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(OP)
Dear Folks,
Assuming that 500KVA 400KW 720A/ph generator .
If the generator attempted to be used for a single phase load ( not possible to divided on 3).what's the maxuim current that can get form the generator ? what's the possible effects of single phase loading on the generator ?

The maximum allowable single phase loading will be 2/3 of the three phase KVA rating.
500 KVA @ 3Ph x 2/3 = 333 KVA @ 1Ph.
400 KW @ 3Ph = 333 KW @ 1Ph.
80% Pf @ 3Ph = 100% Pf @ 1Ph.
The maximum current at single phase depends on the connections and the voltage, both before and after the conversion to single phase.
Is the generator delta connected or star connected?
Will the single phase load be at 400 Volts or at 230 Volts?
The maximum single phase current as it sits will be 720 Amps at either 400 Volts or 230 Volts.
If the generator is star connected, it may be reconnected in double delta or parallel zig-zag to allow 1440 Amps at 230 Volts.

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

Is it a 10 wire or a 12 wire?

A 10 wire set may be connected for single phase with the bar-diamond or Collins connection.
The performance will be equal to a zig-zag connection.

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

Bar-diamond/Collins connection also leaves 3 points at 120° separation and 208 volts (120 to neutral) so the AVR, if it is three phase sensing, need never know that the connection has been molested.

I used to run into a few of them about 15 years ago.
A lot of 10 lead sets were originally built by Delco for telephone back-up sets.
The Telcos replaced a lot of them and there were a lot of ex-telco 10 lead sets on the surplus markets.
A 120/208:480 Volt set could be converted to a true 120/240 Volt single phase set with the Collins connection.

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

Quote (Waross)

10 wire set may be connected for single phase with the bar-diamond or Collins connection

I’ve worked on many of the old military 10 and 12 lead generators converting them to single phase.
I can draw out what I think I want the connections to do and the math seems to work. I’ve just never really had the nerve to try it with a generator that’s working fine...

Please, go into this bar diamond/ Collins connection a little farther.

The zig-zag is the safest connection. It may be done safely even if the number tags are missing from all of the leads.
Assume that the set is wire for either 120:208 Volts or 277:480 Volts and you want to change to 120:240 Volts or to 240:480 Volts.
No numbers are left.
Arbitrarily tag the leads "A", "B" and "C".
Tag the star point leads X0
Pick any one.
Disconnect the neutral connections.
With a continuity tester find the X0 end of the "A" winding(s)
Tag that X0A.
Reconnect the other X0 leads to the star point.
Connect X0A to either "B" or to "C", tape up the connection and forget it.
The two remaining leads will be L1 and L2 and X0 will be the neutral.
Or, said another way, disconnect the star point and do a continuity check.
reconnect the star point with one lead or pair of leads left out.
Make and insulate one connection.
Done.

Collins, 120:208 Volts to 120:240 Volts only.
You have a permanent star point with leads 7, 8, 9 and 10.
Now you see a zig-zag,
Now you see a zig-zag that is a mirror image of the first zig-zag.
Naming.
If you draw a vector sketch with the #1-#4 and the #7-#10 windings horizontal and on the left, the #1-#4 and the #7-#10 windings form a horizontal bar.
The right hand side of the sketch forms a diamond.
Hence Bar-Diamond.
An engineer and his friend Mr. Collins were having coffee together.
The engineer was preoccupied with some generator conversions and was absent mindedly doodling sketches on a napkin.
Mr. Collins, an old cowboy, looked at one of the sketches and remarked;
"That looks like the old Bar-Diamond brand."
Hence; The Collins connection.

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

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

That like what I had envisioned.
Thanks for the reassurance.
I know where a 120/208 400hz unit is coming off and being replaced with a 60 hz unit.
I’ve called to ask about the timeline. I’m going to reconfigure it this weekend and check it out.
I see quite a few 10 wire on the old military generators.
Now the Collins on the other hand...
Still working..

Edit..
I have T1&T4 paralleled with T7&T10
T2 connected to T3, T5 connected to T9, T6 connected to T8.
Bar diamond? Looks right on paper anyway.

That looks right for the Collins connection.
Before making the T2-T3 connection, do a voltage check.
You should have:
T2 to T3> zero volts.
T10 to T2> 120 Volts.
T10 to T3> 120 Volts.

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

I have used the Collins connection on an old 10 lead 25 kw Japanese unit, and it worked for many years like that, until the machine wore out from age and use.

I saw a fellow do the same for a 6-71 Detroit set and he said it worked well.

I have not seen a 10 wire head on anything modern, though. The Japanese unit was from the early 90s and anything with a 6-71 is much older than that.

A certain well-known generator manufacturer (that undercuts the competition) now seems mostly to build their heads with exactly the number of leads they need for the voltage configuration ordered.

The cynic in me says that this limits the utility of these units on the secondary market, which may be the desired effect.

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