No amps on RPC?

[woodhick]

I have  a rotary phase converter that I built some time ago.  It runs my shop machines fine router, planer,etc.  I never balanced the voltage on my "wild" leg as I probably should have.  I helped a friend build on last week and he checked his rpc and voltages are as expected 118,116,180 volts.  But when he checked for amps he had no amps on the third or wild leg.  He came over this evening to check mine and same thing amps are as expected per load on lines 1 and 2 but the third leg going to load has no amps at all? Can anyone explain?  Thanks.

[iain]

Nope not me



iain

[Modat22]

Don't quote me on this but I think the current on the wild leg of a rotary phase converter is dependent on both the voltage created by the converter and the inductive reactance of the motor connected to it.

So unless your load is connected to the converter and is under load itself you won't be able to measure much in the way of current in the wild leg.

I saw this at a print shop I worked at once when we purchased a piece of used 3 phase equipment and a converter to go along with it. The press wouldn't work and we thought it was the converter due to a current reading like you described. In the end is was a control wire in the press not the converter.

[Don P]

I've never checked mine when it wasn't under load, makes sense though, that leg is doing no work unloaded. You got the pressure but no flow . I did balance mine under load, by amps not volts. I kept adding run capacitors till it was as even as I could get it but a little on the low side on the wild leg. Somebody told me it was better for it to be to be lagging rather than leading. I don't know enough to know if any of that is true or not.

[D._Frederick]

My 2 cents worth is: that the 3 phase system is so far out of balance that it appears that load of one phase  is seen as high impedance resulting in no current flow.

In a balance 3 phase system, the load and the source are the same impedance which results in equal current flowing in all three legs.

Using a motor to generate the missing leg of a three phase system using single phase source does not result in a balanced  system.  The phasing angles are all goofed up, instead of being a WYE with the angles being 120 degrees apart,  You have a TEE with the angles being 180, 90 + or - some angle, and  plus the remaining angle part to add up to 360 degrees.

That is why the mfg of converters try to balance the phases. This help to reduce the effects of the different poles fighting each other.

[woodhick]

Thanks for the responses.  I understand it's not perfect three phase but many  many shops are running equipment this way with no problem.  when I checked my "load" motor which was my 24" was started but only running I wasn't planing so there was no real load.  I will go back and check under load.