Q: When tuning speed regulators, at what speed the drive should be stepped and how large a step should be applied?
A: First of all, the speed regulator is tuned very early in the life cycle of the mechanical equipment. Check with mechanical to ensure lubrication, alignment, pressures, vacuums, temperatures, and guarding are all in place and set correctly. There may be mechanical restrictions on the top speed of the drive roller (ex. running in seals). There may also be restrictions on rolling backwards (ex. doctor blade against a soft roller).
Speed regulator tuning primarily compensates for the rotational inertia time constant of the roller (seconds to accelerate to rated speed at rated torque). This time constant is independent of speed. Secondary effects such as friction increase load at higher speed and may actually smooth the drive at higher speeds. Other effects such as backlash in the speed reducer or couplings may result in natural resonances in the operating speed range (probably 80 to 85% of design speed).
Do NOT tune the speed regulator at a resonant speed. You will recognize this as a speed in which it is almost impossible to dampen out the resonance (hunting or oscillating).
Otherwise, speed tuning can be done at any speed. You will barely see any change in the time constant of a step taken at low or high speed.
The size of the step is also determined by the inertia of the roller. Select a step size that produces a noticeable jump in torque (minimum 20% of rated torque). Ensure the step is not large enough to put the drive into torque limit (100 or 150 or 200% of rated torque). Once the drive is in torque limit, all regulator theory based on linear systems goes out the window. Print or paste the trend of the step response into the startup report.
Once tuned, be sure to run to top speed. Record any resonant speeds that should be avoided (advise mechanical). Initiate an E-Stop from top speed. The drive should not fault out. Archive your work.