So I'm diving into cleaning up / fixing up some of dad's older stuff, and there's a Pennsy EP-5 (8551) with an open frame AC motor. It's a pretty bare-bones, no-frills single motor / no sound / no nothing engine that is frankly kinda boring.
So I thought I'd spice it up with one of my surplus PS1 boards, and just so happens a=I had an EP5 sound chip to boot. I researched using a PS1 board with an AC motor, and I understood that the field needed to be isolated from the DC input in order to prevent the polarity from switching when the DC output switched polarity to the motor. There have been previous posts on this forum that even had a sketch, although it took me a while to figure out that the description of the sketch wasn't exactly correct. In the image below, the comment states that the (+) and (-) go to the motor lead outputs of the reverse board, but that's not really what needs to happen. When I wire it like below, all sort of bad things happen, relays click, horns blow unexpectedly, direction changes back and forth...horns stop blowing and bells ring...all sorts of nonsensical shenanigans are afoot:
Anyway, I now have the pickup rollers feeding one side of the ~ bridge input. Then frame ground going to the other ~ input on the bridge. The + / - outputs of the bridge feed directly to each side of the field winding (the field winding wire itself, the shiny copper one, no longer is connected directly to the motor frame ground like it used to be, instead it's connected to the "-" output of the bridge). This allows the polarity of the field to be full-wave rectified DC, always one polarity, right?
Then the QSI PS1 board is connected to the roller pickups and frame ground like normal (red and green wires) with the two motor leads going to each brush. So I have something like this:
So I feel like everything is wired correctly. When I power up, everything works exactly as expected...direction control, horn, throttle...it's all good. But slowly, after about 10 laps at 13-14 volts to the track, it starts to slow down, then sputters for a lap or two, then there's a short and it trips the internal breaker in my KW transformer.
Hmm...I reset the transformer, startup again....everything is normal, then I go in reverse a couple laps...everything's good...then go forward again...I can do 6 or 7 laps OK, but if I sneak up to 17 volts on the transformer, it sputters, coughs and shorts again. Or if I run for a while, it veeeery gradually slows down more and more until I trip a breaker.
It's definitely not my track or transformer, I run other PS1 engines and PW engines smoothly for >1 hour with no issues. So I took the shell of the EP5 off, and the motor is REALLY HOT.
Does the field winding rely on the resistance of the series circuit from the armature to reduce voltage to the field, so am I applying too much power to the field with it wired directly to the pickups through the bridge? I thought the bridge would provide a bit of voltage reduction.
Do I need to install a resistor in line with the field? I've also seen people talk about installing capacitors at the brushes to reduce electrical noise, but that doesn't seem to be my issue.
I've checked all internal wiring for possible exposed connections, shorts, etc. I really think it has something to do with the motor getting stupid hot, drawing too much current after it heats up, then tripping the KW breaker.
FWIW, my bridge to the field is bolted face against the frame with an aluminum bracket backing it up, so it can get rid of heat pretty good.
So maybe the original sketch more more correct than I thought, and I'm really messing up by just isolating the field by itself with the bridge?
Couple of photos if anyone cares to throw something at me. Thanks!