building rotary phase converter

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

You will need a rotary phase converter of at least 1-1/2 times the HP of the load motor. There was a good article in the Nov/Dec, 2001 issue of "Home Shop Machinist" describing Rotary Phase converters. That article has good recommendations for start and run capcitance. Capacitors are available on the surplus market, and new from AC supply houses.
Bob Swinney
application.

The size of your largest load motor will determine the size of the idler motor you need. The size of the idler motor will determine the amount of run capacitance. The rules of thumb developed on this NG over time are:
Idler motor: 1.5x the largest load motor's horsepower (e.g. 3hp load->5hp idler) Run caps : 12 uf (microfarads) per horsepower of idler motor Start caps : 70 uf per horsepower of idler motor
These numbers have been used many times and are reliable. When you're looking for 3 phase idler motors, take a good quality multimeter with you and check the ohm reading from L1->L2->L3->L1. All should be about the same, and very low readings, like 0.2 ohms. No open circuit. Then check each leg to the motor's case. All should look like an open circuit. Then spin the shaft and listen to the bearings. If you feel any crunch be ready to spend $40-50 on new bearings. Inspect the motor case closely. Many times motors are scrapped because of mechanical damage usually from being dropped. If the case is clean or at least undamaged, and the bearings are good, and the meter readings are OK, then consider the motor as a good candidate for an idler motor. The slower it spins the better in my opinion (less noise). Also, if it has sleeve bearings it will be quieter but you will have to oil them periodically.
There is a good source for used run caps in Seattle. I go there a lot. Email me if you want me to look for some.
Grant Erwin Kirkland, Washington To email me, see http://www.t> I recently bought a baldor 10 inch 3ph grinder with dust collector.

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

Is all the capacitance in one place or is it divided across legs? One set of plans used 5uf across L1:L3 and 12uf across L2:L3 Input is L1 & L2. Generated leg is L3.
All across L1:L3 right?
I found a good 10HP motor. Seller claims it runs on real 3ph power, so I think that means its good. Using your formula I would need a 120uf of run caps and 700uf start caps. Will these caps cost a lot?
I think a 10HP idler is way overkill for a 1.5 HP grinder and 1/2HP dust collector. I am concerned that I will need massive contactors and wiring. I am also concerned that the caps will cost me a small forturne.
chuck

I'm about to assemble one for myself (3 hp idler). Grainger has starting caps at a decent price ($10 for the one I need), but has run caps at about 1/4 the Grainger price.
I also bought the HSM issue that Bob Swinney mentioned, and it has some good information in it. Also check out some of the old threads at (see VFD and converter topic area). Then, if you really want more information, there are a number of good r.c.m articles from 3-6 years ago that you can find by a bit of google searching. Look for Bob Swinney and Fitch Williams as authors. There were many other contributors, but google will get you to the threads and you can move back and forth from there.
Pete

Buy new start capacitor(s) because those are electrolytic and dry out. You can scrounge run caps safely. Split the run caps about half between L1-L3 and other half between L2-L3. As you've seen, some guys get way into tweaking these things and add a little more on one leg or the other. The big reason to use run caps is they make the converter run much more quietly. As long as the load runs smoothly and the converter isn't buzzing you're fine. Mine has half/half. All the start capacitance goes L1-L3, yes. And you do have to do something to take the start cap out of the circuit. I use a potential relay, but you can also use a regular relay wired for momentary contact, plus a pushbutton. The relay has to be rated to break the full starting current, which can be much higher than the full load amperage.
A 10 hp idler is way overkill. Loud, big, wasteful. OTOH a buddy of mine dragged over a real nice Toshiba 15 hp motor the other day which we started up in my shop and it ran real quiet. I'd look for a 3hp idler. - GWE
Charles A. Sherwood wrote:

If you already have some VFD's in your shop, why not use some of those relays to switch the output of a vfd to the grinder?
The reason that the various plans don't agree on the run caps is that different motors require different values. So the values are a starting point for something that is not very critical.
Try looking at places that do air conditioning work. Most single phase airconditioners use run caps. And a three phase compressor might make a reasonable motor.
Dan

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

My machines are not physically all together. The mill and lathe are in one room and the grinders are in a different room. The grinder needs a big VFD which are on the machines in the other room.
The bench grinder is right next to the surface grinder because I want to use the dust collector for both machines. The surface grinder has a 208V motor so the VFD needs to be configured for a lower voltage output. Too many variables for changing things around.

Starting surge on a 10 hp idler is substantial. I built a 20 hp rotary, and starting surge was horrific. To avoid that, I eliminated the self- start feature (and capacitors) and simply spin it up with a modest 1 ph motor before applying power. Starting surge is negligible that way, and I only need permanently wired run caps for phase balance, no starting caps or cap start circuits required.
The control circuit uses a spring loaded DPDT center OFF switch. In one position it applies power to the little 1 ph motor to spin the assembly up. In the other position it closes the contactor to throw 240 to the idler. Note that this removes power from the little pony motor once the rotary is started. It then just freewheels along for the ride.
Start procedure is to hold the switch to the pony side until the motors come up to speed, then flip it the other way momentarily to close the contactor and throw 240 to the rotary.
You could use two NO push button switches if you don't have a spring loaded DPDT center off switch, just make sure you don't hold them both in at the same time. If you do, and the two motors' synchronous speeds are different (they will be slightly), the little pony may overheat and fail. The spring loaded DPDT center off switch avoids this possibility.
A safety relay's coil is wired to be held in by L2 (the manufactured leg). Its NO contacts are in parallel with the start switch. So once the rotary starts making L2 (this occurs in a fraction of a second after you hit the rotary with 240), the relay pulls in, holds the contactor closed, and you can release the start switch.
If the power is interrupted and the idler stops, the safety relay drops out, the contactor opens, and will remain open until you work the start switch again. This prevents the idler from attempting to restart on its own (which it can't do), and burning up its windings after restoration from a power failure.
The stop circuit is just a NC push button wired in series with the contactor coil. Push it, the contactor drops out. Let the rotary wind down, then release. The converter won't restart until you operate the start switch again. Safe, simple.
On the subject of sizing breakers and wiring for this project, they only have to be rated slightly heavier than the *load* you're going to put on the converter. If the converter is properly balanced and power factor corrected with capacitors, it won't have high circulating reactive currents through the primary wiring and breaker. The only parasitic power draw is the small amount needed to overcome windage and bearing drag. All other real power will be consumed by the load(s), and so the primary wiring and breaker only need to be sized to accommodate load consumption.
For my converter, I actually sized everything to handle a 20 hp load, because I might run several large motors from my converter at the same time. But that's not necessary if you're only going to have a max load of 2 hp on the converter.
Gary

Up here in the great white north, the average house wife not only can change the engine on the family Buick, but can also come up with ways on how to save cash with the bat of an eye lash.
First, your values are a good ball park figure on your caps, of course you'll have to do some tweaking for balancing the the phases. The actual values to balance the phases actually vary between different makes, models and winding, so line balancing is a must.
I presently run a 5 hp rotary phase, home built to power the shop equipment . I run a Machine shop, and it is a legitimate business, up here where Poly Phase is no where to be found. To run a branch from the nearest city was quoted to me at $ 27,000.00 + $ 4000 per pole. Cost of my rotary converter: $ 25.00 I have a couple of schematics for a rotary PC that might be of some help. P.S. I've been running mine steadily for 4 years now, and not even a whimper! contact me
Charles A. Sherwood wrote:

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.

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

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

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

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.

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

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