zohar golan wrote: > > Hi all, > > I am designing my own controller for an electric scooter. What is the > frequency the controller should vibrate?
"Should" is a matter of opinion and compromise. >From the motor's point of view, you want a low frequency, because it reduces motor and controller losses (improves efficiency). But, you don't want it too low, or the motor's ripple current will be too large. Usually (to save money), the motor's own inductance is the only inductance in the system. You can measure motor inductance, get it from the manufacturer's data sheet, or measure actual ripple current while in operation. In any case, you generally aim to keep motor ripple current less than 10-20% of its DC current. This usually results in a PWM frequency in the low 100's of Hz. The other concern is audible noise. Motors can act as loudspeakers, so you can hear the PWM frequency. Some people are annoyed by this, especially for frequencies in the low 1000's of Hz which sound like high pitched squeaks. If you want it silent, you have to run a much higher frequency. 15 KHz is high enough for most people (a TV set's horizontal oscillator is about 15.75 KHz). Or, go above 20 KHz and nobody can hear it (but dogs and bats :-). There's another trick. Rather than a fixed PWM frequency, you can use a random noise generator so it dithers around. Instead of a whistle, it sounds like a hiss. Much less annoying. -- Lee A. Hart Ring the bells that still can ring 814 8th Ave. N. Forget your perfect offering Sartell, MN 56377 USA There is a crack in everything leeahart_at_earthlink.net That's how the light gets in - Leonard Cohen
