Partial throttle harder for ESC?

[befu]

I have seen it said here several times that running an ESC at partial throttle is very hard on it and might be the cause of some failed ESC's

Why would running a brushless motor at less than full throttle be harder on an ESC?

Doesn't make sense to me and I have not been able to find any technical explanation for this. I personally think it would be the oppsite, full load and rpm is the hardest part.

The only possible explanation I would be able to see is less water flow due to less speed, but this is a setup issue, not the esc issue.

Thank you in advance to anyone who knows the answer to this.

Brian

[Rumdog]

I would like an answer also. Seems it doesnt hurt the esc on brushless cars/trucks. I know the amp load is less, but it shouldnt matter. I can and do run partial throttle on my boats and never damaged an esc doing it.

[bwells]

Count me in also for looking for an answer!

[Make-a-Wake]

This is a direct quote form aquacraft in the UL-1 manual - "If you operate your boat a less than full power for the full duration of the run, you might likely experience an ESC failure"

http://manuals.hobbico.com/aqu/aqub20-manual.pdf

[befu]

"Having said this, if you operate your boat at “less than full power” for the full duration of the run, you might likely experience an ESC failure. The extra voltage the batteries deliver is being managed by the ESC and can cause the ESC to get hot enough to fail."

What extra voltage? A good battery should deliver fairly constant voltage. I know it will drop some with amperage, but I do not understand this. Was this written by the chinese?
Do they mean the wattage the battery will deliver, the ESC will overheat when operated for that long? That is a design problem with the esc/cooling system, not proof that the esc works harder at partial throttle than at high throttle.

Why do people say this, still doesn't make sense.

Gotta be an electrical design engineer somewhere on this board.

Brian

[Xfactor]

The mosfets absorb more voltage mid throttle therfore they get warmer but a properly designed esc's failure rate shouldnt increase as a result as long as the fets are properly cooled

[befu]

The mosfets absorb more voltage mid throttle therfore they get warmer but a properly designed esc's failure rate shouldnt increase as a result as long as the fets are properly cooled

But why? So you have three banks of FETs for each phase. Each bank turns on and off as it senses the back feed emf from the motor. With the motor spinning slower, the FET's have to turn on and off less, under less amperage. Or, is the amperage still high, but the FET's shut it off early due the "low throttle" signal from the Rc. Maybe instantaneous amperage is very high, but average amperage is lower?

So when you say the FETs are absorbing more voltage at mid throttle is the because they shut off the power limiting the draw, but the amount absorbed by them sutting it off is higher as compared to them shutting off as the amperage curve lessens naturally (back side of the semi sinusoidal <sp> shaped current curve) during a full throttle run?

Would take a scope to actually see it I guess and I do not have one.

Brian

[video200]

hopfully no one wil tell my helipoter to have esc failure when run at 70% trothle all the time.
dont think i ever ran the helicopter at full trothle ever.

[Raydee]

hopfully no one wil tell my helipoter to have esc failure when run at 70% trothle all the time.
dont think i ever ran the helicopter at full trothle ever.

You might want to drop a tooth in pinion then and try to get in the 90% range. I notice a big difference in temps when I run a lower throttle curve on my Logo 500.

[video200]

my trex clone runs nice and cool at around 70% but im not at the advanced 3D stage yet.
so far i can only crash like a master

[m4a1usr]

To quote the guy who really knows speedos,

To understand why low speed throttle is bad, you need to know a little about how ESCs work.

In general, with a constant load a motor's RPM changes as a result of the average voltage it sees. Since a battery is a (relatively) fixed voltage, we need some way to vary voltage seen by the motor. A microcontroller is a digital device - it can generate only "on" and "off" conditions. How does one take a fixed input (battery) and an on/off control and make an analog (variable) output?

The answer is by going on and off very quickly, varying the relative amount of on and off pulses. The longer you are "on" and the shorter "off", the more average voltage the motor sees. This is called "Pulse Width Modulation" or PWM. The on-time is called "duty cycle" and is repeated at a certain rate (times per second) called "frequency." The duty cycle is controlled by the transmitter, the frequency is controlled by the speed control.

