Phase converters

J

Jeff Saylor

Well-known member
I'm looking into purchasing a bandsaw with a 5hp 3 phase motor.  I don't have 3 phase in my shop and want to know what's the difference between rotary and static phase converters.  Is one more reliable than the other?  Any drawbacks to phase converters vs. switching out the motor?
Any advice will be greatly appreciated.
 
Well...... I know a good bit about this stuff but need a little more info. Just what saw are you looking into? If it's direct drive (wheel right on the motor shaft) swapping out for a single phase is pretty much out of the question! If this is the case by far your best option is a vfd (variable frequency drive), second choice a rotary convertor. If this saw is big enough to need a 5hp motor it's pretty safe to assume it has large wheels which means a long, hard start cycle which even the best static convertors can't handle for very long! Statics are ok for a machine that starts up quick and isn't heavily loaded and you'll only get about 2/3 of the hp on the tag!  Even a single phase motor can only handle a short start cycle without it's lifespan being drastically shortened. My 42" direct drive bandsaw took about 15 seconds to come up to full speed, no single phase could ever handle that! Vfd's are great! Not only will the right kind give you 3 phase and electric braking but they protect the motor in many ways: over current, under voltage and a bunch of other good stuff. And the 2 best features of all.... you get nearly infinetly variable speed on ANY 3 phase motor and they actually save you electric! You can set them to come up to full speed slowly so you don't "brown out" your house when you turn it on!
Hope this helps, Mark
 
Mark-  with the VFD, In what situation would you want to reduce RPM's?  Is that the only advantage? 

I went thru this same dilema quite a while back when I set up my shop.  From what I understand, with the static converters, it is only for one machine and you loose 1/3 of the power.  I purchased a used 10 horse rotary converter that I had installed professionally to run my entire shop- well, only 3 machines are 3 phase.  All 3 legs are pretty uniform in voltage which is good.  If you are ever going to run  sensitive digital type equipment the converter has to be good. 
 
Jeff,
I made my own converter from a used 3 phase motor and some capacitors. I Worked from an article in an old FWW magazine(black and white, # 26 I think) . My neighbor has one he made from a used 3 phase motor and a used single phase  motor. Once you get the 3 phase converter motor running it generates the third leg which will enable you to start your machine. A VFD for your 5 HP machine would run $650. This is what is on the larger commercial variable speed lathes like my Powermatic. Here is a link http://www.metalwebnews.com/howto/ph-conv/ph-conv.html that will take you to an article on the web. There are several articles out there. Many old metal working machines are also three phase so the topic gets discussed there too.
Mike
 
The most common reason to reduce speed is the lathe as Mike suggested. Any time you need a speed other than what you can get by changing pulleys is fair game. Some shapers won't go low enough to run large raised panel or tenon cutters. Even something as simple as a bench grinder might benefit from a different speed and avoid burning a tool. I've used them to help find and eliminate weird vibrations too. Don't think that a vfd will only slow motors down. Like I said if something coasts too long they can work as a brake. The protection is better than any fuse or cicuit breaker. I'm putting one on a delta shaper to speed up router bits so I'll have something more heavy duty and accurate than any router table. The spindle on cnc routers is actually just a 3450 rpm motor but balanced for the higher speeds and better abec class bearings. The vfd is what speeds them up. Fine Woodworking's book on ww machines has the article Mike mentioned. Like him, I too made my own rotary years ago and it served the purpose on a 12" jointer, two shapers and a 12" grinder for years but they're just not the same. I used to run the convertor and bigger shaper to get the jointer runnning.  Without just the right run caps for power factor correction it won't be true, balanced 3phase. You may get lucky(I did) for decades in a home shop situation, maybe not. I picked up over half a dozen vfd's at a local auction for less than the cost of one new static box! Check on ebay, they always have some. Just be sure it will take single phase input, not all do!
 
I agree with Mark, a VFD is the way to go. My company bought one for a metal lathe form this company http://web5.automationdirect.com/adc/Overview/Catalog/AC_Drives?source=google&keyword=vfd&type=search&gclid=CLqYsM2A6ZoCFQQVswod2nx5CQ.

The guys in the shop have been very happy with it (it's 3-phase to 3-phase they wanted the speed control). So much so that one of the shop employees bought one for his home shop and sold his rotary converter. He wired his in place of the converter, so he is running his whole shop off one VFD. He said it's no problem but if you leave the speed low you notice, and have to walk over and turn the speed up. He said he can run more than one machine at once but if the speed is reduced it's reduced for all of them.

If I ever buy a piece of 3 phase equiptment I would buy a VFD. It would be cheaper than a motor swap (assuming you buy something like a baldor), and you can use it for all you machines. Also I've noticed 3-phase stuff at auctions seems to go for a lot less.

The bigger lathes like the Powermatic and the oneway lathe all use a 3 phase motor with a built in VFD for thier speed control.
 
Thanks to all for the info.  As it turns out, I'm not going to buy the bandsaw after all.  It's huge, but only has 13" resaw capacity and I already have 12" with my current saw.  But the info will be good to have for the future- who knows what's out there waiting to be bought!
 
Jeff, too bad about the saw.  I don't have any experience with VFD's, but I do run about 2/3 of my shop on a 10HP RPC.  It's just a box full of capacitors with a switch that supplies enough juice to start the 3 phase motor.  Regular 220 will turn a 3 phase motor, but won't start one.  My previous set up was a 5 hp motor.  No capacitors, just a rope to turn the pulley to get up enough momentum to start the motor.  The plug was run right from the motor.  This was definitely low tech and kind of a pain in the *ss to start, but it worked until I found a better rig.  The point is, it is entirely possible to run 3 phase motors as long as you have an additional motor with about 2/3 more HP than the largest motor you want to run.  Not sure if the rope would work with my current 10 hp motor, but it did the trick with the smaller motor, with a bit of practive.  Also, you can always find a ton of more specific information on owwm.com. 
Rob
 
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