Hi Allan,
yes its quite dramatic.....when I was cropping the screenshot to post I inadvertanly cut off the frequency scale off the bottom of the FFT pic.
The full picture showed significant harmonic energy to 100kHz and certainly measureable out to 1MHz.
I suspect that OP will find that the speed control is still at the root of his problems, whether he knows it or not. What else can produce the sort of hash this speed control
produces?
One feature of this speed control is that the DC produced is not isolated from electrical earth, at least in New Zealand where we have a MEN electrical system. That means
that the insulation resistance of the motor OP is using is the only thing which prevents line level voltages from getting into the frame of the machine. Additionally there
is a good possibility of the so called, and dreaded 'homopolar' current.
The 5 phase steppers I use on my mill are subject to this phenomenon. They have good insulation to the frame as measured by a Megger. The drivers are direct feed from the AC
mains to a DC link voltage of 150VDC. The magnetic coupling between the stepper coils and the frame mean there is leakage to the frame, several mA at least. Without earth bonding I have
measured 130V on the frame of the machine......a great surpise to me I can tell you. I have investigated quite thoroughly and it turns out the voltage induced into the frame
is in the kHz range, ie phase switching frequency so the current is induced in the frame rather than resistive leakage.
Homopolar current is not common, there is little mention of it in the literature and that is seems to fly in the face of Kirchoffs Law its little wonder that it gets missed. That's what happened to me
anyway!
Craig