How many "starts" per minute - static converter

I have been running my Hurco CNC mill with a Cedarberg static phase converter for several years. I was even able to do tap cycles with it. This past week I had a job where I needed to do a grid of 1/4" tapped holes one inch apart (81 holes). The program uses a "loop & repeat" set of blocks, so once you start you can't stop or pause - only hit emergency stop. Anyhoo, as you can probably guess, about 35 holes into the tapping routine, the static phase converter goes pop. I open it up and there is black goo splattered inside the converter box. I disassembled everything, cleaned it all up, and ordered a new starting cap from McMaster-Carr (it was only a 125vac 270-324uf). I also ordered the 250vac version, but could not fit it inside the case.

A tapping cycle begins with the spindle rotating clockwise and then traveling down until reaching the programmed "bottom" of the hole (thru hole in my case), at that point the rotation "plug reverses" and the spindle retracts until well clear of the hole. It then shuts down the rotation and allows the spindle to coast to a stop before powering back up in the clockwise direction for the next hole. I'm guessing a single hole tapping sequence takes about 30 seconds (1" tapping depth thru a 1/2" steel plate). Sooo, in a one minute period we're seeing 2 starts from a dead-stop and 2 "plug reverses".

#1 - a simple statement: these things will blow up if you exceed their design limits.

#2 - a question: How many starts per minute (per hour?) should these things be good for? and, would the 250vac capacitor increase that number? Is the number of starts cumulative (like the converter is only good for 1,000 starts and then the cap needs to be replaced)?

I have a rotary phase converter for the entire shop and I just wired up a pigtail and plugged in the Hurco so I could get the job done. I don't like to do this because I need to keep power on the controller pretty much all the time; so, I feed this particular machine single phase and use the static converter inside the power cabinet.

I have never had a job where I had to tap more than a few holes before, and now I understand one of the limitations of my setup. Still, it hard to argue with the fact that I have been getting by for several years like this.

Most start capacitors are rated for 3 second long starts 20 starts per hour. If you go much above that, they blow up.

If you want to use single phase, but need to have more starts per hour, replace the start caps with several run caps connected together to get the same uF. If you do that you will have infinite starts per hour, but they will take up more space. (Might need an additional box)

-Joe

this problem is not uncommon

Jim,

I did a similar thing (popping the capacitor) with my static phase converter (and so did someone else), as reported in this thread: http://www.practicalmachinist.com/vb/showthread.php?t=100791

I don't know what the Cedarburg phase converter looks like, but maybe there is a way of increasing the box size for the higher rated capacitor. The Phase-A-Matic brand have enough empty space for a bigger component inside the box.

-Jon