Stepper motor control

The ideal frequency is as fast as you can switch the MOSFETs - this minimises the volume of the filter network. If you switch the MOSFETs too frequently, the heat dissipated during switching can become significant. The last time I tried it - more than ten years ago now, I settled for around 200kHz because that put my switching dissipation at about the same level as the resistive dissipation when the MOSFETs were carrying current. Modern circuits seem to use higher frequencies.

If you are relying on the inductance of the stepping motor coils for your filtering, you may want to use a much lower frequency - the motor magnetic path usually contains a lot of soft iron, and high frequency current through the motor coils could induce eddy currents in the iron, heating the motor to no useful purpose.

The very high frequency components will - of couse - see the motor coils as capacitors (of the order a 1nF or a bit lower) and the leads to the motor can then carry quite a lot of high frequency current which can be an inconvenient source of electromagnetic interference. Ferrite beads don't have an parallel capacitance worth worrying about, and can presnet anything up to a few hundred ohms quasi inductive impedance at high frequencies.

------------ Bill Sloman, Nijmegen

----------- Bill Sloman, Nijmegen

On Mon, 29 Aug 2005 19:33:44 +0200, Norleif Slettebø wrote in Msg.

robert

Hello Norleif,

Bill and Robert have outlined much of what's important. Bottomline the typical frequency would be well above the audible range and below where conducted EMI and stuff like that could bite you. WRT to the audible range keep in mind that animals can hear much higher frequencies than we can.

Regards, Joerg