KVA or KW

Why do we worry to specify the KVA or KW?...If the equipment is manufactured by a established company, the rating should be mentioned in KW,KVA or KVAR accordance with the type of equipments. However,Your answer is applicable, if we are manufacturing the equipment.

Ok, we are discussing or designing a project. Why do we have to deviate from the standard and to discuss/specify other terms.??. Did you ever seen any motor rated in KVAR? or PFCs in KW?...

I would like to add the point also -

for the power factor varying cases, kVA is always preferable. For example, when the Supply Companies release the notifications the load limits are always expressed in kVA, for example in India 50kVA, 100kVA, 150kVA. etc. are having crucial roles.

One more point, I would like to add that use kVA than KVA --- k - Kilo; kVA & kW are better (subject to others comments!)

This has been covered too many times to be comfortable.

There is a difference in specifying the instrument rating in KW and KVA stop.

It depends on the equipment which of the one you should.

If you can tell me which instrument, it will be a quick answer. Else we will be back to the theory that has been repeated ad nauseum here.

For transformers HT/MV/LV - manufacturers are using Kva as specified in nameplates.

Any motors - in Kw and HP

Capacitors - Kvar

Both Transformers and motors has nameplate power factor ratings.

Whichever is given (Kva, Kw or Kvar), then any one of them can be determined in theoritical approach because these three are in one piece power triangle.

Transformer Power Factor:- With the above comment by hentoyk, I would like to clarify that the power factor mentioned with the name plate of transformer might be the load power factor in which the transformer works in full efficiency and not that of Transformer. But, in case of Motor name plate, it is the inherent power factor.

Equipment rating depends on type of Equipment, if it is UPS you may have to say in KVA whereas if it is Motor it is expressed in KW.So, purpose of equipment is to be taken in mind with the extract of Input Power. Say UPS will take normal 3ph or 1ph supply but delivers leading PF whereas Motors delivers Lagging PF. So, these things are to be considered.

for dc machines we use KW rating,for ac machines we use KVA rating

The main intention behind this variation in the nameplate rating of kVA and KW is..

that for Generating (electrical power) source equipment such as Generators,UPSs,DG sets, Transformers(i/p and o/p also electrical power) etc.. its power capacity is mentioned in terms of KVA which is its capability of power generation.

Normal PF can be decided by the type of the load connected to the source. it is the deviation of cos of angle between the current drawn and voltage applied. This deviation of angle is decided by load(inductive or capacitive) only.so when designing these electrical machines(source), manufacturers usually gives its optimum capacity that the machine can generate the power irrespective of the load which is connected to the machine.Because capacitive and inductive loads can be also connected to the same generator at same time.

The PF which is also mentioned on these machines says that if this machine generates power(kVA) at the given nameplate PF, machine is working at better efficiency. As you all know that generator PF can be varied by excitation.

But motors are nothing but load equipments.

If you look into some books, it is mentioned that the reason behind this is heat losses of machines which is independent of pf so machine rating should be in kVAs. I agree with this to a little extent only.

Available power from a source is normally specified in KVA.If the load applied is totally resistive, the KWA and KVA are equal.If there is an inductive load, such as a motor,etc.the actual KWA will be reduced by the power factor below 100%.

A supplier of power(transformer,utility,etc.) does not know the possible applications, so they specify the output as KVA, which is based on a power factor of 100%, or unity.

A low power factor increases losses,due to current lagging voltage, so the total usefull power is reduced.

If a motor is rated at 1KW, and has a 90% power factor, it will require 1.11 KVA to provide the Minimum power required to operate it(not including starting current, line losses, Service Factor of the motor, etc).Normally, 25% extra is allowed for calculating conductor size.

Simple, eh?

HTRN

Dont worry about kVA or kW but just take care of power factor in each section and for each equipment.

with regards

H.E

So if I buy a genset that has 9kva stamped on it , how many 5kw appliances can I run ??

Usually generators are designed to have their maximum efficiency at 0.8 pf load. So, the full load it can carry is 9 x 0.8 = 7.2kW. But, it should carry 20% extra load, so the full load capacity becomes, 8.64kW. So, the generator can carry only one 5kw appliance.