Minimum speed of brushed DC motor? - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: April 02, 2014, 07:30:21 pm
content: I'm working on an application which needs to vary the speed of a motor down to as low as possible.We are pretty much stuck with brushed DC motors, 12V rated. We will use PWM will be in the range of 10Hz to 500Hz, probably more likely to be lower than higher.Will we be able to achieve a low speed? We want to use a 315rpm motor (unloaded speed) and go down to at least 1/20th this speed.http://uk.rs-online.com/web/p/dc-geared-motors/4095232/We have the option of allowing the motor to free-wheel or brake on each PWM cycle. I think free wheeling will allow for lower speed, but I'm not certain.What we're worried about is if we lower the speed too far, the motor could stall?

author: IanB
author_rank: Super Contributor
timestamp: April 02, 2014, 07:42:49 pm
content: The whole answer to this question depends on the load the motor is driving and the torque characteristics of that load.I think you could address the problem in a general way by putting a speed sensor on the output shaft and then varying the power input to the motor in a feedback loop in order to maintain the desired speed.

author: IanB
author_rank: Super Contributor
timestamp: April 02, 2014, 07:45:46 pm
content: If you are stuck on this kind of question in general (how to select a motor and match it to a load) you could consult a mechanical engineer.

author: mjkuwp
author_rank: Unknown
timestamp: April 02, 2014, 07:47:47 pm
content: certainly depends on lots of factors.What is your feedback for speed?I don't think this question can be answered in a forum but some things you'll want to check- inertia of the load as reflected back to the motor- friction?If you have a lot of inertia and do not need to change the speed very quickly then low speed is easier to achieve.A stepper motor is better at low speed.

author: tom66
author_rank: Super Contributor
timestamp: April 02, 2014, 07:57:43 pm
content: Quote from: IanB on April 02, 2014, 07:45:46 pmIt's funny because this project is run by a team of mechanical engineers. I'm the electronics engineer -- in charge of making the movement actually happen! I asked around and it seems I probably need amechatronicsengineer, as the team leader didn't know what a brushed motor was or how to apply it.Luckily it's a university with all three disciplines. If necessary I'll ask around.Quote from: mjkuwp on April 02, 2014, 07:47:47 pmWe are driving a very light load - less than 10% of the motor's rating. The load will be a small model bicycle wheel (4in across at most) - it is an artistic display. Speed changes will be slow, from minimum speed to maximum over the course of about a minute, and then back again.Stepper motors are still an option, we are still in the research phase. It does seem like cost may be a limiting factor though.No speed feedback is necessary or available.

author: mjkuwp
author_rank: Unknown
timestamp: April 02, 2014, 08:08:29 pm
content: so it is a wheel that spins - and you look at it? Does it do any work?A brush motor will run at a speed proportional to the voltage (or proportional to Voltage X PWM duty cycle) and it will draw current proportional to the torque.I have a good feeling about the application of a brush motor in this application from what you have said so far. 20:1 is a relatively high ratio but if you do not have random loads applied to it then it should run nice and steady.disclaimer: of course, given what you have said so far...note: I am a ME

author: tom66
author_rank: Super Contributor
timestamp: April 02, 2014, 08:17:44 pm
content: Quote from: mjkuwp on April 02, 2014, 08:08:29 pmAside from against air resistance, no.It might get stuck and stall (if for example a kid tries to grab it - it will be on public display), however, this is not a concern: if this occurs, the current limit of the driver should be hit and that motor will be restarted when the stall clears.There are going to be about 20 motors in total (running different wheel sizes too), all doing the same; it's quite an engineering challenge, to say the least!Quote from: mjkuwp on April 02, 2014, 08:08:29 pmThis is useful information; it seems like the best results will come from experimentation with the actual motor.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: April 02, 2014, 08:24:48 pm
content: Quote from: tom66 on April 02, 2014, 07:57:43 pmWould that be UoM ?

author: tom66
author_rank: Super Contributor
timestamp: April 02, 2014, 08:46:26 pm
content: Quote from: Mr Smiley on April 02, 2014, 08:24:48 pmBetter. Leeds.

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: April 02, 2014, 09:14:19 pm
content: Quote from: tom66 on April 02, 2014, 08:46:26 pmNot for long. There was a nob end that ran a department at UoM, dragged it down to the gutter and then buggered off to LEEDS. He'll do the same there, you have my commiserations.

author: FJV
author_rank: Regular Contributor
timestamp: April 02, 2014, 09:59:37 pm
content: Quote from: IanB on April 02, 2014, 07:45:46 pmActually electrical motors are part electronics and part mechanical, so you need to be a bit of both to do the calculations.This link will show some basic DC motor calculationshttp://www.micromo.com/motor-calculations.aspxThis link will show some calculations on how much torque the DC motor gets direct at the drive shaft.The effect of gears on the inertia the DC motor "sees" / how much inertia is reflected back to the motor.http://leadshine.com/Pdf/Calculation.pdf

author: FJV
author_rank: Regular Contributor
timestamp: April 02, 2014, 10:41:44 pm
content: Extra linkhttp://zone.ni.com/devzone/cda/ph/p/id/46Also note that the links I provided are calcualtions for dc motorswithoutan Iron core.Cannot find calculations where the iron core is also included.

author: Rufus
author_rank: Super Contributor
timestamp: April 03, 2014, 01:58:20 am
content: Quote from: tom66 on April 02, 2014, 07:30:21 pmBrushed dc motors are quite capable of 'running' at zero speed. The question is how bad will the speed control be if you are trying to run the motor open loop. That depends on the motor the load and the speed.

