How fast does PWM actually need to pulse....

… to fool the device into thinking it’s getting voltage lower than the maximum, as opposed to getting a series of discrete ons and offs?

If I hook a motor up to a battery through a manual switch and switch it off and on every 30 seconds, in theory that’s PWM but too slow to fool the motor. The motor will run for 30 seconds at whatever speed it gets to for that battery, then go off for 30 seconds.

I suppose it depends on the device?- an LED might get to full brightness in a much shorter time than a device with inertia like a motor would get to full speed.

Just curious…

You are right with motors. LED is lighting up instantly, but your brain is slow to notice it for a few milliseconds, so PWM frequency does not have to be very high.

Also, starting a motor takes a higher duty cycle than keeping it running. Start speed of a motor or valve has always been higher than the minimal running speed of that motor or the smallest opening of a valve for the uses i have seen yet. This however has little to do with the frequency, which is what you asked for.

JimboZA: I suppose it depends on the device?- an LED might get to full brightness in a much shorter time than a device with inertia like a motor would get to full speed.

Just curious.....

I think you answered your own question. Optimum rate will depend on the device.

fungus: I think you answered your own question

Qualitatively, yes...

If I hook a motor up to a battery through a manual switch and switch it off and on every 30 seconds, in theory that's PWM but too slow to fool the motor. The motor will run for 30 seconds at whatever speed it gets to for that battery, then go off for 30 seconds.

but, you have to admit, the average RPM over that time will reflect the supply duty cycle ;)

but, you have to admit, the average RPM over that time will reflect the supply duty cycle

True

I have worked with some motors that produced over 25,000 HP and took more than 2 minutes to come to a stop after the power was removed. It took about 10 seconds to go from 0 to 1200rpm with full voltage applied to it.