Over Propping - It Works For Me

[Martin Harris - Moderator]

At the end of the day, the real limitation is motor temperature.  The guidance given to users is an attempt to simplify the process of selecting a motor - you should have an idea of the power required and they give you simple guidelines to select your motor, prop and cell count. 
 

A clubmate regularly operates motors, batteries and ESCs at multiples of their ratings - but limits motor runs to no more than 2 seconds (with one notable exception!). The key here is heating time…

 

To max out a motor to its maximum potential would require a safe sustained operating temperature figure and working with suitably rated power input components to achieve this by propeller selection.  Mind you, you’d also need to take ambient temperature into account!

[Andy Gates]

5 hours ago, PatMc said:

That would mean that the power of all motors are being restricted to 2/3 of there potential. 

In fact there is no reason that more than 3 cells could be used so long as the max current is not exceeded & a practical size prop can be used.

Sorry Pat but you need to be careful about statements like the one above.

 

You seem to have forgotten about the BEC which also has its limits as I have experienced. I have used a 4S ESC in a plane but the BEC on board had a lower current limit at 4S than it did on 3S and I missed it causing the loss of control resulting in  a rekitting of the model.

[PatMc]

Andy, I was specifically dealing with the power limiting factors of the motor.

You're right that the BEC current limit is also a factor varying with the number of cells if it's a linear device, it also dependant on the type/current draw & number of the servos. However surely it's the users responsibility to check this from the ESC's instructions.

[RottenRow]

10 hours ago, PatMc said:

The true power readings can only be taken at max throttle, if you have restricted the throttle to when max rated current is reached the power reading is not true, it's too high.

 

I don't doubt your wattmeter's accuracy in the manner it's designed to be used. However none of our wattmeters are able to measure the power input to the motor except at full throttle.

This is because they measure the voltage at the ingoing battery side of the ESC which is not the RMS voltage that's being supplied to the motor via the 3 wire outgoing side except at full throttle. The current measurement is accurate as it's common at both sides. The wattmeter derives the wattage by multipling the voltage & current readings it detects.

 

This isn't correct I'm afraid.

 

The wattmeter, connected in the battery to ESC leads, actually measures the motor power (watts) plus the power losses in the ESC (that cause it to heat up). This is true at any throttle setting. As the losses in the ESC are comparatively small, this means that the wattmeter will indicate close to the motor power or wattage. (In practice there will also be a small amount of power being fed by the ESC BEC to the receiver and servos, which the wattmeter will also include in its reading).

 

Whilst it is true that the wattmeter is measuring the 'ingoing' battery voltage, it is also measuring the 'ingoing' current to the ESC. This means the wattmeter will read correctly.

 

The actual current in the motor windings will be greater than the DC current flowing from the battery, especially at part throttle settings. As we can't measure the actual motor voltage or current (it's three phase for a start, and constantly varying), the best that we can do is to measure the input voltage and current.

 

The arrangement can be considered to be an electronic transformer....ie  power in = power out.

 

For an older brushed motor and controller, it would be true that the current is the same on both sides (of the ESC), but not for a brushless motor and controller.

 

Brian.

[gangster]

I bet James Watt spins in his grave ever time he hears the word wattage. No such scientific definition 

[Dickw]

12 hours ago, RottenRow said:

..........................................

The actual current in the motor windings will be greater than the DC current flowing from the battery, especially at part throttle settings. As we can't measure the actual motor voltage or current (it's three phase for a start, and constantly varying), the best that we can do is to measure the input voltage and current.

..............................................

Just to clarify the above, ours are not actually three phase motors but are DC motors with the commutation carried out electronically by the ESC instead of by brushgear. Only one motor coil (pair of wires) is live at any one time so he current going into the ESC (less any BEC current) is the current circulating through the winding. I agree that the input current is constantly changing as the motor winding rotates through the magnetic field, so I assume (always dangerous) that modelling type ammeters and wattmeters are designed to deal with that and give a reasonable rms value.

