building rotary phase converter

I recently bought a baldor 10 inch 3ph grinder with dust collector. I use VFDs for most of the machines in my shop, but it seems to make more sense to use a rotary phase converter (RPC) for this application.
I searched the web and found various different articles on building a RPC. However they don't always agree on things like the run caps. Can someone recommend a good set of plans.
Also where is a good source for capacitors and used 3PH motors? I'm in a chicago suburb (naperville).
I think I have plenty of relays
thanks chuck

I found a 10HP motor but I think there are some disadvantages to using a big idler motor.
- hard to start - big starting currents - require large start caps - inefficient
Should I keep looking for a smaller motor? thanks chuck

sherwood.ih.lucent.com says...
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For a bench grinder I wouldn't overlook a static converter, either store bought or home brew. I have VFDs on one lathe and the mill, but despite also having a rotary converter, prefer to run the 3/4HP bandsaw, 3/4HP belt sander, and 1/2HP grinder on a static converter.
Ned Simmons

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I guess I could buy a static phase converter to run the grinder and then connect the dust collector to the running grinder. This might work fairly well. I should be able to test this out with a few caps. If I'm not happy with it, I'm half way to building a rotary phase converter.
chuck

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I agree. The idler in a phase converter is basically a 3-phase motor running on twophase power with runcaps, or singlephase power if run caps aren't used. The main advantage to having an idler is the ability to reverse -- but you don't reverse a grinder often, right?
A popular "rule of thumb" is that 3phase motors operated on singlephase power must be derated. This is true for sustained operation at full load because of heating, but does NOT mean that the motor can't deliver full rated torque intermittently, albeit with perhaps a very slight reduction in run speed -- a percent or two. This may seem contrary to intuition or "conventional wisdom", but Jerry Martes has actual dynomometer data that clearly supports this assertion.

Don Foreman wrote:
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This makes sense to me, Don. Isn't the issue unbalanced current in the windings (hence too much current in one winding) as opposed to not being able to produce the torque?
Steve Smith

On Wed, 16 Jun 2004 21:50:49 -0500, Don Foreman
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That's interesting Don, I used to run my bridgeport with a static phase converter. After switching to a rotary it was obvious that the power was increased. The machine would bog down on heavy cuts with the static converter and not with the rotary. I had repeat jobs so was able to really tell. Was this difference in power more because the caps in the static unit were not matched well enough to the motor? ERS

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It's true that static converters (start and run capacitor systems with no idler) can deliver the full rated power of the motor for surprisingly long periods but that is not the whole story.
A converter of this type is basically a capacitor/inductor phase shift system which produces an open vee 3 phase system. This phase shifter is a series resonant circuit and when it is set up to give the 60 deg phase shift it is working a long way below its natural resonant frequency. 60 deg is of course the correct phase angle between the two legs of an open vee system.
The motor(s) is the inductor in the system and unfortunately the apparent inductance of the motor changes with rotor speed. For any particular rotor speed greater than about 90% of synchronous speed (the lower limit varies a bit with motor type) it is possible to choose a capacitor combination which produces a pretty close approximation to balanced 3 phase at the motor terminals.
For near the full load rated speed of the motor, large run capacitance is needed with most or all of it as a single capacitor feeding the phantom phase from supply live. At light load the speed of the rotor rises and if the capacitor value is chosen to achieve the right phase angle the phantom phase voltage will be excessive. This could be corrected by feeding the capacitor from a lower voltage single phase source but this would mean feeding it from an auto transformer across the supply.
It is much simpler (and of course everybody does this) to use two capacitors arranged as a voltage divider to simultaneously achieve the correct phase angle and phase voltage. The effective capacitance of the two capacitors connected in series across the supply is the sum of the capacitances because the source impedance of the supply is zero and this effectively parallels the two capacitors. Because the they also act as a voltage divider, this sum capacitance is effectively fed from a voltage of supply voltage times C1/(C1+C2) where C1 is the top capacitor and C2 is connected phantom phase to neutral.
Because it looks nicely symmetrical there seems to be a tendency to believe that C1 and C2 should be equal and any inequality in their optimum value must result from some strange second order effect. This is NOT true. There is nothing magic about equal C1 and C2. It simply results in a capacitor of value C1+C2 fed from half the supply voltage. At this low effective supply voltage it is only possible to get close to balanced operation at no load or light loads which enable the rotor to operate close to synchronous speed. As the load increases with consequent slowing of the rotor speed the total capacitance needs to increase with both more in C1 and less in C2. By the time full load is reached the optimum value for C2 is usually zero.
These effects are very noticeable if you're using a single motor on a variable load up to near rated full load power and some compromise necessary. The saving grace is that industrial motors are surprisingly tolerant of reasonable overvoltage when operating at light loads so the trick is to size the capacitors for at or near full loads and to accept some overvoltaqe at light loads. This increases the motor losses at light load but the total motor losses still remain below the losses at rated full load so temperature rise is acceptable.
Summing up - if you need to cope with heavy loads on a static converter throw away the bottom capacitor and be sure to choose C1 for operation near full load.
None of this helps with starting torque - this is inherently poor with the static converter arrangement however large the starting capacitor. This is because correct low speed phasing requires the capacitor to be fed fed from a voltage many times the supply voltage.
Jim

