My build is being used as a router and this is where I need to squeeze out all possible holding torque out of steppers.
I’m using drv8825 and 17HS19-2004S1 steppers [2A/phase @ 2.8V; 59Ncm(84oz.in)]. I’m using auto-squaring edition (5 drivers are employed).
I am setting Vref to 0.97 and it should provide 2A on the stepper. I am breaking into one of the 4 wires between stepper and driver with ampermeter, engage steppers and see from 0.9 to 1.45 A depending on which stepper I test. The current is constant in the same stepper but is varrying from stepper to stepper. Holding torque on each stepper is around 42N*cm which is 70% from declared.
Moving further. I turn Vref to maximum (~1.2-1.25) and the current on steppers becomes 1.25 to 1.8 also depending on which stepper I test. Holding torque is now 59N*cm (90N linear force with pulley radius 0.635 mm).
I see the following problems to which I have no answer and seek for advice:
- Why a usual Popolu formula of A = 2*V doesn't work here?
- Why different current with same Vref and same type stepper?
- Why drivers still cannot deliver 2A to any of steppers?
- Why drivers started to fry one by one on Vref=max while they did not even provide 2A? (I use heat sinks with thermal grease and I have a nice 12cm fan for cooling)