HP equal to KVA rating
- Thread starter eestudent
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mayanees
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- Westminster, MD
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- Electrical Engineer and Master Electrician
Why is it that HP rating is equal to KVA rating when it comes to
Short circuit calculation? Are there any exception to that practice? Thank you...
It's really an approximation of the input power required to generate 1 HP at the shaft of a motor.
1 HP = 746 Watts at the shaft.
Resultant VA = 746/0.8 (typical motor pf) = 932.5 VA at the shaft.
Motor efficiency is on the order of 90%.. or for the purpose of the KVA=HP approximation = 93.25 % - which necessitates 1 kVA input power to the motor.
An exception would be if you had accurate data on efficiency or power factor for a motor to use in the VA = HP/pf/eff equation.
John M
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Yes, that gives you an approximation to rated current if pf times efficiciency is about 0.75 which is not unusual.
Not so sure what it tells you about short circuit calculations unless you are maybe taking the motor contribution into account.
Short circuit calculation? Are there any exception to that practice? Thank you...
I'm a little concerned whether eestudent's question was answered... the 1HP/KVA is a pretty decent approximation to estimate the full load amps (FLA) of a motor for the reasons pointed out by previous posters. It works for single-phase or 3-phase (as long as you handle the root(3) properly. The estimate is good for 1- 30 HP motors and above that you should really have more information anyway. What bothered me is that eestudent asked for "short circuit calculation" not full load amps.
An important responsibility of an engineering design is to determine the available short circuit capability at a given location. If a fault occurs, motors act as generators for a short period of time and supply current to this fault. If for example, you have determined that 28,000 Amps fault current occur at a location (given transformers, wire impedances etc.), size your equipment for an interuppting capability of 30,000 Amps, but ignore some large motors that can supply 5000 Amps of fault current, you can have a catastropic failure when your equipment cannot clear the fault.
Back to the topic, the FLA (calculated from 1KVA/HP or off nameplate) is NOT a good estimate of the short circuit contribution supplied by a motor. It depends on transient reactance of the motor and shouldn't be guessed at but calculated. People here with a lot more experience than I have may have a better answer but a multiple of 4X to 7X the full load current is probably a better estimate... but remember I said, don't estimate it, get some data and calculate it.
mayanees
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- Westminster, MD
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- Electrical Engineer and Master Electrician
... true story.
I think the OP was referring to typical Power Systems Analysis software that automatically translates HP to a KVA base. Those programs then use the base kVA as the multiplier of the per-unit xd'' subtransient reactance to develop the Fault Contribution, which is on the order of your suggested (4 -7) times the fla. At least that's the way SKM does it.
John M
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