Changed Z-motor wiring from Parallel to serial. It stopped working

Hey mates,

My Z-motors worked flawlessly (well, almost) when I had them wired in parallel, but now I've wired it exactly like the picture in this page: [reprap.org]

One of them runs smoothly as expected, but the other one stutters and cannot complete a full rotation.

I'm using a Sanguinololu with Polonos (obviously) and I've tried fiddling with the current-limiting screw thing but it doesn't seem to help. I am using printrun to run and NEMA 17s for motors. The firmware is sprinter.

Help/suggestions would be much appreciated

Since they're wired in series, here's what I'd check right off:

1) increase drive current (apparently tried to adjust current limit pot unsuccessfully)
2) mechanical binding in failed drive. Check for obstructions by manually rotating shaft with system UNPOWERED
3) wiring error on "failed" motor (possibly accidentally swapped phases?) Check your connections again.

Alright, trying those now, but I take forever to do the little bits.

I'm unclear on how the pots work. Do they have a single 360-degree setting, or will it allow more current if I wind it counterclockwise about 5 full rotations?

EDIT: I've taken off all the pieces that connect to the motors. When I set them to go 10x slower (aka 20mm/min) they seem to work properly, and have enough grunt that I cannot stop the shaft with my fingers. The problem is, it was able to function normally at 200mm/min when they were wired in parallel. Perhaps it's the amps or volts that doesn't like that.

IIRC, In parallel the motors have same voltage but divide current, while in serial the motors divide voltage and have the same amperage. Could I boost the voltage with the pololu or Sanguinololu?

Edited 1 time(s). Last edit at 09/06/2012 05:57PM by mlaerin.

it's less than 360 degrees, it's just a ring of carbon with a pickup on one end and a pickup on the rotating part.
turn it gently anti clockwise, you should feel the stop, that's the minimum
If it an actual pololu board you'll end up turning it between 1/4 and 1/2 a turn most likely.
If it's one of the chinese knockoffs with the wrong resistor it'll be <1/4 turn.

Edited 1 time(s). Last edit at 09/06/2012 06:08PM by Polygonhell.

if you had it running fine with the wiring in parallel why change it?

As polygon said, the full rotation for the trimpot is less that 360 degrees! you should feel stops at either end of motion, when the wiper is at the supply terminal or at the ground terminal (the last terminal, and I'm guessing it's labeled 3 in my pic, is where we get the ref voltage), although offhand I don't know which terminal is which on the board (see attached photo and pololu reference schematic R9)

"IIRC, In parallel the motors have same voltage but divide current, while in series the motors divide voltage and have the same amperage. Could I boost the voltage with the pololu or Sanguinololu?"

Correct, When wired in parallel, they run off the same voltage, and the drive current requires is essentially double that of a single motor (teensy differences assuming they're the same part number motor) (technically, the current is V_i/Z₁ + V_i/Z₂, where Z₁ and Z₂ are, for the purposes of this discussion, each motors' resistance, although that's not 100% accurate.

Also correct...When wired in series (not serial), they share the same drive current, and act as a voltage divider, similar to two resistors wired in series, so if they're the same part number, they should split it 50/50

The main result of this changeover (assuming that your input voltage for the pololus' motor drives VMOT pins is actually 12V per the Sanguinololu schematic) is that when you first transitioned from parallel to series connection, your driver current for the affected pololu was set at ~ twice the value it should be set at for series operation. If the trimpot is still functional, you *should be able to clamp off the drive current until neither runs (unloaded), then slowly increase it as you apply the mechanical load by reconnecting the screws to assure proper operation. I don't know if you'll need to lower your max throughput. No experience to say.

Since you're already operating at 12V, I would not recommend increasing the supply voltage.

I can't seem to make the Z motors give it more torque. I've tried setting the trimpot to 8 different directions with seemingly insignificant change between them.

Also I transitioned from parallel to series because I thought it would become impossible for either motor to move without the other one, resulting in less Z-carriage tipping. Unfortunately, this transition can currently be likened to smashing my car to save it from being scratched at a parking lot.

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Also I transitioned from parallel to series because I thought it would become impossible for either motor to move without the other one,

IME Series has the exact opposite effect, when in parallel if you turn one motor without power the other one will try to turn with it.

I figured that when in series, if there is current going through the wire, it must go through both motors, and if there was a broken connection then the motors would both be unable to run so that it would be impossible for one motor to go crazy and send the carriage off the rails.

Well, I'm out of ideas, I guess I'm going to put it back to parallel. Thanks though.

I find when in parallel if you stall one motor the other stalls as well. In series if one motor stalls the other continues to spin.

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