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Guys, the SW X ESC’s Max out at 255, because they are using 8-bit A/D converter. So the 220A ESC will only indicate 255 max. I use the 200A, 8s ESC but I have about a 75% headroom to the ESC’s max current capability. I try and take advantage of HV, lower current setups...
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I think something is wrong looking at the datalog. That motor should rev more than 35000rpm with the 645. I think I saw another thread last week with a similar boat that passed 35000rpm, maybe 36000 rpm early in the run, in the first minute then his rpm went down hill after that.
Something is wrong with this set to only rev that low.
Maybe the long runtime cause the heat to buildup and breakdown the solder causing poor supply of power to the motor.
Resulting in the low rpm you are getting. I wouldn't run for 5 minutes with this setup.
If your connectors are hot your solder has broken down.Comment
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Imagine that you hit 62mph with the 652 prop and you not getting your full power. It would be interesting to see how fast you get with good cells and better connection.Comment
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I did get my 8s set up up and running, thanks for the help.
tp4060 1750kv
300 sf
648 cupped
8s series
4 minute run
74mph
It has more top end but, still learning how to set it up for speeds over 69mph. It does pretty good but, at 70 and beyond you know its on the edge of take off and crash.
The temps are 100 to 125 across set up.
amps draw at 100% hits 250 to 265 on this prop in this boat
I need two 8s lipos next season to run them parallelComment
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Andy explained it best back in 2010. It's what first alerted me to it.
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.
And here is a real life test from Bruce over on RC Groups..
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: 29ºC
End ESC temperature: 45ºC
75% throttle for 1 minute:-
Wattmeter readings: 10.6V, 4.0A, 5895rpm
Start ESC temperature: 29ºC
End ESC temperature: 75ºC
The Search button will find you many articles & threads on the subject.
I will add though, that the closer the voltage & amps are to the limits of your ESC at full throttle, the more pronounce the heating effects of running part throttle are.Last edited by 785boats; 09-23-2018, 02:37 PM.See the danger. THEN DO IT ANYWAY!!!
http://forums.offshoreelectrics.com/...hp?albumid=319
http://forums.offshoreelectrics.com/...hp?albumid=320Comment
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Great info.
I don't understand all of it but, in my set up am I better off lowering voltage to 7s or buying a lower KV motor?Comment
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This is a conservative 90 seconds of data but, it shows that there is so much power in the lower rpm that I don't have to give it full throttle to do what I want .
I want to run in 60s but have the ability to lay down some 70 plus passes.
Im there now, may have too much on the topend for this hull anyway.
sept23.jpg
sept23current.jpg
sept23rpm.jpg
sept23temp.jpgLast edited by Crash; 09-23-2018, 04:03 PM.Comment
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Let me ask this if I may...if partial throttle is bad and makes esc's run hot, how come this isn't the case with aircraft? Are the esc's made that differently? I only have the throttle on my jets smashed to the wall. The other planes are only at full throttle for take off, then backed down to 3/4 to 1/3 for flying...then a lot less for final approach and landing. Never had an esc fail yet.Comment
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8S in a Proboat Voracity ??
And you are not afraid ?Comment
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The esc's in all my aircraft are buried in the fuselage...some air flow around them, but not a lot of cooling action at all.Comment
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