 

Dick

[gangster]

23 minutes ago, Dickw said:

Just to clarify the above, ours are not actually three phase motors but are DC motors with the commutation carried out electronically by the ESC instead of by brushgear. Only one motor coil (pair of wires) is live at any one time so he current going into the ESC (less any BEC current) is the current circulating through the winding. I agree that the input current is constantly changing as the motor winding rotates through the magnetic field, so I assume (always dangerous) that modelling type ammeters and wattmeters are designed to deal with that and give a reasonable rms value.

 

Dick

Yes we are trying to simplify what is in fact a very complex set of circumstances. I guess we are verging on AC theory here a motor is not a pure resistive load. We are switching into an inductive load, back emf and all that. Of course the power and current consumption on the ground will be different to that when the motor unloads in the air. We are not looking for precise measurements, the meters we buy for this purpose are probably accurate enough for what we need. Just give yourself some headroom, ie choose an esc and motor that is rated 10 or 20 % above what you need. Also bear in mind most of these are cheap items and this max figures might be a bit tught

[MattyB]

On 15/09/2023 at 12:11, gangster said:

This issue has gone round and round with regards calibrating the esc to 100% and then limit the max throttle throw to say 75% to reduce max speed. Is there someone out there who can give a definitive answer and not just a guess.

one school of thought is that it is bad for the esc, Why would it be so, an esc surely works by switching not voltage drop.

The second school of thought, which is the one I subscribe to, is that this is perfectly fine as it is no different to flying with the stick 3/4 up.

Please only answer this if you really know and can supply a technical reason rather than repeating an old wives tale as we so often see on social media

 

There are two parts to this answer...

• Does running a brushless ESC for extended periods at part throttle cause the FETs to do more switching and generate more heat? The fact that ESCs run hotter and are less efficient at part throttle is well understood, and easily tested on the bench should you wish to do so (I did just that about 10 years ago when I was getting into larger, higher power electrics). If you don't believe me, fair enough, but the likes of Castle Creations (who've made an ESC or two - see below) and Steve Neu (of Neu Motors fame) have posted extensively about this over the years - additional links here.

 

 

• Is this likely to cause the ESC to fail prematurely? This is much harder to answer, as the way the ESC is run is only a single variable amongst a sea of others (peak and average sustained current, installation, ventilation, time between flights, etc.) that will affect it. I suspect the chances of ESC failure due to sustained mid throttle running are pretty minimal these days, but the probability is not zero. Because of that I will continue to optimise my powertrains without touching the throttle endpoint; it isn't hard to do, and there isn't any downside (it just requires some basic modelling in eCalc before you buy your components). FWIW, I suspect the most likely model type for this sort of failure is something like a Junior 60 - that will pootle around on low throttle ratings for long periods on sunny low wind days, entombed in a poorly ventilated glasshouse of a cockpit!

Edited September 18, 2023 by MattyB

[RottenRow]

In other words... don't reduce your full throttle end point to (say) 75% in an attempt to reduce the current, but fit a smaller prop. (smaller diameter, pitch, or both) to actually reduce the current.

 

Brian.

[Futura57]

Yes RottenRow, I conceded this a while back. That's what I've done. Very interesting MattyB. I understand the role PWM plays in controlling the motor as you explain it. I'm wondering if there is some other way ESCs could operate, maybe at the low and mid speed range, which reduced the voltage presented to the motor (poles) thereby reducing it's RPM and thus current draw. I guess I'm thinking more a hybrid analog / digital algorithm. I'm happy to be told I'm talking rubbish.

[MattyB]

1 hour ago, Futura57 said:

Very interesting MattyB. I understand the role PWM plays in controlling the motor as you explain it. I'm wondering if there is some other way ESCs could operate, maybe at the low and mid speed range, which reduced the voltage presented to the motor (poles) thereby reducing it's RPM and thus current draw. I guess I'm thinking more a hybrid analog / digital algorithm. I'm happy to be told I'm talking rubbish.

 

Short answer - I've no idea, and won't pretend to have! I posted above because gangster had indicated he was sceptical that the effect of running an ESC at part throttle could cause issues, and I had some reasonably well qualified sources that indicate the heating effect (and the resultant loss of efficiency) absolutely is real. I have no further personal knowledge beyond that, as my knowledge of how ESCs actually work is minimal.

