Capacitors and Three-Phase Conversion

"when I try starting the motor it just hums and the shaft barely turns."

Assuming you mean when you start the single phase motor. What you are describing is usually a symptom of the centrifugal start switch in the 1 phase motor being open. It is what applies the start capacitor to get the motor spinning when you start it up. As the motor comes to speed, the centrifugal force opens the switch and drops out the start cap. If the switch fails to reclose when you shut it off, the start cap is not there and the motor just hums.

You may be able to repair it if you are good with mechanical parts, but I have tried and failed, it's trickier than it appears. I would take it to a motor shop, they do that all the time and will not likely charge you much.

He's not running a single-phase motor. He's running a three-phase motor to generate a third leg. Most phase converters use caps for this along with a timer/starter switch to charge the caps enough to provide a current to the L3 of the converter motor just long enough to get that motor turning and generating an L3 through induction. That humming noise he's describing is classic single-phasing, so he's looking at either a bad capacitor, a bad motor, or possibly insufficient current on L1 or L2.

That VAC rating will, IIRC, make a difference. When using a 125VAC cap in a 220VAC circuit, cut the farad rating in half. Some meters have the ability to test a capacitor; if you don't have one, an A/C repair shop might be able to help you out. But I'm pretty sure if it was bad you'd know it...the lid would be blown out and the interior's packing material would be leaked all over the place.

So -- check those caps. Heck, replace them if you're worried about it. I don't think they're that expensive. Meg that motor and check your line voltage. If all else fails, get you a VFD (they can generate an L3 for you) and ditch that phase converter. That's what I finally did with mine, and believe me, it was the best money I ever spent.

Thanks for the great advice. You seem to know what your talking about and understand My question.

One other question . What is a VFD?

Thanks

oilcan13

Happy to help, believe me. I had a phase converter -- two different ones, actually -- on a lift station of mine for years. Had to baby that thing like you wouldn't believe some times. It felt like all I had to do was look at it at times and it would blow caps. When I was finally able to get rid of it, I did and never looked back.

A VFD is a variable-frequency drive. These, through varying current frequency and voltage, allow motors to be run at variable speeds. They can also generate a third leg, just like a phase converter but without having to worry about caps. Note that if you go that way, you'll need to look at a drive with a horsepower rating twice that of the motor; i.e. a 3HP motor will need a 6HP drive. Still, worth it in my opinion.

Sounds like the capacitor is open. Time to replace it. The capacitance value sounds high for 3 HP but the voltage is too low. You need at least a 250V cap. the higher voltage rating, the better. You should have a potential relay in that circuit also, you need to check that it is functioning properly also.

This is a little off subject but, when running a three phase motor on single phase, is there any derating of the horsepower of the three phase motor? I know that to use a VFD for this it needs to be increased.

Dear Guest, I would expect that running a three phase motor from a single phase supply (once you have managed to start the three phase motor) it would only generate about 2/3 of its rated power since you are only using two out of the three windings.

However if you are using a VFD then the VFD will generate the three phase from the single phase so I would not expect a decrease in power from the three phase motor.

Not sure if this answers your query or not.

These are just my thoughts and I am liable to be incorrect. Perhaps someone can either confirm or correct my thoughts.

Kind regards

Mr. W.A Snow

Thanks for the input. All the info I had on converters was That a static converter would generate 2/3 the power needed but that a rotory converter would generate the full amount. i.e. a 3hp 3 phase motor will start and run a 3 hp 3 phase motor where as a static converter rated at 3 hp would only start and run a 2 hp 3 phase motor.

is this correct thinking?

oilcan13

You will get only 2/3 of the rated hp. In this case, he is using the motor to generate or manufacture the missing third leg in order to supply 3 phase to other equipment.

I replaced the capacitor and it now works.

However I replaced it with one similar that I had which was 750-800 mfd @120 volts.

My thinking might be wrong on this but the capacitor is in a 120volt circuit suppling voltage to the third leg of the three phase motor that is why I was hesitant about putting in a 220 volt capacitor. Would a 220 v capacitor work in this situation?

Any comments are welcome.

oilcan13

The 120 volt cap should work for a while. When it goes, replace it with one rated for 250v. In this case, the voltage rating has nothing to do with the amount of energy passed, only the strength of the internal insulation (electrolyte). That capacitor is working in a 230 volt circuit and should have a rating of 250 volts (or more). Hope this helps.

Dear Guest, is this what you have built http://www.metalwebnews.com/howto/phase-converter/phase-converter.html or something like it. Perhaps this article might assist you with your current issue.

Regards

M.r. W.A. Snow

Thanks:

I went to the web site and found the explanation very explicit, however the sketches left somthing to be desired, I could not read the notations on the sketches.

Thanks anyway

oilcan 13

oilcan13, yes the scetches were very poor but I though that the explanation was quite OK.

Regards

Mr. W.A Snow