electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
autotransformer sanity check
- Thread starter electrofelon
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MAC702
Senior Member
- Location
- Clark County, NV
I'm thinking yes, too.
What do you mean "if you didn't?" If you didn't connect the neutral, you wouldn't have one. What am I missing?
What do you mean "if you didn't?" If you didn't connect the neutral, you wouldn't have one. What am I missing?
I think he means not connecting the incoming neutral.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
For step-up:
You do NOT connect the X0 terminal on the 208V side.
You DO connect the X0 on the 480V side.
You do NOT connect the X0 terminal on the 208V side.
You DO connect the X0 on the 480V side.
mbrooke
Batteries Included
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- United States
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- Technician
Then how do you deal with zero sequence currents? Or preventing distortion of the L-N voltage vector?
The neutral of the 208 and 480 volt systems need to be tied solidly together while the EGCs do not connect to the X0/H0- just the chassis. This is not a separately derived system. The 480 is just a continuum of the 208.
mike_kilroy
Senior Member
- Location
- United States
Agree, tie both together. I think jraef is thinking of the more common back feeding a wye (secondary) isolation xfmr as primary.
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mbrooke
Batteries Included
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- United States
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- Technician
I wold say so- a wye delta. In that case DO NOT ground the primary. Very bad things will come of it.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Geez, autotransformer was even in the title... I gotta get new glasses.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Ok, yeah I am aware of that one and perhaps that is why I am paranoid about connecting the 208 neutral. I was expecting to receive an isolation transformer for this job so the auto threw me for a loop.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
The transformer would make a neutral so it seems plausible that you wouldn't need to supply one, that was my thinking.
mike_kilroy
Senior Member
- Location
- United States
And I agree with you too. It would have made you a good 480 neutral, but seems more robust tying. Great question.
Some (most?) 208Y DO tie X0 to EGC so not sure why to keep it separate? Doesn't every 120v circuit get the X0 tied to EGC in each breaker panel? Maybe worth more discussion on that?
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It's an autotransformer.
The 208V and the 480V are taps on the same winding. "Sides" don't really come into it.
What am I missing?
mike_kilroy
Senior Member
- Location
- United States
posts #6 and #8.
kwired
Electron manager
- Location
- NE Nebraska
but if you have an unbalance as a result of 120 volt load on one phase - the neutral voltage may shift as a result of load balancing - having primary neutral tied to center of the wye will stabilize neutral voltage regardless of current imbalance - within a reasonable range anyway.
Separate windings like a isolation transformer has is different.
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
They only provide a single lug for the xo/ho terminal which is odd.
kwired
Electron manager
- Location
- NE Nebraska
Technically it is only one conductor of the system tied to that point. Nothing is separately derived with this setup.
Maybe we both need new glasses.................
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Right but you will have an incoming and outgoing neutral, seems they would give you two terminals or a double lug
electrofelon
Senior Member
- Location
- Cherry Valley NY, Seattle, WA
Maybe we both need new glasses.................
It has been established that jraef missed that it was an auto not an isolation. That has been settled.
mike_kilroy
Senior Member
- Location
- United States
We use DIN rail mounted dead front terminals on our transformers. Perhaps they did too? If so, YOU could buy one and slip it in on the DIN rail to make your connections nicer? On the same note, aren't TWO wires allowed per terminal per NEC/NFPA, etc? Remember the X0 is generally not a current carrying point (unless an old SCR drive system, etc.) so might be able to be smaller gauge? I am not sure on this point for your case - just bringing it up as idea in case you do not add a second terminal.
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