It is interesting to compare 2 motors with the same thrust performance, but different size and other properties, like inductance, resistance, mechanical and re-magnetization (iron) loses. About an year people noticed that replacement of 0603 with 0802 motors increases flight time. My side-by-side comparison of 0603, 19kV and 0802 19 kV motors shows that 0603 has significantly lower efficiency compared to 0802 with regular BHeli_S firmware. If BHeli_S is modified to remove damping, the efficiency of 0603 gets similar to 0802 without damping, at the same time FW with no damping does not change performance of 0802.
Since that time tiny whoop community accepted that 0603 motor is bad because it has 30% less flight time compared to 0802 motor (with regular BHeli_S). In response (and some other reasons), manufacturers tend to produce almost any modern tiny whoop equipped with 0802 and bigger motors.
Many people believe that 0603 motor is less “powerful”. This is NOT TRUE. According to the theory, 19kV motors regardless of their size should have the same performance (neglecting difference in resistance, mechanical and other loses). Internal resistance of 0603 motor is 0.26 Ohm, and 0802 motor has 0.30 Ohm (0603 should be even slightly more efficient than 0802)
Recently Joe Lucid (joelucid) developed new firmware with elevated PWM frequency and also provided several other options (like no damping). People noticed that elevated PWM frequencies significantly increase flight time. I also checked performance of 0802 motors at 24kHz, 48kHz (damped, undamped) and found that efficiency can be enhanced by ~30% , and developed some explanation of this finding.
Now I want to check what happens to 0603 19kV motor at high frequency (HF) PWM.
BetaFPV 0603 19kV motor and HappyModel 0802 (19kV) motor were used at exactly the same setup: 75 mm frame, Crazybee F3, BB21 (“H” ESC FW), BT 2.0 connector, 40mm 4 blade propellers (gemfan), URUAV 300mAh. No RPM filtering.
JESC 2.2 24kHz, JESC 2.2 24kHz no damp and JESC 2.3 96 kHz were tested. The same FC was used in a test stand measurements. Note, FC, prop size are different from what I used in my previous test.
I’ve recorded the flights to everyone can check current, mAh and time.
0603 has 60% more flight time compared to regular damped 24kHz version! And even 17% more than the best of 0802!
Now let us compare 0603 and 0802 performance with my test stand.
Fig. 1 Thrust, rpm, current vs throttle position.
We can see that thrust dependencies of both 0603 and 0802 are very similar and does not depend much on PWM frequency.
To compare real performance we should plot current vs thrust:
Fig. 2 0603 and 0802 efficiency, current vs thrust.
Efficiency curves are in a good agreement with what we got from comparison of hovering time (video above). Moreover their shape is in an agreement with my theory (see fig.2). With higher frequency it gets close to the shape expected for DC current (though in my explanation we use torque and not thrust, they are not the same, and actually I did not even expected such good coincidence).
Here are current and voltage snapshots at 30% throttle.
Fig.3 Current and voltage measured at the coil with my sensor (Hall-sensor). From top to bottom: 24KHz, 24kHz undamped, 96kHz
Looking at current traces we can see that current in 0802 motor is more “integrated” because of larger inductance, as it was discussed in my previous post. And as per my theory this leads to stronger dependence on frequency for 0603 motor and also explains why 0603 is worse than 0803 at low frequency. Larger inductance for bigger motors is also expected, because inductance is proportional to square of coil diameter. Though it cannot explain everything, e.g. why 0802 is less efficient than 0603 at 96 kHz.
This is also in agreement with the reports in rcgroups.com that 25kV motors have more efficiency improvement than 17kV and 19kV motors, to me it happens because of lower inductance of high Kv motors, similar to what we see in the case 0603 and 0802.
Also we see that 96 kHz has no damping (braking) even at 30% throttle, as it was predicted by me in my previous post.
I believe in this post we have enough proves to say that dependence of PWM frequency is due to phenomena in LR circuits (e.g as it was in my explanation in previous post) but not dead times, switch loses, vibrations, etc common in ESC discussions.
1. Welcome back 0603! 0603 has 60% more flight time with JESC 96kHz compared to regular 24kHz version. It has also 17% more flight time than the best of 0802 motor (also at 96 kHz)
2. Thrust dependencies and maximal thrust does not change significantly with PWM frequency, no drawbacks found for HF PWM
3. Efficiency curves are in agreement with my theory, I believe LC circuit properties is the major reason of efficiency improvement.