Home built 20 hp phase converter

Don Gitzel · May 3, 2020

So I just completed a 20 hp converter using a teco westinghouse TEXP 6 pole motor. I used mostly new parts save the start caps which I salvaged from a converter I had purchased for parts. On initial start one of the start caps blew like a Roman Candle bellowing black smoke. I immediately cut power. Upon further investigation I found the Capacitors were too low in value for the rated hp. I replaced the start Capacitors for 4 (400-480) caps. In my calculation 90 mfd per hp for starting equates to 20x90= 1800mfd. Before energizing I decided to add an additional Capacitor rated at 408-480 to the circuit in a temporary wiring mode. But I wired this cap wrong. It was wired not to disconnect from the circuit. When I energized a second time after 4 seconds there was a remarkable explosion and I ended up on my bum. I was working on the tailgate of my truck and the top of the capacitor that I added went about 30 ft in the air and landed on the hood of my truck. Well after insuring my nosy neighbor that I was good, I removed the temporary connection to the 5 th start Capacitor and energized the converter once more. All went well.

Phil in Montana · May 3, 2020

Did you have a recording amp probe to see the inrush amps? a 20 hp with 1800 mfd you will bend the pointer....Phil

thermite · May 3, 2020

Don Gitzel said:
When I energized a second time after 4 seconds there was a remarkable explosion and I ended up on my bum. I was working on the tailgate of my truck and the top of the capacitor that I added went about 30 ft in the air and landed on the hood of my truck. Well after insuring my nosy neighbor that I was good, I removed the temporary connection to the 5 th start Capacitor and energized the converter once more. All went well.

ROFL! First, and CLOSE TO the last - time I did that, the cap in question used a materiel called "phenol furfurol" for it's insanely high (for those olden days, anyway..) dielectric value. The separator was some sort of paper.

"Fur for ALL" fit the results!

Filled a whole ell-wing of my basement with a stinky confetti resembling yellowish milkweed on steroids when the f****er detonated!

Glad you survived it!

Blowing up caps is like riding two-wheelers.

You are JUST NOT a "rider" until you have had to lay one down, get the underside leg out and up before yah hit the jungle, ride the screaming sparky wreckage to a halt ... and be able to dust-off yer leathers - or leatherish scarred legs, ankles, and ASS - and get right back up on two-wheels again, same damned hour, no delays nor excuses.

jim rozen · May 3, 2020

thermite said:
You are JUST NOT a "rider" until you have had to lay one down, get the underside leg out and up before yah hit the jungle, ride the screaming sparky wreckage to a halt ...

I usually just use the brakes and stop - less dramatic. We're not counting that trying to drive on snow thing though.....

DaveKamp · May 3, 2020

Great story, Don!

Uh yeah, capacitors make great uh... electrically detonated explosive devices... best demonstrated by remote control.

For a bigger RPC, a better choice is to either spin the idler up with a pony motor first, or build a smaller self-start (like... 3hp) and then switch in another smaller idler (like... another 5hp, with it's own balancing caps), and then use that to grunt the 20hp up to speed. A pair of time delay relays on contactors does a nice job of staging the starting. It's a lot easier on your mains, as Jim noted, inrush currents are indescribably high...

thermite · May 3, 2020

jim rozen said:
I usually just use the brakes and stop - less dramatic. We're not counting that trying to drive on snow thing though.....

I sure as hell AM counting a 2 ayem LARGE pizza run in hilly Morganhole, West By God Virginia when a smart alec Collitch kid thot he could manage a grip-clutched 5-speed rotary up a twisty hill.... and hold the damned pizza box "loose" atop the fuel tank with t'other hand.

Long story short, a Bridgestone 90 was only 140-odd lbs avoir, I was about the same, somebody's lawn about ten feet DOWN over the embankment was WET with dew.

The bike bounced-off a tellypole about eight feet away from where I skidded easily 20 feet, thoroughly "greening" my over-confident ass LONG before "AOC" came along to sell it as a good idea!

Straightened the steering machinery and mirrors, one shattered, but they were cheap, and that was all the damage.

Pizza even remained in the box, was still warm and was well received back at the party. "Green ass" even got me enough girly attention ISTR I "got lucky" as well. Not that anyone could "keep score", those days.

