OH OH I exploded my 1st attempt at a static phase converter.

I have a 10 hp 3 ph motor and a band saw with a 1 hp 3 ph motor.
I assembled a 4 capacitor circuit in an attempt to construce a "balanced" static phase converter.
Two of the capacitors are 3200uf electrolytics that were connected in series - Cap + to + cap - fashion to protect them from the 220 volt AC.
The other two capacitors were 150uf each and 450volt NON elyctrolytics. All were new. The run capacitor values were taken from a chart containing different HP values provided by someone on this website or on the "allaboutcircuits" site. This was done prior to a computer crash and I lost the info and it's source. ANYway!!

I had the band saw wired into the circuit to pick up it's 3 ph input from the 10 hp windings. To begin the band saw power switch was OFF. I applied 1ph 220v power to the circuit and the 10hp motor. It started and was running fine for a minute or two. As soon as it started I disconnected the start capacitor and it contintued to run.

Then I turned the band saw on and It started up fine, ran for about 5 to 7 seconds, and then the two run capacitors exploded and burned. I immediately turned off the saw and the power to the circuit, grabed a fire extinguisher and put out the small blaze.

OBVIOUSLY, I SCREWED UP!!! But what was my downfall? Perhaps, one or two run capacitors are a bad idea when the motor driven by the 1ph input has its windings wired in parallel with another 3ph motor to drive it.

Can you experts educate me regarding my mistake and what action to take? Recently, I read someones advise that if two run capacitors are used on different legs that one of them should be about half the value of the other and both sized according to motor hp BUT that does not, it seems to me, to be the source of my problem.

It occured to me that adding the saws inductance to the circuit when it was turned on might have, when combined with the capacitance values, caused a parallel resonant circuit. However, that seems unlikely. Please help. Regards. Ed

You don't say, so I assume your 3200uf electrolytics were the start capacitors and the 150uf 450volt non-electrolytics were the run capacitors.

Any idea what the power rating on your run caps was before they fried themselves? Were they smallish electronic components (small box of matches size) or largish power components (good fraction of a coffee mug size)?

Can you provide a schematic (a napkin sketch would do) of how you had the caps wired, and in particular where you were taking off power for the bandsaw?

Bill, OKAY, I'm getting the impression that you are not fond of a " static crapconverter"!! haha Good humor! But yet, you say to get 2 high voltage ripple-rated electrolytics to improve the balance of the 10 hp. Allow me to extract a little clarification. What capacitance value electrolytics? Since you say "balance" so I assume you intend to use these as run capacitors. Correct? I also assume, you are saying to wire these in series either -++- or +--+ to protect their electrolytic nature. Correct? And then, once installed I can use this configureation to drive the 1 hp motor? Does not sound too far from whar I did and got flames!! So to what exactly, do you attribute my explosion? To low of a voltage rating on the run capacitor(s)? The fact that I had 1 on one leg and another on another leg? Pardon my ignorance of the term "pony motor". Do you mean a moter started with a pull rope instean of a start capacitor?

"airey-fairey feather-farter" haha No, dat's not me! But I am known to destroy things!!

If you are saying that I am better off using a 3ph "idler" motor to drive my band saw then what hp idler do you recommend if I were to use my 10 hp motor for a different application?

I have a 26" diameter carbide toothed blade that I would like to spin with that 10 hp motor plus some pulleys and belts. Of course, that puts me in the same pickel of how is best to develop 3ph to drive that application. Another idler of some size???? My 10 hp is a huge heavy, but reconditioned, old one.

Hello steadberg, Yep, the elictrolytics (new and British made) were the start capacitor and the 150uf 's were a capacitors with one in 2 different legs of the 10 hp motor. I don't know of a power rating but they were each about 1.5" diameter and about 4.5" tall. Asian made, I think. Each leg of the band saw motor were connected to each leg of the 10 hp idler motor once the band saw was switched on following the idler motor comming up to speed with it's start capacitor disconnected. Regards Ed

It's amazzzzing how many misspelllllled words I can produce.

OOPS!!! I meant SFRIEDBERG!!!!

machapungo said:

OOPS!!! I meant SFRIEDBERG!!!!

Click to expand...

No harm done.

Well, a couple points to look at.

1) 150 uF capacitors about the size you mentioned sound as if they were not properly rated for the application.... just based on size.... And obviously there WAS a problem, since they didn't work. You about canNOT get 150 uF into that space at 450V and a reasonable long term AC current rating. I'd also be suspicious of something "only" the size of a soda can.

A decent AC across-the-line capacitor of that rating should be expected to be roughly a 5 x 5 x 5 inch cube, or an equivalent/similar volume of some other shape.

2) Someone mentioned using AC rated electrolytic capacitors. I'd not do that, if I were you. You want film or oil and paper type AC capacitors, rated to be connected across the AC line for power factor correction (motor run may also work OK).

Most electrolytics you find will be AC rated only for very short times, used for motor starting, just as you have yours. Not at all what you want for "balance" (which really means power factor correction of the generated leg).

Wonderous things to learn about

All this time, I've thought of "static converters" as starters for 3ph motors when energized with 1ph supply.
In my mind, all the caps would drop out when the 3ph motor got up to speed. after that, the third leg was just a ghost. (but available to any other motor that might be in circuit)

At any rate, caps in circuit with a static converter just seems like extra complication, and a smoke maker at that!

JST, OKAY, I now understand that the 150uF "film" type capacitors that I used were not up to the task. Thanks. Since the explosion did not happen until I turned the saw on I thought, perhaps using run capacitors on the 10 hp "idler" motor was improper when driving another motor. But, your information, I think, suggests that the circuit I used was OK for this application. Correct?

