- #1

MechatronO

- 30

- 1

The reason is that I'm about to pick switch frequency for a motor, and don't know what characterics I can expect if I alter the switch frequency. Papers and publications regarding this are very much appreciated, as I haven't yet found any that answer my questions.

I succeded in coming up with a formula for the max current through a switched RL circuit with a freewheeling diode(essentially a DC motor with the emf substracted from the input voltage), related to PWM period and duty as

I

_{max}= [itex]\frac{E}{R}[/itex] (1-e

^{-RDT/L}/(1-e

^{-RT/L})

Where R is the circuits resistance, L is the inductance, D is the duty ranging from 0-1 and T is period time.

The circuit would look like this, with the load replaced by a resistor:

The output current would look like the non-square wave in this diagram:

I think its correct, at least the endpoints are as they should. If somebody is interested I'll can post the derivation for this one and a similar formula for Imin.

I was about to determine a formula for Iavg as well, however I decided to instead look for established info.

**So, publications, formulas and personal experience regarding DC motor characteristics related to switch frequency are very much wanted. I'm especially interested in a formula that relates Iavg to switch frequency and duty cycle.**