Transformer Rating

Because a transformer does not consume power (I know, except for loss, but let's ignore that) but it does have to have a max power rating.

Excellent reply.

I think, the transformer if rated for KWH then also KW which is peak power is still to be rated else drawing more power than that will sure not permitted for proper functioning. KWH will only mean, how long the transformer can be used or its life time. I think this parameter is interesting one to predict the total usability of the transformer as we have working hours or predicted life span for the bulbs etc. Hence, only usable working hours separately can be listed for continuous use of transformer at peak power but giving KWH may be misleading at times.

Very good question.

Transformer is rated in KVA as it is rated for the maximum possible connected load to the particular transformer.KW cannot be given as rating as KW depend on the Power factor of the connected load which is variable and unknown.IF the load is Inductive or capacitive KW will be less depending on Cos φ (power factor ) If power factor is unity the KVA will be equal to KW . KW = EI Cos φ. (depend on L and C of load )of connected load . So if power factor is less than or more than unity the KW will be less for same KVA rating. KVA rating is not variable. So the Transformer is rated in KVA.not KW.

The power consumed into a network has two type: Active power (maded by the resistive characteristic of network and mesured in KW) and Reactive Power (maded by the inductive/capacitive characteristic of network and mesured in KWR). The real power consumed is a combination betwen the two (have a value calculated with the Pitagora's Law), and named Apparent Power (mesured in KVA). This is the real power abbsorbed by a electrical network of LowVoltage from a HighVoltage network. According this, the transformer is rating in KVA.

there are two typeas of losses in a xmer 1. core loss which depends on voltage 2. ohmic loss or copper loss which depends on the the current Now as these losses depend on the voltage and current and are almost unaffected by the load power factor the rated output is expressed as va and not in watt also the raing given is applicable to both primary and secondary because the primary p.f. and secondary p.f. are equal

As i think a transformer never consume any power just only the internal losse like core losse and cupper loss and as its name says transformer means to transform power so it transform power to the load maybe Reactive load or Resistive load we use kw unit for resistive load and va for inductive of capcitive load the this is a standar to use kva for transformer to include both resistive and reactive loads

if there is new objection according to my discussion please tell me

thanks

Dear Asker,

The rating of transformer is in VA, kVA or MVA because of the basic fact that the nature of load transformer is intended to supply is not known. It may be inductive, capacitive or resistive or mixture of all. This results in load power factor.

Therefore, KW = KVA X PF

And kVA = (1.73 X Line Volts X Line Current)/1000 for a three phase a c system.

When kVA is used, one needs to know the power factor or nature of load and then can decide the actual kW output from the transformer. In any case, the kVA remains constant while kW is directly proportional to the nature of load or power factor. Hence, kVA / MVA are the best qualitative units for selection of transformers.

Hope, you are satisfied with my reply.

Jayesh Shah / Surat / India

Transfomers are intended to deliver maximum power, both arparent, useful and wasteful and real and no one can really tells the nature of the connected load by the users ther/4 rated in kva/mva. whereas, one should strictly restrict machines at the load end to real power by correcting the power factor to reduce the stress on the transformers since only the useful power is what every engineer wants to work with (KW).

Are you sure?

Look, why transformer differ in core size for greater KW power for same input and output voltages. Wall adopter of 12V and welding transformer of 12V differ too much in size just because of load capacity.

Mr. Shyam ,

Addey is correct. KVA rating of a transformer is independent of transformation ratio.Transformer with higher KVA capacity is of higher size and with lower capacity is of lesser size.Should not get confused with transformation ratio and KVA capacity. KVA capacity of a transformer is the maximum permissible load current which can be connected to that transformer with out damaging it .

KVA or power rating in transformer is mainly due to core size and frequency of the power source. Damage to the transformer is due to current capacity of the windings and use of lower frequency or DC current beyond the current capacity of the core windings.

I have a program written for automatic design of the transformer, which takes these parameters like input and output voltage, power, and frequency and it computes the core size, turns in primary and secondary and the gauge of the wires to be used.

Dear Shyam ,

What ever you have mentioned in detail about designing of transformer rating is welcome. Your software for automatic transformer design also appears to be good. Here the original question is why transformers are rated in KVA and not KW. and the simple reason is manufacturers cannot know what is the customers load condition weather Inductive Capacitive or resistive.The actual KW will depend on actual power factor of the load . At 0.8 PF for a 1000 KVA transformer the Usable power is only 800 Kw .and for 0.6 PF maximum usable power will be only 600 KW .So Transformers are rated in KVA and not KW .

Current rating of the coil is critical as it is fixed due to the type of wire gauge is used. This parameter varies drastically when you change the frequency and reduce the core size and when turns in the transformer fall to very low and force one to use thick wires and obviously current in such transformers also does not change and only inductance changes. This is a serious problem at low frequencies, when they work at 50Hz/60Hz where wire thickness is almost selected for DC current rating for the applied voltage. If you apply DC current to high frequency transformers then they will short circuit but low frequency transformers will become more like a heater.

Hence, not only VA is enough but also its response to low frequencies and higher frequencies is equally important. Many inverters apply square wave to the transformer and then you get that near DC heating mechanism into action. SCR/TRIAC switching is another problem for the same transformer. Some time your power source come from over loaded transformer ans you get clipped power source to your transformer. You can not afford to lose your transformer so they are designed to take some DC load also and then temperature sensor placed in the windings comes into action to turn off the input power source to the transformer. I intend to use PTC for this purpose.

Load impedance matching is something very specialized field. I am right now looking for 13.56MHz transformer 1kW to 10kW with load impedance matching (50 ohms -25 ohms to +150 ohms variation) with dynamic control in real time as my load is unstable. Do you think you can handle this?

Thanx 2 all 4 very discriptive n intresting discussion.