[SOLVED]Motor transistors keep dying on me

[EmilFrid]

Ok I know why the measurements differed: the first one I made with motor connected, second one with motor disconnected (duh!)

 

[EmilFrid]

Alright, will do!

 

[EmilFrid]

Alright, interesting. Take up jumps to a high voltage at start up but creeps down and stays at 150 VDC collector - emitter with motor plugged in. Creeps down to barely anything with motor unplugged. Supply motor circuit is the culprit methinks

 

[map2112]

can you plug the either motor plug into either socket to see if symptoms follow one motor?

 

[EmilFrid]

With take up motor collector disconnected, supply motor too creeps to 150 and stays like that (which I think is normal) but it doesn't discharge if I unplug the motor, instead it discharges if I disconnect TAKE UP motor. Something must be wrong with the wiring

 

[EmilFrid]

@map2112 I've done this but saw no difference

 

[map2112]

With take up motor collector disconnected, supply motor too creeps to 150 and stays like that (which I think is normal) but it doesn't discharge if I unplug the motor, instead it discharges if I disconnect TAKE UP motor. Something must be wrong with the wiring

Is there any chance the 100 VAC or the 0VAC connections might have got swapped, like perhaps 100 VAC (1) is using 0 VAC (2) as it’s return?

Has there been any rewiring done in the PSU connections or on the the motor controller, or the possibility of a short on a connector in the power circuits.?

 

[EmilFrid]

No rewiring anywhere, except for the dual run cap, and all is fine there.

Disconnected the rectifier diodes for supply motor, and the constant voltage went away, so the issue is definitely pre-rectifier

 

[EmilFrid]

So at least we've narrowed down the issue to happening post motors and pre rectifier. Will continue troubleshooting

 

[RoadrunnerOZ]

What voltage are the run caps - I hope the momentary high voltage is not exceeding their rating? The two 5uF ones should be replaced anyway in case one or both are failing and the wiring to the motors checked as well

 

[EmilFrid]

Can't find any info on maximum peak, but the maximum rating continuous is 450VAC

Yes, I will be replacing those. Busy for s few days but will be back at it come weekend

 

[RoadrunnerOZ]

Sounds like they’re hitting the breakdown voltage then once the transistor is feeding. How are the transistors connected to the motor control board - does the pin connection wiring match the original wiring? I’ve also had TO3 transistor pins shorting to the collector mounting plate where they’ve not been aligned properly or the holes were too small in the metal and also where two separate transistors have had collectors joined by both making contact with the mounting plate as the insulator kit was not fitted properly when they’re supposed to be fully isolated - there was supposed to be an insulating sleeve for the mounting bolts plus the mica washers and the client had left off the bolt sleeves - bang

 

[EmilFrid]

The transistors are wired like it's always been, I even double checked so no wires had gone loose.

Hah yes, that has happened to me too. A very unpleasant experience

 

[EmilFrid]

So I was an idiot and wired up the run cap wrongly. Now it works, no more dead transistors, BUT the take up motor is quite weak. I guess time to change the other caps as well

 

[RoadrunnerOZ]

Yes - it’s also worth rechecking the diodes you replaced for that weak motor in case any of them have blown

 

[EmilFrid]

That is a very good idea. I think it is a cap because the motor makes some grunting noises when turning, perhaps indicating it doesn't get the correct phase shift

 

[EmilFrid]

Good thing I found a new motor cap in my collection, bought 5 months ago for another project that never happened

 

[RoadrunnerOZ]

The grunting noise could also be a diode(s) breaking down but definitely change the cap as it’s likely suffered overvoltage

 

[EmilFrid]

Will do, and will definitely check diodes

 

[EmilFrid]

She's rolling again! No blown transistors this far.