Transformer

It can be, in one hour a 1000KVA or 1MVA transformer will give 1000KWHr at full load!

Unlike motor which is defined based on active power requirement transformer is equipment which ment to transfer the power to loads I.e. Supply of both active and reactive power (losses). Hence, transformer, generators are rated in VA apparent power. You might be well aware of power triangle. Ref-BL Theraja-a text book of Electrical technology. Chapte-Transformers.

Then what about Power Plant Generators, Mr. Salunkhe? Why are they rated in MW?

If you discount the losses, the transformer without load would have a rating of zero.

What would that tell you?

Is our transformer (equipments) made to run on no-load for it's life time? No. It's just a case of temp. Operation and transformer is always on-load. To clarify more, we do not select motor rating to run on no-load, but consider on -load rating with duty s1.

You are misinterpreting, I fear! kVA is square root of the sum of kW square plus kVAR square. Even if you discount losses on a transformer without any load connected, it would still draw a very high reactive power (PF almost 0.5 lagging or lesser). Thus, it would still draw kVA and it is very correct to rate it in kVA.

I believe that what you intend to ask is:

" Why transformers are rated in kVA or MVA, not in kW?" (kWH is kilowatt-hour...is a measure of power consumed in a certain length of time)

Power is computed as voltage(V) x current(I). 'Watt' will only be applicable to purely resistive loads. Considering almost all types of loads will have reactive component, thus, transformer power capacity will be rated in VA.

1. This aspect has been covered, in great details in earlier CR4. I could give the link but then at least some effort must be put to search answer. There is atleast one thread on this and most likely several.

2. It looks like a class room doubt. Why don't you check back the text book?

3. Just think about what the transformer does. If you write the transfer equation of it you would find out the answer (BTW the same is true for any machine, motors, generators...).

It will show you what is transferred from input to output and what are the parameters affected (and what it does not affect at least in theoretical machine/equipment (in practical it may matter just a tiny bit)

kVA is the unit for apparent power. Apparent power consists of active and reactive power. Active power is the share of the apparent power which transmits energy from the source (generator) to the user. Reactive power is the share of the apparent power which represents a useless oscillation of energy from the source to the user and back again. It occurs when on account of some »inertia« in the system there is a phase shift between voltage and current. This means that the current does not change polarity synchronous with the voltage. But the heat generated in a winding as well as the eddy current losses generated in a transformer core depend on the current only, regardless of whether it aligns with the voltage or not. Therefore the heat is always proportional to the square of the current amplitude, irrespective of the phase angle (the shift between voltage and current). So a transformer has to be rated and selected by apparent power.

Vinu_Answers Sure_Answers

"KW" has to be specfied alongwith power factor (PF).But a transformer can not have a fixed PF since it will depend upon the PF of various loads it feeds ,which will vary in the course of time depending upon the need as well as the type of loads.

However, rated MVA will remian same irrespective of the type of loads fed.