Transformer Wiring

[7JLAman4]

Is it possible to rewire a single phase isolation transformer to provide a 1:2 voltage output ratio.

 

[waross]

YES.
That is if it is a type of transformer that may be rewound. If so,it may be rewound for a 2:1 ratio.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7JLAman4]

Yes I have access to the primary and secondary wires. The transformer is small and only rated for 6A at 117V.

 

[waross]

Do you mean rewind or reconnect? I mistakenly read your post as "rewind".
What are the ratings on the transformer?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7JLAman4]

I was just typing as it came across. I noticed you mentioned rewound, and I was thinking for rewire! It is an isolation transformer, 117/117V, single phase, 6A, with a shield. Looking to use it to get 220-240V to an outlet. Not a permanent installation, just a shop toy.

 

[waross]

You may be able to use it as an autotransformer but it will no longer be isolated. In addition you may have 240 Volts to ground where a normal 240 Volt circuit has only 120 Volts to ground.
Connect one primary lead to one secondary lead. Connect this point to the grounded circuit conductor. (This will avoid 240 volts to ground if done properly.)
Energise the primary.
Check the voltage from the single primary lead to the single secondary lead. If the voltage is zero, reverse the two secondary leads. You should now have 240 volts.
BEWARE THE OUTPUT IS NO LONGER ISOLATED.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[oldfieldguy]

waross nailed the answer I was going to post.

"Not a permanent installation, just a shop toy".

In the years I spent testing, troubleshooting and commissioning power systems, I often found myself having to 'lash up' equipment I had on hand to provide a function that my equipment did not support but that I needed.

Turning an isolation transformer into an autotransformer is a good example. Just remember that when YOU do this, YOU know it's no longer an 'isolation transformer' but something entirely alien to whatever the nameplate might say. It is therefore dangerous to somebody who doesn't know about your modification.

This makes for a very unsafe condition, so be very careful about leaving such a useful little beast laying around for somebody else to work with.

old field guy

 

[7JLAman4]

I have a 3-Ph motor controller for 200-240V. I only have 120V 1-Ph available. In an effort to program the inverter prior to installation, the inverter is able to function with 240V across L1 and L3 terminals. Not looking to connect motor yet, just preset the drive parameters.

I had cut an extension cord, and rewired to the transformer primaries. The green ground wire is connected to the shield cable of the transformer. I connected an outlet in series with the secondary.