I did do research and my circuit schematic did comply with what has been called "balanced" if that means using two run capacitors of the proper value with one in each leg not directly driven by the 1 phase input voltage. As a previous post of mine said, I found differing opinions about the uF size of the run capacitors and so far no one has opined which is correct. Do you have an opinion on whether both should be the same uF size or if one should be about half the uf size of the other?

My research, to date, did not run into AC ratings. You mentioned "current" so I assume that I should be looking for an amperage rating. Correct? For my 10hp motor what minimum amp rating should I look for? WOW !!! A 5"x5"x5" cube is a BIG capacitor!!! No wonder people don't seem to want to use run capacitors. These must be expensive!!! Besides Ebay, do you have any suggestions as to where I might find a good deal on these critters?

I have a general understanding of circuit power factors but I'm not fluent. I do understand capacitive and inductive reactance and their equations. Since my run capacitors did not explode when the 10 hp motor was runung without a load, would I be correct in thinking that the inductive and capacitive reactance in the two legs with the capacitors was closer to providing a ballanced load to the incoming line power UNTIL I applied more inductance in parallel with each leg by connecting the saw motor? This notion suggests that trying to ballance this dual motor configuration is not a trivial matter. What do you think? Thanks and Regards Ed

Hello Calg,
It did occur to me that RUN caps were probably not a good idea (especially with a motor generating phases for another motor) when the dang thing burst into flames.
The start cap seems to be another story and less at risk since it is removed once the "idler" comes up to speed and the saw motor has not joined the circuit.
Thanks for your input. There seems to be a growing consensus that the run caps should be eliminated and let the power company eat the power factor. Regards Ed

At this point, for your, and our edification, you probably ought to draw up and post a schematic of what you were doing.

That will help figure out what the issue was, at least, and possibly help prevent further problems.

3200uF run capacitors? You failed to mention the rated voltage. I have never seen a 3200uF run capacitor rated 110V or better. Also, you NEVER connect the capacitors to two legs of your current...start and run should involve only L1 or L2...not both. Since you are new at this, I have to question your definition of the capacitors...the start capacitors (electrolytic?) are made with a plastic case...the "run" capacitors are steel and relatively heavy. I assume that you burnt the start capacitors...the plastic round ones. This makes sense, because after 7 or 8 seconds, more depending upon their rated voltage capacity, they would explode and burn...the "run capacitors, the metal ones, would not.

I currently have over 50 machines running on static converters that I made and operate. Never have had an issue...after 20+ years.

Hello elysianfield,
I just double checked and the two electrolytics are 3300 uf each. 20% tolerance and 450 VDC. Made in the UK. As I said, wired back to back -++- which drops the capacitance in half to 1650 uf and their cas is plastic. The burnt up 150uf caps I used for run were also plastic case and were described as "film". I had one run cap betweem L1 & L3 and the other between L2 and L3. The input 220 ac was applied across L1 & L2. The 1650 uf start was between L1 & L3 with a switch in series to allow it to disconnect once the 10 hp motor started.
You mentioned making 50. Are you the person that made a chart available that relates capacitor values to motor HP? If so, please point me to it. Regards and Thanks Ed

Hello again Thermite,
My initial posting did say the run caps were 450V. "F's sake" Your memory must be fading in the heat of your thermite manners and smart A attitude. Lighten up, you'll live longer.
Yea, I know, everyone is an incompetent dummy except you. I'm certainly ignorant of many things but dumb I ain't. My IQ has ben tested at 147! What';s yours?????
I(f this post is the best you can do to be helpful then I would be grateful if you took your "Mike Foxtrot" attitude elsewhere. Regards and have a good day. Ed
Yes, I did neglect to mention the 450v rating of the two caps used for a start capacitor.

machapungo said:

Hello elysianfield,
I just double checked and the two electrolytics are 3300 uf each. 20% tolerance and 450 VDC. Made in the UK. As I said, wired back to back -++- which drops the capacitance in half to 1650 uf and their cas is plastic. The burnt up 150uf caps I used for run were also plastic case and were described as "film". I had one run cap betweem L1 & L3 and the other between L2 and L3. The input 220 ac was applied across L1 & L2. The 1650 uf start was between L1 & L3 with a switch in series to allow it to disconnect once the 10 hp motor started.
You mentioned making 50. Are you the person that made a chart available that relates capacitor values to motor HP? If so, please point me to it. Regards and Thanks Ed

Click to expand...

Ed,
Try this. To start a 10hp motor, use a 900uF+ start capacitor. Wire L1 directly to T1. Wire L2 through a switch to T2. Take a line off of L2(switched) and run it through a momentary switch, and then to one connector on the capacitor. Run a line from the other connector on the capacitor to T3...done.

I have never seen electyrolytic capacitors used, and I think you are just complicating things using them. Also, don't worry about using run capacitors to "balance" the 3rd leg unless you need more power from the motor. Don't overthink this...it is as easy as I have just described.

I'm seconding Thermites "polarization" notice.

I have never used DC caps for anything AC, but smoothing ripple. or snubbing contacts in DC circuits they work just fine. (Watch the voltage! ;-)

I think you may have a problem there.

elysianfield,
Other than the run capacitors (which I will now exclude) my configuration was exactly as you recommend, except the start capacitance used was different. I get it. I've got it.
Thanks for your straight forward and well mannered response. Regards Ed

CalG,
OKAY!! I will be taking this polarization notice seriously. As you know, the DC caps DC voltage rating was a tad more than twice the 220 ac RMS line voltage.
I will always "watch the voltage". Thanks for your help. Regards Ed