Doubling Small Generator Voltage By Increasing Excitation?

[JohnMcNutt]

I work on generators in the [roughly] 20 kW range. Recently, an academic question occurred to me: how far would it be possible to increase the voltage on one of these if a customer needed it done?

These generators typically come wired 120/208Y, parallel main windings. The schematic for the model I have in front of me shows a series connected pair of stator field windings which of course goes to a rotor mounted rectifier and the actual field. In 120/208Y, the voltage across each winding is 120V, of course. If I wanted the set to produce 240/480 delta or single phase, what would have to happen on the field to make this occur? Double the current? Double the voltage? Not ordinarily possible?

 

[electricpete]

If you could increase flux forever without reaching saturation, you could do it.
But iron saturation will limit the voltage you can output.

=====================================
(2B)+(2B)' ?

 

[waross]

As Pete said, saturation. A 10 lead or 12 lead generator may be re-connected for 400/480 Volts. With a 4 lead or a 6 lead machine you won't get above about 130 Volts line to neutral, 225 Volts line to line or 260 Volts with a zig zag connection for single phase.
Try an auto-transformer. Doubling the speed will also allow double the voltage at the same excitation level without saturation but that is rarely practical.

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

 

[waross]

Re-reading your post, it seems a little confused.

These generators typically come wired 120/208Y, parallel main windings. The schematic for the model I have in front of me shows a series connected pair of stator field windings which of course goes to a rotor mounted rectifier and the actual field.

[highlight #FCE94F]These generators typically come wired 120/208Y, parallel main windings.[/highlight] That would be the lower voltage connection of a 12 lead machine.
[highlight #FCE94F]The schematic for the model I have in front of me shows a series connected pair of stator field windings[/highlight] That would be the higher voltage connection. (400/480 Volts.)
Some confusion here. Is this brushless? slip rings? is there an AVR?
There are a lot of 20 KW sets available that will generate either 208V 240 V or 480V. Buy one of these. There should be no price difference. With some set builders the 12 lead dual voltage set is the standard. A dual voltage set may even be a little cheaper than a single voltage set.

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

 

[ScottyUK]

Can you post the schematic, or link to it? That would probably resolve some of the uncertainty above - at the moment I share Bill's confusion.

 

[JohnMcNutt]

This is just a hypothetical. I am just trying to satisfy my own curiosity at this point, and to be prepared for a similar question from a customer in the future.

http://www.marathonelectric.com/generators/docs/manuals/SB504.pdf

See page 7, high delta.

Now suppose you wanted to get 480 volts off that delta instead of the 240 it shows there. As has been stated, I suspect that the 2 problems you would run into would be (1) saturation of the rotor and (2) current capacity of the exciter and/or field windings.

I realize that I could simply get 480 volts by connecting it in wye instead.

Would it be a likely scenario that most alternator ends would saturate before achieving the double voltage that I am describing here?

 

[burnt2x]

When your desired application goes beyond what your equipment was designed to do, you're like throwing away whatever warranty left on the said equipment.

Why not buy a 240/480V transformer or a genset with dual-voltage output (as recommended by others)?

 

[waross]

REconnect in Wye. End of story. 139 volts is the highest listed output from a nominal 120 Volt winding. You won't get much more than that before saturating.
I have run into an area and heard of some other areas where 240 Volt four wire delta is common. It does a good job of feeding 240 Volt three phase loads combined with 120:240 Volt single phase loads. You get a full 240 Volts for three phase motors and true 120:240 Volts single phase.
An example three 100 KVA transformers will supply 300 KVA the htree phase loads, but only 200 KVA to single phase loads.
If a 240 Volt delta set is reconnected as a 208 Volt wye set the KVA drops. a 100 KVA set at 240 Volts will drop to 87 KVA if reconnected at 208 Volts wye.
There is still enough prime mover power to supply 80 KW. The new rating is 80 KW, 87 KVA, PF 92%
Back to the original question. There is no expectation for 480 Volts and 120 volts from the same generator so just reconnect a twelve lead delta generator in star. The 240 Volt windings will be pushed up to 277 Volts so the KVA will jump up 115%
The KW will depend on the power of the prime mover.

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

 

[JohnMcNutt]

Thanks for helping me think it through.

I guess the answers would be, if cost was no object, to either:

1. rewind it with twice the turns and half the wire size on the stator winding
2. double the engine speed and reconfigure the poles for that

Neither of which is practical under normal conditions.

But it satisfies my questions.

Except for one thing: an ancient 6 kW unit that we used to have, until we wore it completely out, was originally plated for 600/347 but when we got it, it was setup for 120/240 single phase. Is the original 174 volts per single winding on this unit not a normal capability of a genset? (Not that I think I will ever have need of 600 volts around here.)

