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04-23-2009 01:26 PM  9 years agoPost 1
Phrogger

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Gloucestershire, UK

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Hey guys,
I have a question about brushless speed controllers.
If a given installation calls for a 25amp speed controller, I have always believed that I am doing the right thing by going with a controller well above the stated amperage. I would tend to run a 30-35amp controller in such a situation.
However, a flying buddy suggested to me recently that this is not necessarily the case and that speed controllers actually work better when operated at their full capacity. He seemed to be saying that the controller is having to work harder to deliver say 50% of its rated capacity than it is to deliver say 90% of its rated capacity.
Can anyone advise if this true please?
If so, am I doing any of my equipment any harm by going for higher rated controllers or is it just a case of buying needlessly expensive gear?

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04-23-2009 02:01 PM  9 years agoPost 2
concept1

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Youngstown, OH

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this is true but not on the rating of the ESC amps, but throttle! you want to run the throttle above 80% to gain efficency. if you are flying at 50% then you are really burning a lot of voltage off and wasting energy and creating heat in the ESC, if you are running 100% they you are really using the esc as a on off switch and the esc is doing about nothing. you need to match gearing to headspeed and throttle curve. genarally above 85%. and then match the rated esc's amps to your application, generally higher is ok depending on how much weight it adds. putting a 35amp esc on a Trex 250 would be a over kill, but a 35 on a 450is not or even a 45. I beleive generally a larger esc can be more efficient then a smaller one although don't have any real proof other then what I see in heat on a 25amp esc to a 35 in a 450. I just beleive if a esc is hot it is losing or burning energy someplace, if it is cool you are ok and probably have nothing to gain! but gearing and throttle curve plays the biggest role

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04-24-2009 02:15 AM  9 years agoPost 3
Rob_T

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If you're using good quality ESCs you're just buying more ESC than you need. I don't trust all ESCs to keep working at their rated current, but well known brands like Castle and Kontronic will. (In fact I've run Castle ESCs well beyond their ratings with no problem in short bursts).

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04-24-2009 05:07 AM  9 years agoPost 4
DKNguyen

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THat's one comment I never understood. I'm going to have to pull out me EE card here and say that I've taken several courses on this and did a lot of research into how switching drives work and everything I've learned is contrary to this statement. An ESC does not "burn off" excess voltage as heat". This applies to linear ESCs, but not ESCs or switching BECs. Personally, I see no reason that running an ESC at 50% is any different than at 90%. There are losses incurred every time the ESC switches, so a lower switching frequency will produce less heat. If anything, it will run cooler at lower duty cycles (as long as the RPM is fast enough so the sensorless BEMF still works properly) because the conductive losses in the ESC are less because they are less current.

As best I can figure, this myth originated as a carry-over from the mechanical resistive speed controllers that used to be used in RC, and from linear BECs.

THat said, the motor has to be able to tolerate the battery voltage, because the ESC does not reduce the voltage seen by the ESC from the battery. It makes the motor see a different average voltage than the battery voltage, but it does this by applying the battery voltage to the motor for short periods of time, so the motor peak voltages are still equal to the battery voltage.

In addition, the ratio between peak and average currents is highest (the ESC is noisiest) as you run the ESC closer to 50% duty cycle (whether it is higher or lower). It is noisiest at 50%. When you run an ESC, what is happening is the ESC allows the current to ramp up through the motor inductance when voltage is applied, and then voltage is shut off so the current ramps down. The center-line that it ramps up and down around is the average current while the peaks are the peak currents. In this aspect, the losses due to peak currents will be increased when running at a lower duty cycle, but overall the average current is lower and this has a greater effect so the ESC should be cooler.

But it is better to run a lower kV motor and run the ESC at a higher duty cycle than to use a higher kV motor and run the ESC at a lower duty cycle since you are reducing peak currents and noise, and you are reducing mechanical wear.

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04-24-2009 05:34 AM  9 years agoPost 5
DKNguyen

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I just noticed you were asking not asking about duty cycle but about rated amp capacity. THe answer is still no, there is no problem with running an ESC that has been overrated for current. In fact, it would run better up to a point. There are two types of losses, those that occur when you switch the transistor and those that occur because the transistor is conducting through it's resistance. Optimally, you want the switching losses and conductive losses to be balance out.

All things being equal, high resistance transistors have more conductive losses but less switching losses because they switch faster. Lower resistance transistors are the opposite and tend to have lower conductive losses but higher switching losses because they switch slower. Very high resistance transistors will run noticeably cooler as the current is reduced, while lower resistance transistors will not have as much temperature change as the current is reduced because most of their heating is due to the switching.

