Why Transformer Rated In kVA, Not in KW? | Top Q&A

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Transformer At all times Rated In kVA as an alternative of KW

Because the title suggests, a transformer solely transfers the facility from one circuit to a different with out altering the worth of energy and frequency. In different phrases, It may solely step up or step down the worth of present and voltage whereas the facility and frequency stay the identical. A common date on the transformer nameplate is printed for additional particulars, resembling score in VA, single section / three section (energy or distribution transformer), step up / step down, connection and many others.

Proper to the query, In easy phrases,

Studying: why transformer score in kva

There are two sort of losses in a transformer;

  • first. Copper Losses
  • 2. Iron Losses or Core Losses or Insulation Losses

Copper losses (I²R) rely on present which passes by means of transformer winding whereas Iron losses or core losses or Insulation losses rely on Voltage. ie complete losses rely on voltage (V) and present (I) that are expressed in Volt ampere (VA) and never on the load energy issue (PF). That is why the transformer score could also be expressed in VA or kVA, not in W or kW.

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Why Transformer Rated In kVA, Not in KW

Let’s clear the score of the transformer print VA as an alternative of W with a solved instance.

Losses of the transformer will stay the identical so long as the magnitude of present / voltage is identical. It doesn’t matter what energy issue of the load present / voltage is.

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Instance:

Suppose for a single section step up transformer

  • Transformer score in kVA = 11kVA
  • Main Voltage = 110V
  • Main Present = 100 A
  • Secondary Voltage = 220V
  • Secondary Present = 50 A.
  • Equal resistance on Secondary =
  • Iron losses = 30W

Print first situationIf we join a resistive load to the secondary of the transformer at unity energy issue = 1,

Then complete losses of transformer could be copper losses + iron lossesie:

I²R + Iron losses

Placing the values:

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(502 x 5 ) + 30W = 12.53kW

ie losses on major and secondary of switch are nonetheless the identical. (See under instance for secondary losses as nicely)

The transformer output might be:

P = V x I x Cos

Once more, placing the worth from secondary (Identical worth if we put the values ​​from major)

P = 220 x 50 x 1 = 11kW.

Now score of transformers:

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

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Now, In second situationjoin a capacitive or inductive load huge the secondary of the transformer at energy issue = 0.6.

Once more, complete losses of transformer could be copper losses + iron losses, ie:

I²R + Iron losses

Placing the values:

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(502 x 5 ) + 30W = 12.53kW

Therefore proved that losses in each major and secondary are the identical.

However the output of the transformer might be:

P = V x I x Cos

Once more placing the worth from secondary (Identical worth if we put the values ​​from major)

P = 220 x 50 x 0.6 = 6.6kW.

Now score of the transformer:

kVA = VA / 1000

kVA = 220 x 50 / 1000 = 11kVA.

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It means, 11kVA transformer score means it may well deal with 11kVA. It’s our flip to rework and make the most of the 11kVA as 11kW (we are able to do it by bettering the facility issue to 1 in case of pure resistive load) which isn’t predictable and even very laborious to get in case of inductive and capacitive hundreds the place the facility issue would have completely different values.

From the above instance, it’s clear that the score of the transformer is similar (11kVA) however completely different output in energy (11kW and 6.6kW) as a result of completely different energy issue values after connecting completely different type of hundreds which aren’t predictable for transformers manufactures the place the losses are similar in each instances.

So these are the precise causes for transformer score in kVA as an alternative of kWA.

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As like transformer, score and capability of Alternators & turbines, Stabilizers, UPS, Transmission traces are additionally rated in VA as an alternative if W whereas energy plant capability, AC (Air situation) and motors are rated in W (watts) quite than VA (Volt -Amperes).

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