RPC Run Capacitor Size Calculations

I recently purchased a RPC with a 3HP Lincoln motor. The unit has a Dayton 5X847M start relay, a GE 6X550 voltage sensing relay and (2) start capacitors wired in parallel with a single bleed resistor. The RPC starts right up and runs quiet. The unit will power my 2HP Bridgeport mill just fine but causes my 1.5HP band saw to trip the manual switch after about 10 minutes of operation (the heaters are hot to the touch). The unit will not start my 1.5 HP YAM lathe - just growls. The output voltages are 240, 205,205. A scope shows two phases at about 180 and the third at 60. I think some run capacitors to balance the voltage and phase shift might help the situation.

I would appreciate some general direction on run capacitor sizing. Any other insights are welcome because this is my first experience with the RPC.

Thanks in advance,

Under what conditions did you get those voltages?

What was the reference point (s) for voltages?

Under what conditions did you get those phases?

What was the reference point (s) for the phases?

(1) no load
(2) L1 - L2 240 VAC, L2-L3 205 VAC, L1-L3 205 VAC
(3) no load displaying 2 phases at a time on TECH 422 analog scope
(4) display one phase above the other

Voltages are about the same under light load - just the spindle running about 1200 rpm

"A scope shows two phases at about 180 and the third at 60."

Smart money here says you are checking each hot leg phase with respect to neutral.
That's about what you'd see. A more instructive (and much more difficult measurement)
is to measure the phase angles between each hot leg, with respect to the utility-supplied
incoming waveform, again hot to hot.

You'd probably see them all 120 degrees apart.

Your voltage balance however is poor, and the name of the game is to bring the manufactured
leg up to close to the utility leg. Your best source for run capacitor tuning is to seach for the
fitch williams converter tuning instructions.

If you check vs neutral, you will get the phasing of a "Scott-T", which is 180 and 90..... 2 phases 180 from each other and the third at 90 deg to that (ideally).

Actually a goodish way to check with a 2 channel 'scope, I will have to remember that for some time when I have to check a "wild leg" system or RPC.....

I don't quite see how you CAN get 60 deg unloaded and without any capacitors..... the motor mechanically forces the 120 deg relationship. If you said 85 deg, a few degrees between friends would be no cause for alarm, but 30 deg off is significant....

Does it already have some run capacitors?

And you gave the reference points for the voltages, but not for the phases..... did you measure across each, or from a common point?

I suppose it would be good to ask if you measured with a 4 channel 'scope, so that all were visible at once on same screen. if so a good clear photo of that would be good.....

Really a four channel analog scope only gives you two waveforms, because you need to run
A-B and C-D to do that. So you'll only get two traces. Also the peak to peak voltages involved mean
you need a X100 probe for most scopes.

I cheated when I did those measurements and used a good digital scope from work, where you
can simply clip the probes on each phase and then use the math functions to display what you
want on the screen. A-B, B-C, and C-A.

Some you can do floating measurements on up to 480VAC lines..... we have one at work, I forget the TEK number.

And you can always go from neutral to the lines in a wild leg system. Not with a delta.

He shouldn't have equal voltages from each line to the manufactured leg and 60 degrees phase shift. Something is goofy there. As Jerry says, it should look like a Scott-T. Measuring from the single phase line neutral, when you get the phase angle 90 degrees, the leg to leg voltages balance themselves. In any case, if the unloaded voltages and phase angle are right, almost automatically the loaded ones will be wrong.

Bill

Thanks for the input.

There are no run capacitors in the unit, only the two start capacitors wired in parallel: (1) 233 - 280 MFD 220 - 250VAC, (2) 216 - 259 MFD 220-250 VAC. I think the large values on the capacitors caused the unit to start almost instantly.

Because am not an electronics person, I may have misread the phasing on the scope.

I have read the Fitch Williams info many times and am pursuing some run capacitors at this time.

