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Motor protection after idler in RPC?

leeko

Stainless
Joined
Jun 30, 2012
Location
Chicago, USA
Hi all,

I just purchased a phase-craft 5hp RPC panel, and am getting ready to hook it up to my 3hp 1140rpm idler. In the instructions, it recommends to install:

1) a fuse or breaker box between the panel and the equipment for overload protection

2) overload protection going to the idler motor


My questions are:

a) is (1) satisfied by a magnetic starter on the load machine? A breaker box will be in the plan once I branch out to more than one machine, but is it reasonable to wire an outlet to the output of the RPC, and connect a plug directly to/from the mill's mag starter in the meantime?

b) The RPC idler is an 1140rpm 3hp 3ph motor, with an FLA of 10.6a @ 220v. I have a 30a rated fusible safety switch that I plan to hook up between the RPC panel and the idler. I don't plan to use it as a switch; I'll leave it "on" and just use it for the fuses it houses. What size of fuse should I install? 10amp fuse, based on the FLA? Or does it need to be oversized because of the load from connected motors?

Thanks in advance,

Lee
 
The protection is actually separate for the converter and the motor. See the phase converter section in the NEC, Article 455.

The load motor(s) should be protected as any other motor of the same rating. No difference due to being run from a phase converter.

The wire from the phase converter and the direct wires going to the load should be protected per their ampacity.

The phase converter should be protected on the single phase side by overcurrent protection at not more than 125% of the nameplate full load amps of the converter, which would be the unit if a commercial part, or the idler motor if shop- assembled.

If it is a dedicated unit for a single specific load ONLY, then the rules change a little bit.

That's all assuming the idler is supplied by its own wires, and contributes ONLY the manufactured phase to the loads, so that the single phase "pass-thru" wires to the load are not also supplied by the converter wiring.
 
I find interpreting NEC code difficult.

One thing I have learned from reading this forum over the years, studying the NEC and building my own three phase rpc/three phase distribution system is that circuit breakers and fuses are there to only protect the branch circuit wires and that each motor (unless it has built in thermal protection) should have overload protection to protect the motor.

I believe that there should be a circuit breaker assembly (two breakers with the switches pinned together) providing branch circuit protection (the wiring)to the RPC and Overload protection (I prefer a magnetic motor control with overload protection) at the RPC to protect the idler motor.

Between the single phase circuit breaker and the RPC control there should be a circuit disconnect.

The output of the RPC to the three phase device should also have a circuit breaker (in this case a three phase breaker) feeding the three phase device to protect the wires and right before the device a magnetic motor control with overload protection (to protect the three phase device motor).

You said that for now your RPC would be dedicated to a single device but you intend to expand to multiple devices in the future and at that time install a three phase circuit breaker.

If dedicating to a single device at this time the output to your three phase device should still hace branch circuit protection (a three phase breaker or three fuses [one on each phase]) to protect the wires and overload protection (preferably a magnetic motor control with overload protection)to protect the motor.

I have attached a photo of my own system in case it should be of any help. The photo shows the wall of my shop where the single phase circuit panel is. This panel is located to the left of the window. You can see a conduit exiting the single phase breaker box at the right lower left of the breaker box. This is the circuit feeding my RPC. The circuit runs into a disconnect device (the red switch) and then into the RPC controller. Wiring comes out of the three phase controller to the idler motor below it and up to a three phase circuit breaker panel where I have a breaker for each of my three phase devices. What is not see in this photo are the magnetic motor controls, with integral overload protection at each device.

Vlad

RPC Wall.jpg
 








 
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