 

[waross]

That may have been a Canadian set. Now it is common to use a 480 Volt set and a 480V:600V transformer.
Answer 3. Reconnect in wye. Quick and fairly simple.
Look at all the available voltages in the link you posted. The same winding is used for every voltage from 110 Volts to 139 Volts. Make the proper connections and then turn the adjustment on the AVR until you get the voltage you need.
There are several ways to generate true 120:240 Volt single phase from a three phase generator. All reduce the KVA rating to 2/3 of the original rating.
1. Zig zag
2. Double delta
3. Four wire delta
4. Bar diamond or Collin connection (Used on 10 wire sets).
5. Not true single phase but some owners just use two phases for 120:208 Volts.
By the way.

I work on generators in the [roughly] 20 kW range.

I installed and serviced so many sets from 14 KVA to about 35 KVA, working as a one man shop in Central America that F.G. Wilson invited me to participate in a Beta test program of a new ATS control board.
F.G. wilson was #3 in the world in gen sets at the time.

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

 

[JohnMcNutt]

Bill,

Is there any way to private message on this board? I have some stuff for you that is not really part of this discussion. Your work experience is very interesting to me.

I did a quick search for bar-diamond and/or collins connection and did not find anything other than references you have made. Can you describe at least what leads go where, or do you have a drawing?

 

[waross]

Years ago Delco made a great many gensets that found their way into telephone exchanges as back-up generators. Eventually they were replaced because of their age, but most still had quite low hours. These sets were mostly ten lead sets.
If you look at the high (series) wye diagram on the Marathon link you will see that leads #10,#11, and #12 connect together to form the wye point or neutral. On a 10 lead set, this connection is internal and only one wire, #10, is accessible.
For a normal zig zag connection lead #12 must be reconnected but a 10 lead set has no lead #12.
Draw a wye labeled #7, #8, #9 and #10 as the wye point.
Connect T1 to T7 and T4 to T10.
Now T7 is at 3 o clock, T8 is at 11 o clock and T9 is at 7 o clock.
Connect T6 to T8
Connect T5 to T9
Connect T2 to T3
T2-T3 is line 2
T1-T7 is line 1
T10, is the neutral
T6-T8 are insulated
T5-T9 are insulated
This is sometimes drawn as a mirror image so that Line 1 is on the left.

Looking at the vector diagram;
Line 1 to the neutral forms a bar.
T10,T6-T8, T2-T3, T5-T9 form a diamond.
Mr. Collin is a cowboy who looked at some doodles his electrical friend was making of generator connections and said;
"That looks like the Bar-Diamond."


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

 

[JohnMcNutt]

Bill,

That makes perfect sense. Why didn't I think of that years ago? Why isn't that done all the time? (I guess it might be more confusing.)

 

[redlinej]

If you connect the generator in series delta and try to run it to 480 voltage you would burn the winding because of the volt per turn.

 

[waross]

That is correct redlinej. series delta is used for three phase 240 Volt applications and for 120:240 single phase and 240 Volt three phase combinations.

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

 

[JohnMcNutt]

I made a diagram of what you propose.

 

[waross]

That looks correct. T10 is the neutral for 120:240 Volt usage.
Most manufacturers use re-connected three phase machines for single phase above about 20 or 25 KVA @ 1800 RPM.
The connection that I see most in new sets is the double delta connection.
For reconnecting in service machines, I prefer the zig zag connection as only one one pair of windings has to be changed. There is a lot less chance of a mistake on an old dirty machine when everything is dirty and some of the wire number marks may be in doubt.
You will have six generator leads connected together forming the neutral point. You will have three groups of two leads which will be A phase, B phase and C phase.
To reconnect in zig zag, open the neutral connections and find a pair of leads with continuity. There will be three pairs with continuity, it doesn't matter which pair you choose. Reconnect the remaining two pairs as the neutral.
The winding ends you have removed are now connected to one of the other phases. Doesn't matter which one. This connection is now insulated.
Now you will have two groups of two leads. One will be L1 the other will be L2. You already have the neutral.
The fact that this may be accomplished with a continuity tester, and only one set of connections must be opened makes it a safe and simple way to quickly change a three phase machine to a single phase machine. It may be done safely even if all the wire markers are missing from the generator leads.
The Bar-Diamond connection was originally drawn horizontally with the bar on the left when it was named after the Bar Diamond brand. The bar diamond connection was unique to the old 10 lead sets. Considering the flexibility and wide choice of connections with the 12 lead generators, I doubt that 10 lead sets will be manufactured again.

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