Waveform of a Brushless Motor ESC.

[ReddyWatts]

This video shows how an ESC uses a PWM waveform to control motor speed. It simulates an AC sine wave using positive and negative DC pulses. Notice how it changes the DC pulse widths but keeps a constant peak voltage during the complete throttle range. The constant peak voltage remains at your battery pack voltage.



Why it is best to run at full throttle.
A speed controller controls power to the motor by turning full throttle current on and off really fast, 11 to 13 thousand times per second (Pulse Width Modulation or PWM). The percentage of each on/off pulse that is off compared to the part that is on determines how much power the motor sees. I.E. With a pulse that is 50% off and 50% on the motor will see 50% power*. Because each on pulse is 100% of full throttle current, a system set to pull 20 amps at full throttle through a Phoenix 10 will not last if you are throttled back to the point where you only see 10 amps on a wattmeter. The ESC in this case is still switching 20 amps, which it can’t do for long. Because an electric motor will always to try to pull as much power as is available to get to its rpm (volts times Kv), when you are running the motor below its Kv speed by switching power on an off, each on pulse will actually be way over the full throttle amp draw. That is why ESCs work harder at partial throttle than full throttle and why we underrate our ESCs. We underrate not so they can handle more current than their rating at full throttle, but so they can handle extended partial throttle operation with no problems. CC

I hope this helps to clarify what PWM means and why it is better to run it wide open, than half throttle. Less work, stress and heat on the FETS at WOT

[properchopper]

My amp goes to eleven........

[domwilson]

Reminds me of my days as a bench tech.

[steve-b]

My amp goes to eleven........

Is that one louder then ten?

[ReddyWatts]

The simple of it is: The motor will draw a lot more current for the slower speed - narrow pulses than the full speed - wider pulses. You will not see this on the ESC input side. This is for the ones that like to know "why". Hope it helped?

Proper - everyone knows you only operate at one speed - wide open. ha

Dom- same here. Worked in a tv repair shop while going to college.

[keithbradley]

a system set to pull 20 amps at full throttle through a Phoenix 10 will not last if you are throttled back to the point where you only see 10 amps on a wattmeter. The ESC in this case is still switching 20 amps, which it can’t do for long. Because an electric motor will always to try to pull as much power as is available to get to its rpm (volts times Kv), when you are running the motor below its Kv speed by switching power on an off, each on pulse will actually be way over the full throttle amp draw. That is why ESCs work harder at partial throttle than full throttle and why we underrate our ESCs. We underrate not so they can handle more current than their rating at full throttle, but so they can handle extended partial throttle operation with no problems.

I dont see how this accounts for load. If it takes 100A to turn my prop at 30,000RPMs, it doesnt take 100A to turn it at 15,000RPMs. I get what you're saying about the 20A draw throttled back on a 10A esc, except for the idea that it would be switching 20A. What if I were to throttle back to half throttle, but increase the load on the motor to a greater amount than full throttle...Are you saying that the esc would still be switching 20A? I kind of see what you're saying, but I dont understand how that number can always be 20A, regardless of load.

[keithbradley]

It is the sum of the current that you see on the input. The short bursts will use less over all than the longer bursts.

It is like when you spuirt a water hose with short bursts. The short bursts go farther/higher than when you hold it wide open. It is the sum being used that you see on the input.

What I am gathering is that the esc would be most efficient at full throttle and pulse more current relative to the input current than it would at full throttle. Being that the input current most likely will not be the same at part throttle due to lower speed/decreased load, it may not necessarily be "harder on the esc", right? I mean its definitly is harder on the esc relative to the load experienced at that given speed, but not necessarily overall.

Would you agree? Or am I missing something...

[ReddyWatts]

Yes, it is still drawing at least 20 and more peak amps at half throttle on the motor side of the esc. It is just in smaller bursts.

It is harder on the ESC at slower speeds because of the higher amp bursts on the fets. You just do not see it on the input side because you are looking at the sum of the current being used.

[bigwaveohs]

It almost sounds like the power input from the battery remains constant in this type of a switching system; so at full-throttle all the power gets dumped into the motor but at half-throttle, say roughly 50% of that power has to be absorbed by the ESC as heat. Does that make any sense?

[keithbradley]

It almost sounds like the power input from the battery remains constant in this type of a switching system; so at full-throttle all the power gets dumped into the motor but at half-throttle, say roughly 50% of that power has to be absorbed by the ESC as heat. Does that make any sense?

No, the power input from the batteries definitly varies. I don't agree that the esc somehow knows to pulse a certain amount of current regardless of load. Voltage maybe, but not current. In the given example, if the load were increased, the current would increase beyond 20A and on an unloaded or less loaded motor it would decrease.

[ReddyWatts]

An amp is measured by how many electrons pass a point on a wire in one second. The narrower pulse current that is used by the ESC output during half throttle will have a higher peak current but read less on the input ammeter because it is seeing less passing electrons for the same period of time as it would at full throttle using wider pulses.

[m4a1usr]

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

[ReddyWatts]

I was beginning to wonder if anyone was going to give a better explaination. thanks.

[785boats]

Thanks guys.
This discussion has cleared up a couple of issues that restricted my understanding if this concept. Now it all makes total sense to me.
Cheers.
Paul.

[domwilson]

I think this may also make it clear the difficulties in trying to parallel two ESC's to one motor.