Watts, Volts, Amps, and Step-Up Transformers for the Shop

Here's a discussion of using power transformers to supply machines that need higher voltage. July 31, 2009

I'm looking at purchasing a used widebelt sander that runs on 480V and would require a step-up transformer. How would the electricity usage compare to a running a machine with my voltage (208)? I assume there's a loss from the inefficiencies of the T-former, but does the higher voltage machine make up for it? Would it be possible/more cost effective to change the motor? (Single head machine with a vacuum). Any insights on using a T-former greatly appreciated.

Forum Responses
(Cabinetmaking Forum)
From contributor B:
Do you also have a phase issue?

From the original questioner:
No, I have 3 phase.

From contributor B:
There is a small amount lost in the transformer, given off as heat. Not enough loss to really care about. Step transformers sometimes hum, which bothers some people, but again not a big deal. The decision should be based on the x-former cost vs. cost of a motor. Most motor shops can change the voltage by changing the windings, or some will take a motor as a trade. Look at those costs and what shape the motor is in.

From contributor A:
We ran a 36" Yates planner for years on a step up transformer. 30 hp head motor so it was pulling plenty of amps. The transformer was fairly cheap, around $1,500 or so. No problem, just make sure you have the amps required, double what the motor says plus a safety margin.

From contributor S:
For all practical purposes, a high voltage motor will not use less electricity than a low voltage motor, as both will consume the same wattage. A "watt" is the basic unit of measure of electrical power that you are being charged for.

The formula reads: Volts X Amps = Watts

According to my motor chart, a 30hp motor at 208 volts will draw 92 amps at full load, and so will consume 19,136 watts. At 480 volts, a 30hp motor draws 40 amps, or 19,200 watts. Motors typically can tolerate variations in voltage up to plus/minus 10%. Since the high voltage motor draws fewer amps, you can get by with smaller diameter (less expensive) wire in the feed circuit; also a smaller breaker, smaller disconnect, smaller conduit, etc.

These savings can be significant, especially if your sander is a long distance from your service entrance - you (or your electrician) will just have to "run the numbers". Any time you "transform" one voltage to another, you will lose energy - you can't get something for nothing. The amount of loss depends on how efficient the transformer is, and all such transformers will have an efficiency rating.

From the original questioner:
Hey guys, thanks for the input. Thanks contributor S for looking at the consumption in terms of wattage. I always think of amps first because of the demand charge, which I guess is where the main impact will be, plus the loss at the t-former. It seems like it's no big deal.

From contributor L:
Since the transformer will be after the meter it will have no affect on the demand charge (a bit of additional power for the inefficiency of the transformer.) We are on a 208V system with several machines having transformers. 480 for the router (my electrician gave me a transformer he took out of a hospital during remodel), 400V on the bander, 380V on the panel saw and contour bander. I just bought a used widebelt that runs on 480V so I've got to get another transformer. The upside to higher volts is smaller wire etc. The downside is it's really lethal. If you change the motor to 208V you will need to change the controls and maybe the wiring from the panel to the motor.

From contributor R:
Contributor S did an excellent job spelling it out. The only other thing to consider is the size of your starters and control circuitry. You'll need larger overloads and contacts running at a lower voltage and you'll need to look at the control voltage. There may be a transformer on your machine that is lowering the 480vac to 220vac, 110vac, or even 24vac. That voltage will obviously be incorrect if you input 208vac.

From the original questioner:
Contributor R - thanks for answering my next question. I figured changing the motor would be more involved than a simple motor exchange, but thought I should ask around. Contributor L - do you get charged a flat rate for your demand? We have a demand meter, so it will definitely impact my demand charge. The widebelt will need 2-3 times the amps as my next largest motor. Is there anything to look out for in getting a transformer?

From contributor L:
What I meant by "no affect" is that regardless of what voltage you feed the sander the power consumption will be about the same. Different utilities vary the way the charge for demand, some average it over a longer time period some almost instantaneous.