Stepper Motor high frequency sound

Question

I am using a stepper motor for a gantry setup. When I run the motor above certain speeds, in this case 4rev/sec, it produces a high frequency sort of whinny sound. It is a 4.5A motor and I am running it with 24V.

If I run the motor at 48V, I am able to reduce the high frequency sound quite a bit, in fact I have to go up to 6rev/sec for it to be at audible/annoying levels.

My initial guess is that at 48V the stepper is able generate more torque since the current in the coils is able to overcome back EMF and rise to higher levels than in 24V. Is this a correct assumption or is there more to it?

I do not believe the motor is defective since I tried a few different motors with the same 4.5A rating and they all have a point in the speed where they begin to produce the high frequency noise.

Answer 1

Most stepper drivers limit current by "chopping" their drive output at some frequency. Whether or not the actual frequency would seem to be slightly out of the range of human hearing, you can often hear it as a whine. You could verify that the current chopping is related by applying a torque (i.e. grab the shaft with your fingers!) and seeing if the characteristics of the whine vary. The fact that it varies with speed is a good indicator too.

Your controller may have some tuning for chopping frequency (on older controllers it was set by an external RC that was driven by an internal oscillator). Some have a direct input for this (to chain multiple controllers together, but you can also drive it yourself). Changing the current limit or the input voltage will result in different chopping behavior as well.

Answer 2

OP::My initial guess is that at 48V the stepper is able generate more torque since the current in the coils is able to overcome back EMF and rise to higher levels than in 24V.

My comment: net power delivered to the shaft is dependent on the back emf voltages and coil currents only. Torque is dependent on the product of coil currents and relative change in mutual inductance between the stator and rotor. In any case, if your dc link voltage is lower than the peak of your back emfs, the controller will have to work in a region known as overmodulation, and phase currents won't look pretty (high THD, lot's of harmonics) and that could easily produce noise at audible frequencies. Higher voltage gives the drive more bandwidth to play with and better regulate currents.

Higher motor speeds generate back emfs with larger magnitude and higher mechanical/electrical frequency. Harmonic spectrum of the currents that could deliver average torque is thus shifted to higher frequencies where induced mechanical resonance does not longer bother human ears.

Answer 3

If you have 1.8 degrees per step, that means 200 steps per rev. At 4 rpm, the switch between phases is at 13.3 Hz, which is too low for normal hearing. However, overtones of this switching may be audible. Thus, if the problem is not the chopping frequency, and the problem is not the microstepping frequency (often audible in CNC driven equipment when holding position), then the problem may be overtones of the fundamental function of stepper motors.

Is there a particular reason you're using stepper motors, rather than DC motors with encoders? It may be that geared DC motors can run with a different audible profile.