# Rating of transformer and alternators

Discussion in 'General Electronics Chat' started by MKY, Mar 20, 2015.

1. ### MKY Thread Starter New Member

Feb 18, 2015
12
0
Hello,
I want to know the reason as why transformers and alternators rating is in KVA (kilo volt ampere)or MVA instead of KW(kilo watt) or MW?
(one reason i know is that the output losses do not depend on power factor of the load for a transformer...but i also need the justification for this reason too....)..can you please help me out???

2. ### GopherT AAC Fanatic!

Nov 23, 2012
7,112
5,772
Yes, phase is often off-set (power factor issue) and, therefore, volts x amps = watts at any given instant do not accurately represent the power output by the transformer. Therefore, volt*amps is more common.

3. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,576
1,580
In Electrical Engineering, the term "watt" has a very specific meaning. It is the power an electrical signal would dissipate in a pure resistance. In DC circuits it's pretty simple, and equals the product of the voltage times current. But in AC circuits the phase angle between the voltage and current can be shifted to something other than 0 degrees. The shape of the signal doesn't matter; it can be a sine wave, triangle wave, audio, video, whatever. What matters is the phase angle between the voltage and current. If there is some non-zero phase shift, the product of the measured current and measured voltage will not be the same as the heat produced in a resistor. The phase shift comes from the electro-physics of inductance and capacitance. Very geeky math.

ak

4. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
the kva rating gives the output of the alternator or transformer into a resistive load. there is no way for the people rating the transformer or alternator to know how reactive your load is, so they rate it with resistive load (kva or kvar).