Electronic – DC motor speed vs PWM duty cycle

This will work with a brush motor BUT only because the motor is highly inductive.
Doing it with a resistive load may or may not work - in a significant % of cases it would damage the load - especially as Vin/Vrun ratio rises.

It will not work directly with a "brushless DC motor" and would probably cause damage and let out the magic smoke.

With a brushed motor it will work because the motor forms the inductive element in a buck converter - BUT you will need to provide a diode across the motor that is reverse biased when power is applied and which then provides a path for motor current when the PWM is off. The diode should be rated at a minimum to be able to handle the motor current at the PWM duty cycle (eg 50% on in your 1st example), but it makes sense for it to be able to handle motor current permanently. So use a 2A diode. Or 2 x common 1N400x diodes in parallel would work OK as long as PWM frequency was not too high.

Like this. Ignore component values. Diagram below from here

Dont try this at home - How **NOT to do it.**

Supply is labelled Vcc so power level may be low. FET will required to carry motor current by breaking down under inductive spike from motor when PWM turns off. In low energy circuits this may happen withou damage and without people being aware of it. In your circuit destruction would probably occur.

SUPERB RESOURCE

Many many many circuit ideas for PWM motor control courtesy Google. Links on Google page from photos are live.)

Google for eg PWM motor and then select images and you get -

and much more ... .

Let me preface my answer with a disclaimer: All of my experience PWM'ing motors is with speed control of cooling fans. Your application will certainly differ, at least a little bit.

The way I find the minimum duty cycle, I look for the minimum that will allow the motor to start spinning. With standard muffin fans, this is about 40%. Then I add a small margin just to be safe. I find this by applying 0% duty cycle and slowly ramping it up until the motor starts spinning.

This startup power is higher than the amount of power required to keep the motor spinning once it has already spun up. With fans, this is somewhere in the 20-30% range. In other words, if I wanted to spin a fan really slow, I would have to apply 40% to get it moving and then I could back down to 20-30%.

Normally, just to be safe, I do not go below the startup power. That way I can be sure that the motor is spinning, although it does limit the minimum speed that I can do.

There are problems with this, however. Many things can affect the startup power requirements. Temperature, motor loading, age, dust, different motor lots, etc. You have to take all this into account, and built in some power margin.

Alternatively, you have to monitor your motor through a tachometer or something similar. Then have some motor control software do the appropriate thing if the motor is spinning too fast or too slow. Good motor control software will automatically take into account the startup power and other things.

If you don't want to write motor control software then you have little choice but to empirically measure what the startup power requirement is for your system and then add some more for margin. And hope that you added enough.