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Old Jul 28, 2012, 09:40 AM
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ESC Throttle Position vs Heat: Is This Guy Right?

Quote:
What you describe above is exactly the opposite of what all of us who run 3s on the stock electronics in the Beast 3D & MiG have seen. Higher throttle settings always make the ESC run hotter than lower throttle settings. Also - you get far more flight-time at part-throttle than at full-throttle. Every PWM ESC I own runs cooler at lower throttle settings than at higher throttle settings.
http://www.rcgroups.com/forums/showp...postcount=3415

My understanding: http://www.castlecreations.com/suppo...eral.html#gen6
Quote:

Quote:
6. Can I control how much current passes through my speed controller by limiting full throttle travel on my transmitter with endpoint adjustments/trim? In other words can I use a 25 amp speed controller with a motor that will pull 45 amps but lower my top throttle endpoint on my transmitter so my watt meter only shows 25 amps at full throttle? Will this be OK?
NO!!! 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. Actually it is worse than the simple example above. 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.

* Actually, electric power is not linear as in this example, but you get the idea.
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Old Jul 28, 2012, 10:06 AM
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Another good article that confirms partial throttle ops generates more heat in the FETa. http://www.rcuniverse.com/magazine/a...rticle_id=1344
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Old Jul 28, 2012, 10:27 AM
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Originally Posted by sirzeppu View Post
Another good article that confirms partial throttle ops generates more heat in the FETa. http://www.rcuniverse.com/magazine/a...rticle_id=1344
Yep, as I thought:

Quote:
One of the paradoxes of brushless motor controllers is that partial throttle operation actually generates more ESC heat than full throttle operation.
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Old Jul 28, 2012, 05:22 PM
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I've seen it mentioned a number of times that the Castle info is wrong.

ESC operation theory isn't my strong suit, but I think there's more to the story. For example, why should the motor's current draw remain the same regardless of throttle position?

I've read a few discussions on this topic and I never seem to come away with any better understanding.
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Old Jul 28, 2012, 05:38 PM
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Originally Posted by Gerry__ View Post
Yep, as I thought:
Quote:
Originally Posted by C₄H₁₀ View Post
I've seen it mentioned a number of times that the Castle info is wrong.

ESC operation theory isn't my strong suit, but I think there's more to the story. For example, why should the motor's current draw remain the same regardless of throttle position?

I've read a few discussions on this topic and I never seem to come away with any better understanding.
Because the ESC is switching on and off.
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Old Jul 28, 2012, 06:18 PM
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Gerry, the issue is that the folks saying Castle is wrong are well-respected names around here. They're the people who really do know this stuff.

I won't mention names at this point; maybe some of them will chime in here.

As far as the ESC switching on and off, that doesn't really explain it well enough for me. If I have a blank piece of wire plugged into my ESC, then applying full voltage switching at 50% duty (e.g. half throttle) and measuring the average current will show results that fit with Ohm's law. This is obviously assuming that the wire's resistance is enough to make heating negligible, current is safe for the ESC et cetera ad infinitum.

A brushless motor works differently, though. If our motor draws 20A on 10V at full throttle (100% duty at the FETs), that's 200W input. At half throttle (50% duty) if it's still drawing 20A at those 10V during each on-cycle, then it's taking in a total of 200/2 = 100W average. This can't be right because the prop is demanding so much less power at the lower RPMs.
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Old Jul 28, 2012, 08:11 PM
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Perhaps both views are right.

Partial power generates more switching load and thus more heat.

Full power generates more power and thus, also more heat.

Perhaps the partial power issue is more about decreased airflow over the FETs and that allows more heat to build.

I am just shooting in the dark here.
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Old Jul 28, 2012, 08:58 PM
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The ESC can make more heat at at some partial thlrottle percentages because while switching, the FETs are in linear mode and are dissipating power for a greater portion of the duty cycle..

At full throttle they are always on (except for commutation).

This is why the battery in your watch can last 2 years and a Pentium computer cannot.

In a watch, because of the slow clock speed, the switching time is only a very small portion of the duty cycle.

The text below is from another thread in which a similar question was asked.

At half throttle (50% duty cycle) the peak current is estimated to be 1/4 max current.

This is only an approximation.. In real life, from conservation of energy, the RMS value of current at 50% therottle would be 1/4 max, but because of the inductance of the armature, the current likely rises from zero to some larger value, resulting in a sawtooth current waveform.

