Electronic – arduino – What’s happening to the stepper motor at high RPM

You don't say it, but your problem may be that you're trying to start the stepper at full speed. If so, there is a maximum speed (which varies somewhat with load) beyond which a stepper will not accelerate, and this speed is normally well below what you can reach with a gradual increase in speed. Google on "stepper motor torque curve".

The problem is that, with a 4-phase stepper such as you are using, if the shaft angle lags more than 2 step angles behind the commanded angle, the torque reverses and the motor sits and vibrates and makes horrible noises. This is not, technically speaking, a stall condition, where the shaft does not move at all.

The torque-speed curve for your motor can be found at http://www.kelinginc.net/KL34H2120-42-8AT.pdf and indicates that the motor can be run at 5000 steps/second, which suggests that your problem is your attempt at fast start.

If, on the other hand, you've already tried a slowly increasing step rate while running, then you probably do need a heftier motor. However, my calculations for a .144 hp motor at 10 rps gives about 2 Nm torque, and the linked torque curve for your stepper is about the same, so I wouldn't expect a problem. Unless, of course, you've modified your mechanical setup somehow.

Okay, turns out that there was an issue with a wiring too and the labeling was indeed incorrect on the motors. A big thanks to Andy for spending his precious time in assisting me.

How I solved the problem

Update: After discussion with one of the commenters below, it is learned that there is no difference in the order of wires connected to the driver, therefore, the order of wires labeled in my question and in the answer below don't matter as much as setting the value on the variable resistor itself.

These drivers have an onboard tiny variable resistor that limits the current to the motor. I just had to tweak it to get everything working. Now the motor runs in ultra low speeds, although with some mild vibrations (in full step mode).

After enabling the micro-step mode provided by the chip, I was able to dampen the vibrations by a huge margin and got everything working butter smooth.

For anyone else who may have this motor, here are some references:

Orange and Brown wires constitute one coil of this motor, with the black being the center tap.

Likewise, Yellow and Red constitute the second coil, with the white wire being the center tap.

The way you connect it to a Polulu DRV8825 is as follows:

I hope this helps anyone else who may have the same issue.

Cheers.