author: pa2ees
author_rank: Contributor
timestamp: April 03, 2014, 03:36:50 am
content: I did a project once with a small PIC as a driver and feedback control for a 2 wire DC Brushed motor.Essentially I had a PWM driving the motor driver, and it would run at a few tens of KHz. Then I would shut it off for a few milliseconds, and read the back emf voltage (which was filtered and conditioned with some passive components) using the ADC on the PIC.Then I increased or decreased the PWM duty cycle based on how fast the motor was freewheeling (proportional to the back EMF voltage). Thus if the motor stalled, the PWM duty cycle would increase until it was full on. I got it to run quite slow, without stalling, even under changing load conditions.If you're interested, I could get you more info on the setup and some pseudocode.- Erik

author: oldway
author_rank: Super Contributor
timestamp: April 03, 2014, 05:10:10 am
content: Your problem is very easy to solve: your speed control must have a R x I compensation so output voltage increase when current increase.I suppose your motor is a brushed permanent magnets DC motorU = E + RxIU = output voltage of your speed controllerE = FEM which is proporcional to the RPMR = internal resistance of the motorI = current of the motor.With such a compensation, you will succeed a good speed regulation without feedback and motor will run smoothly at low speed.

author: Kremmen
author_rank: Super Contributor
timestamp: April 03, 2014, 12:52:02 pm
content: Quote from: tom66 on April 02, 2014, 07:30:21 pmMy 2 cents. Recalling back to the days when DC motors ruled the universe and my job was to design drive system for paper machines, steel mills, various traction applications and whatnot, i can still recall some of it. Now the motors i was playing with were anything from 100 kW up to 20 MW so a bit biggerStill the same rules apply.For a separately excited ideal DC motor the speed is directly proportional to armature voltage and torque is directly proportional to armature current. The constants of proportionality are partly determined by the mechanical structure of the machine and partly by the field flux density provided by the separately supplied field coil (or permanent magnet field for smaller motors).This means that an _ideal_ motor will produce any torque while standing still - it just means that the armature voltage is 0 but there will flow an armature current sufficient to generate the torque against the field flux. Same with the motor turning under no load: the speed is defined by the armature voltage while no current flows.A real motor naturally falls short of the above due to imperfections such as winding resistances etc. As others have already noted you need to account for the resistance of the armature coil. For a practical motor the armature current is defined by a simple equation relating the supply voltage, armature resistance and back EMF generated by the motor.V = E + RI, or I = (V - E) / R, whereV = supply voltageE = back EMF from turning motorR = armature resistanceI = armature currentFurther, E*I will be the mechanical output power while R*I^2 is the "copper loss". The proportionality of RPM vs armature voltage is given by the motor constant Kv (RPM/V).The best way to control a DC motor is a cascade controller with an outer speed control loop and an inner torque control loop. The torque loop can be a simple PI type analog controller implemented using a single opamp and a current shunt or, it can be a sophisticated FPGA based all digital wonder. It all depends on what exactly you need. Whatever else, it has to be fast, step response settling times should be well under a millisecond in every case. The speed loop also works nicely as a PI type controller, rarely do you need a D term and generally it does more damage than good.The ultimate controllability depends on several factors; how the load torque varies as a function of time and angular velocity, what quality motor do you have (i.e. how well they are characterized and if they have gross or minor nonidealities), and finally how clever are your control algorithms including the actual power stage driving the motor itself.I would start by implementing the above cascade controller with an analog torque loop and simple feedforward to compensate for the largest motor nonidealities, primarily the armature resistance. The feedforward term should be calcualted from the basic motor equations to be as spot on as practical, thus minimizing the corrections needed by the controller.If you succeed in implementing a good controller and the load is at least reasonably well-behaved, 1:20 speed ratio should be a piece of cake. Whatever else you wish to say about brushed DC drives these days, the fact still remains that as far as controllability is concerned they are second to none.Finally, addressing your specific concern regarding stalling at low speeds; it will not happen in a control scheme such as what i outlined above. You do need a reliable and reasonably accurate velocity and current feedback to give the controller a chance to do its job, and you need to tune the control loops at least decently but once you do, the control system is guaranteed to maintain speed within the envelope possible for the motor/load combination. For the record, the best precision digital speed controller implemented in the team i worked at the time was accurate to within 22 ppm of nominal 1500 RPM motor speed and the motors had no problem following the guidance using exactly this control scheme. To be sure, the torque control was a very fast 4 quadrant analog circuit but much less is enough to get a good result in a normal case.

author: Kohanbash
author_rank: Regular Contributor
timestamp: April 03, 2014, 02:54:30 pm
content: HiYou should also think about the gearing on the motor as a way to reduce your output speed.As others have said you might want to consider having feedback (ie encoder) on your motor. At low speeds (and many other cases) it is hard to control the motor with a constant velocity with no feedback.http://robotsforroboticists.com/motor-selection/

author: calexanian
author_rank: Super Contributor
timestamp: April 03, 2014, 03:49:50 pm
content: Captain empirical here. Just connect the motor of the model to a variable power supply and vary the voltage to determine armature currents required for the different speeds, or to see how slow it can go. Seems simple enough!

author: babysitter
author_rank: Frequent Contributor
timestamp: April 03, 2014, 06:45:27 pm
content: Watch the temperature - some motors have gimmicks to force some cooling air thru them. Might not work right at low rpm.