3D printer z axis motors not responsive to full current

I am running a Prusa Mendel I2 3d printer through the software and firmware of Repetier Host Mac. I have flashed the Arduino MEGA 2560 with the latest Repetier 0.83 firmware. I have complete control of x and y axis in both + and - directions as well as being able to control the extruder and the hot bed temperature. I can expel filament when the temperature is right for the extruder. My problem is that when I try and send a signal to the z axis, the motors receive a signal from Repetier but all they do is whine. No motor action is observed. I have tested each motor on the y axis and they operate as expected. I have adjusted the POT for the z axis full close to full open while the signal is running and the motors are whining. It definitely changes the response of the motor but even at full open the motors just whine. I don’t know if my configuration settings in the Eeprom are not configured for 2 z axis motors or if my steps/mm are set wrong. I am at the end of where my troubleshooting knowledge can take me. Any help would be greatly appreciated.

Shaun

What do you get if you leave the Z-axis steppers in place but decouple them from the threaded rods that move the carriage up & down? How about if you decouple and just run one of the Z steppers?

My stepper motor drivers run hot - it took some heat sinking and fans to keep them running.

I have decoupled the motors from their threaded rods and run them independently and found that they still receive the signal to whine but not actually do any work. Like it is just receiving reactive load signals to build its field but no real go function to spin its rotor. Each motor worked independently as it should’ve when connected as the y axis motor. Is this a configuration error in Repetier?

I happen to have a nearly complete prusa, but I'm using a different toolchain to you so I'm out of suggestions. The Arduino forum is probably not the most fruitful place to be seeking advice on this topic I'm afraid.

The two Z axis motors are wired in parallel so check two things. 1) that the same wires are commoned upon each motor. 2) your driver can supply double the current.

Have you tried running the motors separately?

I think you would put the two motors' windings in series so each gets full current?

MarkT: I think you would put the two motors' windings in series so each gets full current?

When I made my Prussa the motor windings were put in parallel.

Try placing the z axis driver board in the y axis slot, then just try jogging the y axis. If that works put the y axis driver in the z slot and see if it still doesn't work. I'd bet money on a problem in the software, micro step size, speed settings, lots of little things to check.

Grumpy_Mike:

MarkT: I think you would put the two motors' windings in series so each gets full current?

When I made my Prussa the motor windings were put in parallel.

I suppose it depends which axis has to move fastest, but I'd have though Z-axis is slow and needs more torque so windings in series for constant-current drive, parallel for constant-voltage drive, so as not to perform worse than the other axes for torque?

I suppose it depends which axis has to move fastest,

Indeed the Z axis moves up very slowly, it just moves up the slice thickness of the extrusion once every print plane, torque is not very important as only the extruded moves. The thing is that the voltage is more limited ( most run on 12V ) than the current in these designs so parallel is best.

whining means the stepperspeed is too high

Is the whining at the PWM frequency or the step frequency? Is it possible your motors are on and the noise is just the current regulator doing its thing but no steps are performed? Is the step pin connected to the controller properly? If you put a resistor+bicolour led (reverse-parallel red+green) across each stepper winding, do you see both the LEDs changing color at the step speed?

If the whining is the step frequency, the problem is they're just stalled. Slower! Since you have a low supply voltage, the current in the steppers can't react very fast (dI/dT=V/L) so you can't achieve a high step rate, and your torque curve will drop right off at pretty low speed.