Q.1. What is, in brief, the idea of operation of a
3-phase induction motor?
Ans. High revolving magnetic flux which is produced
when a 3-phase stator winding is fed from a 3-phase supply.
Q.2. What factor determine the direction of rotation
of the motor?
Ans. The phase sequence of the availability lines
and therefore the order during which these lines are connected to the stator
winding.
Q.3. Enumerate the possible reasons if a 3-phase
motor fails to start?
Ans. Anybody of the subsequent reasons might be
responsible:
1. One or more fuses
could also be blown.
2. Voltage could also
be too low.
3. The starting load could also be too heavy.
4. Worn bearing thanks
to which the armature could also be touching field laminate, thus introducing
excessive friction.
Q.4. A motor stops after starting it fails to hold
load. What might be the causes?
Ans. Anybody of the following:
1. Hot bearings, which
increase the load by excessive friction.
2. Excessive tension on
belt, which causes the bearings to heat.
3. Failure of short
cut-out switch.
4. Single phasing on
the running position of the starter.
Q.5. what's meant by ‘single-phasing’ and what are
its causes?
Ans. By single-phasing is supposed the opening of 1
wire (or leg) of a three-phase circuit where upon the remaining leg directly
becomes single-phase. When a three-phase circuit functions normally, there are
three distinct currents flowing within the circuit. As is know, any two of
those currents use the third wire because the return path i.e. one among the
three phases acts as a return path for the opposite two. Obviously, an circuit
in one leg kills two of the phases and there'll be just one current or phase
working, albeit two wires are left intact. The remaining phase attempts to hold
all the load. the standard explanation for single-phasing is, what's generally
mentioned as running fuse, which may be a fuse whose current-carrying capacity
is adequate to the full-load current of the motor connected within the circuit.
This fuse will blow-out whenever there's overload on the motor.
Q.6. What happens if single-phasing occurs when the
motor is running? And when it's stationary?
Ans. (i) If already running and carrying half load
or less, the motor will continue running as a single-phase motor on the
remaining single-phase supply, without damage because half loads don't blow
normal fuses.
(ii) If motor
is extremely heavily loaded, then it'll stop under single-phasing and since it
can neither restart nor blow out the remaining fuses, the burn-out is extremely
prompt.
A stationary
motor won't start with one line broken. In fact, thanks to heavy standstill
current, it's likely to burn-out quickly unless immediately disconnected.
Q.7. Where may be a 3-phase induction motor used?
Ans. 3-phase AC induction motors are widely utilized
in industrial and commercial applications. These are of two type, cage and
connection motors. Cage motors are widely used thanks to their rugged
construction and straightforward design. Connection motors require external
resistors to possess high starting torque.
Q.8. How can the motors be protected against
single-phasing?
Ans. (i) By incorporating a combined overload and
single-phasing relay within the control gear.
(ii) By
incorporating a phase-failure relay within the control gear. The relay could
also be either voltage or current-operated.
Q.9. What are the inductions of winding faults in an
induction motor?
Ans. A number of the inductions are as under:
- excessive and unbalanced starting currents
- some peculiar noises
- overheating
Q.10. what's the quality direction of rotation of an
induction motor?
Ans. Counterclockwise, when looking from the front
i.e. non-driving end of the motor.
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