Sizing a Phase Converter for the Load

Advice on matching the capacity of a rotary phase converter to the demands of the equipment it serves. May 28, 2010

I am days away from purchasing my first widebelt (bout time!). It is an 18 hp machine. I was thinking of getting a 25 hp rotary phase converter. Will a 20 hp be enough? This is the only 3 phase machine I own. Someday I may have more, but I can not afford to get a ginormous converter. Once I have the correct size, is it easy enough to hook up myself, or should I get an electrician over here? I'm no slouch, but I'm also not a master electrician. I don't want to screw this up.

Forum Responses
(Cabinetmaking Forum)
From contributor G:
I have pretty much the setup you are looking at. 18hp sander, along with a bunch of other 3ph machines. I bought a 30hp rated converter along with the sander. It is a heavy load at startup, so size your system accordingly. It won't cost much more to do it right. As a bonus, you can run the rest of the shop on it as well as you grow. I would have an electrician wire up a sub-panel for the 3ph system. This is a lot of power, so unless you really know what you're doing, leave it to the pros.

From contributor S:
I agree with contributor G. I have 3-phase in my shop now, but I used to run a phase converter and it would always be the initial startup load that was the issue. I think once you turn it on, you're good to go. I would go with the 25hp minimum, and go a little larger if you can afford it.

From contributor K:
We run a 6000' shop with tons of equipment with one 60hp phase converter without any real issues, but yes - sanders take a lot of amps during startup, so oversize the converter if you can afford to.

On the flip side, our converter pulls 70 amps constant from our single phase service 8 hours a day constant plus various short term spikes during machine startup, but if you're only going to be running the converter during the time you're going to run your sander, oversize the converter.

From contributor P:
I've got a 20hp converter that starts my 12hp widebelt fine. I think they suggested 50-100% oversize for the starting load. If you've got the capacity, go larger rather than smaller - 20 hp definitely won't cut it, and 30 hp gives you a comfort zone.

Be sure you have sufficient ampacity on the converter's input side to allow for starting - I sized the wire run to my shop for the running amperage of the converter, so needed a workaround to get it sufficient starting amperage.

Wiring's not a big deal - I'm no electrical whiz, but comfortable around panel work and the like, and my installation was trouble free. I bought mine through American Rotary, and they were a lot of help with sizing and walking me through the install. Great support, and highly recommended.

From contributor Y:
Also a phase converter can be set up so it can be started with the start button on the machine it will be running. There is a delay timer that will start the sander once the phase converter is up to speed. This saves you having to start the phase converter separately each time you start the sander.

From contributor P:
I was told that the PC should be started and stopped with no load - not even lighted buttons on machines - so important to have a delay before the sander turns on and off. Noise probably won't be an issue with the widebelt, but my converter runs all day, so I installed the control box in the shop and put the motor below in the crawl space, so all I get is a low hum when it's running.

From contributor D:
Rule of thumb is, the converter should be twice the size of the smallest motor to be started. I run a two head Timesaver with a 25hp and a 20hp motor, therefore I installed a 50hp converter. Not a problem starting any or all machines in my shop.

From contributor N:
We have used rotary phase converters for several years. We have 5 of them in use. Mainly because of the high cost of getting 3 phase run to our shop. They are all Arco Roto-Phase units.

From the original questioner:
The sander is a Butfering 109. Says it runs on 50 hz at 230 volts. The converter is a 50/60 hz at 240 volts. Is this a major problem or does the converter sort it all out?

From contributor N:
We have several European machines running on phase converters and just make sure control voltage is not running on the manufactured phase or it will do crazy things. I have learned a lot over the years about how not to hook up a phase converter. Ask for Tim Drake at Arco and he can lead you through the installation.

From contributor S:
Have a look at the digital phase converters. I have a 30 hp Phase Perfect unit, and it runs all my machines without issue. They are more expensive than rotary units, but provide true balanced 3-phase. If you check the output of most rotary converters, the generated phase is much higher voltage. That is why they have a minimum motor to run. This could cause issues with the small motor that is used to raise and lower the head, if the main motor is not running.

The converters are sized based upon maximum load, and if you get a rotary from a well known company, then a 20 hp will start up your sander. I used to have a Butfering 109, and my rotary started it just fine (I used a 15 and a 7.5 hp Kay converter back to back). I switched over to the Phase Perfect after having issues starting my Martin shaper (2 speed motor) and could not get it into the second speed using the rotary converters. The Phase Perfect works great. I now have a Kundig sander with a 25 hp motor and dual belts. I also checked the amount of amps the rotary pulled when idle. It was huge. The Phase Perfect uses 750 watts of power when idle.

Phase converters do not change the Hz. We are on 60 Hz in USA and if you are buying a sander intended for USA 230 V 60Hz you are fine. They are typically the same motor for 50 or 60 Hz, and the motor will spin faster at 60 Hz, but since there are pulleys that are sized, the head speed is the same.

If you do go with a rotary converter, be sure that you use the 2 phases from the utility to run all control circuits. The third phase only goes to motors. When the sander is installed, they should be able to figure this out. With a digital converter you do not have to worry.

By the way, all the phase converter is doing is offsetting one of the phases so it is 120 degrees from the others. A rotary converter works by running a 3-phase motor and using the output of that motor to create the third phase. A standard motor can generate power if an external source is spinning it. A digital converter does not have moving parts; it electronically generates the third phase, and thus is much more power efficient. I found that my machines ran cooler and smoother starts after changing to the Phase Perfect.

From contributor G:
What did the 30hp Phase Perfect run?

From contributor S:
I think I paid $5000. Obviously a lot of money, but far less than the machines it is powering.