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
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Is the rockwell mill three phase or single?
bob g.
Charles A. Sherwood wrote:

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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
snipped-for-privacy@w-sherwood.ih.lucent.com (Charles A. Sherwood) wrote in message

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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.
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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.
Bob Swinney

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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
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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:

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Are the motors truly in parallel? As in; all three legs of one motor connected to all three legs of the other motor? If so, you should see some 3rd leg current between the motors. Is there a separate switch on the 1/2 HP motor? If they are in parallel you should be able to switch on and start the 1/2 HP motor *after* the 1.5 HP motor is up to speed. The larger motor will be generating some unbalanced 3-phase power by transformer action. It is acting like an idler motor in a rotary phase converter configuration.
Bob Swinney

machine
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They are both on the same contactor and start at the same time. However they are connected together before the overload controls. One contactor suppling power to seperate "heaters" for each motor. The third wire of the static converter connects to the third wire of the motors through the heaters. Seems like the logical way to wire it (at least to me).
I could connect the third legs together before the heaters, but I really don't think that would make any difference.
I see no current in the third leg with a clamp on ampmeter after the motors have started. The voltage in the third leg is 208 and 224 with respect to the other two legs. I guess there is no current because the voltage is low.

Probably could, but the motors are hard wired together.
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    Where are you applying the clamp-on ammeter? If between the junction of the two motors' wires and the static converter, there *should* be no current once the motors are started.
    However, if you clamp around one or the other of the motors, you should see some current at least. (Mostly the amount would be based on the horsepower and the loading of the smaller motor.) Perhaps if both motors are equally lightly loaded, they are each generating about the same voltage for the third phase, and as a result, are generating very little current between them. Put a serious load on one motor or the other and see whether there is a visible upswing in the current.
    Good Luck,         DoN.
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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.
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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
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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

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wouldn't
choice."
for
In addition to the RPC plan I used (http://www.frugalmachinist.com/rpc.html ), 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.
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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.
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Source of 3 ph alternator and price?
bob g.
Charles A. Sherwood wrote:

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I found mine in the newspaper. I think he got them at an auction when an importer was scrapping a bunch of stuff.
chuck
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On 25 Aug 2004 19:18:36 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
Thanks for all the replies. I am interested because I see 3PH stuff for peanuts, and single phase stuff is often limited in power.

These were my thoughts, too. Alternators are available, and many have themselves set up to generate power as bought. There certainly are engines with governors that maintin a very good constant speed.
However, I have supply, but not 3PH. I was looking at using an electric motor to drive the alternator to get 3PH.
The reason I see the alternator as better is that RPCs and VFDs seem to have a rep for having trouble with load balance etc. The idea and construction is simple, but they seem to be "picky". I have never used one, but read a lot of stuff here about the problems getting them to behave under various load conditions.
They also do not seem all that efficient, and seem to need a far larger motor to act as the supply than the load. ***************************************************** I have decided that I should not be offended by anybody's behaviour but my own......the theory's good, anyway.
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Its cheaper, simpler, and smaller. But I still think the alternator will generate more balanced 3ph power.
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