On the Prusa Mendel it uses 2 stepper motors on the Z-Axis, one at each end.

I am having a problem that the left-hand feed occasionally has problems feeding upwards. It stutters and loses sync with the right-hand. This often means that when it goes back down it crashes into the platform and the Z axis position is not repeatable.

I am using a gen6 with stepper motors from camiel, but looking at the spec each motor requires 1.48 amps, however in the DRV881 spec it will only supply a max of 2.5 amps.

I have the steppers wired in parallel.

A) Has anyone got a Prusa to work like this?

B) Is it possible to rewire the motors to be in series, if so which wire should go where.

Cheers
Geoff

Mmm. I don`t like these solutions. Especially connecting in parallel will give a non-uniform distribution of current through your windings.

Connecting in series is much better for your current distribution, however you have to take care: you will probably need more volts to drive your motors, especially at higher speeds. Now this is your Z axis so I can understand that you can live with lower Z speeds.

I suggest you do the following:
- Connect steppermotors in series (I'll explain later how)
- Turn down the current of your stepper driver to approx 50% (abs)
- Try to run with your current power supply (I guess that you are using 12Vdc)
- Use not a very high Z feedrate

If you experience problems with low torque, switch to a higher voltage PSU (24Vdc). Make sure your hot-end is suited for this voltage too!


How to connect your motors in series:

You have the following wires:
Blue A+
Red A-
Green B+
Black B-
(for each motor)

Now connect like this:
Gen6 pin1 (blue) ---> Motor1 A+ Blue
Gen6 pin2 (red) ---> Motor2 A- Red
Gen6 pin3 (green) ---> Motor1 B+ Green
Gen6 pin4 (black) ---> Motor2 B- Black
Motor1 A- (red) ---> Motor2 A+ Blue
Motor1 B- (black) ---> Motor2 B+ Green

Please note that this is at your own risk.

Brilliant explanation, thanks.

I will give a try sometime over the next few days.

Cheers
Geoff

Thanks for this, Elmar. I'm using the SY42STH47-1684B motors which have the colours going to different pins (A+ is black, A- is green, B+ is red, B- is blue), so they changed a bit for me. But working fine after testing. I went:
Pin 1 -> A+ motor 1 A- -> A+ motor 2 A- -> Pin 2
Pin 3 -> B+ motor 1 B- -> B+ motor 2 B- -> Pin 4

Anyone care to comment on connecting the motors in parallel but feeding each connection through a diode so that the motors can't circulate currents between themselves? I know this has been done in similar cases with DC motors connected in parallel (including a device used on the Apollo spacecraft 40 years ago).

I hasten to add that I'm not primarily an electrical type (I'm a retired Mechanical Engineer) so I could be dead wrong in this instance. I am curious though.


.... Sid

Just completed my Prusa to the point where I can test it. Everything works well except the z-axis.

Right now I have the z-axis mechanical joints slackened slightly to be sure that misalignment isn't a problem. The two z-axis motors are connected in parallel and they sit there and buzz. Occasionally either motor may rotate a little then stop and buzz again. This happens in either direction.

- If I disconnect either motor, the remaining motor will run quite happily in either direction.

- If I connect either the X or Y-axis motor to the Z-axis output, that motor runs fine. If I connect the Z-axis pair to either the X or Y outputs the problem is the same.

- I am using a 12V supply. RepSnapper software.

The strange thing is, I'm talking by email to someone else who has essentially the same setup on his Prusa and has no problem at all with the motors in parallel.

I'd like to try the series connection outlined above but I don't know how to reduce the current as recommended. Can someone explain that to me?

My problem with the Z axis ended up not being with the motors, so I never rewired them in series, it was to do with alignment.

Have you checked the motor current limit on the Z-axis drivers? If it is too low you would get the symptoms you are seeing.

Cheers
Geoff

Geoff, I did check the motor currents. All motors were set about the same so I left it. It did occur to me that, with the motors in parallel the z-axis pair would each receive half that current so maybe should be set up a bit but I didn't try it.

I too noticed an alignment problem. When I slackened off some of the joints to free things up, the threaded rod was all over the place. I threw out the printed couplers and made solid metal ones after realigning everything, there was very little run-out.

The (parallel) motors still just sat there buzzing with occasional attempts at movement.

So I wired them in series. They still sat there buzzing with no attempts at movement.

At this point, while looking through the Printer Definition options in Repsnapper, I chanced on the one called "Enable Acceleration" which was off by default. So I turned it on. Suddenly the z-axis worked just fine. The weird thing is though that I later turned it off again and it still worked just as well. I've searched high and low for a bad connection somewhere and can't find a thing. But for now it's working. Homing to the wrong corner - which I can fix -but working nonetheless.

Well done, I am glad you managed to sort it out.

I have been far to busy with work etc. to do anything on my Prusa for a while, but I now have some Stainless Steel threaded rod to replace the Z-axis drive bars, and my next job is to strip down the Z-axis and realign it.

Cheers
Geoff