# [EVDL] Autotransformer for 230V charger off 115V plug



## EVDL List (Jul 27, 2007)

Hello Steve,

For a autotransformer, transformers, fuses, circuit breaker and any 
electrical devices, it is best to not go over 80% of its name plate rating.

On a 20 amp device, it is best to only draw 16 amps at a continuous rating. 
Some line type oil cool transformers that we install can go to 250% over the 
KVA rating for a short time.

Let say the autotransformer has the input line taps mark :

| | |
L1 N L2


You can connect 240 volts AC between L1 and L2 and then tap off L1 and N 
for 120 volts.

You can also tap off N and L2 for 120 volts.

You can also connect 240 volts AC between L1 and L2 and a incoming neutral 
to N.

Or can connect 120 volts AC between L1 and N and tap off L1 and L2 for 240 
volts.

The neutral and one of the lines may have been connected together in the 
last time it was use where the higher voltage may have a common ground. A 
480/240 volt transformer may have a 240 volt tap as a common ground which is 
a neutral point between the Line conductors.

In your case, the neutral should not be combined with any line wires.

I have a auto transformer that has a 120 volt input, that has outputs of 
208, 240, 280, 310 volt that was use with my old charger. I connected 120 
volts input to L1 and N and the output was connected to L1 and the 280 volt 
tap to a full wave SCR bridge for 180 volt battery pack.

Roland




----- Original Message ----- 
From: "Steve Skarda" <[email protected]>
To: "Electric Vehicle Discussion List" <[email protected]>
Sent: Friday, August 21, 2009 6:40 AM
Subject: [EVDL] Autotransformer for 230V charger off 115V plug


> I purchased a 2000 VA step-up autotransformer that boosts 115V to 230V for
> $20 on Ebay. My 230v charger has adjustable current so I can dial it 
> back
> to keep the AC line to 15A on a 20A receptacle. I'd like to use that for
> charging from a 20 amp, 115V plug at work. That should be 1800VA. Any
> reasons why this would be a bad idea? When an auto-transformer is rated
> 2000VA, can I assume that is a continuous rating? I tried it and it 
> really
> gets hot. I guess thats normal? When I look for an updated spec sheet, 
> it
> doesn't look like they make this particular model or size step-up
> autotransformer anymore.
> http://s227.photobucket.com/albums/dd106/skskarda/?action=view&current=12409469780143.jpg
> Thordarson 2000VA TSU
>
> Taking this a step further, I'd actually like 230V and 115V available for
> different functions while charging. At home, I used a 4 prong plug and 4
> wire cable so I have both phases, a neutral, and a ground. When I am
> connected to 115V, I will have a neutral and 115V on one side of the
> autotransformer and 230V on the other side. I guess I should 
> theoretically
> be able to use both the 115V and the 230V at the same time on different
> devices. It just seems odd to have my 115V neutral and one phase of my 
> 220V
> tied on the same line together which is essentially what I'll have. Oh,
> all of my devices are isolated so that is not an issue.
>
> Steve
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## EVDL List (Jul 27, 2007)

Thanks Roland. You helped clear up how I was thinking about the
autotransformer regarding how to think about where the neutral resides in
the circuit. ( I was thinking the neutral was on the bottom of the coil and
the 120V was in the middle). I think I get it now. I am no longer
scratching my head. Not exceeding 80% is good guidance. I wasn't even
really thinking about the fact that I was running it at 90% when it was
putting off pretty good heat.

I assumed that this would work but I had never heard of anyone doing it
before. Sounds like you have used that on an SCR bridge. My charger is
actually an 230V SCR based analog DC drive that I am using as a charger.

Steve
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