Autotransformer differences

On the weekend I picked up a new/used autotransformer for fairly cheap. But I was a little confused when I first opened it, never seeing this style of winding.

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The Transformers I have dealt with in the past, usually look like this.



What is the difference? They are both three phase autotransformers.

In all 3 phase transformers, the sections are independent, so having separate transformers, or a one-core assembly is equivalent.

Now, he one is standard, the other appears to have two sections only, there are only wires for two. That is the system of adjusting two of the phase wires, and not the third Works OK for delta, messes up the neutral if used on wye.

JST said:

In all 3 phase transformers, the sections are independent, so having separate transformers, or a one-core assembly is equivalent.

Now, he one is standard, the other appears to have two sections only, there are only wires for two. That is the system of adjusting two of the phase wires, and not the third Works OK for delta, messes up the neutral if used on wye.

Click to expand...

Thanks for the reply, I was just hoping to use it on a portable RPC jumping 240sp to 600 3p for field testing. This auto transformer is running about half the weight, and that's what I thought would be ideal for the cart. It is a 15 kVA, and it's going to have a 20 HP RPC. I'm trying to get my 600 to 15 A on a portable cart. I don't intend to use the neutral.

It looks like your two winding autotransformer is intended to be used in a buck/boost (really buck or boost) mode. For three phase, you only need two transformers if the voltage change is small compared to the input voltage.

Factories don't typically put voltages on transformers like that, they should have a more generic nameplate that states the connection diagrams and less specific variables like "High" and "Low". My suspicion is that this was a motor starting autotransformer with a fixed 40% tap that someone re-purposed as a buck or boost transformer and put their own voltage tags on it based on what they were doing with it.

The only potential down side of that is that motor starting autotransformers are sometimes designed to be used in very limited duty cycles, i.e. 15 seconds every 10 minutes max., hence the two coils and low weight compared to transformers rated for continuous duty. Just be cognizant of that risk when you use it as a test bench device. Often times (but not always) the transformer will have a thermal switch embedded in it that you would run the contactor coil voltage through on a RVAT (Reduced Voltage AutoTransformer) starter arrangement so that if the transformer over heats, it disables you from re-starting. That might be an option for you if it's there.