phase converter questions

I recently bought a 1.5HP 3ph Baldor 10 inch grinder with 1/2HP 3Ph dust collector.

I connected them to a Phase-a-matic static phase converter; They start and run ok, but they do seem to draw a bit more current running on single phase than 3phase.

I figured that the big grinder would act like a rotary phase converter for the dust collector, but I don't see any current in the third leg with a clamp on ampmeter.

Input is L1 and L2. I measure L1:L3 208V L2:L3 224V

Should I experiement with some run caps or just leave it alone?

Another interesting fact is that the baldor grinder is rated

1.5 HP but FL current is 2.5 amps. I have 1.5HP on my rockwell mill and that motor is rated 4.6 amps. Is baldor guiltly of over rating like sears?

chuck

Reply to
Charles A. Sherwood
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Is the rockwell mill three phase or single?

bob g.

Charles A. Sherwood wrote:

Reply to
Robert Galloway

The Baldor is rated at 1.5 hp, but unless you are using it the motor is not going to draw much current. The better the motor the less current it will draw. Dan

Reply to
Dan Caster

Reply to
Robert Swinney

With a static phase converter, zero current in the third leg is normal (after the brief starting interval) when you are running a 3 phase machine on single phase current. The third leg is only intergized for starting. After start-up the motor runs as a single phase machine with current through

2 of its 3 sets of windings.
Reply to
Robert Swinney

On 24 Aug 2004 19:25:25 GMT, snipped-for-privacy@w-sherwood.ih.lucent.com (Charles A. Sherwood) vaguely proposed a theory ......and in reply I say!:

remove ns from my header address to reply via email

PMFJI

Why is a rotary phase convert better than a 3-phase alternator driven by a high-current single phase motor?

RPCs seem complex and prone to all sorts of vagaries (with SPC even more so). Why not obtain high current 1 PH power, or several sources of 1 PH power, and run a motor that runs a 3-phase alternator?

***************************************************** Marriage. Where two people decide to get together so that neither of them can do what they want to because of the other one.
Reply to
Old Nick

First reason is complexity. Second is efficiency, I'd imagine, though I'm not an expert.

You think this is simpler than an RPC? I built an RPC from plans on the 'net a few weeks ago, and it was a *lot* simpler than I'd thought. I wouldn't want to bother with a generator setup unless I really had no other choice.

Pete

Reply to
Pete Bergstrom

Pete sez:

"You think this is simpler than an RPC? I built an RPC from plans on the 'net a few weeks ago, and it was a *lot* simpler than I thought. I wouldn't want to bother with a generator setup unless I really had no other choice."

Right on, Pete! Also, it is a lot easier to find surplus 3-phase motors for conversion to RPCs than it is to find alternators. Alternators should be driven at a constant speed, regardless of load - not an easy thing. Alternators need a separate source of direct current for magnetic excitation - also adding to the complexity. Granted, alternators can be "permanent magnet", eliminating the need for excitation power, but those types, AFAIK, are not readily available or cost efficient in smaller sizes. The only example I can think of might be a welding/AC supply "generator" that outputs 3-phase 220v AC. Again, not nearly as economical as a RPC. IMO, the only justification for an alternator would be if there was no commercial power available.

Bob Swinney

Reply to
Robert Swinney

In addition to the RPC plan I used

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I'll also recommend Bob's article in Home Shop Machinist Nov-Dec 2001 "Induction Motors and Rotary Phase Converters". I bought the back issue as part of my research.

I had originally planned to buy a VFD, but since my mill also has a coolant pump I ended up building the RPC to power it.

Reply to
Pete Bergstrom

Well if you do the math 2.5amps @ 240V 3 ph is

2.5*240*1.7 = 1000 watts. No way is that 1.5HP.
Reply to
Charles A. Sherwood

But in this case I have two motors in parallel. The big 1.5HP grinder motor and the little 1/2 HP blower. The big grinder motor is pretty much unloaded and the the blower motor which is always loaded because it is moving lots of air. I expected to see some current in the third leg but I don't see any.

chuck

Reply to
Charles A. Sherwood

Its cheaper, simpler, and smaller. But I still think the alternator will generate more balanced 3ph power.

