Static phase converter for 3HP Unisaw

[Jason Neal]

Guys, a couple of weeks ago I posted a topic about a used Unisaw that I was considering, and you guys gave me some good advice.

http://www.sawmillcreek.org/showthre...deal&p=1688849

I didn't go with that particular one but soon came across a 3-phase model in good condition and at a good price so I picked it up last weekend.

As you might expect, I do not have a 3-phase feed to my shop but I do have 240V. I've read up and understand a rotary converter or VFD would be better, but I want to build a static phase converter to save money and I think I can live with the modest loss of power for the type work I do. I have an electronics background and suitable instrumentation to fine tune the capacitor values and solve little issues that may arise (without electrocuting myself).

Have any of you guys built a static phase converter for a 3HP Delta motor? What capacitor values did you end up using? I'm just looking for a starting point...I realize they will need tweaking since the motor design has probably gone through various changes over the years.

I just didn't want to have to start with a wheelbarrow full of capacitors to be able to try 25 different combinations. I also hoped to minimize the amount of abuse that I subject the motor to while trying those combinations.

[ian maybury]

HI Jason. I did a lot of digging some months ago on the topic of DIY converter (and got some good inputs from here), but in the end bought a used rotary from a well known UK maker which works very well - even winding up the big heavy cast band wheels on a 24in band saw.

Google will bring up several sites on how to build your own - including information on capacitor sizing. (sorry, but I didn't save the links) Practical Machinist forum has a VFD specific section and quite a few people that seem very knowledgeable too, but if you post there's a bit of pot luck in having the right person read it.

I just didn't want to get into an extended fine tuning cycle, and as a mech guy was a little unsure of what I was getting into. I wasn't too keen on the idea of a static, it seemed possibly to be giving too much away in terms of motor performance. DIY wouldn't have worked too well financially either - I couldn't find a local or UK source for the required type of capacitor at a reasonable price. There do seem to be surplus places doing them cheaply in the US if you know what you are looking for.

A VFD wasn't an option for me as I needed 400V, but they seem to do a good job of limiting the start up current. The less useful aspect is that they basically need you to wire directly to the motor (by passing the saw controls and safety), so that you use the VFD controls instead. They also don't seem to drive more than one machine at a time - unlike a converter.

I guess I'm wondering if you mightn't find a used example too at a decent price?

[Salvatore Buscemi]

Boy, how much are you really saving by making a static converter. A new VFD could be had for around 180 bucks, shouldn't damage the motor, and give you the rated power, and that's not even checking the used market. The only people that I see recommending static converters are the people that sell them with used equipment.

Sal

[David Kumm]

Vfd would give you much better power. Probably more in the $200-300 range used. You need a 5hp vfd if rated based on three phase because you need to derate if running off single phase. Almost a vfd's will run off single even if the plate says 3 in 3 out but need 20 amps in to get 10 out. You can cheat a little on the size though because the saw won't need to run at fla very long and vfd's allow for overcurrent for short periods. Static is a real short term crummy solution. Dave

[Salvatore Buscemi]

Vfd would give you much better power. Probably more in the $200-300 range used. You need a 5hp vfd if rated based on three phase because you need to derate if running off single phase. Almost a vfd's will run off single even if the plate says 3 in 3 out but need 20 amps in to get 10 out. You can cheat a little on the size though because the saw won't need to run at fla very long and vfd's allow for overcurrent for short periods. Static is a real short term crummy solution. Dave

Wrong, they have 3hp rated single phase input vfd's for under 200, check dealerselectric if you don't believe me. IIRC if you have a 5hp motor, you need to go to a larger size and de-rate, but that's not necessary for a 3hp motor.

[David Kumm]

You are correct. they are cheaper than I thought. I think if constant torque and speed control are a big deal you have to go into the $200+ range but still a deal. I was unaware that no derating is necessary on small vfd's. Hope someone can explain why. Even more reason to use. The mag starter will have to be bypassed but still a better result.

[Jason Neal]

Okay, so I'm a cheapskate. My brother has wholesale access to motor start and motor run capacitors so I'd pay probably 1/4 to 1/3 of retail.

Previously I came across someone recommending dealerselectric and I eyeballed the sub-$200 3HP VFD and I assumed it would become a sub-$200 paperweight in under 5 years. Can anyone vouch for the quality or reliability of the Teco brand?

But I wanted to try the static phase converter not just because I'm a cheapskate but because I like electronics and I get a kick out of making something work on a shoestring budget.

You know, I got into this whole woodworking thing so I can get rich quick. Did I have the wrong idea?

[Salvatore Buscemi]

Can anyone vouch for the quality or reliability of the Teco brand?

I believe TECO is made by Westinghouse, which is not a fly-by-night company. I have one, and like it, but I haven't had it for 5 years so I have no idea how long it will last. I will say they are made to work in a industrial setting, running 24/7. So the intermittent use it will probably get in a small shop might be less taxing. That being said, it is a piece of electronics, and they seem like they would be more prone to fail than the rest of the saw.

