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The transformer rating is given in kVA, but the load connected is always considered in watts. Why?

Eventhough, the load active power is represented in watts it has some reactive power, right?

Is the total power consumed by load (active + reactive) equal to the power supplied by the secondary side (secondary voltage × seconadary current) which is apparent?

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4 Answers 4

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"Watts" of a load signifies the Real/Active power. It is the useful power consumed by it. Reactive power is not useful. Consumers are normally charged by the utilities only for the real power.

$$P_{real} = P_{app} cos \phi $$
\$ \cos \phi \$ is the power factor of the load,

While KVA of transformer signifies its rated apparent power. It defines the maximum current you can draw from the primary/secondary for given voltage. Above which, copper losses cause windings to heat up.

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    \$\begingroup\$ And why should core losses be affected by load current? I trust you don't think saturation is caused by secondary currents? \$\endgroup\$
    – Andy aka
    Mar 21, 2018 at 17:06
  • \$\begingroup\$ Yea thats right. So its Just copper losses . \$\endgroup\$
    – Mitu Raj
    Mar 21, 2018 at 17:14
  • \$\begingroup\$ @Andy Strange, I have seasoned fellow EEs at work who does the same assumption as both Mr Raj and Tony. Something wasn’t taught in school around the world it seems. \$\endgroup\$
    – winny
    Mar 21, 2018 at 17:26
  • \$\begingroup\$ @winny Actually In my country, They have separated Electrical and Electronics into two different fields of study. I am graduated from Electronics. We were hardly taught about transformers, power transmission stuffs and all. Ha could be the root of my "assumption" lol. \$\endgroup\$
    – Mitu Raj
    Mar 21, 2018 at 17:34
  • \$\begingroup\$ @winny don't worry , sometimes I make misteaks \$\endgroup\$ Mar 21, 2018 at 18:28
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The transformer rating is given in kVA, but the load connected is always considered in watts. Why?

If the load on the secondary is purely reactive then using "watts" does not account for this type of load. However, "VA" accounts for any type of load on the secondary from capacitive through purely resistive to inductive.

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major re-write

In a transformer, losses are of two types

  • Constant losses or core losses - These depend on V
  • Variable losses or ohmic (I2R) losses - These depend on I

Hence total losses depend on V and I.

  • Since rating of transformer depends upon losses,
    and losses depends upon V and I,
    hence the rating of transformer depends upon V×I,
    which is also termed as VI rating, in units of VA, kVA, MVA.

  • As losses are independent of the power factor of load,
    the rating of the transformer is also independent of load and can be only decided based on losses.

    That’s why transformer is generally specified with apparent power rating (VA or KVA) and not in KW

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    \$\begingroup\$ Why would it take more peak magnetic flux to provide more VAr? Primary side exited the core with very constant volt-second area and your VAr only takes up available current headroom in your windings and power lines. \$\endgroup\$
    – winny
    Mar 21, 2018 at 16:26
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    \$\begingroup\$ Secondary load current does not affect saturation of the core. How old are you @Tony LOL? \$\endgroup\$
    – Andy aka
    Mar 21, 2018 at 17:04
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    \$\begingroup\$ Too old, I’d rather be golfing \$\endgroup\$ Mar 21, 2018 at 17:19
  • \$\begingroup\$ Even after your edit, eddy currents in the conductors increase, but the frequency is too low for this to matter. Magnetic losses stay virtually unchanged in the transformer with increased load just like dielectric losses. What you end up with is “just” increased resistive losses. \$\endgroup\$
    – winny
    Mar 21, 2018 at 17:24
  • \$\begingroup\$ @Winny as you know , the difference between VA supply rating and W load rating is due to network losses: {mainly Electric( conduction, skin effects) and Magnetic: core(hyst&eddy) not necessarily related to customer W load \$\endgroup\$ Mar 21, 2018 at 18:07
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Actually, if the load is highly reactive and near the rating of the transformer, then ignoring the reactive component can be dangerous. Technically, the transformer load should be evaluated in VA rather than watts.

If, in an industrial environment, those designing the setup are actually ignoring the VA of large reactive loads when specifying a transformer then they should have their sliderules, er, calculators, er, smartphones taken away from them.

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