Generator 3-phae and 1-phase output power

[veritas]

Hi

I need a small generator for some testing. The supplier said he can give me an 8kVA, 3-phase gen set rated at 230V/400V, 11.5A. The same set can also give me 21.6A, 5kVA singlephase. I am puzzled by this. Where does the 21.6A and 5kVA come from?

According to my calcs, 8kVA translates into 11.5A per phase as the line current assuming the generator is star connected. Power per phase is 8kVA/3 = 2.67kV/phase. Assume the gen is now delta connected. Then the phase current is 2667VA/230V = 11.6A which translates into a line current of 20A. This is the closest I can get to 21.6A.

If 1-phase load is now connected between two phases, I get the power in the load is 230V * 20A = 4.6kVA. Not quite 5kVA. Puzzling indeed!

Can anyone shed more light on this? Is my assumption correct that the generator is changed frm star to delta connected for the 1-phase load? Is it possible for a 3-phase gen to operate as a 1-phase gen without adverse effects?

Thanks.

 

[7anoter4]

It is not so a big problem. The current supplied by the source will be-in a cable- always:
S[KVA]*1000/sqrt(3)/VL_L in a three_ phases system.
So 8000/sqrt(3)/400=11.55 A
In a single-phase system-230V -live to neutral
S[KVA]*1000/)/VL_N
So 5000/230=21.74 A

 

[veritas]

7anoter4

I understad the calcs but I do not see the link between the 3-phase and siglephase case. Should it not be possible from the 3-phase parameters to be able to determine what the generator can deliver as a single-phase source?

 

[waross]

The current is limited by the capability of the windings.
Your windings are capable of 11.5 amps. Hence 11.5 Amps at 400 Volts.
For single phase the windings are in parallel for 23 Amps. (rounded off to 21.6A to make an even 5 KW)
The generator may be converted to single phase by either a zig-zag connection, a double delta connection (becoming popular with set manufacturers. or the bar-diamond or Collins connection (to convert 10 lead generators to single phase)
You have neglected to consider phase angles in your calculations.
Three phase to single phase at 100% PF; KVA and KW = 2/3 of three phase.
Three phase to single phase at 80% PF; KVA = 2/3 of three phase, KW = 2/3 x 1.25 of three phase. Allowable PF becomes 100%
Above about 15 KW at 1800 RPM and about 25 KW at 3600 RPM almost all single phase sets are reconnected three phase sets.

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

 

[veritas]

What do you mean by "For single phase the windings are in parallel for 23 Amps."? If the 3 windings are in parallel will their output not sum to zero volts?

 

[waross]

I mis-spoke. When a generator or transformer is reconnected in parallel for the lower voltage, the windings are in parallel for twice the current rating.
However, if you want 400 Volts single phase, connect the windings in delta. The in-phase winding will carry rated current, 11.5 Amps.
The other two windings may be compared to an open delta transformer connection. The voltage across the open side will be in phase and equal to the voltage of the in-phase winding. The current rating will be 11.5 amps.
I am assuming from your numbers that you want 400 Volts single phase. For a a single phase load on a delta transformer or generator, the in-phase winding supports 50% of the load and the out of phase windings together support the other 50% of the load.

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

 

[ScottyUK]

veritas,

Have a look on page 2 of this link:

http://igor.chudov.com/manuals/12-Lead-Zigzag-Generator-Connection.pdf.

The connections Bill was describing are shown there.

 

[waross]

Thanks Scotty. Nice diagrams. I'll make a note to a note:
The note indicates about 50% capacity when going from three phase to single phase. That may be true when a conversion to single phase requires the voltage to be reduced but:..
The most common conversion that I see is 120/208 volts to 120/240 Volts. In that case the new KVA rating is 2/3 of the original. On an 80% PF set, the single phase connection allows 100% PF and the new KW rating is 2/3 x 1.25 = 83%
Your conversion from 230/400 Volts to 400 Volts will allow 8 KW x 2/3 = 5.33 KW/KVA at 460 Volts. (Assuming a 100% PF set)
If you have to dial the voltage down to 400 Volts the new KW/KVA will be %.33 KVA x 400/460Volts = 4.64 KVA/KW.
But the note says.... I have learned the hard way never to trust a sales engineer when it comes to re-rating three phase to single phase conversions or 60 Hz to 50 Hz conversions. Too many sales engineers have given me the wrong information too many times.
I never see the bar diamond or Collins connection in print.
On the 10 lead PARALLEL LOW WYE connection make the following modification;
Connect T5 to T7, Connect T4 to T8, Connect T1 to T2.
L1, N and L2 are now L1:T9-T3, N:T10 and L2:T1-T2
This will convert a 120/208 Volt three phase set to a 120/240 Volt single phase set.

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