# How does frequency affect KVA rating of Transformer

Discussion in 'Electrical | Electronics | Communications' started by yeshwenth, Sep 23, 2006.

1. ### yeshwenth Newbie

Can u explain what is the effect on the KVA rating of a transformer when frequency is reduced (say 50 Hz to 25 Hz)......VOLATGE REMAINING CONSTANT......????

#1
2. ### Jerry Apprentice

Simply raising the frequency applied to the transformer primary won't increase the voltage. But assuming that such a voltage increase is made, keeping a constant ratio of volts/hertz, the magnetic flux in the core will stay the same. A transformer is not a load but a coupling device magnetically linking primary and secondary circuits. With a constant magnetic flux, the coupling will be unchanged, and so will the kVA capability.

#2
3. ### summit.sehgal Newbie

Ok let's diagnose the problem in steps:

1) The rating of an equipment is the maximum value of some parameter(s)(Power or Current or temprature) it can be subjected without getting damaged.

2)Now if the supply frequency is decreased, it directly implies lesser cycles of magnetization & demagnetization in any given time.
which means lesser hysterisis losses,
which further implies the power handlig capablity (i.e.KVA) MUST INCREASE.

Let's diagnose the other part of the problem:
1)in normal circumstances,If the supply system frequency is decreased the supply voltage should also decrease as well, due to the direct relation between them...

2)Now if one's keeping the voltage constant, it means for the same power less current should be reuired at reduced frequency....

3)Which means lesser current through the transformer primary & Secondary

4)Which implies lesser Ohmic losses...
The result, again Power handling capablity (i.e. KVA rating) SHOULD INCREASE

So ,My Verdict:
KVA rating should increase with decrease in supply frequency irrespective of voltage remaining constant PROVIDED the power delivering capacity of the system remaining the same...;-)

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4. ### hemant Apprentice

transformer dose a freqency always same it dosent change the freq
i m in ec

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5. ### plantate Newbie

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The KVA rating or transformer will depend on the cross sectional area of the core,the amount of iron will depend on frequency,if you drop the frequency the KVA rating must reduce.
This is why hf transformers are preferred becuase they are so small and light for the VA output.hence the use of 400c/s for aircraft supplies (when not DC)
Also if you drop the frequency,you need more turns in the windings,or drop the voltage to the primary.
With higher frequency,there would be higher hysterises loses,but this is offset by the smaller core,but for HF, cores are usually ferrite instead of laminated iron.

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6. ### deepak singh Newbie

its ok that kva rating will decrease bt how can u calculate the exact kva while frequency is 25hz?

#6