277/480 Generator for a 240/480 Service?

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Tuna Fish Electric

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Hi there,

I have a lead where someone is asking for Generator Backup at a Municipal Water Pump Station. According to the prints, the service is rated 240/480. The Generator Supplier makes Generators that are rated 277/480, but not 240/480. There is also a step-down transformer that transforms the power to 120/240. If most of the 480 loads are 3 phase motors, using a 277/480 Genset probably wouldn't be a problem, however, I would imagine the Transformer is Delta-Delta and it would throw off the voltage for the down-stream loads.

Your thoughts?
 
Tuna Fish Electric said:
There is also a step-down transformer that transforms the power to 120/240. If most of the 480 loads are 3 phase motors, using a 277/480 Genset probably wouldn't be a problem, however, I would imagine the Transformer is Delta-Delta and it would throw off the voltage for the down-stream loads.
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The primary to the transformer would still be 480 volts so there is no issue.
 
If the existing service is 480/240 high leg delta, but only L-L 480V loads are served, then a 480/277 wye source will work just fine.

But if anything is connected 240V L-N you might have a problem.

Jon
 
There is lots of slang out there.

My assumption is that when the OP said 240/480 and then made it clear that it was a three phase system was that they had a high leg delta 480V system. This gives 240V L-N on two of the phases, and 416V L-N on the 'high leg'.

Such a system _might_ be used to supply 240V loads, but generally this is used to supply pure 480V three phase loads, and the neutral grounding of one of the legs is simply a convenient grounding point.

Standard practice would be to use a 480/277V wye system, ground the neutral but otherwise not use it.

-Jon
 
I have never heard of high leg 480. Nor have I heard of a split phase 240/480 (If that is what is described). I wonder if there is a miscommunication.
 
Tuna Fish Electric said:
Hi there,

I have a lead where someone is asking for Generator Backup at a Municipal Water Pump Station. According to the prints, the service is rated 240/480. The Generator Supplier makes Generators that are rated 277/480, but not 240/480. There is also a step-down transformer that transforms the power to 120/240. If most of the 480 loads are 3 phase motors, using a 277/480 Genset probably wouldn't be a problem, however, I would imagine the Transformer is Delta-Delta and it would throw off the voltage for the down-stream loads.

Your thoughts?
Click to expand...
Got to be a Genercrap! LOL! We had one left over from a job, never could find a place for it, because all of our customers that have 480 volt services buy their own. Finally had a transformer show up from a job, was going to use it at the old boss’s airplane hanger, (120/208) but never got to it before I left.
 
Around here 240/480 single (split) phase is used in rural areas that do not have 3 phase or when costs more to bring it in than an oversized VFD.
480V L-L single phase can supply a oversized 480V VFD for motors, pumps etc.
240 L-N is used for small fractional HP motors, yard pole lighting and a 240:24V controls transformer for the pumps.
The service would generally be a isolated lift station or irrigation pump where there is not a need for 120V stuff.
 
Not really.

A 12 lead generator has 6 separate coils, 2 for each phase. You can get a max of about 140V per coil or adjust the voltage regulator down to below 120V. This lets you get a bunch of common utilization voltages, 120/240V single phase, 208/120V wye, 240/120V high leg delta or 480/277V three phase.

To make 480V _delta_ you need to be able to connect your coils to get 480V across a single phase, which you can't do with a normal 12 lead generator.

You could certainly design a 12 lead alternator which could do either 480/277 or 480/240 high leg, but this would lose the ability to do 208/120. Or you could design an alternator with even more leads brought out, raising costs.

Probably the simplest thing to do, if you really need 480/240 high leg, is to use a 480/277V genset, not connect the wye neutral to anything, and then use a single phase grounding transformer to derive the 'mid leg neutral', and ground that.

-Jon
 
Just an update on this -
Spoke with another Generator Contractor and he said he has dealt with a lot of those situations - 240/480 pump stations.
He recommended asking the Manufacturer for "Special Engineering" and to "Isolate Neutral" - still ground the neutral for clearing fault current, but it would not be used for any loads. 480 Motors and step-down transformers would still work fine.
 
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