Your frequency is not low, so what happens is this. When the h-bridge goes = HI-Z, the energy stored in the inductance dumps back into the supply rails = and your net motor voltage is 0 and the current waveform is based on the in= ductance and frequency. Current means you have some motor torque, but no vo= ltage means no speed. But when you raise the duty cycle to 50% and above, remember your supply ra= ils clamp the voltage through those back diodes. The voltage is the same(yo= u use lossless transistors and diodes, right?) as when ON, just the polarit= y is now reversed. Being an inductor, when you go Hi-Z the current can't i= ncrease, it ramps down. There isn't enough time before the next ON cycle fo= r the inductor current to go to zero, and current will build up over subseq= uent cycles. In order for things to be happy, the average motor voltage will move away f= rom 0, and your motor will actually spin. So what you have is a voltage profile that is not a straight line starting = from 0% duty cycle such as you would have with an ON/shorted PWM cycle trai= n. Either way works, it's just the result that's quite different. You need goo= d strong caps across the supply rails close to the H-bridge either way. The third way is to not use Hi-Z or braking, just drive the H-bridge for AC= out and then you get 0 voltage at 50% duty cycle. That method is easy to c= alculate, but losses are continuous so you will want to shut off the H-brid= ge when you are not running the motor. Hope this helps. Bob ________________________________________ From: piclist-bounces@mit.edu on behalf of Jason = White=20 Sent: Tuesday, July 3, 2018 1:26 PM To: Microcontroller discussion list - Public. Subject: Re: [EE] DC Motor PWM, Average Voltage? Supplementary information: PWM frequency =3D 25kHz Motor Wattage 15W, 20V (I suppose low wattage could imply lowish inductance, I don't know the actual motor) Duty Cycle is always between 50% and 100%. -Jason White On Tue, Jul 3, 2018 at 4:17 PM, Bob Blick wrote: > Hi Jason, > > At high frequencies and <<50% duty cycle, yes, the ON/Hi-Z cycle will > average 0 volts. > > It depends on duty cycle, your PWM frequency and the motor inductance. > > The motor is an imperfect inductor. > > But generally what you say is entirely accurate. > > Friendly regards, Bob > > ________________________________________ > From: piclist-bounces@mit.edu on behalf of > Jason White > Sent: Tuesday, July 3, 2018 12:48 PM > To: Microcontroller discussion list - Public. > Subject: [EE] DC Motor PWM, Average Voltage? > > Hello PICList! > > I am currently design my first piece of avionics hardware. For my > application I am driving a DC motor with an H-Bridge. I aim to (very > loosely) regulate the average voltage supplied to the motor to 20V over a > supply voltage of 20V to 40V using open-loop PWM. > > I would like to know if my thinking is correct. > > The H-Bridge contains two "Off" states: High impedance and braking. > > With a 40V supply: If I PWM the H-Bridge between "on" and high impedance = at > a 50% duty cycle the average voltage will not be 20V because of the back > emf of the DC motor. > > Instead to achieve an average of 20V with a 50% duty cycle the H-Bridge > must be toggled between "on" and braking. This will eliminate the effect = of > back-EMF on the average applied motor voltage. > > Correct thinking? > > Thanks, > Jason White --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .