6. What is the principle of motor?
Ans:
Whenever a current carrying
conductor is placed in an magnetic field it produce turning or twisting movement
is called as torque.
7. Name different types of motors and its uses.
Ans:
Universal motor - Vacuum cleaners
Hysteresis motor - Phonographic
appliances
Shaded pole motor - Vending
machines
Capacitor split phase motors -
Refrigerators
Cumulative motors - Rolling mills
DC shunt motors - Lathes
Synchronous motors - Power factor
improvement
8. Which motor has high Starting Torque and Staring current ?
Ans:
DC Series motor has high starting
torque. We can not start the Induction motor and Synchronous motors on load,
but can not start the DC series motor without load.
9. What is slip in an induction motor?
Ans:
Slip can be defined as the
difference between the flux speed (Ns) and the rotor speed (N). Speed of the
rotor of an induction motor is always less than its synchronous speed. It is
usually expressed as a percentage of synchronous speed (Ns) and represented by
the symbol ‘S’.
10. Why star delta starter is preferred with induction motor?
Ans:
Star delta starter is preferred
with induction motor due to following reasons:
• Starting current is reduced 3-4
times of the direct current due to which voltage drops and hence it causes less
losses.
• Star delta starter circuit
comes in circuit first during starting of motor, which reduces voltage 3 times,
that is why current also reduces up to 3 times and hence less motor burning is
caused.
• In addition, starting torque is
increased and it prevents the damage of motor winding.
11. Why series motor cannot be started on no-load?
Ans:
Series motor cannot be started
without load because of high starting torque.
Series motor are used in Trains,
Crane etc.
12. Why AC systems are preferred over DC systems?
Ans:
Due to following reasons, AC
systems are preferred over DC systems:
a. It is easy to maintain and
change the voltage of AC electricity for transmission and distribution.
b. Plant cost for AC transmission
(circuit breakers, transformers etc) is much lower than the equivalent DC
transmission.
c. From power stations, AC is
produced so it is better to use AC then DC instead of converting it.
d. When a large fault occurs in a
network, it is easier to interrupt in an AC system, as the sine wave current
will naturally tend to zero at some point making the current easier to
interrupt.
13. What is the difference between delta-delta, delta-star transformer?
Ans:
Delta-delta transformer is used
at generating station or a receiving station for Change of Voltage (i,e)
generally it is used where the Voltage is high & Current is low.
Delta-star is a distribution kind
of transformer where from secondary star neutral is taken as a return path and
this configuration is used for Step down voltage phenomena.
14. What are the various kind of cables used for transmission?
Ans:
Cables, which are used for
transmitting power, can be categorized in three forms:
• Low-tension cables, which can
transmit voltage upto 1000 volts.
• High-tension cables can
transmit voltage upto 23000 volts.
• Super tension cables can
transmit voltage 66 kV to 132 kV.
15. What is vaccum circuit breaker?
Ans:
Breaker is normally used to break a circuit. While breaking the circuit,
the contact terminals will be separated. At the time of separation an air gap
is formed in between the terminals. Due to existing current flow the air in the
gap is ionised and results in the arc. various mediums are used to quench this
arc irrespective CB's. but in VCB the medium is vaccum gas. since the air in
the CB is having vaccum pressure the arc formation is interrupted. VCB's can be
used upto11kv.
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