Don't give me that jargon.

October 31, 2009 11:35 AM

Do I need a rotary phase converter or a variable frequency drive for my three phase bandsaw?

I just bought a beautiful 36" Oliver bandsaw from 1905. Naturally, it has a three phase motor and there is no way we are getting three phase power out here.

I've ruled out a static phase converter since I don't want to lose a third of the horsepower so my options are as above.

I understand what the RPC does and think I know what the VFD does. The former generates a new leg of power which doesn't seem terribly efficient. The VFD, I think, digitally splits the 220 into 3 phase 220 which is what the motor needs.

Is there something about the VFD that I don't get? Why does my fathers electrician friend say that "they are very specialized. You don't need it." I actually think I do, and when my massive lathe comes in I will definitely want the VFD to control the speed.

BTW, at no point will I be using both bandsaw and lathe simultaneously.

posted by mearls to home & garden (10 comments total)

In a sense both kinds of converters are doing the same thing. The rotary converter converts the incoming power to rotary motion, then generates 3-phase AC from that. The VFD converts the incoming power to DC electricity, then generates 3-phase AC from that. The VFD is only possible because of reasonably-modern semiconductors.

A VFD can, as the name suggests, alter the frequency of its output. This is useful since the kinds of AC motors you're probably using are synchronous— that is, the rotation of the shaft is locked to the phase of the power-line. To adjust the speed of the lathe, then, you either need belts/gears/etc. after the motor, or a variable-frequency power-line feeding the motor.

Another option is to replace the 3-phase motor in your bandsaw with a motor that can run off of the power you actually have (2-phase 220). This is probably the most efficient option since any conversion of power loses something. It'd probably also be cheaper than a converter. But if you're going to end up with a VFD anyway, then I can't think of a reason not to use it for the bandsaw as well.

Disclaimer: I'm coming from an EE standpoint, not a shop-tools standpoint.
posted by hattifattener at 12:39 PM on October 31 [2 favorites]

If you want to put a VFD on your system to control the speed you will need to get a motor rated for VFD use. An older motor will not has the correct type of insulation on the motor windings to handle the high frequencies that a VFD produces, you will quickly burn out the motor.

If you want to switch between two motors you will either need a very fancy VFD that knows how to talk to two motors (something like a Control Techniques Unidrive SP) or you will need two motors of exactly the same specs (HP, Voltage, etc.).

I think the 'two motor' option is only available on the larger sizes of Unidrives, but I've never used that feature so I'm not sure.

Feel free to me-mail with VFD questions, I've set up a few dozen of them on the machines that I work on.
posted by Confess, Fletch at 1:19 PM on October 31

You might find some of this helpful.

Are you sure you need full power? Those 36" Olivers typically have power to spare.

Hattifattener, replacing motors on some of that older equipment is well nigh impossible, as the motor housing is sometimes integral to the rest of the machine.
posted by jon1270 at 1:21 PM on October 31

Huh, my (extremely limited!) experience has been that older machines' motors are usually designed to be easy to replace, or at least remove for maintenance or whatever. But it's quite possible I've seen a non-representative sample.
posted by hattifattener at 1:49 PM on October 31

Huh, my (extremely limited!) experience has been that older machines' motors are usually designed to be easy to replace.

Yeah, that's the case more often than not but I've seen some exceptions. Oliver, Tannewitz or Yates.. I'm not sure which; they're all blending together.
posted by jon1270 at 2:04 PM on October 31

This was originally a line driven machine and I don't know when the motor and mount were installed but it is a really nice motor with attached brake motor. Which is what exactly?

Hattifattener, thank you for the very simple description of the VFD. DC power...OK. Also explains why the slightly older electrician described it as magic. I think the 220 motors are cost prohibitive which is one reason there are so many three phase motors around.

Confess, Fletch - I think you are suggesting also that I need to pick my VFD with great care not just any old 3 HP VFD. At this point I don't think anyone is suggesting a RPC over the VFD which is about as close to consensus as I can hope.

Jon, Those guys on woodweb are serious sawyers. The max HP on the Oliver is 4, so I don't want to drop it as far as 2HP. I have maple and cherry that I'm cutting and cogging would not be fun.

Thanks.
posted by mearls at 4:51 PM on October 31

A VFD that can handle two motors will be much more expensive than two VFD's for one motor each. Any old cheap and nasty VFD should be fine for your application as long as it's sized correctly. Here's a 3 HP one that isn't too pricey and will probably do anything you care about. Those Unidrives have all kinds of crazy features like plug-in PLC modules and Ethernet control which I doubt you need.

My main concern is that you will quickly damage the motor that you have with a VFD.
posted by Confess, Fletch at 5:14 PM on October 31

I would expect a new 220V single phase 3 HP motor would be cheaper than a VFD. You could also probably find a used one very cheap. A used VFD not so easily.
posted by JackFlash at 6:01 PM on October 31

Right, the best option is to put a different motor in. There are going to be power losses in any other solution besides getting three phase run out to your shop.
posted by gjc at 6:04 AM on November 1

A thread here:
http://www.woodenboat.com/forum/showthread.php?t=75736
is talking about the same thing. In Post #16 is talking about a 10HP phase converter, to run multiple pieces of equipment (not simultaneously).
You can make one yourself
*Disclaimer - I did not read this article. When dealing with electricity you could kill yourself. IANAEE (Electrical Engineer).
posted by defcom1 at 2:37 PM on November 1