Transformer Question

[Motorwiz]

I am working on a draw bridge dedicated in 1935 that has 115v 3PH power powering two air compressors, one of which died. Is there any reason I could not use a 230v input to 460v output transformer to achieve 115v to 230v? The motor is only 1.5HP.

I wanted to just install 115V 1PH motor but the state electrician does not.

 

[burnt2x]

The effect will be deriving a 230V, single phase output(115 x 460/230) from the 115V source. The only problem is that your transformer will be operating at a lower flux level which will mean a bad regulation, meaning - the secondary may not be 230V at all when the load is thrown ON. The motor may not be able to start properly with too much reduced supply voltage. Please ask your electrician if the transformer you got have its center taps brought out of the casing (both primary and secondary) and arrange for a change into 115/230.
Else, ask for a new transformer fit for the purpose.

 

[ccjersey]

I would look at converting the supply transformers now and install new 3 phase compressors/motors. This thing is giving you warning!

Too big of a project? Then I see no reason not to use the 115 Volt single phase motors on both compressors assuming the existing transformer, starters and wiring can deliver the power or can be upgraded to deliver it.

Why doesn't the electrician want to use single phase motors?

 

[waross]

You may use 240 to 460 Volt transformers.
You may also use 120 to 240 Volt transformers at about 1/2 the price. 120/240:240/480 Volt rated transformers are readily available and will be half the rating of 240:480 Volt transformers.
There are also two auto-transformer schemes which may be used to boost the voltage 200%.
If you have a set of suitable 240:480 Volt transformers in stock and available go ahead with them.
At 50% voltage the flux curve is pretty linear. No problem with regulation. There are a number of metering schemes where 120V transformers are used with normal circuit voltages of 69 Volts. The accuracy is adequate for both revenue metering and for protection.

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

 

[Motorwiz]

We have not bought a transformer yet. I have contacted three supply houses that I normally use and the only answer back has been a 15KVA transformer that is bigger and weights more that the whole compressor skid.

I was trying to find a ~5kva transformer that could be put on the skid so that the bridge crew has a complete package that could be picked up and moved to any spot needed, as the main power run over the bridge is 115v 3ph.

 

[waross]

It sounds as if they don't move many transformers and want to sell what they have in stock rather than order the correct xformer.
My book rates 1.5 HP/230 Volt 3 Ph. motors at 5.2 Amps. A 2.5KVA three phase transformer will do one motor.
If both motors are on the same skid then you are right on at 5 KVA.
If you can live with an auto-transformer connection, you can run one motor on two 750 VA transformers. You can run both on a pair of 1500 VA transformers. The difficulty may be getting 120:120 Volt transformers.

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