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Power Factor Correction

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LAN_403

Potluck

Sat Feb 16 2008, 09:34AM

Registered Member #1322 Joined: Sat Feb 16 2008, 09:11AM
Location:
Posts: 6

I've done a search on this board for the topic but didn't find much. Well, not what I'm unterested in anyway so I'll pose my question.
I've seen others using motor-run caps for PFC. While this is primarily used to reduce the power drawn by a transformer my main interest is simply getting the phase in sync. The voltage is 240vac.
Although this isn't really tesla oriented is seems the field of tesla coils is about the only area I've seen that addresses PFC outside the industrial arena or utility company concerns. Not much info for the Joe-hobbyist out there if you know what I mean.

Anyway,
As I understand it motor-run caps are used across the line to bring voltage and current into phase. My concern is the actual connection... right across a 240vac line. I've looked at motor-run caps, that is where they are used and there seems to always be a coil, a winding, in series with those caps. Motor-run = used in motors.

I have 25 Mallory Motor run caps to play with, 370vac, oil filled, dual 15 and 20uF. These aren't small caps. They measure 7 1/2" tall by about 2" X 3 1/2" oval. I have a stack of Staco variacs, 6 - 2510s with paralleling chokes, that power up initially with a 250W 9 ohm resistor in series to dampen surge which is then shorted by a large contactor. So surge current through the caps shouldn't be much problem since I want to use them on the output side of the variac stack anyway. That and the fact that the variacs must be a zero output in order to power up. (microswitch senses rotor position).

Now, the last thing I need is an oil filled cap/s this size exploding with 240 volt power behind it from the variacs in my basement or project box/enclosure. The Stacos output up to 280vac so that's the absolute highest voltage I'll be dealing with.

So, has anyone ever had any problems with putting motor-run caps directly across a 240vac line?

GeordieBoy

Sat Feb 16 2008, 05:05PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

Motor run capacitors should work fine for PFC duty. (It is motor-start capacitors of the electrolytic kind that are only rated for low duty cycle that can explode quite violently when used for PFC duty.)

A motor run capacitor is usually rated for more volts than the mains line because it forms a kind of resonant circuit with the run winding of the motor. In the "motor run" application the capacitor is actually creating a phase shift so as to create a rotating field inside the induction motor. This is contrasted with PFC duty where the capacitor is attempting to minimise the phase-shift between the line voltage and the total current drawn from the line. As long as you don't exceed it's ratings the capacitor doesn't care what the application is.

Dr. Slack

Sat Feb 16 2008, 07:09PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Can I suggest an experiment to put your mind at rest. Put one of your motor run caps cross the output of your variac, crank it up to 280v, then go away and have a coffee. When you come back, feel the temperature rise. If it feels warm or less then use them, hot or more then don't. With the rating you have posted, I would fully expect them to laugh at your 280v and ask for more!

I needed a large amount of power factor correction, and turned my attention to my collection of photoflash electrolytics. To test, I connected two in anti-series with diodes, and cranked them up on a variac. The temperature rise test suggested that 4 in series in each leg would be needed for 240v, that's a lot of soldering to make effrectively an MMC power factor correction cap, so I bought some motor-runs!

Potluck

Sat Feb 16 2008, 10:06PM

Registered Member #1322 Joined: Sat Feb 16 2008, 09:11AM
Location:
Posts: 6

I love tesla forums. You can ask about something really out of the ordinary or strange and people on those forums handle it like there's nothing crazy about it at all.

NeilThomas wrote ...

Can I suggest an experiment...

An experiment! Yes!
I love experiments.

Thanks guys!

Hazmatt_(The Underdog)

Sat Feb 16 2008, 10:39PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I hope to finish the article on PFC. I started it, but I also started work recently and its a really hectic 14 hour day, new job training and all that. I might be able to finish it next week, but I dunno. The timing was just bad, but I hope the article will answer some of your questions in the future.

Matt

Potluck

Sun Feb 17 2008, 08:17PM

Registered Member #1322 Joined: Sat Feb 16 2008, 09:11AM
Location:
Posts: 6

I need to mod and install a board for the power center before I can do the experiment. As soon as I get that finished I'll give this a shot and I'll be back with the results.
One other thing I'll be considering though. The 280vac is the RMS value. The peak value would be close to 400v so I'll begin with two caps in series and take some measurements.

Potluck

Mon Feb 18 2008, 08:54PM

Registered Member #1322 Joined: Sat Feb 16 2008, 09:11AM
Location:
Posts: 6

Ran the experiment.
Two in series barely got warm after 15 minutes. A single cap got as warm in no time at all and was getting much warmer. Didn't press the single cap. I'll go with two in series. The voltage peaks measured 378 (one peak - not peak to peak) with 272vac delivered by the variacs.

Thanks all for your help!

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