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RPC design phase - making sure parts will "fit"

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Paul_NJ

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#1
I'm finally getting around to building a RPC, for the present at least to power a Bridgeport M I've been restoring. The BP motor is 1/2 HP, 3P with a nameplate voltage of 208-230 volts.

My workshop power measures 240 volts 1P.

Now that I'm taking a closer look at the "parts" I've accumulated, I have a few issues I'd appreciate someone "looking over my shoulder" on . . .

1. Idler motor is a relatively new 5HP 3P listed as 208-230 volts. I wouldn't think 4% overvoltage would matter, but worth mentioning. (Yeah, I know, greatly oversized for the intended load. First: it was a "gift" and Second: who knows what the future will bring . . .)

2. Motor starters I was planning to use are NEMA 1 (idler) and 00 (BP) C-H's, but both have 208 volt coils. Do I need to change the coils to 240 volt ones?

3. I was planning on employing a start capacitor with a delay timer to drop it out after a few seconds of starting. I've seen designs using an OFF delay timer. However, I've got quite a few ON delay timers sitting around not doing anything (and no OFF's) . . . with NO & NC contact pairs. Can't I use an ON delay, wiring to the NC contacts, and timing them to open to drop the capacitor? Am I missing something? Also, can someone give me a guideline on needed ampacity of the timer relay for dropping a starting capacitor?

Would appreciate any and all comments and suggestions!

Thanks

Paul
 
Last edited:

markba633csi

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#2
No real reason you can't use the timers you have- only real difference is whether the timer's internal relay is powered or un-powered after the timeout I think. What are the contacts rated at? I would say 10 amp would be OK but that's a guess.
Mark S.
ps Not sure about the starters, they may get too hot on 220/240- maybe you could put resistors in series?
 

Keith Foor

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#3
If the idler motor is 5 HP I would advise a 30 amp rated contactor for the start capacitors.
As far as timer control.
I would look at Omron or other 8 pin octal base timers.
The timers are going to have multiple configurations that will close on powerup, then time out of cycle on and off.
Before actually connecting the contactor to it and starting a motor with it, make sure to apply power and verify that it's operating the way in needs to.

THe other thing I would advise is to temp a momentary switch to the start cap contactor and time it with a stopwatch when the motor starts up and you release the button. That will give you a starting point for setting the timer.

On a side note. I typically put in an output contactor to keep power from being applied to the output lines before the RPC is up and running.
This can be controlled from that same timer. Wire the output contactor to have power applied to it's coil when the timer times out and opens the closure controlling the start contactor.
 

Paul_NJ

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#4
If the idler motor is 5 HP I would advise a 30 amp rated contactor for the start capacitors.
As far as timer control.
I would look at Omron or other 8 pin octal base timers.
The timers are going to have multiple configurations that will close on powerup, then time out of cycle on and off.
Before actually connecting the contactor to it and starting a motor with it, make sure to apply power and verify that it's operating the way in needs to.

THe other thing I would advise is to temp a momentary switch to the start cap contactor and time it with a stopwatch when the motor starts up and you release the button. That will give you a starting point for setting the timer.

On a side note. I typically put in an output contactor to keep power from being applied to the output lines before the RPC is up and running.
This can be controlled from that same timer. Wire the output contactor to have power applied to it's coil when the timer times out and opens the closure controlling the start contactor.
Thanks Keith for good, practical suggestions. I will definitely factor them into my design.
 
Last edited:

Paul_NJ

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#5
Thanks Keith for good, practical suggestions. I will definitely factor them into my design.
 
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