driving 3 phase brushless motors

[Simon]

I am looking at this catalogue: https://www.parvalux.com/media/wysiwyg/Resources/Catalogues/Parvalux_-_Brushless_Catalogue.pdf

In particular at the PBL86-91. I've never worked with these motors before but from the information given driving them is as simple as switching the phases high and low for the listed outputs of the sensors? is it that simple? not proper sine waves required?

[MrW0lf]

not proper sine waves required?

You PWM voltage to get semi-sinusoidal current. Thats why they squeal like pigs at low RPM when there is no BEMF to smooth things out.

[Simon]

yea that is what I meant, so basically by high and low they are referring to the polarity of the sine wave.

[MrW0lf]

yea that is what I meant, so basically by high and low they are referring to the polarity of the sine wave.

...and if you get PWM wrong its trouble because impedance of the windings is non-existent. Hardly any inductance at all - was quite surprised when measured them first time. So better start playing at veeery low voltages

[Simon]

even better to get an off the shelf controller but I need one for automotive not a pansy industrial controller that won't take the 250V spikes and 40V surges.

[Benta]

You have to think about it like this:
A BLDC works exactly as a PMDC motor. The only difference is, that the brushes and the commutator have been replaced with electronic switches.
This also means you have the same switching pattern as with a PMDC, meaning only two phases are active at any time.
For speed control, you can start adding in PWM, but that's on top of the basic operation. This is also where the "squeal" issue comes in, otherwise BLDCs are very quiet.

[langwadt]

You have to think about it like this:
A BLDC works exactly as a PMDC motor. The only difference is, that the brushes and the commutator have been replaced with electronic switches.
This also means you have the same switching pattern as with a PMDC, meaning only two phases are active at any time.
For speed control, you can start adding in PWM, but that's on top of the basic operation. This is also where the "squeal" issue comes in, otherwise BLDCs are very quiet.

yep, I've seen a BLDC driver that had a switching voltage regulator feeding the switches instead of doing PWM

[Simon]

Indeed, our motor and driver supplier at work was proposing just such a scheme because it meant that he could do the PWM at the most efficient point and reduce peak currents in the stator coils.

[Benta]

There are two basic schemes for BLDC control:
1: varying the supply voltage to the power stage, either DC or PWM
2: gating a PWM signal into the drive circuit for the power stage.
In both cases, the "Master" signal for the power stage drive is the feedback from the Hall sensors.

The opportunities for adding advanced features on top of this are of course infinite.

[Simon]

Yes but if you are having to approximate sine waves with PWM how do you decide on the cycle length of the sine? do you base it on the last known time taken to go between the previous phases?

The other thing is can't you just send the motor a sine wave and know the rotor will follow it round particularly if it's something like a fan? I can understand that something like a traction motor requires feedback.

[langwadt]

Yes but if you are having to approximate sine waves with PWM how do you decide on the cycle length of the sine? do you base it on the last known time taken to go between the previous phases?

The other thing is can't you just send the motor a sine wave and know the rotor will follow it round particularly if it's something like a fan? I can understand that something like a traction motor requires feedback.

you can run it open loop just like a stepper motor, that's how sensor-less start up

but you have very little torque and you can't go very fast before the slightest disturbance makes gets the rotor out of sync
and it stops turning

[Benta]

Stop, stop, stop!

Please reread reply #5.

Sine waves do not come into basic BLDC drive at all. It's bang-bang commutation, just like PMDC.
Now, you can try fiddling with making pseudo sine waves for a BLDC drive, but it's not really worth the effort.

Sine wave generation comes into play when driving 3-phase PMSM, where all three phases are active at any time. The motor is basically the same, but the drive strategy is completely different. It's an open-loop drive where speed is controlled using a constant U/f ratio. For certain applications it can be attractive (very precise constant speed), but is complicated and a lot of bother.

[Simon]

what is PMSM? The motors I am looking at are 3 phase although as you say only 2 phases are active at a time.

U/f ratio?

[langwadt]

Stop, stop, stop!

Please reread reply #5.

Sine waves do not come into basic BLDC drive at all. It's bang-bang commutation, just like PMDC.
Now, you can try fiddling with making pseudo sine waves for a BLDC drive, but it's not really worth the effort.

Sine wave generation comes into play when driving 3-phase PMSM, where all three phases are active at any time. The motor is basically the same, but the drive strategy is completely different. It's an open-loop drive where speed is controlled using a constant U/f ratio. For certain applications it can be attractive (very precise constant speed), but is complicated and a lot of bother.

doesn't seem much different than running  a stepper motor full-stepping vs. micro-stepping

[MrW0lf]

what is PMSM? The motors I am looking at are 3 phase although as you say only 2 phases are active at a time.

