Phase converter strange vibration

I

I'd be curious to know if the vibration is there when the RUN capacitor is eliminated. If you insist that balancing the RPC output voltages is important, ignor this capacitor removal suggestion.

Jerry

Jerry, I have only one run capacitor between legs 1-3. That's what allows the RPC to start. There is, possibly, too much capacitance. I will experiment...

i

I

My point is -- There is very little to ever be gained from leaving the start caoacitor IN the circuit, especially if the ultimate use of the RPC is to power a compressor with an unloader. Bump the capacitor into the circuit only long enough to get the idler spinning a little then disconnect it. It doesnt add to the power delivering capacity of the RPC. Correct me if I'm wrong or if I misunderstand.

Jerry

Jerry, you were 100% right. After I disconnected starting caps, the loud buzzing noise was replaced with pleasant humming.

I will use a start pushbutton and your solid state relays to switch caps off. (caps will be on only while the pushbutton is engaged).

i

Damn he's good.

Well done Jerry!

Gunner

"Pax Americana is a philosophy. Hardly an empire. Making sure other people play nice and dont kill each other (and us) off in job lots is hardly empire building, particularly when you give them self determination under "play nice" rules.

Think of it as having your older brother knock the shit out of you for torturing the cat." Gunner

I would suggest the 7.5 HP idler motor was originally designed for very high starting torque; maybe for an air compressor. NEMA designates the design of squirrel-cage induction motors in several classes according to their electrical characteristics. The winding configuration of the rotor (generally) is varied from class to class in order to modify starting torque, running current, slip, efficiency and etc. In all liklihood, the design of your motor was for high starting torque - such as class "C". Leaving the start cap permanently in place (self starting RPC) could cause excessive noise in a motor designed for high starting torque.

Bob Swinney

Since you already have the capacitors, I would suggest you measure the voltages that you get now with no starting caps. And then see what reconnecting some of the caps you disconnected and remeasuring the voltages.

Yep it isn't necessary. But I think you will find that some is good, a lot is not. My experience is that with a little playing around, a starting push button and a relay are not necessary. Your mileage may vary.

Dan

Ignoramus19393 wrote:

Thanks Bob. What you say makes perfect sense. As I said, I experimented with

1) more capacitance -- it helped start the motor quickly

and

2) with disconnecting caps after successful startup -- it greatly reduced vibration.

It was interesting to see how disconnecting the cap produced sparks.

What was even more interesting is that, after unplugging, when I shorted my 3 parallel big behemoth vodka bottle sized capacitore, to remove the charge, there was a huge spark and a sound like a .22 shot.

I think that I will add a discharge resistor, too. Any idea how many ohms for every 100 mF is best?

I bought several 220 mF start capacitors on ebay last night, I will use them and Jerry's solid state relays. I want to find nice momentary switches too, preferably sized like household light switches for easy mounting.

I appreciate your input...

i

Yes, experimenting is fun and easy. I will play some more. The problem is this. As soon as I have enough capacitance to get the motor spinning, it vibrates a lot.

If I remove capacitance at run time to reduce vibration, I am left with not enough to restart the motor once it stops.

i

If you multiply the capacitance by the resistance, you get what is known as the RC time constant. The capacitance is normally in ufd or farads times 10^-6. So if you multiply by resistance in Megohms ( ohm times 10^6 ) you get the time constant is seconds. In one time constant, the voltage drops by 63%. So the rule of thumb is to wait about three times constants so the voltage is only 5% of what it was.

For 100 mfd, try 10,000 ohms. (100 * 10^ -6 ) times ( 10,000 /

1,000,000 ) or ( 100 * 10^6 ) times (1/100 * 10^6 ) or one second. So waiting three seconds for that size discharge resistor would be prudent.

With a 10 K resistor and 240 volts, the current thru the resistor would be 24 milliamps. And the power in the resistor would be 5.7 watts.

So maybe a two watt 100 K resistor would be a better choice. It would mean you ought to wait at least 30 seconds before assuming the capacitors are safe.

Dan

Ignoramus677 wrote:

approximately to e^-1 of the original charge, right?

I like that better, actually. I can wait 30 seconds.

Thanks Dan.

i