So what will happen with an ESC that is seriously overrated for current is that it will run at a very constant temperature (with the majority of it's heating due to switching, not conducting). This temperature might be a noticeably higher than room temperature, but it won't change much as the current increases, but neither will it decrease very much as the current decreases. Higher minimum temperature, but lower maximum temperature.

Between you and your friend, I think he was talking about throttle (duty cycle) and you were talking about the amp rating of the ESC. So probably a miscommunication there.

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04-24-2009 07:04 AM  9 years agoPost 6
Rob_T

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Personally, I see no reason that running an ESC at 50% is any different than at 90%. There are losses incurred every time the ESC switches, so a lower switching frequency will produce less heat. If anything, it will run cooler at lower duty cycles (as long as the RPM is fast enough so the sensorless BEMF still works properly) because the conductive losses in the ESC are less because they are less current.
From personal observation I have to disagree. ESCs most certainly do run hotter at partial throttle. I once burned the heatshrink off an ESC while setting up my Eco-8, but the same ESC ran reasonably cool once I realized that my throttle curve was set too low - that was on a sensor equipped motor with a sensor ESC (Castle Dragon series - anyone still remember those?) so back EMF sensing was not an issue.

I also remember reading an article, probably more than 10 years ago, that did dynamometer testing of different ESCs with a motor at various throttle settings. As I remember it, the bench test showed reduced efficiency at partial throttle with some ESCs - but some ESCs did better than others, even though at full throttle all of the ESCs performed about the same.

I'll have to see if I can rummage through my old magazines and find the article.

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04-24-2009 08:19 AM  9 years agoPost 7
Phrogger

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Gloucestershire, UK

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Thanks for the responses guys.
Between you and your friend, I think he was talking about throttle (duty cycle) and you were talking about the amp rating of the ESC. So probably a miscommunication there.
I think you are correct with this comment. I mainly fly nitro models and I am not very clued up on the sparky stuff, so these posts are particularly useful.
It would appear that I have got my setup right by accident rather than design! I do run the throttle curve on my Mini-Titan over 80% for most of the time and by the above explanations, this is why the controller is always cool at the end of a flight.
Thanks again for the helpful responses.

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04-24-2009 08:21 AM  9 years agoPost 8
DKNguyen

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DO you have any idea why this might be? It just seems to go against my understanding of motor drives...unless those peak currents and their I^2R losses are really making a big difference. They might, I haven't sat down and done careful math on it.

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04-24-2009 08:39 AM  9 years agoPost 9
DKNguyen

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An ESC running at a lower duty cycle will be less efficient, yes because of higher peak currents relative to the average current. So yes, the partial throttle efficiency tests you were talking about make sense.

At this point, I want to point out efficiency and heating are not the same thing. (ie. As output power increases, efficiency might increase slower or faster. If efficiency increases slower as power increase, then an ESC running at low power, low efficiency is cooler than the same ESC running at high power, high efficiency because the actual watts dissipated as heat is higher.)

BUt, right now, the heating does not. Just to double check you are saying that an ESC running at 90% throttle putting out 1500W is HOTTER than the same ESC running at 60% throttle putting out 1000W? Even though the power output is less? Is it less? Or much less? Any idea how large the differene is between power outputs when you see the temperature increase?

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04-24-2009 11:03 PM  9 years agoPost 10
Pistol Pete

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Seffner, FL

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FWIW...some good info here page 4.

obtained from here issue 1

~~Enjoying the hobby one flight at a time~~

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04-25-2009 01:23 AM  9 years agoPost 11
DKNguyen

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Okay, I think I know what's going on now. I wrongly assumed that the inductance in the motor was the same size as the inductor in a switching BEC (relative to the voltage and switching frequency of the system). So the current ramping up/down during the on/off times of the ESC are faster (instanteous at worst case between +V and 0V). So a lot more of the actual current conduction is in the form of the peak current rather than the average current. If the current is assumed to ramp down to 0V instantly during the off-time then all current conduction is in the form of peak currents and the average current has no meaningful basis for calculating loss.

Which would mean that the slower you run the motor by decreasing partial throttle, the closer you get to the scenario that you are actually running the motor at full throttle stall (even though the motor shaft is spinning and not stalled.)

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04-26-2009 10:12 PM  9 years agoPost 12
MutantGarage

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Texas, USA

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I think that article is misleading, that implies that if you ran at 10% throttle that you will get the same run time and load on the battery, ESC, and motor as if you ran 100%?

"you are taxing
your batteries, ESC, and motor at full
throttle amperage the entire time
the system is running, regardless of
throttle level."

The motor certainly does not see full current, it's an inductive load, it cant change current instantly. The ESC has input capacitors to store energy from the battery due to the inductance of the leads and battery itself. This filters the current spikes the battery sees from the ESC.

They also do not make a distinction between brushless multi-phase motors and brushed DC motors, they have different PWM switching characteristics.

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