Think he means this?

[langwadt]

what is PMSM? The motors I am looking at are 3 phase although as you say only 2 phases are active at a time.

Think he means this?

that's BLDC, PMSM is similar but the winding are spread out so the back-EMF is sinusoidal instead of trapezoidal

[Benta]

My apologies for using an acronym, I normally tend to avoid this, as it can lead to confusion and misunderstanding. This time I fell into the trap myself.

PMSM is " Permanent Magnet Synchronous Motor".

It is basically the same motor as a brushless DC motor, but the drive strategy is completely different. Three-phase AC is supplied to the motor, mostly in the form of PWM.
Torque/speed characteristics are also completely different, as it's a U/f (or V/Hz) drive.

This as opposed to a brushless DC motor, which behaves exactly as a brushed DC motor.

MrW0lf, your schematic shows exactly the commutation of a BLDC motor. The trapezoidal waveforms come from the motor inductance and are very idealized in your graph, but still representative.

[Benta]

Simon, check out the graph from MrW0lf and you'll see the following sequence from 1...6 (the lowest three traces):

A - B - C

+   0   -
+   -   0
0   -   +
-   0   +
-   +   0
0   +   -

Do you see what I mean when I say only two phases are active at any time? This is the commutation pattern of both a PMDC and a BLDC motor.

[Simon]

I thought PMSM meant that really it's the difference between the two that is the confusion.

So do off the shelf chips exist that take care of everything if I supply the power stage? well I know they do exist but is this something worth considering or is it a custom program on a micro controller?

[MrW0lf]

Here is some old thread I found about BLDCs, maybe something useful:
https://www.eevblog.com/forum/projects/is-there-any-_easy_-way-to-do-brushless-dc-motor-speed-control/

BTW initially when mentioned squealing, and being careful with PWM I was actually thinking PMSM. But was little disoriented because some refer to brushless outrunners also as BLDCs, now if think about it's of course PMSM because BEMF is sinusoidal and drive is crazy PWM. But for example Hobby King never names them PMSM. Did get one little bugger (Turnigy 2730-1300) from drawer and measured: L=~36uH, R=0.37ohm, Z=0.43ohm @ 1kHz. Essentially rotating resistor

[Benta]

I thought PMSM meant that really it's the difference between the two that is the confusion.

So do off the shelf chips exist that take care of everything if I supply the power stage? well I know they do exist but is this something worth considering or is it a custom program on a micro controller?

There are lots of chips out there that will:
1: understand the inputs from the Hall feedback sensors.
2: provide drive for your output stage in the correct sequence.
3: accept a PWM input for speed control.
Many will have additional features like current monitoring, UVLO etc.

A search for "brushless DC motor drive IC" should give plenty of results.
TI is an obvious place to look.

[IDEngineer]

There are lots of chips out there that will:
1: understand the inputs from the Hall feedback sensors.
2: provide drive for your output stage in the correct sequence.
3: accept a PWM input for speed control.
Many will have additional features like current monitoring, UVLO etc.

Not all such motors have Hall effect sensors. Some use the BEMF from the inactive phase to track the rotor's position and time the commutation. It's a juicy little problem because the BEMF sensing circuitry has to be able to withstand the MUCH higher driving voltage.

Yes, there are off-the-shelf IC's that handle BLDC motors, including those without integrated Hall effect sensors. Many/most are primarily logic/timing, and expect 1-3 driver chips (or at least discrete FET's) between them and the motor to handle the higher voltage/current side of things. This can be a deep rabbit hole... very interesting but set aside a fair amount of time if you really want to get into it!

[Benta]

IDE, if you'd care to read the original post, it already links to a motor in question, which has Hall feedback...

Just trying to raise the signal/noise ratio.

[IDEngineer]

IDE, if you'd care to read the original post, it already links to a motor in question, which has Hall feedback...

Agreed, but "understand the inputs from the Hall feedback sensors" could mislead someone into presuming that all such multiphase motors have integrated Hall effect sensors. I was trying to add data - raise the S/N ratio, so to speak - not contradicting you. 

[james_s]

There are several open source firmware distributions for speed controllers used in RC aircraft, lots of schematics out there too. Most of these motors are sensorless but the sort used in RC cars typically have hall sensors. Looking at that firmware should give you some clues on how it works.