In article <110r8g9q6jd8qe5eqaon4duq5hctscvs6a@4ax.com>, jlarkin@highlandsniptechnology.com says...> > The brushes might wear out or get hot from friction if run long-term. > Starter brushes are big and crude, with strong springs. > > >Brushes are designed for a certain current density (think that is the correct term but not sure). Too little current can be as bad for the brushes as too much as far as brush wear.
PWM for High-Torque Motors
Started by ●May 1, 2021
Reply by ●May 1, 20212021-05-01
Reply by ●May 1, 20212021-05-01
On Sat, 01 May 2021 09:20:51 -0700, jlarkin@highlandsniptechnology.com wrote:>On Sat, 01 May 2021 16:07:08 +0100, Cursitor Doom <cd@nowhere.com> >wrote: > >>On Sat, 01 May 2021 08:02:05 -0700, jlarkin@highlandsniptechnology.com >>wrote: >> >>>On Sat, 01 May 2021 14:48:14 +0100, Cursitor Doom <cd@nowhere.com> >>>wrote: >>> >>>>Gentlemen, >>>> >>>>Is it feasible to PWM a car starter motor or is there just too much >>>>reactance in them? >>>> >>>>TIA, CD >>> >>>It should work. It might help to drive the field and armature with >>>separate PWMs. I think classic starters were series motors, optimized >>>for starting and nothing else. >> >>Yeah, they were wound such that which ever polarity they were >>connected they would always turn in the same direction. Some had the >>direction of turn stamped onto them, too. >> >>> >>>Starter motors are brutes. >>> >>>Reactance helps! >> >>How does it *help* exactly?? > >Motor inductance lowpass filters the ripple current out of PWM, so you >don't need separate inductors. At modern switching frequencies, a >motor winding should have lots of inductance, but not enough to affect >motion dynamics. > >What do you want to do with a starter motor?I'll email you tomorrow.
Reply by ●May 2, 20212021-05-02
On Saturday, May 1, 2021 at 6:48:21 AM UTC-7, Cursitor Doom wrote:> Is it feasible to PWM a car starter motor or is there just too much > reactance in them?For starting a car, you don't need any control except on/off. And, you don't make the motor good at keeping itself cool. So, what's the non-automotive application that benefits from PWM?
Reply by ●May 2, 20212021-05-02
On Sat, 1 May 2021 22:20:19 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Saturday, May 1, 2021 at 6:48:21 AM UTC-7, Cursitor Doom wrote: > >> Is it feasible to PWM a car starter motor or is there just too much >> reactance in them? > >For starting a car, you don't need any control except on/off. And, you >don't make the motor good at keeping itself cool. So, what's the >non-automotive application that benefits from PWM?Proprietory ideas abound. Why not use your imagination? There are many possible applications for a high-torque motor operating in short bursts. I found mine, now it's over to you.
Reply by ●May 2, 20212021-05-02
On 05/01/21 14:48, Cursitor Doom wrote:> Gentlemen, > > Is it feasible to PWM a car starter motor or is there just too much > reactance in them? > > TIA, CDNo reason why not, though they are usually series wound, whereas a separate field might be better from a control pov. Pwm for higher power dc motors has been done for years and things like exercise treadmills often use 1-3HP permanent magnet motors running at > 100 volts. The higher the voltage, the lower the ir losses, all else being equal...
Reply by ●May 2, 20212021-05-02
On Sunday, May 2, 2021 at 2:02:38 AM UTC-7, Cursitor Doom wrote:> On Sat, 1 May 2021 22:20:19 -0700 (PDT), whit3rd <whi...@gmail.com> > wrote: > >On Saturday, May 1, 2021 at 6:48:21 AM UTC-7, Cursitor Doom wrote:> >> Is it feasible to PWM a car starter motor or is there just too much > >> reactance in them?> >For starting a car, you don't need any control except on/off. And, you > >don't make the motor good at keeping itself cool. So, what's the > >non-automotive application that benefits from PWM?> Proprietory ideas abound. Why not use your imagination?Classically, for a burst one uses a regular-old-motor and a flywheel. An electric clutch doesn't need PWM, and doesn't dim the lights when engaged.
