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contactor/timer for starting capacitors for RPC

taiwanluthiers

Cast Iron
Joined
Dec 6, 2016
Location
Xinjhuan District, New Taipei City
So I have a RPC that right now I must manually shut off the start capacitor when I start it. The problem is when I shut it off the switch arcs a LOT. Like spark flies, IT'S ALIVE!!! kind of spark.

I want to use a timer and a contactor, but I am concerned about the contactor arcing and being welded shut, which is a disaster.

This is why I haven't done such an upgrade because I'm very concerned about this.

Is there a safer way I can automate the shutting off of the start capacitor?
 
As an RPC should be run up off load it ought not to need a start capacitor. If it does I'd be inclined to suspect either the design, component values or health of the idler motor.

When I built a few static converters I automated the start capacitor shut off by using a contactor connected between the wild, generated, leg and the centre point of a couple of same value capacitors connected across the other two phases. The capacitors made an AC potential divided so when the generated leg voltage got sufficiently close to the mid point voltage between the other phases the contactor would drop out due to insufficient current in the coil. Drop out on mine was when the wild leg got to maybe 2/3 rds its proper voltage. Which worked well enough.

You can buy relays / contactors made for this sort of job (potential relays?) but I used what I had.

Side benefit of switching at lower voltage is less current so less spark.

Clive
 
My RPC is basically a static phase converter with a couple of idler motors. I can start it without the start capacitor but it makes really bad sound for a couple of seconds before starting. I'm concerned about damage to the motor if I keep starting it this way.

Or if I put a centrifugal switch on the pony motor would this be better?
 
Diode snubber network?

Any interrupted current will arc. You can break it when the current is instantaneously zero, or you can provide an alternate path (snubber).
 
With no data on the RPC in question...

1. I use a mercury wetted relay. I got a couple of these for $30 ea in a surplus store.

2. Or use a three or four way contactor to divide the current and use a MOV across each set of contacts.

Had a open style relay in use for years. The arcing was noticeable with some pitting. It worked like that with MOVs.
The mercury relay is enclosed and should out-last anything in it's current rating class.
 
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Well, I wired the contactor in parallel across all three contacts (the start cap uses only one wire) to spread the current out. No stuck contact so far.

I did however manage to blow a breaker because I was confused by the design of the contactor, instead of one lead on top and one on bottom, both leads are on top AND bottom and leading to a short circuit.... Why can't they make all contactors the same??

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Should have read instructions before throwing the switch. Now I got one damaged (possibly) contactor and stuff. But I set it up now so I just throw the switch on the RPC and the timer/contactor does the rest.
 
"Women don't like guys who likes to machine stuff..."

Too bad you gave up.

Mulligan.jpg
 








 
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