240 and 480 from same alternator at the same time

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Aleman said:
We recently had a vendor bring a demo unit for a process we use. We rented a diesel generator from a yard. I hooked it up, the demo unit need 480V at 30 amps. The generator had
pretty much everything available...120/220, 208 and 480 3P. Why not just do that? I don't know what it cost to rent the thing but for doing equipment demonstrations
it might be easier to just rent what you need at the site.
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Were each of those voltages available at same time? If so at what capacity of total kVA rating of the generator? It is easy to tap into windings to get different voltages, but you can only ground one point of the system. 120 volts single phase is achievable in multiple configurations from a typical 12 lead unit while providing other voltages and number of phases and is common, you are limited to capacity of the winding you are connected across less any capacity it is already supplying to other circuits. The base voltage of each winding is 120 volts. But to arrive with 208 or 277 volts you need a wye configuration or those values will just not be present, outside of the high leg only of a 240 volt delta connection, 120 volts is harder to achieve with 480/277 configuration though because voltage excitation needs boosted a little to get to 277 and will be about 138 volts across each winding.

Some of those generators that have both 120 and 480 volt outputs on them very likely have separate sets of windings that are independent of one another to get these voltages.
 
kwired said:
Were each of those voltages available at same time? If so at what capacity of total kVA rating of the generator? It is easy to tap into windings to get different voltages, but you can only ground one point of the system. 120 volts single phase is achievable in multiple configurations from a typical 12 lead unit while providing other voltages and number of phases and is common, you are limited to capacity of the winding you are connected across less any capacity it is already supplying to other circuits. The base voltage of each winding is 120 volts. But to arrive with 208 or 277 volts you need a wye configuration or those values will just not be present, outside of the high leg only of a 240 volt delta connection, 120 volts is harder to achieve with 480/277 configuration though because voltage excitation needs boosted a little to get to 277 and will be about 138 volts across each winding.

Some of those generators that have both 120 and 480 volt outputs on them very likely have separate sets of windings that are independent of one another to get these voltages.
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I didn't look too far into it just hooked it up. The voltages had their own breakers. My guess is that it did have essentially 2 units on board.
 
141012-1256 EDT

kwired:

The contactors when unenergized can default to the 240 setup. No power required. The contactor coils can be 240 V AC, and power to energize the coils to switch to 480 can be derived from one 240 V generator winding.

.
 
gar said:
141012-1256 EDT

kwired:

The contactors when unenergized can default to the 240 setup. No power required. The contactor coils can be 240 V AC, and power to energize the coils to switch to 480 can be derived from one 240 V generator winding.

.
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Do we want them transferring while it is energized? If so do we need to watch for certain "breaks" before certain "makes" can occur? Not saying it can't be done, just more possible complexities.
 
Actually one could do this without any switching. Use a 50KVA 12 lead alternator. Set the regulator to 138.5V/coil - probably the 480V setting. The output would be either 480/277Y or 240/139Y.

Connect for 480/277Y, grounded neutral. Take that to the 480V output 25A CB and 480V, 4 pole receptacle - 3phases and ground, no neutral.

Bring out each Wye leg centertap to the 240V, 50A CB and then to the 240V 4pole receptacle - again 3 phases and ground, no neutral.

If there is a concern with being able to overload the alternator if both CBs are on, get the CBs with an NC aux contact. Parallel the two NC contacts in the engine run circuit. At least one of the CBs will have to be OFF for the engine to run.

Could be the money one would save not getting the switches would pay for the extra alternator capacity.

And if you are assembling the driver/alternator, you will only need a 40 hp driver, same as if it were a 25KVA alternator.


Tell you what - be a sweet science fair project.

ice
 
iceworm said:
Actually one could do this without any switching. Use a 50KVA 12 lead alternator. Set the regulator to 138.5V/coil - probably the 480V setting. The output would be either 480/277Y or 240/139Y.

Connect for 480/277Y, grounded neutral. Take that to the 480V output 25A CB and 480V, 4 pole receptacle - 3phases and ground, no neutral.

Bring out each Wye leg centertap to the 240V, 50A CB and then to the 240V 4pole receptacle - again 3 phases and ground, no neutral.

If there is a concern with being able to overload the alternator if both CBs are on, get the CBs with an NC aux contact. Parallel the two NC contacts in the engine run circuit. At least one of the CBs will have to be OFF for the engine to run.

Could be the money one would save not getting the switches would pay for the extra alternator capacity.

And if you are assembling the driver/alternator, you will only need a 40 hp driver, same as if it were a 25KVA alternator.


Tell you what - be a sweet science fair project.

ice
Click to expand...
That crossed my mind, though the OP didn't mention the need to supply anything 120 volts, seems like there always is something 120 volts that needs powered. One could separately derive that or even use an autotransformer I guess.
 