The RMS value for a sawtooth waveform is1/ (root 3) = 1/ (1.73) times peak current and for this reason at 50% throttle the peak current would likely be close to 1/4 X 1.73 = 0.43 max current.

Average voltage and currents cannot be used to accurately calculate power using Ohm's law. RMS values must be used.

Here is a link to some measurements.

http://www.peakeff.com/beta/PostDetail.aspx?PostID=9

================================================== =========

The important difference here is that we are talking about what happens with a motor and a propeller load, not a resistive load.

For a propeller load, Power is proportional to RPM cubed. Power ~ RPM^3

RPM is proportional to average voltage or duty cycle.

At 50 % duty cycle, RPM is 1/2 max.

If RPM is 1/2 max, then for a propeller load, power is 1/8 max.

Because the ESC is on for only 1/2 the time, it means that 1/4 max power must be drawn from the battery during this period of time so that over the total time 1/8 max power is drawn from the battery.

That means that at 1/2 throttle, the peak current must be 1/4 max, not 29A or what Castle Creations states on their website. This discussion has been going on for years and I am surprised that Castle Creations has still not corrected their website.

This information is only true for propeller loads and high PWM rates and or high armature inductance.

At low PWM rates and low armature inductance, the motor RPM is not proportional to average applied voltage (duty cycle) but is proportional to the RMS applied voltage.

For this case where the armature inductance is not storing energy which can be distributed during the off time, the peak currents will be higher.
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Old Jul 28, 2012, 11:15 PM
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Quote:
Originally Posted by Martyn McKinney View Post
The ESC can make more heat at at some partial thlrottle percentages because while switching, the FETs are in linear mode and are dissipating power for a greater portion of the duty cycle..

At full throttle they are always on (except for commutation).

This is why the battery in your watch can last 2 years and a Pentium computer cannot.

In a watch, because of the slow clock speed, the switching time is only a very small portion of the duty cycle.

The text below is from another thread in which a similar question was asked.

At half throttle (50% duty cycle) the peak current is estimated to be 1/4 max current.

This is only an approximation.. In real life, from conservation of energy, the RMS value of current at 50% therottle would be 1/4 max, but because of the inductance of the armature, the current likely rises from zero to some larger value, resulting in a sawtooth current waveform.

The RMS value for a sawtooth waveform is1/ (root 3) = 1/ (1.73) times peak current and for this reason at 50% throttle the peak current would likely be close to 1/4 X 1.73 = 0.43 max current.

Average voltage and currents cannot be used to accurately calculate power using Ohm's law. RMS values must be used.

Here is a link to some measurements.

http://www.peakeff.com/beta/PostDetail.aspx?PostID=9

================================================== =========

The important difference here is that we are talking about what happens with a motor and a propeller load, not a resistive load.

For a propeller load, Power is proportional to RPM cubed. Power ~ RPM^3

RPM is proportional to average voltage or duty cycle.

At 50 % duty cycle, RPM is 1/2 max.

If RPM is 1/2 max, then for a propeller load, power is 1/8 max.

Because the ESC is on for only 1/2 the time, it means that 1/4 max power must be drawn from the battery during this period of time so that over the total time 1/8 max power is drawn from the battery.

That means that at 1/2 throttle, the peak current must be 1/4 max, not 29A or what Castle Creations states on their website. This discussion has been going on for years and I am surprised that Castle Creations has still not corrected their website.

This information is only true for propeller loads and high PWM rates and or high armature inductance.

At low PWM rates and low armature inductance, the motor RPM is not proportional to average applied voltage (duty cycle) but is proportional to the RMS applied voltage.

For this case where the armature inductance is not storing energy which can be distributed during the off time, the peak currents will be higher.
Thankyou for that excellent explanation. I love this stuff. It is quite fascinating...just wish I understood it better.
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Old Jul 28, 2012, 11:20 PM
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Quote:
Originally Posted by Martyn McKinney View Post
The RMS value for a sawtooth waveform is1/ (root 3) = 1/ (1.73) times peak current and for this reason at 50% throttle the peak current would likely be close to 1/4 X 1.73 = 0.43 max current.
To illustrate Martyn's point, here are some photos of the current flow of an AXI 2212-34 as displayed on an oscilloscope. The first one is at full throttle and the second one is at half throttle. The half throttle peak current is a little higher than he calculated, but that is to be expected. This is because motor losses will cause half throttle RPM to be a little higher than half of full throttle RPM.

