VFD question

Because in the case of an RPC, the entire downstream bus in all probability has excessive voltage imbalances when measured phase to phase and even even a few percent of voltage imbalance has a very profoundly negative effect on motor temtrature rise and energy losses to core heating....

Are YOU that stupid?

The voltage at load will sag quite a bit--depending on the second series components' electrical characteristics unless IT also happens to be a purely resistive load.

That's not necessary. Let's keep it civil - it's an RCM tradition. Pretend that you're talking face-to-face.

Bob

PrecisionmachinisT wrote: ...

You too - that's enough. Bob

I'm not stupid, but you may be if you are that clueless as to the context and the point I made about the transformer sizing. If you have for example three 1KVA transformers on hand but no 3KVA transformers, it's a pretty easy choice to use the three you have downstream rather than buy a new transformer to use upstream.

Pete C is completely right!!!

i

Sigh. Are you saying that you will adjust each transformer as you change the load?

OK, if a hack job pleases you.

Remember ......... This multiple transformer idea was in response to a VFD Question. A VFD Question ... GET IT?

The original question was about a VFD. The response was to add transformers to the output.

Do not expect to make it into a sensible discussion. It will not happen.

No, I'm saying you use what you have and if you have three small transformers that will do the job after the RPC, you use those instead of buying a new transformer.

A "hack job" utilizing parts you already have on hand vs. spending money buying new parts will please most HSM folks. We are talking HSM here, not in a factory where you would have real three phase power anyway.

Wait! You're both right! But, I just re-read the original post. Gunner doesn't make it clear whether the overvoltage alarm is coming from the mill or from the VFD. If the latter, then you'd have to make the adjustment on the single-phase input. If the former, you could do the adjustment on either end, depending on what transformers you have on hand.

For the eleventh time... You cannot use transformers on the output of a VFD !

Sigh....

What I'm saying is to balance the RPC output voltage first and THEN apply power factor correction on an as-needed basis at each individual load.

anything else.

I'm gonna give that an "it depends." If the VFD is only being used to generate 3-phase from single-phase and the frequency stays more-or- less at 60Hz, I don't know why it wouldn't work. Of course, you wouldn't want to use 60Hz transformers at 20Hz (or at 400Hz), but at

60Hz, why would it be a problem?

Even within a factory environment it is perfectly sound engineering to parallel transformers.

Fact is, this is very commonly practiced at the utility substation as well as individual customer distribution levels.

Much of the residential distribution in the city of Campbell River, Canada for example is tied together at the secondary voltage level with triplex....240vac--there are literally hundreds of pole tranformers all coneccted to a same triplex buss....with a transformer mounted to every third pole or so depending on the actual number of houses per block.

Indeed, a few years ago they replace all of the pole transformers in my area. B\Nobody lost poer during this operation, as they are all im parallel - primaries and secondaries.

The reason for the swap (and this did require shutting off thje power for about 15 minutes) was the new transformers had switchable primaries. They changed from the existing 8KV to 13KV, which saved them over half of their I2R losses in the HV cable. They brought in over 200 bucket trucks to throw these switches. It was quite a sight.

The net result, as far as it concerns me, is that my power has been a rock-solit 117V, where it used to often drop as low as 95V, and now I don't have that incessant clicking and clacking from the undervoltage trip in my UPSs.

So, you are going to use them on the output to protect the load, but not the VFD or the RPC?

What good will they do on the output, if the three phase motor in your RPC is saturating and smoking, or on fire?

You do know that if the three transformers are identical, they can be run in parallel?

And overvolt the RPC motor? Not very smart, is it?