If you want to make something, why don't you make an RPC. That should still be pretty cheap if you can get the components for less than retail. You wouldnt need to use the variable speed ascpect of a VFD anyway.
Sal
Sal

[David Kumm]

There you go Jason. Find a good heavy frame 5hp motor. There are plans for self made RPC's available through OWWM and others. Then you have the real deal. I don't have TECO vfd's but they also have a big following with OWWM. Those guys are fairly fussy. Dave

[Tom Cornish]

You are correct. they are cheaper than I thought. I think if constant torque and speed control are a big deal you have to go into the $200+ range but still a deal. I was unaware that no derating is necessary on small vfd's. Hope someone can explain why. Even more reason to use. The mag starter will have to be bypassed but still a better result.

Derating is still required if the VFD in question is designed as a 3-phase in (assuming that particular unit can work that way), however there are several 1-phase in 3-phase out VFDs that are inexpensive up to 3HP which is I think what Salvatore means.

[Tom Cornish]

Okay, so I'm a cheapskate. My brother has wholesale access to motor start and motor run capacitors so I'd pay probably 1/4 to 1/3 of retail.

Previously I came across someone recommending dealerselectric and I eyeballed the sub-$200 3HP VFD and I assumed it would become a sub-$200 paperweight in under 5 years. Can anyone vouch for the quality or reliability of the Teco brand?

But I wanted to try the static phase converter not just because I'm a cheapskate but because I like electronics and I get a kick out of making something work on a shoestring budget.

You know, I got into this whole woodworking thing so I can get rich quick. Did I have the wrong idea?

TECO units are good quality. It was mentioned above, but just to reiterate - you need to re-wire the saw's switch to be a control input to the VFD - if you use a switch wired between the VFD and the motor you will very likely damage the VFD's output stage.

A VFD is the best choice for single-motor machines that aren't too huge (cost reasons), and for any machine that may benefit from varying the speed - drill presses, lathes, etc.

A rotary converter is the best choice for multiple motor machines or multiple machines, and may be more economical than VFDs depending on motor size.

A static converter is the last ditch choice you only take when money is pretty much the only factor.

[fRED mCnEILL]

I have a RotAry phase converter that uses a 7HP idler motor. Inside the box is the following. 2 capacitors, one double pole breaker and one momentary switch. If you have a motor the rest should be cheap. Mine works great. The only problem is that I have to replace the momentary switch occasionally (once A YEAR) as the power busn the contacts after a while. It should probably have a relay but switches are cheap so I keep 2 or 3 on hand.

[Jason Neal]

Okay, I've read some more on the subject of RPCs and now I'm convinced that is a good solution for me. Supposedly 3 phase motors and cheap and plentiful, so I can see buying a couple right off the bat. One to build the RPC...something like a 5-7hp. And another to upgrade my bandsaw's little 0.75hp motor.

I just need to find a source for them, and it's not quite as easy as folks make it sound.

Jim Wilson's article http://www.paragoncode.com/shop/rotary_converter/ talks about paying $30 for a 20HP motor at a salvage yard, but because he was impatient he paid a whopping $200 for it at a surplus outlet. Oh yeah? Either one of those sounds great.

Guess I'll ask around at work and keep an eye on Ebay. Craigslist is a bust. Guess I just don't live in a good area...can't get a good deal if I spend $75 in gas to get there and back.

Headed to the FS/FT forum now.

[David Kumm]

Jason, call local motor shops. They have or may know of good old motors that are too odd- or old to be generally useful but may work for you. If not they may give you a lead. Dave

[Chip Lindley]

Okay, I've read some more on the subject of RPCs and now I'm convinced that is a good solution for me. Supposedly 3 phase motors and cheap and plentiful, so I can see buying a couple right off the bat. One to build the RPC...something like a 5-7hp. And another to upgrade my bandsaw's little 0.75hp motor.

I just need to find a source for them, and it's not quite as easy as folks make it sound.

Jim Wilson's article http://www.paragoncode.com/shop/rotary_converter/ talks about paying $30 for a 20HP motor at a salvage yard, but because he was impatient he paid a whopping $200 for it at a surplus outlet. Oh yeah? Either one of those sounds great.

Guess I'll ask around at work and keep an eye on Ebay. Craigslist is a bust. Guess I just don't live in a good area...can't get a good deal if I spend $75 in gas to get there and back.

We must figure that he bought that 20Hp motor for $30 several years ago. But bargains are where you find them--at the right place, at the right time.

There is always FedEx or UPS Freight Jason! I have a couple of big older Reliance and Marathon 5Hp 3Ph motors I would part with. One is perfect; the other needs bearings, but the price is right on both. PM me if interested!