I just want a clarification as I’m fairly un-knowledgeable about stepper motors and drivers. I’m trying to drive a wantai motor 42BYGH-M809 stepper with a standard A4988. The stepper spec calls for 1.7A. So my Vref should be 0.68V.
When I set the Vref, should the motor be attached or does that not matter? Should the stepper be in full-step mode, or can it be in 1/8th mode?
Setting the Vref to 0.68V results in skipped steps, but if I turn the POT down I get smooth motion out of the motor and the choppiness goes away. I’ve got a fan and heat sink running on this trying to keep it cool, but always get skipped steps @ 0.68V. No bueno.
I really need the torque that the motor provides, which is why I’m trying to push it to it’s full potential. Is this a bad idea? Should I target an 80% value for A (1.36A, 0.544V)? Is there a golden ratio that I missed somewhere along the way?
Some other things you should know about the system. Running a 24V PSU for the Motors and 5V for the logic via a RAMPS board.
Should I give up on this configuration and go to either the A4988 black or DRV8825?
I have a NEMA23 rated at 2A that I can use in the place of this motor, assuming I’d have to use the DRV8825 with that. I’d rather not as this would be a bit painful, but if necessary I can swap it out.
I know this is long and a bit rambly, but any help would be appreciated.
It is generally pretty tough to get that much current out of our (green) A4988 stepper motor driver carrier, and the behavior you are describing makes it sound like it is overheating. If you want to get the full torque out of your stepper motor, I suggest you use our DRV8825 stepper motor driver carrier, which a drop-in replacement for the A4988 in most respects and should have no problem delivering 1.7 A per coil if you add a heat sink (you shouldn’t even need a fan).
It does not matter if the motor is connected or what the step mode is if you are measuring the voltage at the Vref pin. However, it is good practice to set the current limit to something appropriate before connecting a motor. If you want to set the current limit by measuring the coil current with a multimeter, you should make sure the driver is in full-step mode (and is not being stepped), and a motor must be connected to the driver with the multimeter in series with one of the coils.
Setting the current limit to the full rated current of the stepper motor is fine as long as the driver can handle it. This should be your goal if you want to maximize torque.
if you’re short on the cash for a new driver, you can also get more out of the a4988 green boards with a little bit of heatsinkng on the top and bottom :). Save money for other pololu boards :p. I use small little copper heatsinks that I cut into 1/3 - 1/4 with a dremel tool. Very thin epoxy adheres them to the chips (superglue softens at the higher heat.)