I have one of these brushless motor speed controllers hooked to an old hard drive. All is working as expected but I am trying to understand what I am seeing.


Here is the output. It is pretty much the same on all three coils. I really can't get the display to sync no matter what I try with the trigger. I would expect to see straight PWM but the display is all over the place. Exactly what am I looking at? Is this some sort of feedback coming from the windings of the motor?



The common winding connector even has an stranger output.

 

Here is just a slightly better shot of the common winding

 

All 3 phases should look pretty much the same, just 120degrees out.
The phase pulses switch under control of the position sensing (assuming you are not just free running an have Hall sensor feedback) then they are "PWMed" to control the speed. The chip may be sensing the back EMF to determine the armature position.
Your scope pictures show that, a 3 phase switched output with PWM superimposed on it.
It may be not sensing the position, just producing the phase signals, and the PWM could be the current limit coming into play.
Do you have details of the board?

 

All 3 phases should look pretty much the same, just 120degrees out.
The phase pulses switch under control of the position sensing (assuming you are not just free running an have Hall sensor feedback) then they are "PWMed" to control the speed. The chip may be sensing the back EMF to determine the armature position.
Your scope pictures show that, a 3 phase switched output with PWM superimposed on it.
It may be not sensing the position, just producing the phase signals, and the PWM could be the current limit coming into play.
Do you have details of the board?

Not sure I understand all of that. Are you saying I might be seeing all the the pulses from all 3 windings on one single winding?

No I do not have details on the board.

 

Not sure I understand all of that. Are you saying I might be seeing all the the pulses from all 3 windings on one single winding?

No, you are looking at one winding if you have a single channel scope. Do you have a dual channel one so to look at 2 windings together?
It is just they will all look the same, just offset from each other. It is a 3 phase motor after all, driven by switched DC instead of AC. The motor would quite probably run ok on 3 phase AC of the correct voltage.

 

I think that is it. I disconnected the leads to one of the windings and get this. I get a pulse of a set of those every half second or so.

 

No, you are looking at one winding if you have a single channel scope. Do you have a dual channel one so to look at 2 windings together?
It is just they will all look the same, just offset from each other. It is a 3 phase motor after all, driven by switched DC instead of AC. The motor would quite probably run ok on 3 phase AC of the correct voltage.

I have a dula channel and I know they will be the same. What I am thinking is I am picking up the pulses from all of the windings on a single winding and they are superimosing on one another.

 

You will get some inductive coupling between windings. This will show up more if the driver is single ended, just switching to 0V then open, as then the open winding will be unterminated. If your driver is a 3 phase "H" bridge, it will more likely have the windings loaded so the inductive coupling will not be so pronounced. But the winding signals will be out of phase, even tho they are the same shape.
What may help is if you can find a spot on the driver board to get a trigger signal from. Solder a test point there so it can hook to the scope external trigger, then look at 2 phases with your probes,

 

Have a look here...
https://electronics.stackexchange.com/questions/335484/are-these-bldc-waveforms-making-any-sense

 

It's notoriously hard to make a scope trigger on a motor drive output, because they aren't consistent. Always adjusting frequency up/down to maintain speed. Always adjusting pulse width to control torque.

Best to freeze frame

 

It's notoriously hard to make a scope trigger on a motor drive output, because they aren't consistent. Always adjusting frequency up/down to maintain speed. Always adjusting pulse width to control torque.

Best to freeze frame

Also, the inductive reactance and back EMF between pulses make for an interesting show of wild traces in the scope...