Bob, I am curious about method (3). Would the efficiency/load/stress be any different than the On-Braking case? This has been very helpful. Thanks, -Jason White On Tuesday, July 3, 2018, Bob Blick wrote: > Your frequency is not low, so what happens is this. When the h-bridge goe= s > HI-Z, the energy stored in the inductance dumps back into the supply rail= s > and your net motor voltage is 0 and the current waveform is based on the > inductance and frequency. Current means you have some motor torque, but n= o > voltage means no speed. > > But when you raise the duty cycle to 50% and above, remember your supply > rails clamp the voltage through those back diodes. The voltage is the > same(you use lossless transistors and diodes, right?) as when ON, just th= e > polarity is now reversed. Being an inductor, when you go Hi-Z the curren= t > can't increase, it ramps down. There isn't enough time before the next ON > cycle for the inductor current to go to zero, and current will build up > over subsequent cycles. > > In order for things to be happy, the average motor voltage will move away > from 0, and your motor will actually spin. > > So what you have is a voltage profile that is not a straight line startin= g > from 0% duty cycle such as you would have with an ON/shorted PWM cycle > train. > > Either way works, it's just the result that's quite different. You need > good 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 > calculate, but losses are continuous so you will want to shut off the > H-bridge when you are not running the motor. > > Hope this helps. > > Bob > > > ________________________________________ > From: piclist-bounces@mit.edu on behalf of > Jason White > 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 impedanc= e > at > > a 50% duty cycle the average voltage will not be 20V because of the bac= k > > 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 effec= t > of > > back-EMF on the average applied motor voltage. > > > > Correct thinking? > > > > Thanks, > > Jason White > > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 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 .