PING Tim Wescott

Oh damn -- this is something that I've just sort of picked up over the years, without ever having one comprehensive reference. So, uh, my brain?

You care about the motor design to the extent that it affects the circuits -- mostly you care how much leakage inductance there is, and what the back-EMF waveshape is (nominally if it's a BLDC it'll be trapezoidal, with the term BLAC or synchronous machine reserved for a sinusoidal back EMF -- but you can't count on that).

Beyond that, there are a LOT of app notes floating around from processor vendors on how to run BLDC motors. Many of them are connect-the-dots- with-our-software, but some of them actually explain the whys and wherefores.

Den torsdag den 9. februar 2017 kl. 01.00.34 UTC+1 skrev Jim Thompson:

power driver or controller?

I designed a chip for this one, nearly 30 years ago...

"DC_Motor_Electronically_Commutated.jpg", on the S.E.D/Schematics page of my website.

But I cheated, I had the original discrete circuit, so I simply ran it and observed the commutation waveforms and adapted the circuitry for safe behavior on a bipolar chip, fixed up the start-up oscillator and found a way to make sure it always started with the right rotation... it was a fan motor in a Mercedes A/C system and wind could cause it to be spinning backwards.

Also added acceleration controls so the whine wouldn't offend the sensitivities of Mercedes drivers >:-}

But now I'm seeing a need to design chips for modern BLDC's, so I need to bone up ;-) ...Jim Thompson

Den torsdag den 9. februar 2017 kl. 01.52.06 UTC+1 skrev Jim Thompson:

3*half-brigde, current sense and possibly 3*comperator, and ofcourse an MCU ;)

Yup. That MCU is just essential for any Jim Thompson chip.

good luck trying to do something like

That motor design I pointed to had all kinds of bells and whistles... all Analog.

The world DID manage to get along before uP's >:-} ...Jim Thompson

Den torsdag den 9. februar 2017 kl. 02.38.36 UTC+1 skrev Jim Thompson:

ditto before the steam engine or electricity ;)

I don' think uPs are used because the old way was just "too good"

Y'know, I don't see anything there that couldn't be done with analog, if you were doing a dedicated chip. It might not be a very _versatile_ chip, but I might be underestimating Jim's ingenuity.

If I were going to do a controller at board level I'd do it with a processor, because there aren't chips made that do that job effectively. But if you challenged me (and someone _paid_ me) I could do it in analog. I'd probably need close to a hundred square inches for the electronics, but it would all be op-amps, diodes, and resistors. Jim would take it from me, throw away the unnecessary half, and put the rest into a custom chip.

I would suggest, if anyone wants to go there, that getting a new textbook for an electric machines course and working through it (or taking an online class if there's one offered for free) would probably be a Very Good Thing. I was the only "circuits" guy taking electric machines at Portland State (in among a bunch of power guys); I've never worked on a project involving motors where that information didn't prove immensely useful.

tto:

BLDC with or without rotor position sensors (hall probes, encoders,...).

Anyway Freescale (NXP) has good ref design for HW and SW. For the HW part for example:

For SW: sensorless:

Bye Jack

PS: also other vendoes have more or less the same kind of ref designs (Atme l, Microchip, ...)

A brushless DC motor is a synchronous motor, and needs roughly sine-wave cu rrents through the windings.

With two-phase motors the sine waves are in quadrature, with three phase mo tors the three phases are separated by 60 degrees.

If Jim-out-of-touch-with-reality-Thompson doesn't care about the motor desi gn, he may produce a sub-optimal motor drive circuit.

In the devices I worked on, the back EMF in the windings was a lot bigger t han the resistive drop, but I was making them spin tolerably fast.

The mechanical engineers involved couldn't quite see why I wanted to - and expected to get away with - putting 60V across a 12V motor, and it took a w hile to explain to them that the 12V was the maximum voltage you could get away with applying to coils of a stalled motor - they dragged me off to tal k to the guys that were selling us the motors, and were a bit surprised to learn that most of the people who bought the motors used them supply voltag es well over the ticket voltage. Weird experience.

Speaking of being out of touch with reality, if you're buying a bare motor, "BLDC" has more or less come to mean a motor whose magnetic circuit is arranged so that the back-EMF is trapezoidal, which, in turn, means that you're quite safe in feeding it in a pattern of either constant voltage (for the flat top of the trapezoid) or off (for the sloped part of the trapezoid). A sine wave drive is actually less efficient.

"Brushless AC" means a motor with a sine-wave back EMF. Not that you shouldn't always check, preferably with an example motor in your hand and an oscilloscope on two or three of its leads.

Apparently in the machine-builder world, "BLDC" means a motor with a built-in drive, but that's a different story.

Slowman can't find his own asshole, even with an assistant holding a flashlight and a mirror, let alone design a motor driver >:-} ...Jim Thompson

yeh basically a BLDC is a DC motor with the brushes and commutator replaced with external switches

Apparently in the machine-builder world, "BLDC" now means a motor with a controller board built in -- so you give it power and a velocity command, and it does the rest.

Which is more fun for machine builders, I'm sure, but less fun for circuit designers.

For this sort of problem I usually just get a good undergraduate textbook and read it like a novel to get the gist, then go through the important bits of the math, which are usually about 1% of the total, so it only takes a day or three to come up to speed (so to speak).

Cheers

Phil Hobbs

Yup. The hard part of this for me is the "good" part -- particularly since "good in a classroom context" can, but doesn't necessarily, mean "good for self-study".

Back in ancient times, I had to take two semisters of Electrical Machinery. It was actually valuable and the labs were fun. I don't know how all those big old motors survived us, or that they could contain so much smoke.

I averaged about 50-60% on tests, which got me an A. Class average was around 16.