Hi Folks,
I made up a rotary phase converter, because the VFD we bought would not drive a star-wound or Y-wound 3phase motor on our drill press or metal lathe.
(The Ebay chinese VFD would only control a delta wound 3 phase motor)
I have a nice professional rotary phase converter, but it was only a 2.2kW version for our cnc router spindle - bought 8 years ago for a small fortune ($2300)
So I made up a 5hp one yesterday.
The 3-phase 5hp motor I picked up had the option of being wired star or delta. All the drawings I've looked at show the idler motor as a star or wye (Y) connection in the drawing, so I set the terminals like that.
The general rule of thumb was about 60 uF per hp, so I put a 300 microfarad starter capacitor in, with a momentary 'on' switch that you held down till the idler was up to speed...
I put 2 x 70 uF in parallel = 140 uF as a run capacitor across the same phase as the starter circuit. The other line had no capacitors.
I put a 100k 1/2W resister across each capacitor as a 'bleed' to discharge them when it was off.
When I went to start it yesterday, the idler motor spun up to speed very quickly, and was actually slower and worse when I used the starter capacitor.
I'm wondering why...
The motor got rather warm also...
and one bleed resistor burned out to open circuit, the other shorted down to 10 ohms - in the run capacitors. Maybe I should have used 2-watt ones instead of half-watt resistors...
However, my query is that when I put the multimeter across the 3 phases while running the idler, I got L1-2 = 245Vac(mains), L2-3 = 123Vac (half mains) , L1-3 = 365Vac (very high) - at idle
When I switched in another 3hp motor to help balance the phases better, then switched in the lathe, and then turned the 3hp helper off, the line voltages are now 246,(mains line in) and L2-3 which had been 123 volts was now 376 Vac when the extra motor was running, and was 407 Vac when I turned the 3hp extra balancer motor off.
(I didnt measure the other phase L1-3)
I realise the varying loads will alter the induced voltages, but I'd been reading where others had tweaked the capacitor sizes to get up to 5% variation between phases.
I have variations of up to 100% between phases at idle, and some darned high figures under load...
Wondering if maybe I should have set the idler motor on delta configuration instead of star, for our Aussie 240Vac mains line in L1-2 ?
and why the momentary starter capacitor works best if I do not use it at all? The idler gets up to speed in 2 seconds or less without it being momentarily pressed.
and why the bleed resistors burned out?
and If I should have a smaller capacitor across the other induced phase line
I'm no electrical expert, (or I would not be asking here), nor a total fool either...(I don't think!)
Thanks!
Any advice or suggestions appreciated!