Another 3 phase issue

graduate_owner · 24 Sep 2015

I have connected a 3hp 3 phase motor in star configuration to my 3 HP static converter and now I get a much more balanced output, as expected. It also starts up my 3 HP Colchester lathe in top speed, something the converter on its own would not do. Fine.
I have another 3 phase converter which came with a 5 HP motor and have managed to get it running after changing some capacitors and un- jamming the motor, although I do know there is one more duff cap in there. Again the output seems reasonable balanced according to my voltmeter, and it starts up a 1 HP polisher with no problems. However when I connect it to the Colchester I get the most horrendous noise from the lathe as though I were hitting the thing with an oversized sledgehammer. It really does bang and clank as if it's about to self destruct. I have to switch off within few seconds in case it does.
It is puzzling, considering the lathe runs fine (as does the polisher) on the 3 HP converter but only the polisher runs on the 5 HP converter.
Does anyone have any ideas on this?

K

wallace · 25 Sep 2015

Not sure if this helps but a static has to be adjusted to the machine in use. If I try and use my lathe and have the setting on the static too high it will run the motor but it does not sound happy. If I leave the static on a low setting accidently and try and start my saw which is 5hp it will make a horrible noise.
HTH

graduate_owner · 25 Sep 2015

Hi Wallace, thanks for replying. It makes me wonder if my converter is not putting out enough juice whilst one of the caps is duff.
I have a few 3 phase motors ready for putting in other kit so I think I will try loading the converter with the polisher and a few more motors to see if it objects to that. I should be able to get a total of 5hp unloaded. I think it is time to buy another capacitor too.

K

Myfordman · 25 Sep 2015

I saw this first thing this morning looking on my phone and can now reply from my lappy where it is much easier to type.
Wallace is spot on that certainly static converters and indeed some rotary converters need adjusting to the load.

The phase shift and hence the balance of voltages is achieved using the inductance of the driven motor and the capacitors in the converter, balanced to an extent by the action of the converter motor. The phase balance will vary with applied load.

When you run the lathe on the part repaired rotary, check the voltages. I bet they will be all over the place.
I don't really see how you can expect it to work fully when you admit there is a duff capacitor in there.

The other thing you must ensure is that the control gear (DOL starter etc) is not fed from the artificial phase but instead is only connected to the output from the auto transformer. This will be the pair of terminals that is closest to the 415v output and does not vary with applied load. Wiring this way, will stop the contactor coils jumping in and out which also contributes to noise.

hth
Bob

graduate_owner · 26 Sep 2015

Thanks Myfordman, that makes a lot of sense. I will try changing around the feeds to the lathe to avoid the phantom phase from being connected to the starter coils - hopefully in the next few days. The converter has 3 banks of capacitors with 3 or 4 in each bank so I thought one duff cap might not be an issue. It is quite a complicated beast compared with my 3 HP one, in that it has about 5 multi pole electromagnetic switches and about a dozen integrated circuit chips.
The transformer has 4 mains leads and about 8 other sensing wires. Still, as long as I can get it to work.

K

graduate_owner · 2 Oct 2015

I have tried changing the connections to the lathe and - bingo. Started up nicely so I am happy. Just need to order a replacement capacitor now and things should be spot on.
Thanks for the tip Myfordman.

K