In a brushed control, this goes directly into the two motor leads, with the coils and mass of the motor acting to average out the pulses into a more-or-less linear value.

In a brushless control, the same thing is going on, with an added twist. Not only are the pulses varying the voltage, but the voltage must be moved between the 3 different wires in order to simulate the plates of the commutator (comm) on the armature.

Now, why is low speed bad? The on/off switching is done with a device called a FET. A FET is a low-resistance switch when 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 the "linear mode." When in linear mode, a FET is like a big resistor - you know, those things inside your slot car handle that get HOT, or like the ceramic thing in your old mechanical speed control. The problem is, FETs don't like to be in linear mode - as 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 you have is a situation that can quickly degenerate into what we call "Thermal Runaway" - the heating of something causes it to heat itself even more.

So why don't ESC designers not run FETs in linear mode? Well, we try real hard not to, but it's a matter of physics - you can't completely eliminate it. If we make the linear mode time very short, it adds bigger capacitors to the board to provide the necessary energy, and people complain about size. They also complain about glitching their radios, as the speed at which the FETs are switched then generates radio frequencies. If we spend more time in linear mode, we can eliminate the glitching in the radio - but then we run into potential heat problems. Engineering always is a balance of competing problems and solutions.

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.

I'd be happy to go into more detail if you need it. Hopefully I didn't give you too much already.

Andy

Clip and paste this for future use or reference as I'm sure the question will once again arise.


John

[befu]

Well, how about that! Excellent answer. Thank you for posting that.

So it is a characteristic of how the FET's function. I knew someone on this site would know the technical reason.

Now, Wouldn't a faster spinning motor make it worse than a slower motor? I guess poles would come into it at that point.

I have not had any problems with ESC's at partial throttle, but I am also running lower amperage setups.

Thank you,

Brian

[Rumdog]

That is a great explanation. Thank you. Now can someone please explain why it doesn't effect my brushless buggy's esc? I can run a Castle/Neu 1515 with a HUGE pinion on 5s at any type of partial/full throttle I want with a 120 amp MMM Castle controller and have no problems with over 6 month of running. Does it have anything to do with less amp draw? It actually has to be pulling a lot of amps as the motor will get quite hot over a 15 minute or so runwith the big pinion. I could never see this esc going poof at partial throttle. Are they using higher quality FET's on the land based esc's?

[mappo]

To quote the guy who really knows speedos,





Clip and paste this for future use or reference as I'm sure the question will once again arise.


John

Could not of explained it better myself! in Lehman's terms, partial throttle will still give you the full amperage at very high switching rate.
At full throttle you will get out of a linear voltage without the constant switching between the field effect transistors "FETS". Higher switching at low throttle will produce high heat in the ESC which leads to greater failure. some manufactures like TEKIN are designing their Esc to run outside linear mode at 1/3 throttle, so that your speedo will not heat up as much. the Technology does exist to produce bullet proof systems, but like everything else, manufactures will argue that they will not be able to sell any parts...??

[ED66677]

I'm not en expert but I have discussed this with a friend of mine who is an electronic engineer, he said :
when you operate a BL ESC at partial throttle, the FET's are triggered in the voltage regulation range, meaning the frequency of the FET trigger is way higher than at full throttle, so unless the ESC is made of very high quality FET's, it might just not resist to these condition of use, understand that it is not the batt voltage or the amp draw that cause this type of faillure, it is just that when a motor spin at 30000rpm at full throttle, the ON/OFF triggering frequency of each FET bank is not as high as if you operate at 90% throttle, because the voltage that is applied to the motor coils do not equals the batt voltage unless full throttle, the FET's are triggered at 8, 16 or 32Khz to regulate the voltage, he added this high frequency trigger creates enormous heat!

[Raydee]

I think the reason why we get away with it on our planes and buggies is because the amp draw is way lower than it is on the boats. I have a plane that I use a Phoenix 35 amps ESC in that draws about 25 amps. When I baby the throttle the controller gets about 30* hotter than when I run it wide open the whole flight.

[ED66677]

Raydee airplane prop rpm is much lower than boat! way much lower! so FET's switching frequency is also way much lower!