Examining the abrushless DC motor (in a weed trimmer, where it’s easy to get at), but scoping it out, I discovered something odd about how it’s driven. It strikes me as a really dumb way to do variable speed.
, https://i.ytimg.com/vi/DEaEjp_Peb8/hqdefault.jpg
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2022-03-20
As with batteries, it might be best to test under load: Without load, batteries exhibit near-to-nominal voltage, even if they are practically empty. With a 50 Ohm resistor as load, things turn out a little differently. I suppose the same goes with electronically controlled motors: No serious load, no usable measurements.
There is indeed some mystery /voodoo in the controller behavior, but it would be a disaster for DeWalt if their run time lagged behind their competitors. It doesn't, so there's method to their madness.
Maybe it should have wood gears?
You can also rotate stator as rotor. Watch my video….
Looks like it’s about max torque.
would it not have to do with the the third set of coils that are not attached, to the scope (not registering a signal, the third set would always keep a magnetic inershia thus prolonging the battery load, tri-lobal magnetic field limits the load requirement to start and maintain the rotational field,basically they are never truly north south , and why the stator clings to the magnetic field when stationary 33.33% of a pole rather than 50%opposition , similar to S.E.G.? lol ,got me thinking of of a AC MOTOR AND ELECTRICAL MAGNETISM course i took in moncton nb in the late 80's
My hunch is that they want to build it as cheaply as possible and nothing else matters.
Kinda like some OLED technology, whereby to show a dim picture, it just ups the refresh rate of the pixels so much so that the brightness is never at max.
Hi Matthias, little late to the party here. But one observation is that shorting the windings doesn't automatically incur high resistive losses in a brushless motor, especially if it has high leakage inductance. If you're operating the motor electrically much faster than the L/R time constant of the leakage inductance, then the rotor flux is mostly excluded from the stator by the opposing currents induced in the shorted windings, and the torque is low (equivalent to exceeding the critical slip speed of an induction motor). Yes there’s still induced current circulating, but depending on the magnitude of the winding resistance, this might not actually embody a large power loss.
Shorting the windings will potentially accomplish two positive things.
First, it could reduce the cogging torque. Cogging torque arises due to the variation in the reluctance which the rotor flux sees versus angle, as it tries to find a path through the stator. Shorting the windings excludes the flux from the stator entirely and so could reduce the cogging torque.
Second and probably more importantly, it will reduce the iron losses in the stator. Is that thing even laminated? At up to 500Hz, the eddy current losses in a solid core will be pretty darn severe. Even in laminations it would be bad. The hysteresis loss is also always there too, but eddy current losses scale as frequency squared. So by shorting the windings and excluding the rotor flux from the stator, you may actually get lower drag torque than if you left the windings open circuit.
This might in fact be literally the most efficient way to drive such a cheap, high RPM motor. When you want to apply torque, you need some of the rotor flux to penetrate the windings. So you allow all the flux through the windings and apply as much torque (current) as possible for a time, so the ratio of injected power to power loss is high (high efficiency). Then you exclude the flux from the stator completely to stop that iron from absorbing tons of eddy current loss and creating a huge drag torque.
Are you going to make another video with milwaukee tool?
It needs a load, the gas ones I use have a large RPM drop with the standard load from thick weeds. I imagine once its in the lower RPM range it will stop the breaking like that.
almost seems like for the "Brake", they are dumping the energy back into the battery. They probably drive the the motor hard so their driver transistor is either on or off and has a minimal power loss across it.
Don't know how you are probing but if find a GND point and you probe two phases at the same time you should see more ON time. What I'm saying is that the motor is driven one phase at the time, so if you probe just one phase you will see a lot of OFF time.
So now I will make a pantograph to weed whack my lawn intricately
Dewalt exposed. Was this the trimmer kit that was $100 in the US around May if 2019? I was close to getting it, but I’m a Makita guy.
Probably needs a load to get it to go to normal duty
I believe there is nothing stupid, this waveform is representing only one phase out of three, so when it is turned off (suppressed) another coil is actually turned on in order to complete the cycle, it's just how 3ph BLDC motors work.
Note: opposite coil are usually connected in series and generate both S and N poles when energized.
Nothing wrong with your basics, your basics are really good.
What type of usb data logger is that? That’s awesome.
is it regenerative braking.i have discover if a bldc esc motor controller is in a motor break possition and i turn the motor with me hand it wil give a charge back to the battery.
05:55 thought its a 6-pole??
I had a DeWalt trimmer just like this. Had it for 2 seasons, in my tiny yard, and never fully uncharged the battery. Then after this past winter I went to use it and the battery was dead. Wouldn't charge up. I call customer support and they tell me it's out of warranty, and that they don't make that battery anymore. Although I could buy a replacement battery at Lowes… for $265.
The trimmer / battery came together for $150. A brand new trimmer / battery was around the same price.
Meanwhile I've had a Ryobi mower, trimmer, and leaf blower and 3 batteries that are absolutely packed with juice and I can take care of my entire yard on less than 2.5 batteries.
Never buying DeWalt battery-powered lawn equipment again. My experience is that they make solid cordless hand tools but I don't trust the brand anymore after they abandoned this whole battery pack after 2 measly years. The kicker was that when I gave the serial number for the battery, the lady at customer support told me it was manufactured in 2016. So I bought a brand new trimmer that came with a year and a half old battery.
Great.
Its these big wigs making that dough, ripping people off. Like you said, "Its IDIOTIC."
I'm pretty sure you've voided the warranty, LOL. I have the same weed trimmer, best I've ever owned.
Also, I don't believe my pulses like that. Possibly its inperceivable?
efficiency is not the whole point of a brushless motor, for an equipment that will be running for countless hours reliability is way more important than efficiency, and of course, it was made to be cheap, so of course they will not use efficient pwm circuit to drive it
Looks alot like a 3 phase motor to me. But I'm a generator tech not a rocket scientist?
It's because DC motors are more efficient at high voltages/currents as the inefficiencies are static and/or diminishing. So tl;dr if running 10W through it gives 1 unit of power out, 100W would give not 10 but 12. So they use this method to get the motor working in it's sweet spot and then it's supposed to idle, which you're probably seeing as back EMF and thinking is lower power input. Great video though. Too bad the loop wasnt very sensitive.