PWM control of a DC motor

--
The point I was trying to make, but didn\'t quite, was that as long
as the duty cycle stays at 75% the average rate of rotation of the
motor\'s armature will remain constant.  With limits as described
later...

Hi All,

I have a basic question about PWM control of DC motors: Can all DC brush and brushless vibration motors, even soleniod-type, (think cell phone and pager vibramotors to alert the user of an incoming call) that are looking for a fixed input voltage for a rated RPM be controlled using a PWM? Would changing the duty cycle of a given PRF change the effective motor RPM? Or is it changing the actual PRF and the duty cycle that controls the speed of the motor (up to the max RPM that is).

As an example, I have an application that requries PWM control of a vibration motor (small DC brushless or even solenoid type motor design) to vibrate at particular frequency, 250Hz nominally). If the choosen motor is rated for 3V / 15000 RPM ( 15000/60=250Hz), then 3VDC should be fine, I believe. If the motor has a higher RPM rating, say, 20000 RPM, would utilizing a PWM at 75% (15000/20000) duty cycle allow me to achieve 250 Hz requirement? I am thinking that since 15000 RPM is 75% of the rated RPM of the motor, then utilizing a 75% duty cycle would allow this unit to ramp up to about 15000 RPM nominally assuming some sort of linear relationship between the average DC seen by the motor. Am I on the right track?

However, its not clear what role the PRF plays here. If I use a pulse period of 3ms (333Hz) @ say at 75% duty cyle, could I not achieve my target vibration frequency for a 3V / 20000RPM vibration motor? So wouldn't this be turning the motor on and off 250 times per second? How does the pulse amplitude factor in? I would believe the average DCV seen by the vibration motor would still need to be whatever the input rated voltage is, in this case 3V. So in continuation of the example above using the 75% duty cycle, then the pulse amplitude should be 3V / .75 = 4V. Thus, a 4V, 75% duty cycle pulse with a PRF of 3ms would work?

Some one was trying to explain this to me using a carrier system anology where the motor is rated for 20000RPM and it tries to achieve that target during the "on" portion of the duty cycle, but it will never quite reach that because your duty cycle is less than 100% and hence you use duty cycle to control the speed. This makes sense to me but wouldn't the PRF also contribute to the resulting vibration frequency? The resulting motor vibration from a 50% duty cycle with 4ms period is different than 50% duty cycle of a 2ms period, right?

Can someone please validate my calculations above and if they are incorrect, please explain what the pulse waveform would look like based on a 3V /

20000RPM vibration motor to achieve 250 Hz vibration frequency and how they arrived at those values?

Thanks in advance,

Mike