Understanding the voltage output of the Basic ESC for the T500

Hi All.

I am trying to understand the voltage output of the Basic ESC for the T500. I am running the ESC at an input of 1580 us which I assume is 20% duty cycle. The output between Phase 1 and the ground is seen in the first picture below. The switching is as expected, but I can not figure out why the baseline voltage seems to be 24 volts and not 0. This input provides a freespin RMP of 2000 rpm. When the thruster is not running, the baseline voltage is 0 indicating the it is not a ground fault.

I have a different ESC (VESC) which is seen in the second picture, where the baseline is as I expect it at 0 volts. The setup and probe used on the oscilloscope are the same. This input provides a freespin RMP of 980 rpm.

The difference in rpm for the same duty cycle is what has caused I have started to look at the signal. I hope some of you can help me understand why the voltage looks like it does.

Hi @SIGURD,

I’m not sure why the voltage between a phase wire and ground is relevant here - the motor coils are energised by current going through them, which a controller achieves by varying the voltages of the phase wires relative to each other (not necessarily directly relative to ground).

The motors are driven by energising the coils with a particular order and timing, so it’s likely more of interest to measure and compare the voltage across different phase pairs over time, to see whether the voltages across each phase are similar or different between the two controllers, or ideally measure the current through the coils rather than voltage across them, to more directly correlate with the magnetic field strength.

@Mark-Belbin’s phase current plots here are perhaps worth a look

Dear Eliot.

The reason I am investigating this matter is because the 2 ESC that I use (VESC and Basic) is not providing the same RPM (Force) for the duty cycles before I get to 100 %. At 100% they provide almost the same RPM (Force). From the plot in the first post, it can be seen that the two ESC I use, uses the same commutation techniques to generate the voltage input of the motor, but it seems that the signal is flipped and moved up.
The reason that this is interesting to look at is because the RPM (Force) of the prop is directly proportional to the average phase voltage that the motor sees.
So I am trying to figure out why 1580 us (Bacis) and 20% duty (VESC) is not obtaining the same RPM.

The “phase voltage” is the voltage across a motor phase, between two ESC phase wires - not the voltage of one phase wire relative to the power supply ground. If two phase wires are both at 24V relative to the power supply ground, then the voltage between them is still 0V, so there’s no induced magnetic field.

You can use the power supply ground as a common reference, where you measure two phase wire voltages relative to ground and then subtract one from the other to get the difference (the voltage across that phase), but it’s not meaningful to measure a single phase wire relative to ground without knowing the corresponding voltages of the other wire(s).

It may be worth looking at the performance curves in our T500 product page technical details - the RPM vs pulse-duration relationship is not completely linear, and the dead-band around neutral means there’s some offset as well. Your VESC may have a different RPM vs input curve and dead-band to the Basic ESC 500 we did our testing with.