3 phase hookup, ground and disconnect

I have a 2.75 hp 3 phase specialized machine which was manufactured in germany. The specifications call for a 208 volt delta power configuration, 3 hot wires and a ground.

I want to run it using a phase-a-matic R5 phase converter with the CNC box, which I added because the voltage on my single phase panel runs high, about 242.

The machine has minimal electronics, the contactors and an electronic timer (all 220, since there is no neutral).

My single phase panel has the neutral bonded to ground. Is this acceptable for this device or would I be better to run an isolated ground (i.e. a seperate wire to a rod in the ground)?

What is the difference between a "3 phase disconnect" and a 3 pole single phase disconnect? I find boxes like this:
http://www.amazon.com/SIEMENS-GF321NR-240-Volt-Outdoor-General/dp/B005G95JX8/ref=wl_it_dp_o_npd?ie=UTF8&coliid=I3TQGFBH1ZE100&colid=563L69A6KZVD listed on one web site as being a 3 phase disconnect and on another site, the same device being listed as a 1 phase 3 pole disconnect. Are they interchangable, or might the single phase 3 pole device have a neutral bond to one of the poles or something? I would guess that would not matter since I will not connect a neutral to the box.

Thank You,
Nataraj

For ground, run ground not neutral! Neutral is for (return) supply current. Ground is for protection, and is not intended to carry any significant amount of current. Ground and neutral should be connected in only one place, at your service.

Assuming your phase-a-matic doesn't qualify as a "separately derived service", there is no need to establish an independent grounding electrode. In fact, that can be undesirable! Your grounding conductor to the new equipment should connect back to your service panel and thus to the existing grounding electrodes, but it doesn't have to be an uninterrupted "home run" if you have a properly bonded subpanel or distribution box.

As for the disconnects, you can generally use a 3-pole disconnect as either a three-phase delta disconnect or a split-single-phase-with-neutral disconnect. You'd have to look at the details of a given disconnect to be certain, but that's the general rule.

I'm not clear on why there is confusion between neutral and ground.....

Sfriedberg has detailed the difference.... BUT YOU DON"T NEED NEUTRAL..... your machine uses Delta input with no neutral.

Generating 3 phase from single phase with an RPC usually produces a 3 phase set of voltages which do NOT share their "true neutral" with the input neutral wire.

You CANNOT use the neutral that comes to the RPC for the 3 phase, there is a voltage difference of around 70V between the input neutral and the 3 phase "true neutral".

I think I am pretty clear on this now. I do understand the difference between neutral and ground and that they are only bonded in one place in my main circuit breaker panel. I was just wondering if the ground coming from my single phase panel was acceptable to use as the ground which gets passed through to my equipment and I'm hearing that it is.

Isolated grounds are something that are more commonly used for large computer installations (they are good for sensitive electronics), or at least there was a time when they were required for certain computer systems. I'm not sure how common they are today, but I have been running my computers off of UPS outlets with IGR's (Isolated ground receptacles) since the mid 80's and have experiences very high reliability from that. I'm hearing that I don't need anything like this for my 3 phase motor device, though I remember back in the 80's being involved in the installation of large 3 phase computer systems that required isolated grounds.

My understanding of 3 phase power is that, if I had a true Wye system instead of a Delta, then the neutral would come off of a center tap on the transformer and would be around 120V relative to each of the 3 hots, but what I'm hearing you say, is that the neutral on my single phase panel is not a proper neutral relative to the phases generated by the phase converter, again, not important here since my equipment requires only delta.

Thank You,
Nataraj

sfriedberg said:

As for the disconnects, you can generally use a 3-pole disconnect as either a three-phase delta disconnect or a split-single-phase-with-neutral disconnect. You'd have to look at the details of a given disconnect to be certain, but that's the general rule.

Click to expand...

Just to clarify one point. The neutral can be switched in single phase system, although not commonly done. If you do switch the neutral, NEC requires that all ungrounded leads be switched.


Tom