Poof, I saw the Genie

[Aukai]

I recently moved my 5hp RPC from a fused 30 amp disconnect, and moved it to a 50 welding outlet. So I started the RPC to run my DoAll bandsaw, and had to do something else, and cut power, I finished what I had to do(not as long as I thought) the RPC still had a decent RPM going winding down, and I hit the start button with a brief hold. That is when there was a pop with magic smoke from the cabinet, and the start capacitor lost it's containment. Luckily I did not shut it down, and I was able to make the cuts without issue. Now the capacitor is 340-408um, did I cause the failure by the restart, or moving to a 50 amp circuit, the RPC has a 30 amp breaker on the box?

 

[homebrewed]

I recently moved my 5hp RPC from a fused 30 amp disconnect, and moved it to a 50 welding outlet. So I started the RPC to run my DoAll bandsaw, and had to do something else, and cut power, I finished what I had to do(not as long as I thought) the RPC still had a decent RPM going winding down, and I hit the start button with a brief hold. That is when there was a pop with magic smoke from the cabinet, and the start capacitor lost it's containment. Luckily I did not shut it down, and I was able to make the cuts without issue. Now the capacitor is 340-408um, did I cause the failure by the restart, or moving to a 50 amp circuit, the RPC has a 30 amp breaker on the box?

It's hard to believe that you did anything wrong, given that the start switch and associated circuitry permitted _something_ to happen. You may just have triggered a failure that was going to happen sooner rather than later. Sounds like a start capacitor on its way out just got a little more encouragement .

It's pretty unlikely that going from a 30-amp circuit to a 50 amp would cause a problem -- as long as the RPC worked fine on the 30 amp circuit. If it was popping the 30 amp breaker.....well, that's a different story.

If you have documentation for your RPC it would be good to see what the recommended procedures are for starting and stopping it. RTFM, eh?

 

[Aukai]

I was advised to give a momentary push on the start button not a quick push, and release, so I do a 1 count to start. It's not a vendor RPC.

 

[rabler]

I agree with @homebrewed that it is likely a pending failure. The start capacitor is only energized during startup so it was going to fail during startup. Having just run the RPC it is possible that capacitor was a bit warmer and that helped move it along the way. I'd replace the capacitor and not worry about it unless a second one fails. The change of connection point would only matter if there was a significant change in voltage, which there shouldn't be.

If you are suppose to hold the start switch, it is possible that the start capacitor is active during that "hold". That would be the only reason I would think of to hold it. You might listen to the RPC idler and only hold it long enough for the idler to get mostly spun up. Holding it longer than necessary would (if my assumption is correct) just stress the start capacitor, they're not designed to withstand extended use. Again, if you're only doing a 1 count, that shouldn't be problematic, so it was likely that capacitors time was up. What could be problematic is trying to start the RPC under load and holding the start button overly long as it struggles to spin up.

 

[graham-xrf]

I am assuming that you switched off the bandsaw, and then cut power to the rotary converter, but I am not sure the exact sequence of what you did.
Nor do I get what happened when you powered up again. Do you mean switching on the still spinning converter with the bandsaw switched off, or was it supposed to get going with the bandsaw already connected?

What I do know is that an open circuit rotational anything still turning with inertia will make a real big voltage as soon as it sees a field coil energized. The start capacitor is supposed to cause the motor to start turning in the correct direction, and the run capacitor section is supposed to shift the phase of the incoming supply to make an artificial version shifted 90° to apply to the other motor winding. I would not expect that any "hold" on the start switch is ever necessary, because the contactor would already be latched on.

The key thing is to know that if you already have a (high) unconnected AC voltage coming from spinning down RPC windings, one via a capacitor, with phase not at all going at 60Hz, and suddenly slam on a contactor joining that to a incoming supply, the relationship between these voltages is such that a capacitor may be being asked to stand the transient current, possibly exceeding short-circuit conditions, until the motor catches up to it's proper phase lag.

It may not be that the capacitor was "on it's way out". Now, given what you have done to it I would think it now is.
Never, ever apply power to motors until they have made it to a safe stop!

 

[Eddyde]

I have a home built RPC the enclosure has a clear front; I notice whenever I restart it while the idler is still spinning it produces an arc-flash from the start relay and there is a longer louder buzzing sound than heard during a cold start. Not sure what that means electrically but there is a distinct difference between the start up modes.

 

[markba633csi]

Just a surge. Probably won't happen again.

 

[mksj]

The capacitor would be an electrolytic 340-408uF, the higher the voltage rating usually uses heavier internal materials and should last longer. The start capacitors like in motors heat up very quickly, and if they stay in the circuit for more than a short period (seconds) will blow. The times they will most frequently fail is when start/stop is repeated in close succession. Most start capacitors are rated for a fixed number of start/stop cycles in and hour, one reason why I recommend 3 phase motors for machines like lathe and mills w/o a clutch system. I wonder if there is some back generation from the spinning RPC motor that may cause an issue with you reengaged the start capacitor. Might as well get two.

TEMCo 340-408 uf/MFD 220-250 VAC volts Round Start Capacitor 50/60 Hz -Lot-1 | eBay

TEMCo ID: SC0083. Uf (mfd): 340-408. "I have always been delighted with the service from TEMCo. About TEMCo. Frequency: 50/60 Hz. Format: Round. Country of Manufacture: United States. Temperature range: -40 to +65°C.

www.ebay.com

TEMCo 340-408 uf/MFD 220-250 VAC volts Round Start Capacitor 50/60 Hz -Lot-2 | eBay

TEMCo ID: SC0083. Uf (mfd): 340-408. "I have always been delighted with the service from TEMCo. About TEMCo. Frequency: 50/60 Hz. Format: Round. Country of Manufacture: United States. Temperature range: -40 to +65°C.

www.ebay.com

 

[markba633csi]

Agree with Mark it probably was some type of back-generation voltage crash - if you can up-size the cap voltage ratings it would help prevent another one-
Higher voltage ratings often means larger size so keep that in mind
A couple kilovolt rated cap would be ideal but each one would be the size of a toaster oven

 

[dwall174]

I recently moved my 5hp RPC from a fused 30 amp disconnect, and moved it to a 50 welding outlet.

did I cause the failure by the restart, or moving to a 50 amp circuit, the RPC has a 30 amp breaker on the box?

I doubt it was caused by moving the power supply! Especially if you have a 30A breaker on the RPC.

I switched my power supply on my DIY RPC from a 30A plug to a 40A hard-wire set-up & I never had any problems.
I did however up-size the controls & wiring to handle 40 amps when I first built it.

Agree with Mark it probably was some type of back-generation voltage crash

That's my thought also!
Is your start capacitor wired straight to the push-button?
My RPC uses a potential relay to control the start capacitors, so if there's any back-generated voltage the caps won't trigger.

Doug