Help needed diagnosing blown lathe motor controller - Page 1
Forum section: electronics / repair

author: CheekyRobot
author_rank: Contributor
timestamp: May 01, 2021, 08:56:44 pm
content: A while ago, the motor controller board failed on my trusty Chinese Lathe. The PCB labelled KPWT-500 is now discontinued which is a shame, and others I see on the market just don't look as good. It accepts a PWM input from the control PCB and drives a 500w mains voltage brushed DC motor.When switched on, it blows the 5A fuse and also trips the house RCD (the latter may be a red herring though, as it trips even if you look at it a bt funny). It blows the fuse even when it's completely removed from the lathe and motor (so, not an earth short or anything on the lathe itself).Can anyone give me some useful pointers as to how to trace the fault? I've had a good look but run out of ideas.There's no obvious damage, no burn marks, bits of metal shorting anywhere - all looks good and clean.From what I can tell, from a few hours of tracing things through, here's how it works (roughly).UK mains comes in, though a bridge rectifier to give us DC which is used to drive the motor.The PWM input drives a IRFP460 mosfet with obligatory flyback diode. The pulsed DC then goes off through two relays. One for start/stop and another for motor direction.A transformer takes the mains to generate some DC for the current limiting circuit, programmable by a couple of DIP switches. The low value current sense resistor (top right) of the picture looks a LITTLE burned - I thought it was just the black/brown value colour coding at first! LOL.Some measurements:L/N IN: 739R (is this the transformer?)L/E, N/E, M1/E, M2/E: o.c (so no shorts in the caps?)Motor Out: 33.7R (hmmm... A bit low?)Mosfet seems okay as far as I can tell with a meter, and no shorts to the heatsink.Huge respect given for any clues! Obviously, a little tricky without a schematic.

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 01, 2021, 10:49:03 pm
content: Your L-N input resistance measurement will be the transformer.Likely you have at least one of these shorted: Diode bridge, IRFP460, freewheel diode.

author: amyk
author_rank: Super Contributor
timestamp: May 01, 2021, 11:42:09 pm
content: Looks like it's only a 2-sided board, not too hard to draw a schematic from it yourself.Depending on the actual specs of the motor, a treadmill controller might be a suitable replacement.

author: CheekyRobot
author_rank: Contributor
timestamp: May 03, 2021, 12:10:23 pm
content: Huh. I posted various thanks but my posts seem to have vanished.Tying to rememeber what I said now - thanks firstly to the answers!Yep, I'll take another look at the rectifier - seems like just about the only thing that can cause a short.I'll probably get another mosfet next time I order some bits just to play devil's advocate and at least I'll have a spare

author: shakalnokturn
author_rank: Super Contributor
timestamp: May 03, 2021, 02:45:45 pm
content: You can actually test each of the three for shorts.Yes most likely is the diode bridge but if it's designed to soft-start to avoid over-current a shorted MOS could throw you a red herring.If the diode bridge is shorted also check the smoothing capacitor to be on the safe side.