First lets look at the obvious issue, it RUNs the 2HP Bridgeport just fine! Your saying the 2HP motor works OK but smaller will have problems?
Then 240VAC, 205VAC and 205VAC, what is your line voltage?? Does you scope have a digital display for voltage? Is the voltage listed Peak to Peak or RMS???
Is your Bridgeport Drum switch controlled? Your other motors have starters since the heaters get hot, is the control voltage connected to the 240 Line? Assuming your looking at the 180 degree shift of the input power the only shift that would be equal to both phases is 90
Frank

Your first statement is correct: it will start one 1.5 hp motor (60's vintage GE induction motor), but after about 10 minutes the switch will trip and two of the three heaters are too hot to touch; the second motor (1.5 hp from Tiwan) will not start - only growls.

The single phase line voltage is 240. The Bridgeport starts quickly and will run at any speed with no problems and will instantly reverse directions if not running above about 800 rpm; above that speed, it trips the magnetic switch. I have not yet tested it with a real working load: large drill or large milling cutter. There is a magnetic switch on the Bridgeport but the motor is drum switch controlled. The magnetic coil (230v) control voltage is hooked to the T! & T2 from the RPC which measures 240.

As for the phase shift, I had a helper who has scope knowledge and after doing probe calibrations, the sine waves on L1 & L2 appeared to be approximately 180 out. The wild leg compared to either L1 or L2 appeared to lead or lag about 60 degrees(I suppose it could have been closer to 90 than 60). The phase shift was about the same whether the machine was under power or not. The scope is analog and does not have a digital display for voltage. I have an old digital VOM and I don't know if voltage readings are peak to peak or RMS.

Keep asking questions and maybe the light will come on.

Thanks

I will get out my scope again as soon as my helper is available and we will make some measurements.

After studying wiring and components of my RPC, I have determined it was made according to the instructions and schematics of one George Carlson. That design does not show any run capacitors so that is probably why mine does not have any. The previous owner of my RPC claims he ran a Bridgeport mill with no problems. Me too. It is these 1.5 HP motors that are giving me the problems.

I purchased some capacitors today so tomorrow it is back to the bench for some trial and error voltage matching per Fitch Williams and Rick Christopherson info.

First I would start checking the wiring of the other 2 motors! If a 2 HP motor runs OK what makes you think adding run Caps will make the 1.5 HP motors work? First start with the 1.5 motor that gets the heaters hot. Was it changed from 480 to 240VAC? Are the Heaters the correct size? If you can borrow a Scope can you get a clamp on Amp meter? I assume the BP has overload protection since it has some sort of magnetic switch. When the magnetic switch trips at above 800RMP reversal does the Heater or OL device trip or does the switch just drop out?

Frank

I have achieved success - somewhat.

My original readings from the RPC output were as follows:

L1 - L2 240 VAC & 4.5 amps, L2-L3 205 VAC & 4.3 amps, L1-L3 205 VAC & about 0.5 amps.
The readings at the magnetic switch on the 2 HP Bridgeport were about the same under light load - just the spindle running about 1200 rpm, however I did not measure the amps at this point. The Bridgeport would run indefinitely.

As for the wiring on the 1.5 HP band saw motor: it is a standard 9-wire motor and is wired correctly for low voltage (230).
The saw would run for several minutes then the heaters would trip the manual switch (2 of the 3 heaters were hot and the low amp leg heater was not hot). I took some voltage and amperage readings at the manual saw switch and they were about the same as those from the RPC. The low amperage reading on the L1-L3 leg looked odd.

I purchased some run type capacitors and began the tuning per Fitch Williams (FW) info. The FW RPC is 10 HP and the one I have is 3 HP. At the end of my tuning exercise, my capacitor values were real close to 1/3 of those for the 10HP model and the voltage / amperage profiles were proportional as well. Good point of reference for future projects.

I plugged the band saw into my RPC with run capacitors installed and sawed alloy steel and tool steel for about two hours without any issues. Tomorrow I will start machining some of these parts on the Bridgeport to give it a load test before I consider this project complete.

