RPC Balancing, Capacitor Sizing and Voltage

So... I've got my 380V 3Ø Schaublin 102 lathe running off my "reconditioned" Arco Roto-Phase MG. The 246V (my panel is only 80 ft from the transformer) single phase input produces apx. 440 V at the motor (440 V L1-L2/L1-L3 and 460 V L2-L3). The transformer is P 220V : S 380V.

The Schaublin is very smooth and quiet when running (just the spindle, at low speed, motor at 900 rpm). The Roto-Phase is very noisy (buzzy, groany sound) when idling and quiets down considerably when loaded by the lathe motor.

This leads me to the idea to convert the Roto-Phase, which is self starting (two 40 µF caps in parallel between L2-L3) to a balanced configuration to reduce the noise during idling (which would eventually annoy the hell out of me). Also, the option would be there in the future to add another idler if I need more system capacity.

I've been going back through the threads, about 30 pages worth so far, and I understand the basics, but I have a few questions still:

1. I've seen frequent mention of a 40:60% ratio in running capacitor sizes. What's the reason for this, and on which side should the larger capacitance be placed?

I could easily accomplish this by placing the 40 µF caps across the two different lines, and adding a 20 µF cap, but this would raise the total capacitance to 100 µF. Is this desirable or should it be avoided?

2. Since my power is already above 240V, I would prefer to use start caps rated higher than 240V for the extra safety margin. Can anyone suggest a source for these?

I checked Surplus Center but they only seemed to have 220V rated start caps. Also, how do I determine the correct amount of start capacitance? Peter H suggested 400 µF (thanks!)

What are the advantages/disadvantages of more or less starting capacitance? Also, I understand that the start cap should go on the same side as the larger running capacitance.

3. The motor, a 1982 vintage Swiss RCB two-speed metric (IEC?) 100L C-Face motor is being run at 16% above rated voltage. Is this a risk to the motor windings? I can't get the voltage any lower since I'm already on the lowest transformer output tap. The only option would be another transformer on the 245V side.

All for now. Thanks in advance and Happy New Year...

-Jorge
CinemaTechnic

The best article I have found for determining start capacitance and balancing of an RPC is
http://home.att.net/~waterfront-woods/Articles/phaseconverter.htm
Somewhere in that article you will find these guidelines-
CL1-L3 = 4 to 5 times the motor horsepower
CL2-L3 = 10 to 15 times the motor horsepower
Cstarter = 40 to 100 microfarads times motor hosepower just for testing
Towards the bottom of that article you will find the rotary phase converter print.
Your idler on your RPC should hit its rated RPM within 1-2 seconds after hitting the start button. I found 100mf times idler HP worked the best in the start circuit.
Hope that helps. K.

HP of idler?

Thanks for the info. I had seen that page before and saved it, but I am taking another look now.

I have to say that I'm still curious about the reason why you want substantially different capacitance on the two "sides". In this case he is recommending a ratio of 4:10 or 5:15 (29:71% and 25:75% respectively).

I'm going to take the author's (Rick Christopherson) advice and buy a bunch of surplus run capacitors in different sizes for trial and error testing. Fortunately they are quite inexpensive.

Since I built this RPC is being built from an old commercial self-starting RPC, I don't know what the HP rating of the idler is.

The Arco is conservatively rated at 1.5 HP "Largest motor started", and 5 HP "multiple motors", although it has no problems starting the 2 HP motor on the Schaublin (which does start at 0.8 HP in the low speed and then go to high speed).

The idler is a 182 frame, and it draws 2.5A @ 220V idling and requires overload protection at 10A. Based on the info I have, I would estimate the idler HP rating at about 2.5HP or 1.9kW. This would give 8.8A at full load, and overload protection at 115% of full load amperage.

Interestingly, I've already got two 40 µF run caps, which is 16 times the idler HP each (two are connected across phases B-C). I could try 40 µF on B-C and 12.5 - 15 µF on A-C.

BTW, can anyone suggest a relatively inexpensive tool for testing capacitors? I wonder if the 30 year old oil-filled caps I am using now are still 40 µF.

I really appreciate this forum and the help I have received from the members here. I wish everyone a happy and prosperous new year.

-Jorge
CinemaTechnic

Idler Size...

I just found the info, going back through the threads in a post around page 37, that most industrial motors 2.8 A @ 220V 3Ø = 1 HP.

In that case, my 182 frame Arco idler must be a 3 HP, which would give 8.4 FLA, and 10 A overload protection would give OL protection at 120% of FLA (which I believe is enough to meet NEC standards).

This is the picture of the Pony motor wiring diagram. But ,again, it might not be the original cover but I’m not sure if these differ much. Also I wrote at the btm L1 L2 L 3 in black when I started the RPC. Thanks y’all