Brushless Motor Sine Wave

[TRSDOC]

I hooked an oscilloscope to two of the three legs of my Helicopter motor to see what kind of a waves it was being fed. Below are two pictures of the patterns. The first is low speed and the second picture is high speed. I don't know too much about reading the waves but it is interesting that the controller can put out a such a really good AC signal. Any input on the following?:
(1) The dotted offshoots, I wonder if this is just noise or some kind of feedback to the control about what the motor is doing.
(2) Both waves have a center horizontal line at reference, is this part of a speed signal or something?
(3) The second signal has something interesting going on at the bottom of the valley's.
Any input on what is happening here?

[Blackhawk3D]

if you look at the bottom of the low speed graph, it looks like you have the saome thing going on, just farther from the wave.

[TRSDOC]

if you look at the bottom of the low speed graph, it looks like you have the saome thing going on, just farther from the wave.
Interesting observation, I didn't note that. Still, I am very curious about what all these phantom traces mean. Anybody know what the scoop is here?

[izmile]

The sine wave that you see is the back EMF generated by the BL motor. Higher the speed higher is the amplitude of back EMF.

And the phantom wave (i guess) is the PWM modulated power that is being fed to the BL motor. I guess, decreasing the time scale of the oscilloscope will show the PWM waveforms more clearly.

Microchip.com website has a good application note (AN857) on how BL motors works.

[simingx]

The "horizontal line" in the middle is caused by the controller switching off both the high side and low side FETs to avoid shorting out the battery. There's a discontinuity when that happens, therefore the "line".

[wingster]

I take it that these sine waves are at the 8khz or whatever frequency the speed controller runs at. Many people are measuring current using clamp on ammeters. The cheaper ones seem to be calibrated only for 50 - 60 Hz household current. Does this mean their current readings are off?

[qv_]

Wingster:

You don't measure the AC current between ESC - BL motor
You should measure DC current between battery - ESC


/jan

[tigrr]

DC current could be just as off due to waveform issues.

The Ampere meter should be capable of showing True RMS regardless of type.

True RMS is required whenever you leave the pure sine waveform.

[TRSDOC]

The "horizontal line" in the middle is caused by the controller switching off both the high side and low side FETs to avoid shorting out the battery. There's a discontinuity when that happens, therefore the "line".
Thanks for the input simingx, izmile, This is what I am interested in. I am not really looking for efficency / amp draw issues as in some of the responses though. Again, I am just interested in "what" the traces mean... Thanks to everyone so far for looking.... :)