Question { L&T, 133529 }

Why a starter is used for a motor?

Answer

at the moment of start there is no rotation of the motor. So there is no back emf present. So the full voltage is applied across the winding so the current is higher( at the moment of start). In this case the starting current is more than 6 times the rated current. So the motor consumes more apparent power( for most of the industrial motors this is measured normally in kVA and metering is carried out for this seperately called maximum demand measuring. This is costed in Rs/kVA and rate is pretty much higher (In sri Lanka Rs 1000/kVA) So due to not using a startor for a motor can cause for much higher electricity bill.

Question { 9033 }

Can anybody explain Why starters are not used for motor
starting in industries?

Answer

In industry for 3ph IMs starters are used..such as start
delta starter..

Question { 3410 }

(1)Electrical power is generated in KV, power consunmed is
in MW and the transformers used are in KVA. How is KVA
converted to MW as power being consumed by the consumers?
(2)What is vector group of a transformer?

Answer

power is generated in MW range at ~11,12 or 15kV terminal
valtage and current in kA range in a generator.
so it gives power output in MWs range.
for the transmission in order to reduce the losses current
is reduced. this is done by step up transformers. it step up
the voltage to 132kV or 220kV or higher so the cuurent is
reduced by that factor.
these transformers have MVA range ratings.
then after transmission(long distance) this high voltage is
again step down to 33 or 11kV and again transmits to urban
areas.
Then again this 33 or 11kV is again step down to 400V (line
to line)For this only trnsformers in kVA are used.

vector group denotes the windin formation and phase shift
between primary and secondary

Question { 3645 }

what is the formula for calculating motor rating? Example:
If we want to rotate a 1000 kg waight disc up to 100 RPM,
which size motor will require?

Answer

we can have the power by taking Tw
so if we know the real power and the power factor we can
have the idea of VA value of the motor

Question { BPDB, 39830 }

What types of motors are used in cranes

Answer

Biki: AC slip ring motor?? I think you mean wound rotor
induction motor as it is the only AC motor which has slip rings?
Biswa: Normally squirrel cage rotors are made of two copper
rings connected by copper bars. So how to make it high
resistive?

Question { 3405 }

In a generator (Synchronous) does the rotor speed
(frequency) corresponds to the speed of the water (in
hydro). does the torque on the turbine relate to the flow of
water?

Answer

when we increase the flow of water (m3/s) by the governor
(controlling the AREA of the water inlets) we change the
torque on the turbine not the speed,right? but can we change
the output frequency changing the SPEED of the water flow ?

Question { MSETCL Maharashtra State Electricity Transmission Co Ltd, 6716 }

why it is unable to birds to be on a 220kV 0r 132 kv lines
like on 33kV and 11kV lines?

Answer

yes thats why birds can be on 11kV or 33kV lines. But they
cannot lie down on a 132kV line or even get close to them..
Is there any reason for this? I have heard the induction
currents cause this ..is it???

Question { 4386 }

what is phase shifting transformer.

Answer

It is simply a 3 phase wound rotor induction motor which is operated at locked rotor condition. Its rotor and stator winding terminal has come out where we can make external connections.
We can connect a 3 phase power supply side to the stator and observe the waveform at other side (rotor) (or vice versa).
There if we change the rotor position manually by rotating we can observe that the  phase angle difference between stator voltage and rotor voltage is increasing or decreasing (depending on the direction of rotation).
In the technical term this is called as "synchro differential transmitter"

Question { 4047 }

if the synchronizing b/w
generator and busbar not
complete and we close the
out going circuit breaker
of generator then what
happen ?

Answer

IF the synchronization does not complete and if you close a cct breaker a severe arcing may occur and generator will trip instantaneously. Because the Generator sides voltages and bus side voltages do not match. To match the volatges following has to satisfied.
1. voltage magnitude
2. phase angle difference - 0
3. phase sequence - same
So at this condition there might be present voltage or phase angles or phase position mismatches. Say for example there exists a voltage magntude mismatch of 1 kV and when you close the cct breaker contact points have that voltage across them and this will cause severe current through contact points of breaker