141013-0845 EDT

The generator size Oregonbythesea is talking about is probably a 20 kW unit. A Kohler 20RESA falls in this category. This has an engine that falls in the range of 27 to 30 HP. To use a wye connection wired for 480 and use the 240 V tap points means doubling the size of the generator.

This is a big size and weight increase. An autotransformer would be less incremental size and weight, and that is considered by Oregonbythesea too much of an addition. Seems to rule out an oversized generator.

A manual drum or cam switch with 50 A contacts at 480 V would be the smallest lightest means. Earlier I indicated the switching can be done with a 6PDT or 9PDT break-before-make switch.

.
 
gar said:
141013-0845 EDT

The generator size Oregonbythesea is talking about is probably a 20 kW unit. A Kohler 20RESA falls in this category. This has an engine that falls in the range of 27 to 30 HP. To use a wye connection wired for 480 and use the 240 V tap points means doubling the size of the generator.

This is a big size and weight increase. An autotransformer would be less incremental size and weight, and that is considered by Oregonbythesea too much of an addition. Seems to rule out an oversized generator.

A manual drum or cam switch with 50 A contacts at 480 V would be the smallest lightest means. Earlier I indicated the switching can be done with a 6PDT or 9PDT break-before-make switch.

.
Click to expand...
The need for doubling the size of the generator because of using the mid point taps would depend on how much load is to be connected. If you need over half the kVA rating of the generator then yes you must increase the generator rating if only using half the windings.

I just realized however that if one sets regulator so you can have 240 volts wye phase to phase at the mid point taps, you will have more then 480 volts at the full winding ends phase to phase. You should have full winding voltages of ~ 555/960 shouldn't you? (4 139 volt sections in series per leg = 556 volts to neutral x 1.732= 963 phase to phase.

Kind of leaves out using that mid point tap application for 240 and 480 at same time, and you just can't do this at all with a delta configuration.
 
kwired said:
The need for doubling the size of the generator because of using the mid point taps would depend on how much load is to be connected. If you need over half the kVA rating of the generator then yes you must increase the generator rating if only using half the windings.

I just realized however that if one sets regulator so you can have 240 volts wye phase to phase at the mid point taps, you will have more then 480 volts at the full winding ends phase to phase. You should have full winding voltages of ~ 555/960 shouldn't you? (4 139 volt sections in series per leg = 556 volts to neutral x 1.732= 963 phase to phase.

Kind of leaves out using that mid point tap application for 240 and 480 at same time, and you just can't do this at all with a delta configuration.
Click to expand...

If I tell you, you will go:slaphead:

Its two 139 volt coils is series per wye leg = 277V L-N = 480V L-L

You're correct, has to be Wye not delta.

ice said:
... Set the regulator to 138.5V/coil - probably the 480V setting. The output would be either 480/277Y or 240/139Y.

... Connect for 480/277Y, grounded neutral. Take that to the 480V output 25A CB and 480V, 4 pole receptacle - 3phases and ground, no neutral.

Bring out each Wye leg centertap to the 240V, 50A CB and then to the 240V 4pole receptacle - again 3 phases and ground, no neutral.
Click to expand...

ice
 
141013-1132 EDT

In post #4 the current requirements were 25 A at 480 and 50 A at 240. We should asssume these are line-to-line voltages. It probably does not matter whether the generator coils are wye or delta unless there is need for the a wye neutral. The same logic applies --- the coils are either connected in series or parallel depending upon the required voltage.

Just looking at one coil pair label the wires X1-X2 for coil 1, and x3-X4 for coil 2. X1 and X3 are in phase.

From a DPDT switch ---

Connect a NC contact between X2 and X4, and another NC contact connected between X1 and X3. This parallels the coils and provides 240 V.

Connect a NO contact between X2 and X3, and the other NO between X2 and X3. This puts the coils in series for 480 V. Clearly NO from X2 to X3 is redundant,

The output terminals are X1 and X4 for a wye or delta configuration. All switches are break before make.

The above requires 6 double throw poles.

Additional contacts are required between the generator and the sockets so that the socket with the unselected voltage is dead. This requires 3 more double throw poles.

.
 
gar said:
... The above requires 6 double throw poles.

Additional contacts are required between the generator and the sockets so that the socket with the unselected voltage is dead. This requires 3 more double throw poles. ...
Click to expand...

Yeah - that's in-line most all of the switchings schemes. Nema 3R, 480V, 60A, $10K - $15K list (maybe).

That's one of the reasons I like my version of the science fair project - Trades 150 lbs and $1500 in alternator costs for $10K in switches

ice
 
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