Question { 3002 }

HOW TO SIZE THE TRANSFORMER OR WHAT ARE THE STEPS INVOLVED IN
FINDING THE SIZE OF THE TRANSFORMER FOR GIVEN LOAD SAY 2000KW
?

Answer

cosq = kw/kva
for 2000kw
cos q *kva
suppose
cosq=.85
2000/.85 =2352.9412
so we need 2500kva transformer...

Question { 10778 }

If two transformers same rating(11KV / 440V)connected in the
same primary line(11KV), can we connect(bus coupler)
secondary side directly or need a synchronization????

Answer

no,if we want to connect the secondary side we need synchronization we have to match the v,phase sequence,f.if we use bc it wouldn't couple because efficency of transformer of same rating will differ, it should need synchronization.

Question { 6746 }

Why we use RYB phase sequence?

Answer

suppose RYB phase sequence is correct,we start a motor it will rotate in correct direction,and motor is safe...if phase sequence is not correct voltage and current will flow and direction will be reverse and it will affect the motor and motor will get damaged.so we should check the phase sequence when we will start a motor

Question { BARC, 38664 }

why induction motor having low power factor at starting time
and high at full load.

Answer

The power factor at no load is low because the magnetizing component of input cur rent is a large part of the total input current of the motor. When the load on the motor is increased, the in-phase current supplied to the motor increases, but the magnetizing component of current remains the same. This means that the resultant line current is more nearly in phase with the voltage and the power factor is improved when the motor is loaded, compared with an unloaded motor which has its magnetizing current as a major component of the input current.