Transformer sizing

I am currently looking for a 230v to 575v auto transformer. Apparently the auto type is reversible.

I have a 50 amp welding plug. 6 gauge wire with a neutral going into my garage on a 50 amp breaker.

I was wondering if i will be alright with a 15kva transformer. The online calculator asks for single or 3 phase to compute. Not sure which variable to put in.

I haven’t got the rotary converter yet. I was thinking a 10-15hp converter.

This set up will be to run a Tos sn50c that i just purchased. Its all 575v . The main , the pump, as well as the rapid traverse motor.

Id appreciate any input on the subject. I haven’t bought anything other than the lathe yet so not committed down one path yet.

Thank you in advance Virgil

Most transformers are "reversible", the "auto" in Autotransformer just means "self", i.e. the true latin meaning of auto, not auto as in a shortened version of automatic. The primary and secondary of an autotransformer are actually still connected to one another as one circuit so the transformer action takes place on itself. The "opposite" of an Autotransformer is an Isolation Transformer where the primary and secondary are not the same circuit, they are only magnetically coupled through the iron core of the transformer. For something like a welder, that's a better idea, in fact you really want a SHIELDED isolation transformer as best practice.

Per the newer edition of the NEC (starting in 2014 I believe), if you use any transformer for "step-up" instead of step-down, the transformer MUST now be identified for that purpose, either on the transformer nameplate itself or in manufacturer's data sheets, which you may need to produce to an AHJ if you are installing it with a permit. Most transformers will qualify, but there are a few designs that say no step-up use.

PS: Just noticed it's a welding PLUG, not a welder, so forget that... Also noticed you are in Canada, so I don't know if the CEC is the same regarding step-up use. I also left out sizing: if a 15kVA converter works for you, then a 15kVA transformer is fine. The real question is what does your load require?

The spindle motor is 575v 8 hp.

Saw Doctor said:

The spindle motor is 575v 8 hp.

Click to expand...

15kVA should be fine then.

Isolation Transformer will be a better choice. I am using 240 to 480 in my shop. Please note that in BC, and maybe other Canadian provinces, 240V is the highest voltage allowed in residential buildings.

Please explain why an isolation transformer will be the better choice? Honestly I haven’t the experience to really know.

Ive read that isolation transformers are friendlier to voltage sensitive equipment. I dont think a lathe qualifies as sensitive but i could be off the mark on that one or everthing! Lol

The cost of an isolation transformer is 50% greater than an autotranformer. I dont mind shelling out extra cash, but hate to do it needlessly.

Again thank you in advance for your valuable input.

Saw Doctor said:

Ive read that isolation transformers are friendlier to voltage sensitive equipment. I dont think a lathe qualifies as sensitive but i could be off the mark on that one or everthing! Lol

The cost of an isolation transformer is 50% greater than an autotranformer. I dont mind shelling out extra cash, but hate to do it needlessly.

Again thank you in advance for your valuable input.

Click to expand...

The reason a 2:1 or 1:2 auto transformer is half the cost of an isolation transformer is because electrically it only needs to be half the size. which means the transformer is about 2/3rds the cost and 2/3rds the weight to get similar efficiency under similar conditions. a 4:1 or 1:4 isolation transformer only saves you 25% mass of copper, so its diminishing returns at that point. But when you need 3mm of insulation around the entire secondary and that insulation takes up 10% of the core volume, a 4:1 auto transformer might save you 35% on the core, and 25% on the copper.


Because its half the size and weight and because there is one coil, the leakage inductance is lower. Leakage inductance is a portion of the total "impedance" printed on the nameplate. However because the transformer is smaller, losses are higher. why? because they can be. less volume to surface area ratio means a higher power density can be achieved within the same xx Celcius temperature rise. So I would not expect to see a significantly lower impedance from an auto transformer than an isolation transformer of identical capacity and identical temperature rise from the same manufacturer.



anyhow a transformers leakage inductance and the physical isolation of the coils is what makes them useful and potentially helpful for some loads. But they should not be needed.

For example a welder should have complete isolation internally from the input (240v, 480vac) to the output. if you have a welder who's output is grounded to the incoming neutral for example and if you have a high frequency start on the neutral side of the output (because you had the polarity reversed or because you didn't want the high frequency start to shock you), you would be radiating that high frequency start into your plant's neutral line. this shouldn't matter btw, but sometimes weird shit happens and just throwing a transformer in line with a welder will fix some problems. but these problems are unknown unknowns until they are investigated and usually nothing gets investigated unless its worth the time.

Do you have a photo of the transformer tag you are considering?

Does the saw have any VFD's included or, are the motors powered across the line?

There can be some grounding implications, depending on particulars, going from a center tapped delta, phase converter output, to a wye type autotransformr, stepping up to 600V.

The proper grounding of a wye autotransformer could be an issue. A full isolation one may be a better choice for this issue, depending on particulars.

SAF Ω

15 kVA 575 Volt to 23 Volt Autotransformer RC15J1-C1

Hope the link works

15 kVA 23 Volt to 575Y332 Volt Isolation Transformer BC15C1-Q1/Z

Looks like the price has gone up about 300$ in the last couple of days.

Its a lathe that i will be powering with a rotary phase converter. No vfd’s

I thought about changing it over to 230v but then you run into wiring that may be too small of gauge, having to rewire the whole machine and troubleshooting etc. Seems like a transformer is the easiest way to go. Should keep the electricians bill down to a minimum.

Either of those transformers will work for providing 600V 3Φ output. However there are some differences in the output with respect to the ground.

A RPC output is a center tapped delta, with a hi leg voltage to ground. If you feed an autotransformer with a hi leg voltage in, you will get a hi leg voltage out, with respect to ground. Phase to phase voltage will be the same, but not the voltage to ground.

On the hi leg input, the voltage to ground is 15% higher (240/208). On your 600V output, 15% higher output, with respect to ground, would put it at 690V. This may be more that what the equipment controls are rated for.

This non symmetrical output can be detrimental to VFD drives as well if you plan to power something else in the future.

Another unwanted feature of the autotransformer is higher impedance to ground for fault currents, when something shorts out to the frame.

The wye autotransformer center point cannot be solidly grounded. Input and output lines are directly connected to the same winding. The input is non symmetrical, therefore there will be a different potential at the wye point than the supply ground reference. If you were to try and ground the center point, a large current would flow from the difference of potential.

Any fault (short circuit) current from the load would be required to flow back though the ground wire to the feeder source, then through the phase converter, and finally back to the transformer, to trip an overcurrent device. This added impedance in the fault current path can be detrimental when a fault occurs.

Wye autotransformers are best applied to a supply system that is fed from a wye system so that both center points can be solidly connected to eliminate the issues.

A full isolation transformer is more expensive, to be sure, but suffers none of these issues when properly connected, and the output wye center is solidly grounded, to the supply system.

If the transformer has to be made, it might be beneficial to have taps added so that the voltage could be adjusted to better suit your site conditions, especially when an PRC is used in the supply source.

SAF Ω