As for my lathe, which I thought was also 1.5 HP, there is another issue. The lathe is against the wall and the motor data plate is difficult to see. After wiping off the data plate and getting a bright light and a clean mirror, I discovered the lathe has a 5HP motor. No wonder the 3 HP RPC only growls and chatters.

In a few days I will begin collecting parts to build a 5 HP RPC. This whole process has been a learning experience and I feel almost qualified to build one from scratch.

Thanks for all your help

Rich

Rich,
If your lathe is 5 hp, you will likely need a 7.5 hp RPC. Lathes can be hard to start at high speed--just a thought.
By the way, the Carlson RPC design has a major fault. The potential relay used references to ground. there are much better ways to do this--specifically the Steveco (white rogers) 90-66 which references phase to phase and works much better. I have helped several posters replace the 6X550 relay with a Steveco. The 6X550 will hande a 3 hp idler directly, a Steveco 90-66 will handle a 5 hp idler directly. For 7-1/2 hp, I use the Steveco to control a 3 pole start contactor rated for 40 or 50 amps and put one start cap one a pole-likely to need two start caps about 270-324 mfd.
send a PM if you want and I will send you a phone number and some circuits.
Bruce

Bruce is correct, you'll need about 7 1/2 HP. You can test it by starting the RPC then the Bridgeport and let it jut run, no load. Try starting the lathe, if it still has problems starting then add your saw motor to the mix. Start it and let it run no load and again try to start your lathe. I prefer Delta wound motor for RPC.
Older frame motors make better RPCs than the newer. I was told by a motor maker the older motors were rated by torque and speed, the new rating are similar to the European type as they are rated by electrical HP. If you want to build a good RPC get an older big frame motor and as I said I like Delta wound better. I built an RPC for a friend using a 3HP Delta motor, later he added a 5HP lathe, it runs fine with no problems on that 3HP RPC, only problem he has is that if he tries to reverse motor direction without waiting for the spindle to stop it reverses the RPC instead.

Frank

Don't be concerned with the current values. The low current value on the manufactured leg
is pretty common, and likely and artifact of the 4.5 amp utility leg values being mostly
reactive current. It's important to achive voltage balance between all three legs by
adjusting the run capacitance. That's the real goal.

In the limiting case where the idler is very large compared with the load motor, you
don't need any run capacitance to get this. If your idler is undersized a bit (sounds
like this is true) then you need to work to get the voltage balance up there, or
move to a larger idler.

"- Probe each phase in turn and log the zero-cross as displacement along the time-base."

This works. Again the caveat is that many modern scopes will have a limited range for
input voltages (and this includes an crippled inputs or trigger inputs) and often the peak
values of line voltage can exceed them. Not everyone has access to a TEK 535 scope.

Also, just counting the zero crossing times works, but you need to keep your wits about
you and understand which is lagging and which is leading - it's not hard to get turned
around and mix up the phases. Done correctly it's a good way to go.

jim rozen said:

Not everyone has access to a TEK 535 scope.

Click to expand...

Not everyone even knows what a TEK 535 is. They are all tubes, for gawd's sake. I used them when they were new and personally wrote the purchase order for a Huges Memotron. Even I have progressed to a pair of TEK 564s.

Bill

Point was, those old tube monsters often had plug-ins that were very happy having hundreds of volts of
signal going to the inputs.

I got my start re-habbing RM45s and other tube type tek equipment. Stuff was bullet-proof.
One year I got three 511s out of huge trash dumpster, after they had been tossed out of a third
story window.

Each of them worked just fine when I plugged them in.

I never used a 511 but put in a lot of time with a 514AD circa 1956. After never having access to anything better than a Heathkit, it was a wonder. Then we got a 535 with the delayed sweep. Over a half century later, they are still very useful. these days, I mostly use a 453 on a cart because it is so convenient.

Bill

PS the early TEKs had the world's greatest terminal strips.