So I am assuming that there is just some fundamental fact about how motors and drivers work that I am misunderstanding.
About 2 years back, I started a robot project, and I have since then purchased 6 25D HP 12V 75:1 motors with 3 dual VNH5019 controllers. I steer each of the dual motor controllers with an Arduino Nano Every. All of this is quite quite the investment. As my project nears completion, the issue I have is getting more and more worrisome.
My issue is that I do not seem capable of running the motors at low RPMs, and by low I mean less than a quarter of the speed which they run on when driving them with 12V. if I try to drive them slower, they don’t start and occasionally stall. All of the below measurements are done with a 12V supply voltage.
This is what I have seen:
If I set the PWM to 1/4 duty cycle or above, all is fine and the motor runs. The speed vs voltage curve is highly nonlinear, though.
If I set the duty cycle to 1/8, then the motor just buzzes at what I assume to be the PWM frequency of the Arduino. If I give the motor a bit of a kick, then it starts spinning. Once started, if I add some physical resistance, then the motor will stall.
As an experiment, I measured the voltage over the motor using a regular voltage meter while driving it at 1/8 duty cycle. What I noticed was weird, to me. When the motor stalls, the voltage drops down to about 0.9V. But if I give it a kick and the motor starts spinning and the voltage goes up to about 3V. If I apply some resistance so the motor slows, I see that the voltage drops significantly. If I stall the motor, it drops down again to 0.9V.
My assumption was that the motor would draw current as I applied more resistance to keep moving. And with the VH5019 rated at 30A, and my input voltage not fluctuating more than a few tens of a millivolt, I don’t see how there could be a current limitation on the supply side at play here.
So what is going on here? With all of these massive amounts of current available, why does the motor simply not run and why does the VNH5019 allow the voltage to drop so significantly?
Here is my original question on the same topic: Am I missing something obvious regarding low duty cycles with this setup?