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Run Capacitor Question

ow2knives

Cast Iron
Joined
Nov 16, 2002
Location
Arvada Colorado
I am balancing the phases on my 7.5hp rotary phase converter by adding capacitance to the B-C leg and the A-B leg using as a reference an article by Bob Swinney in the Dec 2001 Home SHop Machinist.

As supplied the unit came with 2 three terminal caps. They are both marked 24 - 24 - 24 microfarads. They are wired in parrallel between the B and C leg using only one side of each capacitor - giving a total of 48 mf between the B and C leg

According to Swinney I require a minimum of 30 mf per HP giving a total of 225 mf for a 7.5HP unit. I have a lot of capacitance to add.

It seems that one should be able to use the second capacitor in each of the cans? How should they be wired? Common sense suggests that if the B leg is connected to the center terminal then the C leg can be connected to both outer terminals giving a total of 48 mf per unit and 96 for both?

I would really appreciate it if someone would confirm my thinking here.

many thanks.

Owen Wood
 
Hi Owen,

Yup. But they should only be marked 24-24 because the center terminal should be common to the two outside ones and you should have 24 mfd to each outside terminal. So you wire the two outside terminals together and you have 48 mfd between the center terminal and the common wire to the outside terminals.

IN THEORY, you want to put about 60 percent of the TOTAL capacity between A and C and about 40 percent of it between B and C. But practice may not be that way when you actually start measuring the voltages. I found that adding the 3 horse lathe motor partly loaded to the 5 horse idler motor changed that balance CONSIDERABLY.

Don't forget to add more condensers to the line input (A to B) to improve the power factor and reduce the line current. Just don't add these until you get the idler ones correct. And don't add so much to the line that you end up driving the voltage too high on the idler motor when the load motor is not running.

Just as a "fer instance", at the moment (with the stock of condensers I have and with a 5 horse idler and a 3 horse lathe motor), the best compromise I can get is a total of 100 mfd between A and C, a total of 40 mfd between B and C, and a total of 50 mfd between A and B (the line). With that combination, the unloaded idler motor voltages are 247 (A-B), 253 (A-C), 256 (B-C) with the line current 2.4 amps. And with the lathe motor also connected and running the lathe spindle at 1000 rpm the voltages are 244 (A-B), and 237 for both A-C and B-C, and a line current of 9.2 amps. NOTE that this is considerably different than a 60-40 ratio, but the best match of the voltages between A and C and B when the lathe is partly loaded. That's just the way it turned out. If I had more condensers at the moment, I might be able to raise the C voltage with the lathe motor on, but that would also raise it even more when the idler is running unloaded -- maybe more than I want to. I don't know. I intend to buy some more and find out.

Kurt
 
Hi Owen,

The one thing I forgot to say is that in my case, this converter I am building was built to supply power ONLY to the lathe in my backyard "barn". (The rest of my three phase stuff is in my house garage a hundred feet away, but I can't fit another lathe along with two cars in my house garage.) So it's highly unlikely that it will ever supply power to anything else. So I'm interested it balancing it as a complete system including the lathe motor at some (what I figure by wild ass guess method is the) "average" load on the lathe.

If you are intending to use your converter to supply power to an assortment of motors, then what I am doing has no meaning for you and you should just do the standard routine for setting it up with only the idler motor running. Or, I suppose you could balance it for each motor in turn, keep track of what you used, and then sort of "eyeball" an average of them and set it up for that. (If that works. I don't know. I haven't tried that yet on the converter in my garage, but I will when I get this one set up and free up all of my condenser assortment. But even there, it's a simpler task because it's only feeding a 12" Clausing lathe and a Republic surface grinder -- and maybe someday my milling machine if I ever find a two speed three phase motor for it to replace the single speed single phase motor that it came with. And of course, since it's only me, I only run one machine at a time.)

Kurt
 
"It seems that one should be able to use the second capacitor in each of the cans?"

Definitely.


"How should they be wired?"

Series is one possibility, but this is not the obvious choice.

Parallel is the obvious choice, with the total effective capacitance being the sum of the two sections.

However, there are cases where the two sections are quite different in value, one larger, one small, and you may choose to reserve the smaller of the two sections for final "tuning", in which case the common terminal would be the manufactured phase (B), or you may choose to reserve the smaller of the two sections for power factor correction, in which case the common terminal would be one of the lines (A or C).
 








 
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