What is your definition of C1 and C2? Won't things be wrong if you run the load motor the other way? I seem to run my mill backwards a fair amount. I have to admit I have C1 and C2 equal, but that isn't because I can't do it any other way, it's because I don't really know exactly how to tune my converter to be "right" for the entire range of load motors, directions and speeds it sees. I have one rotary converter and about 8 machines it drives, one at a time of course. Smallest motor is a 3/4hp 1140 rpm unit on the vertical bandsaw, then a 1hp 1760 on the surface grinder with another 1hp 3450 (I think) on the tool & cutter grinder. My Bridgeport is 2hp 1760 and my Cincinnati lathe is 3hp 1760. My press has a 3hp 1760 rpm motor. The power hammer has a 1hp 1140 rpm motor. I simply don't know how to optimize across all of these machines. Plus, everything works well and fairly quietly. Thus while there may be merit in fine-tuning in an academic or theoretical sense, my experience (limited though it is) is that it isn't really necessary in practice.
I will freely admit that I don't think I've ever run any of my motors at anything like their rated capacity. I go slower than that, never having worked in a pro shop, even when I'm working for money.
Grant Erwin Kirkland, Washington
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One way you could optimize each machine is to put caps on each machine (on the machine side of the machine's contactor). This way each one could have a different set of run caps. I haven't bothered to do this of course...
Steve Smith
Grant Erwin wrote:
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On Thu, 17 Jun 2004 18:03:44 -0700, Grant Erwin
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Firstly my comments only related to the setup of static converters which have facilities to switch from a large single start capacitor to smaller capacitors chosen to optimise the run performance. These static systems have no rotary idler motor so optimum choice of the run capacitor arrangement is vital.
Setups that that also include an idler motor are much more more forgiving because the idler is always running on no load close to synchronous speed and this reduces the variabilty of the operating conditions. With a sufficiently large idler it is possible to operate as you have done with a variable number and rating of load motors switched in to circuit without change of the run(tuning) capacitors. This also makes them more tolerant to the capacitor setup and they can survive with capacitor setups that would severely degrade a purely static arrangement.
For a purely static setup each motor would need to be set up at least with it's own separately chosen run capacitor and,unless one motor is always running, each will need it's own starting circuit.
To answer your questions.
C1 and C2 are generally referred to as "run" or"tuning" capacitors and are permanently in circuit once the motor has reached operating speed. C1 is connected from the live side of the supply to the phantom phase motor terminal. C2 is connected from supply neutral to the same terminal. Circuit diagrams usually show them bridging the motor terminals which is of course technically accurate. I prefer to show them as two capacitors series connected directly across the supply with a wire joining the central connection to the motor phantom phase terminal. This is electrically identical to the usual depiction but shows clearly that the capacitors are acting as a voltage divider. It also makes it obvious that the capacitors make partial power factor correction i.e. in addition to their primary function, C1+C2/C1xC2 - their series connected capacitance value appears as power factor correction across the supply.
The load motor(s) will run in the direction controlled by the sequence in which its' windings are connected.The back EMF's at the motor terminals will be just the same for either direction so the load seen by the converter will be unaffected.
Your comment that it's difficult to discover how to fine tune a system with multiple loads and maybe it's academic is entirely valid. If you've a reasonably large idler and are not trying to extract the last oz of torque from the load motors, accurate tuning is not going to make that much difference.
Jim

I'm still confused. Most of us run 3 phase motors at 220VAC, right? At my house 220 power is between L1 and L2, in other words you have 2 hot leads with respect to earth ground. Your comments would make sense if referred to 110VAC wiring, but who uses machinery motors at that voltage?
Grant
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On Fri, 18 Jun 2004 14:30:56 -0700, Grant Erwin
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Sorry - I slipped into a UK specific habit. Our 240 AC domestic supplies arrive as a three wire system - Live and Neutral and Earth (Ground). For your supply read "live" as L1 and "neutral" as L2
As far as the motor is concerned it doesn't care which way round the supply wires are connected - reversing the connections to the supply makes no difference.
Jim

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Jim
It isnt likely that anyone in this news group would have need for this information, but, I disconnected one of the windings in a 3 phase motor to evaluate the start torque from a 3 phase motor on single phase. I reconnected that winding so it was like the start winding in a regular single phase motor with a capacitor in series with it across the single phase input to the motor. If the start capacitor is selected to be "just right", the start torque of a three phase motor (reconnected for single phase use) is almost double (200%) the motor's rated max running torque. I dont know where this "reconnecting" of a 3 phase motor for use on single phase would be usefull. But it was interesting to get an idea of how similar 3 phase and single phase motors are.
Jerry

On Fri, 18 Jun 2004 02:44:12 GMT, "Jerry Martes"
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Jerry
An interesting experiment - maybe it's one way of rescueing a motor with a single dud winding.
Jim

Jim
That would work but, with a dud winding, it'd have to be spin up with a pony (or rope).
I went inside the 3 phase motor and disconnected one the *Y* windings and reconnected it to be the start winding for single phase use. I rate this "rewire" of a 3 phase motor for use on single phase as an interesting project with very little usefulness. And, all 3 windings would have to be useable.
Jerry
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single

Thanks again for a post that gives me a better insight. You are one of the people that post on RCM that I make a point of reading. Do you teach electrical engineering?
Dan
snipped-for-privacy@yahoo.com wrote in message
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On 20 Jun 2004 09:04:01 -0700, snipped-for-privacy@krl.org (Dan Caster) wrote:
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Glad to hear you find the posts useful. I've learnt a lot from this group and I try to make some return.
In answer to your question - no - just a retired electronic engineer with a fairly wide range of interests.
Jim

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    I consider that unlikely, because the caps in a "static" phase converter are only connected long enough to spin the motor up to speed, and then disconnect. (Since they are usually motor-starting caps, they aren't rated for continuous duty, anyway, and would let the "magic smoke" out rather quickly.
    Enjoy,         DoN.

On 17 Jun 2004 20:40:58 -0400, snipped-for-privacy@d-and-d.com (DoN. Nichols) wrote:
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Does this mean that the static converter used only start caps? And is this why the motor had less torque than when run with the rotary? The motor really did have much less torque. There was no doubt about that. ERS