Larry
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Old Jul 28, 2012, 11:32 PM
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In terms of heat dissipation and efficiency, an ESC is most efficient and generates the least heat when run at full throttle. At partial throttle the PWM switching losses kick in which generates more heat which in turn lowers efficiency. But, as Martyn and I have shown, as the throttle is decreased the ESC current decreases correspondingly. What this means is that as the current decreases with decreased throttle, switching losses also decrease with decreased throttle. This results in the fact that the amount of heat generated at half throttle will be less than the amount of heat generated at 80% throttle, but it will still be more than the amount of heat generated at full throttle. As such, the very worst environment for an ESC is when it is run for long periods of time at 70% to 80% throttle.

Larry
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Old Jul 29, 2012, 01:03 AM
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Larry, Martyn, thanks very much for the comprehensive explainations.
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Old Jul 29, 2012, 11:38 AM
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An ESC does not necessarily have to be used to control a motor and propeller.

It can also be used to control a resistive load.

I thought that it might be interesting to determine what the theoretical peak current would be at partial throttle with different types of loads.

Early speed controllers that used the transmitter frame rate for the PWM frequency (50Hz) behaved like number 2.

1. Motor-Propeller 50% Throttle
High PWM Frequency
High Armature Inductance
RPM = 1/2 max
Propeller Absorbed Power = 0.125 Max Power

2. Motor-Propeller 50% Throttle
Low PWM Frequency
Low Armature Inductance
RPM = 0.707 max
Propeller Absorbed Power = 0.35 Max Power


3. Resistive Load 50% Throttle
Power Dissipated in Resistive Load = 50% Max Power

The description that Castle Creations gives on how an ESC performs at partial throttle settings applies only to a resistive load.
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Old Jul 29, 2012, 06:11 PM
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Quote:
Originally Posted by Martyn McKinney View Post
The description that Castle Creations gives on how an ESC performs at partial throttle settings applies only to a resistive load.
True, a motor is not a resistive load. It's worse.

The winding resistance is very low, and if this was the only factor then the motor would draw extremely high current. For example, a 50g 1000Kv outrunner might have a winding resistance of 0.25 Ohms, so on 11V it would (if acting like a pure resistor) draw 11V/0.25 Ohms = 44A, much higher than its maximum rated current!

Luckily however, when the motor is spinning it also generates a voltage which subtracts from the input voltage. With no load it can generate enough voltage to almost completely cancel out the input voltage, so the current draw is very low. When a load is applied the current increases, the voltage difference becomes greater, and the motor slows down. Our example motor might draw 8A on 11V, implying that the voltage drop across the winding resistance is about 8A*0.25 Ohms = 2V. 11V-2V = 9V, so it should be spinning at 1000Kv*9V = 9000rpm.

But what happens at part throttle? The ESC switches power on and off rapidly (PWM) to reduce the average voltage and make the motor run at a lower speed, so the generated voltage must also be proportionally less. But the supply voltage is still the same, so the (peak) voltage difference will be greater, and the motor will be drawing much higher (peak) current than at full throttle! For example, at 50% throttle our motor might be spinning at 4500rpm and generating 4.5V. Therefore the (peak) voltage across its winding resistance is 11V-4.5V = 6.5V, and the (peak) current is 6.5V/0.25 Ohms = 26 Amps!

Of course in practice the motor windings also have inductance, which slows down the rise and fall of current (creating a triangular current waveform). If the PWM switching frequency is high enough then the peak current will be limited to an acceptable value. But if either PWM frequency or motor inductance are too low then the peak current will be higher than at full throttle! This is in contrast to applying PWM to a purely resistive load, which always draws the same (peak) current, no matter what the throttle level.
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Old Jul 29, 2012, 06:28 PM
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Enough theory. Here are some real-world test results:-

ESC: Castle Creations Phoenix 10
Motor: AX-2208C-1080Kv
Prop: GWS EP9050
Battery: Bench Power Supply

ESC was attached to the back of my thrust stand, shielded from propwash. Temperature readings were taken with an Extech Infrared thermometer.

Full throttle for 1 minute:-
Wattmeter readings: 10.6V, 8.1A, 7620rpm
Start ESC temperature: 29C
End ESC temperature: 45C

75% throttle for 1 minute:-
Wattmeter readings: 10.6V, 4.0A, 5895rpm
Start ESC temperature: 29C
End ESC temperature: 75C
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