Why transformer rated in KVA instead in KWH?

Answer 1

VA, or volt-amperes, is a measure of the output of the transformer. VA is also called apparent power. Watts on the other hand, is also called true power. If the transformer feeds a resistive load, then watts are equal to VA, because the voltage and current are in phase. If the transformer feeds a reactive load, such as a motor, then the voltage and current are no longer in phase, and the true power (watts) is less than apparent power.

Since the true power, or watts delivered can change depending on the load, it is not very useful as a transformer rating. The VA remains constant irregardless of the load characteristics, and so is a much better indicator of transformer performance.

Answer 2

The transformer has a maximum rated voltage, determined by the size of the magnetic core and the frequency, and, quite separately, a maximum rated current determined by the size of the copper wire in the coils. Multiply the two together to find the VA or kVA rating. The kilowatts are equal to the kVA times the power factor of the load that is connected to the transformer.

Answer 3

I assume this question is asking why Transformers are rated in VA as opposed to watts. The reason is because the transformer doesn't not care about the phase angle relationship of the voltage applied to it to the current flowing through it. All that matters is the magnitude of the current flow, which dictates the level of heating the transformer will experience.

Alternative Answer

A transformer's iron losses depend on the magnitude of the flux which, in turn, is proportional to voltage, while its copper losses depend on the winding currents. As both iron and copper losses contribute to the maximum operating temperature of the transformer, it follows that a transformer must be rated in terms of voltage and current. In alternating current systems, the product of voltage and current is apparent power, expressed in volt amperes.

As a transformer's secondary voltage is kept approximately constant, it is its 'volt ampere' rating that determines its maximum (secondary) load current.

Expressing a transformer's rating in watts (i.e. true power) would be completely meaningless because, with a highly-reactive load, it will be supplying practically zero watts while still having to supply its rated current.

Answer 4

Because a transformer is rated separately for maximum voltage and maximum current. Multiply the two together to find the rated kVA.

Excessive current causes overheating in the copper wire windings because of the resistance of the wire, while excessive voltage causes overheating in the iron core because of eddy current losses.

Answer 5

Transformers has two types of losses. 1) Copper loss or variable loss 2) Iron loss or core loss or constant loss. Copper loss depend on Current similarly Iron loss depend on Voltage. So the Trfr. ratings would be in volt amps.

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As the answer says, there are separate limits on the voltage and the current a transformer can use, so you just multiply them together to get the volt-amp limit.

Also, this covers cases where a load with a poor power-factor is connected to the transformer.

Answer 6

Transformers generally power AC loads which have some reactance and the load is therefore expressed in VA (Volt Amperes). Heaters and light bulbs are an exception as they are nearly pure resistance and would be expressed in Watts. VA is actually the wattage of your load transformers and are rated according to output voltage and current. For a given unit, the volt-ampre (VA) capacity is often specified. This is the product of the voltage and current. A transformers with a 12V output, capable of delivering 10A, would be rated as 120VA (12V x 10A).

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And now to try and answer the question, i.e., whythe 'VA' specification of a

transformer is more important than its 'watts' rating:

The current and voltage exert separate stresses on the transformer, and impose

separate limits on it:

-- The current in the windings determines the magnetic saturation of the core material.

When the core approaches saturation, the losses and heating in the core rise rapidly.

This is regardless of what the voltage might be.

-- The voltage between the windings determines at what point there could be

arcing between them, with a breakdown of the whole operation of the transformer.

This is regardless of what the current might be.

If the transformer is operating in a low-power-factor environment (large phase

angle between the voltage and current), then the power it's handling may be

relatively very low ... deceptively so. Either the voltage or the current may be

approaching the transformer's limit, but you wouldn't know it if you're only

monitoring the watts of power. If you're monitoring the VA, then a high value

of either voltage or current immediately shows up.