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Autotransformer for Delta Load

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Tsemple

Member
Location
Canada
Occupation
Electrician
Hey everyone,

New to the forums, I've been poking around for a similar thread but haven't seen one so please redirect me if I'm asking a question that's already been answered.

I'm in Alberta looking to install a 15kVa autotransformer to reduce voltage from a 600/347v source to a dedicated 480v delta load (air conditioner for a pumphouse). Talking to the wholesaler a Y-Y is the most common configuration but does this have any impact on the delta connection in terms of bonding the secondary? My plan would be to just hook it up delta to delta and ignore the neutral altogether.

Would this work? I'm not really familiar with auto transformers but figured since its a shared coil this shouldn't be a problem as long as all the enclosures/equipment are bonded through from the primary side.

That make sense or am I missing something?
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The thing that makes an autotransformer 'auto' is that there are not separate primary and secondary coils; you simply have a single set of coils that is connected to your supply (primary) and tapped for your output (secondary).

By definition you don't have to consider bonding the secondary, because you don't have a 'separately derived system'. You simply connect your loads. Grounding is determined by whatever the supply grounding is.

I believe you are _required_ to bring the primary neutral to the autotransformer, but I am not sure on that point.

In terms of connecting your 480V delta load, no problem there; simply connect your 'delta' load to your three supply phases, just as you would connect a 'delta' load to a normal 480/277V wye source.

-Jon
 
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paulengr

Senior Member
Auto transformers are wye-wye only. Delta wouldn’t make sense. Think about what your voltages are. You can feed it delta (no neutral) in which case it generates a neutral. Make sure this is grounded or you will be sorry you didn’t. Either continue the neutral of the high side or if it’s delta, start one. The loads can be delta or wye, makes no difference.

BUT do not forget that any fault or harmonics on the 600 V side affects the 480
V side and vice versa. The magic of a delta wye is they are magnetically isolated and ground faults don’t pass through.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
An open delta autotransformer works just fine; you can step voltage up or down; just that the 'vertex' of the V remains at whatever the supply voltage is. Say you have a 480/277V supply, and used an open delta autotransformer to step the voltage down to 240V for a delta load, that 240V would be 'referenced' to one of the 277V supply terminals, and the L-G voltage of the 240V would be 148, 277, 148 with strange phase angles. The 240V load internally would be perfectly fine, but the load to ground voltages would be all strange.

If you set up a _closed_ delta autotransformer, then the lowest voltage you could get would be half of the supply voltage, along with a 60 degree phase shift.

-Jon
 
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