View Full Version : RPC capacitors

07-27-2008, 03:22 AM
Long ago I picked up a Desco 5hp RPC and I spent some time this week wiring it up in a temporary way to test the surface grinder I just bought. It all worked well and I've got a few items on order so I can hardwire the RPC into the shop and run conduit to various machine locations.

What I'm wondering about is whether there is a safety issue due to the capacitors in RPC.

Do they hold a charge after the AC has been shut off?

I've not taken this testing setup apart yet. Electrical systems are one of my many weak areas so I figured I'd ask before I started grabbing bare wires.

07-27-2008, 04:04 AM
AFIK, the balancing caps will be wired across the motor so they will disharge into the windings when power is switched off.

The start caps on the other hand may be dangerous as they are only in circuit for a short while. To be safe they could be wired with disharge resistors.

Before sticking my hands into an rpc, i would make a discharge probe with a low resistance load like a halogen bulb or cooker element and put it across all the caps, cant be too safe.

A dead short is not good for caps.


07-27-2008, 09:25 AM
Are you saying sticking a screwdriver across them like I've always done aint good? :rolleyes: Rick

Paul Alciatore
07-27-2008, 04:40 PM
Are you saying sticking a screwdriver across them like I've always done aint good? :rolleyes: Rick

I have worked in electronics and Radio/TV for my entire career, including high power transmitters with capacitors that held charges in the tens of thousands of Volts range and enough charge to kill. I have never heard of or seen a dead short doing any damage to a capacitor The above mentioned transmitters always had both an automatic shorting mechanism AND a manual shorting rod that both had heavy gauge connections directly to ground. You always GROUNDED all caps before working on anything. No low Ohm load, just a driect short.

A light bulb can burn out and then not provide any discharge of the caps. Any other resistive device, such as a regular resistor can also fail with an open circuit. I have actually seen small resistors used as fuses in some applications and of course, any component can just fail. I would NOT recommend using any such device, just a heavy gauge shorting wire or the above mentioned screwdriver. KISS: Keep It Simple Stupid. Keep it as simple as possible. The a** you save may well be your own.

On the screwdriver thing, if you use it, be prepared to loose it. I once burned about 1/4" off the tip of a screwdriver while shorting a cap - while the power was still turned on it turned out. Bad for the screwdriver but it saved my a**. Oh, and do be sure the handle really is insulated. Believe it or not, some plastics DO CONDUCT.