Gave up 2-wheeling around 40 years of age and many pounds heavier when a buddy's Yammer-hammer dirt-nuke buzzed my eyeballs on washboard clay and gravel so damned badly I couldn't see the road for the blur!

Figured it was jest no longer my game...

Don Gitzel · May 3, 2020

My home made converter

thermite · May 3, 2020

Don Gitzel said:
View attachment 287391

My home made converter

Couldn't "zoom" it enuf to see but

A) .. yah doo have about a 15K Ohm high-grade bleeder resistor in parallel with EACH of those caps? Bag of a hundred is nearly as cheap as dirt.

B) Fuses... are never "inherently" made-up as "Common trip". 3-Pole Circuit Breakers, generally ARE.

Loss of but ONE Phase is not a "good thing".

1-Phase or 3-Phase, I like the Square-D "QO" family for breakers. Won't go near their "Homeline" OTOH, even with a politician's fingers.

Hardly the "lone ranger" on either of those choices.

Don Gitzel · May 3, 2020

Ha ha the bleed resistors are 22 k. If you were able to zoom in you would see the lengths of #14 solid wire soldered across each fuse. I couldn’t find 60 amp slow blow fuses. &#55357;&#56838;

D.D.Machine · May 4, 2020

are you trying for a Darwin Award ?

just go old school with starter motor to get the 20HP up to speed ... I ran a shop off a DIY rotary for years tell I needed more power then I got more RPC`s ,,,

jim rozen · May 4, 2020

Don Gitzel said:
Ha ha the bleed resistors are 22 k. If you were able to zoom in you would see the lengths of #14 solid wire soldered across each fuse. I couldn’t find 60 amp slow blow fuses. ��

Suggestion: if you do use fuses, consider fusing the incoming single phase 240 - two fuses. This protects all the downstream wiring and it cannot
single phase, ever.

Don Gitzel · May 4, 2020

Makes goo sense Jim. This converter is the most well voltage balanced one I have ever assembled. With a load consisting of an old 7.5 hp Lincoln motor plus a 2 hp induction motor the variance between phases is 2.9 %. It is a six pole motor and I am wondering if this is the reason. I have welded a large nut on the shaft of the motor to spin it up with my drill for starting purposes but the motor starts well without it.

thermite · May 4, 2020

jim rozen said:
Suggestion: if you do use fuses, consider fusing the incoming single phase 240 - two fuses. This protects all the downstream wiring and it cannot
single phase, ever.

Mebbe not. But the remaining live leg can still put YOUR lights TF out!

Common-trip BREAKER, Jim. TWO-pole, not three-pole, on the single/split-phase side, but COMMON trip nonetheless!

They are actually cheaper than good fuses. Yah can even TEST a used CB.

Yah can test a fuse, too!

But then again....

DDoug · May 4, 2020

I see you learned that capacitors are very efficient devices....pound for pound, when they "let go" they really put out the power.

I made my RPC cabinet with steel shielding between the start & run caps, as well the rest of the components, just for that reason.

jim rozen · May 4, 2020

thermite said:
...the remaining live leg can still put YOUR lights TF out!

He's a smart guy. If the thing stops he'll figure it out. Doubt he will make that mistake. Many industrial
machines (my hardinge lathe for example) use fuses.
Never understood why folks insist on fusing the manfuactured leg.

My approach for my stone age converter is BOTH. 15 amp common trip in the panelboard, that feeds the
converter via number 12 wire, with two TEN amp fuses in the knife switch. The breaker never trips.
The fuses only blow if the power drops out and the converter spins down when I'm away from
the shop. And then the power comes back up....

Has not happened yet.

jim rozen · May 4, 2020

digger doug said:
I see you learned that capacitors are very efficient devices....pound for pound, when they "let go" they really put out the power.
I made my RPC cabinet with steel shielding between the start & run caps, as well the rest of the components, just for that reason.

I've been told that military batteries have the energy density of TNT. If they develop an internal short, you get the same *bang* effect.

thermite · May 4, 2020

jim rozen said:
I've been told that military batteries have the energy density of TNT. If they develop an internal short, you get the same *bang* effect.

"No current knowledge", but... the batteries in an early cold-war-era MIM-14B (NIKE-Hercules, improved) nuke-warshot anti-aircraft or city-buster - missile were a pyrotechnic. "Ignited" to put out power.

Mind "not for long", given the bugger hit Mach 5 within a few seconds of launch.. ...

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