RPC and Motor Starter question

Chucky55 · Sep 25, 2017

I have set up a 15hp RPC to run a powermatic vertical bandsaw, and for future equipment for my little home shop. I have a question regarding the some wiring options for the magnetic starter.

The RPC is dead simple...pony start, unbalanced. Works like a charm. Because it is oversized for the little 2hp motor on the Powermatic, I am not concerned with balancing at this time...maybe a future project. My generated leg runs about 10% lower than other two legs at idle. Haven't measured at load yet.

I am using a Allen Bradley 709 series size 0 starter...here is the wiring diagram

My question is this...using a 240v coil in the starter, am I OK to use my unbalanced generated leg(L2)for one leg of the coil circuit?

My initial thought was that it would provide protection from single phasing if something caused the RPC to drop offline or if there was any problem. But I am not sure if the lower voltage, and maybe fluctuating voltage, would be an issue for the coil? Will the coil operate at the lower voltage from my generated leg (about 217V at idle), or should I use the two factory legs at the full 240v to power the coil and give up any "single phasing" protection? Or maybe there is another method I am unaware of to provide "single phasing" protection?

Thank you in advance for reading and replys...
Chucky

rons · Sep 27, 2017

A overload block with phase loss detection or a 3 phase monitor is the right way.

Your idea is only concerned with one phase. I assume that the starter you mention is on the Powermatic, not for the RPC motor. As your diagram shows, there are only overloads for L1 and L3. There is no phase loss protection. You can replace the overload module with a modern piece of hardware that monitors all three phases for balance and loss.

What I did was use a modern motor starter on my RPC that will do all the protection and leave all machinery stock.

Monarchist · Sep 28, 2017

Chucky55 said:
I have set up a 15hp RPC to run a powermatic vertical bandsaw, and for future equipment for my little home shop. I have a question regarding the some wiring options for the magnetic starter.

The RPC is dead simple...pony start, unbalanced. Works like a charm. Because it is oversized for the little 2hp motor on the Powermatic, I am not concerned with balancing at this time...maybe a future project. My generated leg runs about 10% lower than other two legs at idle. Haven't measured at load yet.

I am using a Allen Bradley 709 series size 0 starter...here is the wiring diagramView attachment 208778

My question is this...using a 240v coil in the starter, am I OK to use my unbalanced generated leg(L2)for one leg of the coil circuit?

My initial thought was that it would provide protection from single phasing if something caused the RPC to drop offline or if there was any problem. But I am not sure if the lower voltage, and maybe fluctuating voltage, would be an issue for the coil? Will the coil operate at the lower voltage from my generated leg (about 217V at idle), or should I use the two factory legs at the full 240v to power the coil and give up any "single phasing" protection? Or maybe there is another method I am unaware of to provide "single phasing" protection?

Thank you in advance for reading and replys...
Chucky

What Ron said. as to enhancing protection. If.. you really feel the need.

More often, YOU (and yer ears or NOSE) are far the better line of defence. If/as/when s**t goes pear-shaped, shut 'er down and troubleshoot.

Otherwise.. "conventional wisdom" is to NOT use a(ny) RPC's "generated" leg for protective devices, OR powering control transformer(s), OR powering any other ancillary if avoidable. Just the load.

KISS

JST · Sep 29, 2017

You can, it you do not want to go the whole way with it, just put a small light on the generated leg. If the light is out, or dim, then so's the generated leg. Simple, no questions,

I am not super happy with solutions like waiting for something to smell (burnt, I assume). Besides, unless you are really working the machine, you likely won't overload it enough to smell anyway.

RPCs are pretty simple and robust, they don't fail that often. And a few percent low is pretty normal for the generated leg. The generated voltage has to be less than the line volts for the motor to work. A maker of dedicated RPCs can have that set of coils wound for higher volts, and get nominal volts out of the generated leg. Rolling your own, you cannot.

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RPC and Motor Starter question

Chucky55

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