Using 3ph transformer as single pahse transformer

I live in Canada were most industrial machinery is run on 575V. I have five or six such machines in my residential garage. Some of them have
motors that are not easily replaceable such as the Bridgeport and the Swedish gearhead drill press. I used to use a rotary phase convertor in conjunction with a three phase transformer but I got sick of the noise.
I have bought a used 1.5 KVA single phase 220 to 600v transformer which allows me to use small 575V VFDs that I can buy ebay every once in a while. They seem to work fine when fead with single phase instead of three phase.
My problem is my lathe, it is 60 year old 2-1/2 ton monster. It has a 7.5 hp motor tucked under the headstock gearbox. To replace the motor I would have to jack the headstock end of up a couple of feet off the floor and drop the motor. I should probably replace the 575V motor with a 5hp 208V motor that I could buy quite cheaply, but I dont relish the job.
I have been toying with the idea of using a 30 KVA 3 phase transformer that I have to step up 220V single phase to 600V single phase and running the lathe with a 7.5 HP 600V VFD. The problem is that I have not been able to get the output voltage I need. The transformer's primary is 600V and wired in a Y and the secondary is 220V and is wired in a Delta. I beleive the transformer original use was as a step down distribution transformer. When I apply 230V single phase to two of the secondary connections, I measure 500V (phase to phase,no load) on the primary side.
I am thinking of buying a 480V VFD and driving the motor with that, the loss of power (according to my calculations) would be about 2.5 hp which is fine by me. Anyone have any comment on this scheme, if not I'm just going to try it and let you know how it all works out.
stan
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stanley baer wrote:

If I'm reading your description correctly and your 575V Y service is like the 480V Y service in the US then I don't think that transformer will work and I don't think that 500V you read is correct either.
The US 480V Y service is 480/277V as in 480V phase to phase and 277V phase to neutral (square root of three thing). This would put your 575V Y service as 575/332V if I'm correct.
With a Y wired primary on your transformer each transformer primary would be expecting to see 332V across it, not the 575V phase to phase voltage. 575V would be across a pair of the windings, not an individual one.
The secondary winding would be for the 220V since it's for a delta configuration, which would make each transformer section basically 332V to 220V, not accounting for adjustment taps and losses and whatnot.
If you were to rewire and power two of the transformers sections on the secondary side with 220V, making sure they are properly phased, you would most likely get about 664V on the primary side across those two sections due to the fact that you are feeding single phase so the square root of three thing wouldn't apply to give you the 575V.
The 500V reading you got was probably a result of the magnetic coupling to the other transformer sections. It sure doesn't seem like this transformer would do what you want it to do. It's probably best to track down a suitable single phase transformer.
Pete C.
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Pete C. wrote:

Wouldn't I rewire one of the three instead of two of the three. By my calculations the actual wiring ratio is 1.58 ((600/Sqrt 3)/220)which means I would get about 348v from stepping up 220 single phase using a single trnsformer coil.

Thank for the explanation Pete. I did an experiment that I did not mention in my first post that may corresspond to what you are saying. I applied 120 V single phase across two of the secondary connections (the delta ones). I measured the phase to neutral voltage on the corresponding primary coil by exposing the copper bar that links the primary coils together. The voltage reading was about 150V.
stan
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snipped-for-privacy@robustmachine.com says...

Leaving aside the transformer issue, as long as you configure the VFD properly it'll work fine. Tell the VFD it's powering a 480V/50Hz motor, the drive will output the proper volts/Hz, and you'll get 50/60 x 7.5HP from 50Hz up to whatever you set as the maximum speed on the drive. IOW, you only lose 1-1/4HP.
Ned Simmons
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says...

Ned, I'll be the first to admit VFD's aren't my thing. Can you please explain the loss of 1-1/4 HP? Is is related to the square of the voltage difference between 480 and 575 ; that would come out closer to 1 1/2 HP wouldn't it?
Bob Swinney
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snipped-for-privacy@comcast.net says...

Simple open-loop frequency drives deliver constant torque up to base speed (the motor's nameplate speed) by varying the motor voltage with frequency. In order to maintain the proper volts/Hz ratio for the motor I was suggesting that Stan program the drive for a 480V/50Hz motor.
480V/50Hz = 575V/60Hz
Since the motor's torque won't be affected by this change, but its redefined base speed will be 50/60 x nameplate speed, HP will also be 50/60 x nameplate HP, or .83 x 7.5HP = 6.25HP.
Ned
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