RPC problem with 10EE MG

[kvom01]

This morning when I attempted to use the lathe, one of the start capacitors in my RPC panel blew up, spreading its oily guts inside. I contacted the manufacturer, Anderson Converters, and on their advice I purchased a new cap from Grainger and installed it in the box. I can now start the idler motor, apparently without any problems, but I have not attempted to restart the lathe pending advice from this august forum.

The telephone "diagnosis" from the Anderson guy is that the lathe is drawing more current than the converter is designed for, and thus stressing the start capacitors. The panel and idler motor are both 7.5HP.

He basically said the following:

1) I have a 50A breaker on the input power, and he says it should be 30A to match the RPC panel capacity. I could do this if necessary; his opinion is that the lathe would draw more than 30A and trip the breaker. FWIW, the wire between the breaker and RPC panel is 6ga.

2) He thinks the lathe will draw more current at startup than the RPC is designed for.

I obviously would prefer not to have to buy a larger RPC if it's not a necessity. What are people's experiences with RPC sizes and MG 10EEs?

[Cal Haines]

Do you have and RPC with an idler motor (sounds like you do), or just a "static" phase converter (which is usually just start and run caps). If you have an idler, I would think you should start it first, then try to start the lathe's MG. Then there should be no issue with the RPC's start capacitors, as they should be out of the circuit.

Cal

[kvom01]

Yes, there is an idler motor. Since the lathe has the momentary start button, I can't even start the MG before the converter (unless for some reason I tried holding it in while flipping the RPC's start switch).

The combination has run fine for 6 months until yesterday. So the reason for the cap's blowup is "mysterious", unless it's a situation where something gradually degrades the cap over time.

[jwatts]

The RPC for my 10EE is also a 7.5 hp and I had a start cap blow one day as well. In my case the culprit endud up being a defective switch. On my unit the start caps are only in the circuit for a second or so through the use of a momentary push button. The button stuck in the on position and about 10 seconds later the cap blew.

[Cal Haines]

OK, so you start the RPC, then you start the lathe. AFIK, the start caps should only be in the circuit when the RPC starts and should not come back into the circuit when a load motor starts. If they are, that could definitely be a problem.

You said the cap blew up and created an oily mess? That sounds more like a run cap. Can you post a photo of the converter and identify the cap that blew?

Cal

[MichaelP]

The electrolytic start capacitors are, usually, of cylindrical shape and often black color. Oil-filled run capacitors most often are oval in crossection and of silver color. Capacitance and voltage rating, quantity of the caps of each type or a photo would help determine which is which.

Electrolytic caps are not as reliable as oil-filled ones by far and are not designed to be energized in RPC circuits for more than a few seconds. If they are, they will certainly blow up.

So the current your lathe draws has absolutely nothing to do with your run capacitor. By the time you start your lathe the start capacitor must be long out of the circuit.

However, if the load is so great that it stops the idler motor, then, depending on schematics, I could imagine that the relay could re-energize the capacitor and keep it energized for too long. Otherwise, I see no connection between the load and starting capacitor of the RPC. What's your lathe motor horse power rating, anyway?

I suspect that you had relay contacts stuck temporarily. If so, it may happen again. At least, clean the contacts.

[kvom01]

Here's the photo requested:



The capacitor that blew was the center one of three connected in parallel. The run capacitors are the two larger silver ones on the right (one of the terminals is connected to one of the output wires). The white box on the left is the relay that is supposed to disconnect the start caps after the idler starts, and the black box on the top is the power-on start relay.

Here's the installation:



There are four connections to the panel:

- 230V 1-phase from the breaker
- 4-wire to the idler
- 4-wire to the 3-phase socket mounted on the other side of the wall for the lathe
- 3-wire through the wall to a toggle switch next to the lathe for turning on the RPC (the panel has an on-off switch as well, and the idler starts if either is in the on position)

As manufactured, all of the caps are attached together and to the bottom of the enclosure with silicone, and the spade connections are soldered. I'm wondering if I need to solder the replacement cap. It doesn't look as if they would come off.

[MichaelP]

It, definitely, is an electrolytic start capacitor. And no, you don't need to have a soldered connection.

I'd incline to use start capacitors rated for higher than 250V (although 250V rated ones should work just fine for 240V, higher rating will provide a better safety margin for the capacitor). Run a bleeder resistor across the legs of each capacitor.

And clean the relay contacts as I suggested (if they're accessible).

[kvom01]

Relay is a sealed box, so no way to clean the contacts. Lathe's AC motor is approximately 6-7HP. The replacement start cap is not oil filled.

[MichaelP]

Well, in this case you can hope for the better and turn the lathe on. Try to start it unloaded (no spindle movement). And let RPC work for a few minutes before starting the lathe just to make sure the relay worked fine this time. I'd watch the idler carefully for any hesitation at the moment you switch the lathe on. It would be even better if you use a tester to make sure the run caps stay disconnected before, during and right after the lathe is started.

You can test the relay separately, of course, but since the problem can be intermittent such test, if negative, will be non-conclusive.

P.S. Personally, I'd prefer to use a more powerful RPC with a 7HP load.

[Cal Haines]

Something is very wrong here. The starting caps should not be coming back into the circuit after the idler is running. Your first step should probably be to call the manufacturer again and ask what's going on.

I would not run the lathe until you get this resolved or you may have another cap blow up.

One possible work around is to put a switch in series with the start caps so that you can disconnect them before starting the 10EE. But that makes your remote start feature pretty useless.

Can you get a part number off of the white relay?

Cal

[peterh5322]

"Something is very wrong here. The starting caps should not be coming back into the circuit after the idler is running."

That's generally true.

However, if the potential relay starting method is being employed, and there is not a lock-out circuit, one which prevents the starting circuit from being reconnected to the RPC system subsequent to its starting, then it is possible for the starting circuit to be reconnected to the RPC whenever the load results in enough droop in the manufactured phase to cause the P.R. to drop-in.