat up
3. Static friction
could also be large- either thanks to high belt tension or too tight bearings
4. Stator windings
could also be incorrectly connected
5. Field excitation
could also be too strong.
Q.10. A electric motor starts as was common but
fails to develop its full torque. What could it's due to?
Ans. 1. Exciter voltage could also be too low
2. Field spool could also be reversed
3. There could also be either
open-circuit or short-circuit within the field
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