Reply by ●May 2, 20212021-05-02
On 02/05/2021 16:22, whit3rd wrote: <snip>> > Classically, for a burst one uses a regular-old-motor and a flywheel. > An electric clutch doesn't need PWM, and doesn't dim the lights when > engaged.High torque short bursts? You need a Marinov motor... http://www.electricstuff.co.uk/bbmotor.html#article -- Cheers Clive
Reply by ●May 2, 20212021-05-02
On Sunday, May 2, 2021 at 11:38:25 AM UTC-4, Clive Arthur wrote:> On 02/05/2021 16:22, whit3rd wrote: > > <snip> > > > > Classically, for a burst one uses a regular-old-motor and a flywheel. > > An electric clutch doesn't need PWM, and doesn't dim the lights when > > engaged. > High torque short bursts? You need a Marinov motor... > > http://www.electricstuff.co.uk/bbmotor.html#articleInteresting. I'll take the guy's word for it that it works. He says some things that don't make sense. "One can see immediately that the baI1- bearing motor has no back tension because there are no magnets, and the magnetic field of the current in the "stator" cannot induce electric tension in the metal of the "rotor". Thus the firm conclusion is to be drawn that the mechanical energy delivered by the ball-bearing motor is produced from nothing, in a drastic contradiction to the energy conservation law. " There is "back tension" because of the resistance that creates the heating. I assume he means no inductive counter EMF, but that's not what makes a motor work so who cares? The statement that the motion is produced from "nothing" is absurd and violates no energy laws. Maybe he is speaking creatively, intending to mock what others might say? This is a very low impedance device. I suppose the impedance can be increased by making the bearing balls smaller while keeping the radius of the race large. At some point this becomes counter productive to the lifespan of a high torque motor. The author also posts some errors. He claims the two motors have equal ohmic resistance, but says the larger motor draws a correspondingly higher current commensurate to the torque. If the current is higher, would not the resistance need to be lower? If he didn't use the same voltage in each case there is no comparison. Two other statements offered apparently without evidence. --- The driving force is higher for bigger bores, as the curvature of the races is less. The driving force is greater for bigger balls, as their curvature is less. I don't see this. A lesser curvature essentially lowers the leverage at a given distance from the point of contact. A greater rotation is required to accommodate the expansion. This would perhaps allow a larger speed of rotation, but either with no more force or with less force, but maybe I'm not grasping the full effect. I suppose since the expansion interaction area is greater this could result in a higher force. As the result is not so obvious at first glance, perhaps some measurement is in order. A lot would depend on the rate of cooling of the working area. I wonder if a thermal insulating layer might be of use? I'm also curious as to why box bearing don't exhibit this effect. Maybe there is too much area of contact lowering the electrical resistance and raising the thermal conductivity to a point the effect is nearly absent. I wonder if there is any practical use for such a motor? I'm guessing it is not very efficient. It might however, be useful at the very low end of motor size where torque is very important to overcome stiction. -- Rick C. - Get 1,000 miles of free Supercharging - Tesla referral code - https://ts.la/richard11209
Reply by ●May 2, 20212021-05-02
On Sun, 2 May 2021 08:22:47 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:>On Sunday, May 2, 2021 at 2:02:38 AM UTC-7, Cursitor Doom wrote: >> On Sat, 1 May 2021 22:20:19 -0700 (PDT), whit3rd <whi...@gmail.com> >> wrote: >> >On Saturday, May 1, 2021 at 6:48:21 AM UTC-7, Cursitor Doom wrote: > >> >> Is it feasible to PWM a car starter motor or is there just too much >> >> reactance in them? > >> >For starting a car, you don't need any control except on/off. And, you >> >don't make the motor good at keeping itself cool. So, what's the >> >non-automotive application that benefits from PWM? > >> Proprietory ideas abound. Why not use your imagination? > >Classically, for a burst one uses a regular-old-motor and a flywheel. >An electric clutch doesn't need PWM, and doesn't dim the lights when >engaged.I'm familiar with that system but AFAIK it requires the flywheel to be running *all* the time. I know they don't consume much energy to keep 'em running once they're up there at speed, but nevertheless, they do still need to be powered constantly. Plus a flywheel by it's very nature has to be *big* and *heavy* so that's a non-starter for my purposes, I'm afraid.
Reply by ●May 2, 20212021-05-02
On Sun, 2 May 2021 16:38:18 +0100, Clive Arthur <clive@nowaytoday.co.uk> wrote:>On 02/05/2021 16:22, whit3rd wrote: > ><snip> >> >> Classically, for a burst one uses a regular-old-motor and a flywheel. >> An electric clutch doesn't need PWM, and doesn't dim the lights when >> engaged. > >High torque short bursts? You need a Marinov motor... > >http://www.electricstuff.co.uk/bbmotor.html#articleApril the 1st was a month ago. I can't see any basis for this thing working as described.