Reply to
Charles A. Sherwood

Not all the hard. There are governors on industrial engines that do it well. Even the consumer quality generators do a pretty decent job of it.

Modern brushless alternators have the excitor built in and will be self starting and self exciting. Some require an external regulator which uses a tiny amount of generated AC to power the field. For example the field on my 15KW unit is 30VDC at 1 amp. This tiny control voltage is produced by the bassler regulator which is powered by the alternator itself.

Reply to
Charles A. Sherwood

You have to understand that most of the designs you see are highly optimized. Using an oversized idler motor that is started with a pony motor means you don't need any power factor correction, no balancing, and no starting circuitry.

It gets built for free, runs forever, and never has to be thought about. Example is the patented "jim's stone age converter" seen here:

Your idea of using a large single phase motor to power an excited field 3~ alternator is in principle the best way to do this.

However because hardly anyone ever actually purchases a 3~ alternator in the 5 hp range, and likewise 5 hp single phase motors, the trouble is that those two things are incredibly expensive and never available on the surplus market.

5 to 10 hp three phase motors on the other hand are literally given away for scrap value on a regular basis. Likewise the small 1/4 hp pony motors to start the thing.

Folks build what they can afford, and what works. If you advocate the more correct design, be prepared to put your wallet into the breech to implement it. At the end of the day, nobody will be able to tell the difference between the power on your wires, and the power on my wires.

Your wires will wind up costing well over $1K. Mine are under ten bucks.

Jim

Reply to
jim rozen

I assume you are refering to me because I proposed the motor driving alternator design a while back. (but you quoted someone else)

I have looked for surplus motors to build a phase converter but frankly I have not found the oasis where they are available for cheap. The shipping costs are usually the killer.

One of the reasons I proposed building a motor alternator setup is exactly what you quote above. I already have the alternator and a 5HP motor to drive it. So to me it is cheap.

I started experimenting with a static phase converter because I found a used one and a couple people here stated it would be adequate for my needs. So far it seems to be working; but running a 3ph motor on 2 windings rubs me the wrong way; Especially when the run current in those two windings is higher than the full load current spec of the motor.

Someday I will build that motor alternater setup and experiment with it too. But for now, I got lots of projects ..

chuck

Reply to
Charles A. Sherwood

My comments were specific to Nick who proposed an *electrically* driven alternator to create a three phase supply, instead of a rotary converter.

Obviously in your case the 'works well and is cheap' approach to life says you should use what you have available. I would do the same thing if I had a prime mover available, and no other easy way to get an idler motor.

Be aware that not all of that is 'real' power. Do see how much it really is consuming, you need to check the phase angle or have a true rms wattmeter available. It might not be as much as you think from an ampclamp check.

I understand that! I think I'm in the same boat as you on this.

Jim

Reply to
jim rozen

If you're in the Twin Cities (or nearby), Minnesota, I'll tell you where to find a surplus motor. I got a 3HP GE motor for $30 (I countered a $50 original price, figuring that $10/hp was fair). It's running very nicely in my garage.

The surplus store near my house (Axman) has 5HP motors for $200+, but they're NOS rather than slightly used.

Pete

Reply to
Pete Bergstrom

Ever heard of a motor-generator where both functions were incorporated onto one armature? Such things exist. A rotary phase converter is sort of the same thing. It'll give you good, real, three phase power, unlike a static. It can use a few caps to correct for power factor but is more efficient of weight and materials than a separate motor driving a generator. Seems like perhaps the old Lincoln DC welding supplies may have been built this way (don't hold me to this).

rhg

Old Nick wrote:

Reply to
Robert Galloway

That's the way I understood your description in the first place and when you said no current in the third leg, I figured I'd read it wrong. I use a 2hp idler to run a 1/2 hp on an old lathe and there is current in the third leg. If there were no current in the third leg, you couldn't start a three phase motor off a rotary converter. BTW, using the old rope start rotary converter, if you started the converter in the wrong direction (who could be absent minded enough to do that?) The load would also run in the wrong direction. The load certainly found something to use in that third leg.

bob g.

Charles A. Sherwood wrote:

Reply to
Robert Galloway

Source of 3 ph alternator and price?

bob g.

Charles A. Sherwood wrote:

Reply to
Robert Galloway

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