Waveform of a Brushless Motor ESC.

[keithbradley]

There seems to be some confusion about PWM. What it means. Pulse Width Modulation is essentialy the on/ off timing the FET's are being switched at. The measurement of PWM is in frequency. Lower PWM means slower switching. That means longer on/off pulses. The longer average time duration the pulse is on, the more voltage the motor see's. Thats what drives motor rpm. Not current.

You have to understand how an FET works to grasp the task. A FET is a low-resistance switch when its fully turned on. When it's not fully turned on, it has a resistance which varies depending on exactly how turned-on it is. This is called "linear mode." When in linear mode, a FET is like a big resistor. The problem is, FETs don't like to be in linear mode - they warm up, they get more and more resistive. But the motor at low RPM is less and less resistive, meaning it will draw more current.

What ends up happening is things get worse. Heat in the FET's causes the resistance to increase and so more heat is added as time passes. Thats called thermal runaway.

What happens at low throttle is that the FET goes into the linear mode for a larger percentage of the on-time. That means the FET spends more time heating than it should - and hot FETs have more resistance, meaning next time they are in linear mode, they will heat up even hotter. At the same time, the motor is at lower RPM and therefore pulling more amps (a stalled motor will draw the maximum current).

You can get a lower RPM (ie, a lower average voltage) at maximum duty cycle only by lowering the input voltage. The speed control will run better with a lower input voltage even though it may be passing more amps simply because there will be less time for the FETs to be in linear mode. It is not an exaggeration to say that the FET may have 10X or more resistance when in linear mode. Heat generated is directly related to resistance.

Thank you Andy Kunz for very simply explaining this phenom to me so I can share this with fellow RC'ers.


John

That makes a little more sense. I dont get the part about a motor at lower RPM pulling more amps though...What do you mean by "a stalled motor will draw the maximum current". If the load is so great that it is stalled of course it will, but thats not the same as lowering the input voltage. I can see the motor having more resistance at lower RPM, but that would result in lower current draw, not higher.

**By the way, I find the contributors in this thread to be very intelligent people, and I definitely respect your opinion. I'm just trying to grasp some of this stuff that seems to be often presented with only part of the information needed to make an asumption. Alot of this stuff I have heard many times but honestly never fully grasped. I went to school for electronics so I have some knowledge, but I must have missed brushless esc day
Thanks for the info guys

[ReddyWatts]

Good point Dom, If you do not have anything syncing the output pulses from the two ESC's? One could put out a positive pulse at the time the other had a negative. This would be like a short circuit.