Transformer Compatibility: 208/480 work for 220/440 mill?

The guy selling this says it turns 208v to 480v but isn't this tag saying it makes 480v into 208v?
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In theory (and usually in practice as well) you can run a transformer backwards, feeding the secondary and taking the output from the primary.

However, feeding 220 (or 230 or 240) volts into a 208V winding will at a minimum make the transformer run hot. Worst case it runs so hot that it burns up. And since you are feeding 220 into a 208 winding, you'll get something like 508 volts out of the 480 winding.

Next problem: you didn't mention how big the motor that you are trying to run is. That transformer is huge - 45kVA could run something like a 50HP motor when used as intended. Because it is a big transformer it will have big inrush current (tripping the 220V breaker), and draw a lot of no-load current. Not good.

A further complication (and this is the real show-stopper): That transformer is three phase. You have one phase power. You can't feed a three-phase transformer with single phase power. Period. Done. Do not pass go.

In theory you could use a VFD to convert your 220 single phase into 0-208V three phase, then the transformer could step it up to 480 three phase for the motor. In theory. I'm an electrical engineer and spent most of my career designing VFDs, and I would try to find another way before I tried using the transformer on the VFD output. Just too many ways it could go wrong. For someone unfamiliar with the technology I wouldn't touch it with a ten foot pole. And the fact that the transformer is still way over-sized makes it worse.

You want a single phase 240-480 transformer. And not 45 kVA.

You didn't mention the size of the motor you are trying to run. If it's a typical Bridgeport or similar 1-1/2 to 3 HP motor, you could use about a 5kVA to 7.5kVA transformer. Something like these:


Any of those would convert your 220 single phase to 440 single phase. Then use a 480V rated VFD to run the motor.
 
Wow, I didn't realize any of that! Thank you!! It is a 3 phase, 2 speed motor, 1800rpm at 1 hp and 3600rpm at 2hp. It came right off a 440v main so it would just need 440v attached to the lines in the box. The motor is a 6 wire. I just took it off the mill and it spins absolutely smooth as silk.
The link in post#6 above is where the pictures of all the wiring, diagrams, etc are if you could take a look. On these transformers, they all seem to indicate they are 480 to 240 so my ignorance is obvious. How can they make 220 into 440? I think I get the reason and methods to change single to 3 phase but I would love to understand this better.
 
The motor in my YouTube was originally 400v 2 speed. The switch had five positions: two forward and reverse speeds and off.

Your motor has to be wound in a similar way. If you've only got six wires presented externally, you're going to have to delve into it, identify the windings as I did, unsolder the leads from the tails of the windings and rearrange them to suit running the windings in parallel delta, rather than switchable series/parallel star configuration that the Dahlander scheme runs. Its really not as scary as it sounds! Mine has been running beautiful for six or nine months now; something like that. I've asked a lot of it on occasion too.
 
The bottom line as I mentioned in your other post for your mill, you cannot use a VFD to provide power to your control box, it is only designed to power the motor directly. So your only choice would be using an RPC to generate 3 phase, then feeding a 3 phase transformer to step up the voltage. You probably would need a 3 phase transformer with adjustable taps, as line voltage is often 240VAC. Using a 480 to 240 transformer in reverse and adjusting the taps you could get around 460VAC on a 95% tap. Motors/equipment is usually rated for +/-5% voltage variation. Some possible examples below, but I would defer to someone with more experience to give you specifics as to KVA and hookup.

I do a fair amount of VFD installs/work, what I often will see if you are just driving the motor would be to use a single phase step up transformer (like 5-7.5 kVa) to go from 240 to 480VAC or something in that range and feed a 400V VFD (i.e. these are rated for 400-480VAC in) derated for single phase input (so 5 Hp VFD) and the VFD sets the output voltage to the motor (it is directly wired to the motor. The sub systems are run off the transformer tapped for 240VAC single phase input. You CANNOT use the current contactors and motor controls for the spindle motor and the VFD would be directly wired so the motor runs on it's 2 Hp setting. I also mentioned that there are 230VAC single phase input VFD's that output ~440 VAC 3 phase from China and the UK, how well they will work is just a guess, but the ones from China are around $100 for a 2.2 kW VFD. I wouldn't use one on any of my system builds, but it could work. VFD's for the most part will not drive a transformer on their output, they are not made for variable frequency VFD wave output. My understanding that there are specific transformers for this purpose to minimize the current/size of cables between the VFD and motor, but they are very expensive and application specific.

These might be suitable for 3 phase 460Vac conversion from 240VAC 3 phase RPC.
 
As mksj (and others) have pointed out, the phase converter followed by a 3 phase transformer makes a great set up. Most any small transformer that is intended for step down will work fine for step up as well. My shop ran well for several years with the RPC feeding several 240V machines and a few 575V machines. When I got a larger machine I needed a larger phase converter and changed to a Phase Perfect - it also feeds the transformer and 575V machines (both options work great - the Phase Perfect works better, priced higher).

I'm not saying the motor rewire won't work. I can say the phase converter and transformer work great. Strictly speaking the phase converter is not part of your decision, since the motor rewire still needs a phase converter.
 
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