Generator Specification DOUBTS 1

[ThePunisher]

Hi all,

I would like to obtain your opinion. I have this generator specification section which seems vague to me.

"The generator shall be capable of operating continuously at maximum KVA output power at any power factor between the rated power factor lagging and unity AND at any voltage between +/- 5% rated voltage, BUT NOT NECESSARILY WITHIN THE GUARANTEED TEMPERATURE LIMITS"

I found the capitalized portion of the section vague and somewhat sound contradictory to the earlier phrase. Is this a NEMA MG1 statement (do not have a copy right now unfortunately)? I would like to obtain help in understanding what this specification section is trying to attain.

Thank you very much.

 

[HamburgerHelper]

I think they might be making two separate statements.

1. That it can operate at its maximum kva rated power factor to unity continuously.

2. Output any voltage +/- 5% but not necessarily within the guaranteed temperature limits.

I think that they are separate because you can exceed the rotor or stator limits before hitting +/- 5% voltage.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[ScottyUK]

At 95% voltage and rated output the current will be at 105%. The I²r losses will be higher, so it will run hotter than normal.

 

[IBRCAN]

I assume this is a spec for an engine driven generator? Seems like a very odd spec.
The vast majority of 3 phase generator sets are rated at a .8 power factor. In that case, the engine will be overloaded at maximum kva output and any power factor closer to unity than .8.
If the customer actually NEEDS this, it sounds like they need a larger generator set.

 

[waross]

Gen set specs:
KVA; (at rated voltage): The heating limit.
Determine the rated maximum current either from the nameplate or by calculation and don't load past rated current.
kW; (or KW on older sets): Limited by the power of the prime mover.
PF; Typically 0.8
Example; A typical standby set may be rated at:
KVA 100
KW 80
PF 0.8
Voltage 240 Volts
Phase 3
Current 240 Amps.
This machine will be overloaded electrically if the load exceeds 240 Amps.
This prime mover will be overloaded mechanically if the load exceeds 80 KW.
A real world example:
A standby set for a movie theater.
The load was calculated at 50 KVA at 208 Volts.
The set was ordered as such.
The set that was supplied was rated at 240 Volts.
The prime mover had ample power but the KVA rating dropped to 50 KVA x (208V/240V) = 43.3 KVA, Not enough to power the connected load.
May I be forgiven if I suspect that there is a reason why some engineers are in sales? Some are excellent but not all.
This was not the only time this happened.
In all cases, the customer shook hands with Murphy and under a sub-rule of Murphy's law (The customer is always right) ordered the set himself.
This applies to standby rated sets.
Prime power rated sets are allowed a 10% overload for a stated time.
Typically a basic 100 KVA set will be rerated as a 90 KVA (72 KVA) set when marketed as a prime power set.
Why the confusing spec?
I think that Scotty nailed it.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Thanks Bill.

It's definitely an odd spec - the engine is under-utilised except at unity PF. What size is this machine - I'm guessing it's a fairly small one, no one would ever spec a utility-scale generator with such poor utilisation of the prime mover.

 

[waross]

Hi Scotty.
I would also suspect a very small machine.
I don't remember seeing a unity power factor rated machine above about 15 KVA or 25 KVA.
This may be a spec

Bill
--------------------
"Why not the best?"
Jimmy Carter