1 phase to 3 phase?

[barnes_gunworks]

hi all new to forum hope i got this in the correct place. i just bouhgt a mill thats 3 phase an i dont have 3 phase power. my question is do i build a 3 phase convertor out of an old 3 phase motor? if so could ya get me some pointers? or would you use a static convertor?
thanks all
johnathen

[websterz]

Here's my short answer. A static converter is good for one machine, one motor. If you plan to add more 3 phase equipment then go with a rotary converter. Otherwise a static will suffice.

[Richard-TX]

Never ever use a static converter. They are a waste of time and money. They are the cause of more mill motor burnouts than I can count. There for a while one I was seeing about one posting a week where yet another poor fellow fell victim to static converter motor burnout.

You have two choices.

RPC or VFD.

You can build a RPC from an old 3 phase motor or you can buy one built. American Rotary has some very good rotary converters. THe problem is money. RPCs are a little pricey.

If it were me, I would get a VFD. They are fairly cheap and offer many features that a RPC does not such as variable speed. VFDs is what machinery manufacturers put on their upper end machines.

[SteveM]

Here's an example of a VFD:

https://www.surpluscenter.com/item.asp? ... 0&catname=



115v or 230v 1 phase in, 230v 3 phase out.

Steve

[Richard-TX]

Here is a 1 hp VFD that takes 115 volts single phase in and produce 230 volts 3 phase out.



http://www.factorymation.com/s.nl/it.A/id.193/.f

$120.00

I have a few of these in my shop. Never a single problem.

[steamin10]

Static converters are simply a capacitor thrown across the lines of the single leg, to imitate the missing pulse. The motor will run fine with the low leg. The problem is the HP generated is way off to the low side, and running out of phase creates heat in the motor windings, that increases as the motor is forced to do more work. That is where the problem becomes serious. The motor will OVERHEAT or even stall at anything aproaching the posted HP. A home made rotary is better, but turning on a small 3phase motor on the same line as the Mill motor is a trick used to bring up the phasing and power of the low leg.

There are better ways to go, but the money is a ways up there, and if not real 3 phase still short on power. The electronic drives are getting pretty good now. I dont have one. I have five 3 ph motors that run from one capacitive supply, at diferent times.

[JackF]

As I understand it VFDs are for one machine only, you need one for each machine. Good if you have just one machine or if money is no object, one for each machine. A RPC can be wired to feed more than one machine and can run more than one machine at a time, up to a limit. I made my 7 1/2 HP RPC from a scrap motor (paid $10 for it) and used capacitors and have less than $50 tied up in it. I see used 3 phase motors on craigs list quite often and if you Google "home made 3 phase converters" you can find several plans to do it yourself.

Jack.

[lakeside53]

Basically what Steaming says....

but.. burning up of a motor is not the fault of the static converter - it's just the motor being used in excess of it's capabilities. Static converters can be implemented reliably, but most are not.

A motor with a static converter can only have a max load placed on it that is less than 66% (in theory, and as low as 50% in reality) of the rated HP. The motor supplies power based on demand, so the user has to limit the load; try to consume the original HP and the motor will fry. On a home mill or lathe it's possible to "get away with" ignoring this - you'll rarely use the max power and if you do it's "peaky". Putting a static converter on a constant (when needed) device like a pump or compressor without changing pulley sizes is a recipe of cooking.

The problem is not limited to "static converters". Exactly the same thing happens with a 3 phase motor runing on true three phase - if you place a load on it in excess of the motor rating, things get hot quickly. Good installations have some form of motor overload protection - either "heaters" (thermal relays on the input contactor) or electronic overloads.

You can solve the static converter problem by installing motor overload circuits to limit the HP to 2/3 (maybe less) of the plate rating. It gets a bit tricky to guess at the optimal point because you really only have two legs to sense current from, but setting it to the plate rated current is close enough. If you already have overloads, a static converter can be just fine. You can buy a used contactor/heaters ("motor starters") for $10-$35 on Ebay. I have a box of them left over from VFD installs - they are not requried.

Me? Never static converters - I'm all VFD's.. but I'm now putting in a 15hp rotary converter... to power bigger 3 phase VFD's

VFDs are far more than phase converters - soft start, variable speed, braking, motor protection and a bucket of other useful features.

[barnes_gunworks]

very good info guys, between a VFD and building a RPC which is the cheapest option? ive got a lead in houston on a AO Smith Centruy E-Plus 7.5 3-phase motor for $50. is that the way to go? my hp is only 2 on my mill so 7.5 is way more than enough. ive looked for plans on building a RFC and im having no luck. everyone wants $ for em.
thanks again

[JimGlass]

I have all three systems in my shop. I have had a static converter on my surface grinder for ten years with no problems.

I have a VFD on an old Walker/Turner drillpress. This works great because the VFD converts 240 single phase to 230 three-phase. Becase of the VFD the drillpress is electrically variable speed and my installing a foot switch to the VFD, the drillpress is reversable. Great for tapping.

I also have two three phase rotary converters. One runs my surface grinder for times I need full HP. The other runs my Bridgeport.
Rotary converters are cheap and easy to build. Used three phase motors needed for an idler are cheap and readily available. I think I made the rotary converters for about $40 each.

Jim

[Richard-TX]

when it comes to building a RPC you have a choice:

Good
Fast
Cheap

pick any two.

I have seen guys scrounge scrap yards and have come away with everything they need for about $30. I have seen other guys buy new RPCs and spent $700.

Then there are the 3 phase infrastructure costs. A lot depends if you have to meet the NEC.

Since this is a mill, a VFD is what I would put on it. Variable speed on a mill or lathe is so insanely useful that you will fall in love with it.

Once you use it for a while you will know why manufacturers put VFDs on the high end machines. Even Grizzly puts VFDs on their high end machines.

So what does a VFD cost? Certainly less than a used Kurt vice. It is about the same a Mitu Series 293 Digital micrometer. Maybe two OSG 1" endmills.

The RPC is nice because you can power multiple machines from one.

The VFD is nice because you have variable speed. and other features.

[Marty_Escarcega]

hi all new to forum hope i got this in the correct place. i just bouhgt a mill thats 3 phase an i dont have 3 phase power. my question is do i build a 3 phase convertor out of an old 3 phase motor? if so could ya get me some pointers? or would you use a static convertor?
thanks all
johnathen

Do you have the electrical skills to build the RPC? For a 2hp mill, I might consider just buying a Variable Frequency Drive. If I thought I might buy more 3 phase equipment, then I would do the RPC.
You can get a static converter and put it on an "idler" motor and you have an RPC.

I believe if you found a 5hp motor it would do everything you wanted it to. I built one and it ran my 3hp LeBlond lathe and everything else in the shop just fine. I have a 3hp now running my Clausing lathe with a 2hp motor no problem.

Marty