This article reports measurements on a brushless drive used for an RC quadcopter.
Dead time is the time allowed between turning one FET in a half bridge off and the other FET on to ensure that they do not both conduct simultaneously, even for an instant, as very large currents may flow. The dead time specified in ESC configuation is an ‘additional' delay added over and above circuit delays.
Measuring a stable properly synchronised drive system at minimum speed can expose the effects of insufficient dead time on total drive input current. At minimum speed, very little power is delivered to the propeller and most drive input power is lost in various system losses, so drive current becomes very sensitive to system losses.
The test drive system comprises:
- 12V.0 regulated power supply;
- Turnigy D3530-14 1000kv Brushless Motor;
- Hobbyking 40A ESC 4A UBEC 9261000003;
- SimonK ESC firmware (commit 02bd8e4ca36a06722efe51bc7cd5130d72a184b8); and
- HK Gemfan 11×4.7 two blade propellers.
Current drawn by the drive from the power supply when armed but not rotating was 0.05A. There is no BEC load.
|DEAD_LOW_NS||DEAD_LOW_NS||COMP_PWM||Total drive current at minimum speed (A)||Comment||Notes|
- This drive draws about 10A at 7000RPM (full power), we might expect the component of current input delivered as propeller power at around 1500RPM to be around (1300/7000)^3*10=0.06A. The balance of the input current, 1.2A, seems excessive and hints insufficient dead time.
- The reduction in current when COMP_PWM is disabled hints insufficient dead time. (COMP_PWM increases switching events and so exacerbates losses due to insufficient dead time.)
Excessive drive current due to insufficient dead time causes heating of the FETs. In the example given at 300/300 with COMP_PWM, around 12W of additional heat is dissipated in the ESC with the motor barely spinning.
Some take home messages:
- COMP_PWM exacerbates the effect of insufficient dead time, if you intend using COMP_PWM then verify that you have sufficient dead time. Best COMP_PWM benefit will accrue from using no less, and no more dead time than required.
- Total drive current (without BEC load) at minimum speed should be very low, if it isn't or if the ESC shows signs of heating at minimum motor load, something is wrong and dead time should be reviewed.
- ESC compatibility is not just about pin compatibility, but also hardware dependent critical parameters which include dead time.
The most valid comparison of low speed current is one done at constant speed. That was not done in this case as it seems many if not most RC enthusiasts do not have means of reliably measuring rotating speed.