In my previous posts I’ve found that 0603 motors (both 17kV and 19 kV) are under braking on every PWM cycle by algorithm of BHeli_S firmware. This leads to 30% less flight time. If the braking is removed by modification of the BHeli_S, the flight time can be significantly extended. That time I thought that this excess braking is a feature of BHeli_S and is applicable to any motor. Recently many people noticed that tiny whoops powered by 0802 motors can fly longer compared to 0603 motors (even when kV are the same). Theory says that same kV motors (assuming about the same resistance of the coils) should perform similar. So I decided to make clear why there is such a big difference between 0603 and 0802 motors.
I compared 0603 (19kV, betafpv) and 0802(19kV, happymodel) loaded with 4 blade 31 mm prop (betafpv) and powered by Crazybee F3 controller (UR65, red). The setup is similar to the previous.
Fig 1 shows rpm, thrust and battery current of 0603 and 0802 motors with the original O_L_5 BlHeli_S ESC firmware and the firmware where any braking is removed by firmware modification (so called MSH_mod).
Fig. 1 0603 and 0803 comparison. 0603 original fw and MSH_mod are shown with magenta and cyan lines, 0802 orig. and MSH_mod are shown with blue and red lines.
With the original O_L_5 fw both motors show exactly the same dependence of rpm and thrust on throttle position. But current consumption is significantly bigger for 0603 motors. The rpm and thrust curves have very different shape when original firmware is replaced with the firmware where braking is prohibited. We have discussed this behavior earlier, the reason is the original ESC algorithm try to hold rpm proportional to the throttle position, while motor without braking rotates on its own, similar to the brushed motor.
Fig.2 shows snapshots of current and voltage at the 0603 motor’s coil at 30%, 70% and 100% throttle position.
Fig. 2 Current and voltage at the 0603 motor coil. Left plot is the original O_L_5 ESC firmware, right plot is modified ESC firmware where braking is removed. 30%, 70% and 100% throttle from top to bottom.
With the original firmware we see the picture as before, negative current corresponds to non-regenerative braking of the motor at the every PWM cycle, even at steady rotation. When this excess braking is removed by MSH_mod in full agreement with the expected, there is no negative current any more (plot at right), and this leads to 30% increase of flight time.
Fig.3 shows similar plots for 0802 motor.
Fig. 3 Current and voltage at the 0802 motor coil. Left plot is the original O_L_5 ESC firmware, right plot is modified ESC firmware where braking is removed. 30%, 70% and 100% throttle from top to bottom.
In contrast to 0603 motor there is no negative current (at least at steady rotation) and MSH_mod does essentially nothing to current consumption. Sure the shape of rpm and thrust curves (Fig.1) are different in the case of unmod and MSH_mod, this can be explained by ESC algorithms which can use braking shortly only when needed, but not all the time we had in the case of 0603 motor.
All the above is confirmed by hovering time. The same build with the only difference of 0603 vs 0802 motors give the following hovering times (65 mm frame, 31mm prop, 250 mAh 1s battery):
1. CrazybeeF3 pro (Mobula7) FC, 19kV 0603, (Betafpv). S_H_50(orig) 3:02, MSH_mod 3:35
2. CrazybeeF3 pro (Mobula7) FC, 19kV 0802, S_H_50(orig) 3:25, MSH_mod 3:36
Similar results are obtained with UR65/US65/UK64 Crazybee FC (O_L_05 fw /O_L_05_MSH_mod).
We see that hovering time of 0603 with MSH_mod is about the same to the hovering time of 0802 motor (with or without MSH_mod) and is 30% longer than unmodified 0603 fw.
Looks BLHeli_S firmware has some bug for 0603 motor. It may be related to lower inertia or some other reasons.
- 0603 and 0802 are exactly the same from the point of view of dependence of thrust on the throttle position. The difference is in current consumption, non-regenerative braking at each PWM period (BHeli_S bug) makes current significantly bigger in the case of 0603 motor. That is why 0802 hovering time is 30% longer.
- 0603 and 0802 can have the same hovering time if MSH_mod is used for 0603 motor. This mod removes braking and makes the shape of thrust curve different (more soft) but still with the same maximal thrust.
- 0802 does not require any fw mod. I myself use MSH_mod for the 0603 motors, it flies a little bit different, but still well with significant time increase (see video in my previous post).