 

If there are ESCs that operate in a different way and don't have these issues I am not aware of them, but that doesn't mean they don't exist. However, in the last 10-15 years I if there had been a revolution in ESC technology it would have been well publicised in the modelling (and wider) press given it's applicability in all sorts of industrial and commercial applications. Has that happened? I can't personally remember it, but again, that doesn't mean it didn't occur.

[gangster]

Surely the very full explanation quoted from Castle creations answers a totally different question. It confirms that you cannot use an 18amp esc on a 35amp motor even if the max DC current does not exceed 18A. I don’t believe that was ever the question at any point in this thread . If reducing the end point below 100% is likely to stress the esc then we must never fly for more than a very short period with the throttle stick anywhere other than fully up

[Dickw]

36 minutes ago, gangster said:

Surely the very full explanation quoted from Castle creations answers a totally different question. It confirms that you cannot use an 18amp esc on a 35amp motor even if the max DC current does not exceed 18A. I don’t believe that was ever the question at any point in this thread . If reducing the end point below 100% is likely to stress the esc then we must never fly for more than a very short period with the throttle stick anywhere other than fully up

I believe the explanation from Castle Creations was quoted simply to explain why part throttle can be harder on an ESC than full throttle - a point which some have queried.

OK the example given doesn't fit exactly in with this thread, but the logic and the physics are still applicable and surely of some interest.

Dick

[MattyB]

49 minutes ago, gangster said:

Surely the very full explanation quoted from Castle creations answers a totally different question. It confirms that you cannot use an 18amp esc on a 35amp motor even if the max DC current does not exceed 18A. I don’t believe that was ever the question at any point in this thread . If reducing the end point below 100% is likely to stress the esc then we must never fly for more than a very short period with the throttle stick anywhere other than fully up

Hmmm, ok… I have gone back to the OP’s post and re-read it, and it’s true the thrust of that was more about any negative effects in the motor rather than the ESC. I do agree that, from the perspective of the motor, there should be no issue with running it with a reduced throttle endpoint to keep the peak wattage and current below the motors maximum limits. Apologies if I caused any confusion there. 
 

However, if you do that the ESC will generate more heat and be less efficient than if the motor were propped to allow its max power to be generated at the 100% throttle setting. Whether that could cause it to fail in the long run though is more complex - see my previous post on that point. Since no-one is going to have any conclusive data to say how likely that is, I just choose to avoid doing it in  case it could cause a failure, no matter how unlikely that may be. YMMV. 

[Futura57]

Hmmm, so are ESCs any better than the good old rheostatic resistor which controlled current to a device by dissipating (wasting) unwanted supply power as heat 🤔

[Philip Lewis 3]

1 hour ago, MattyB said:

However, if you do that the ESC will generate more heat and be less efficient than if the motor were propped to allow its max power to be generated at the 100% throttle setting. Whether that could cause it to fail in the long run though is more complex - see my previous post on that point. Since no-one is going to have any conclusive data to say how likely that is, I just choose to avoid doing it in  case it could cause a failure, no matter how unlikely that may be. YMMV. 

 

But going back to my previous post even if you propped the motor to allow it's max power to be generated at 100% throttle how much of an average flight would be flown using 100% throttle (or full power) anyway?

 

Surely even in a properly set up system as you desribe most of the flight is going to be flown at part throttle anyway?

Edited September 18, 2023 by Philip Lewis 3
Spelling

[Dickw]

22 minutes ago, Futura57 said:

Hmmm, so are ESCs any better than the good old rheostatic resistor which controlled current to a device by dissipating (wasting) unwanted supply power as heat 🤔

Yes.

[gangster]

9 hours ago, Philip Lewis 3 said:

 

But going back to my previous post even if you propped the motor to allow it's max power to be generated at 100% throttle how much of an average flight would be flown using 100% throttle (or full power) anyway?

 

Surely even in a properly set up system as you desribe most of the flight is going to be flown at part throttle anyway?

My point entirely sir

[gangster]

9 hours ago, Futura57 said:

Hmmm, so are ESCs any better than the good old rheostatic resistor which controlled current to a device by dissipating (wasting) unwanted supply power as heat 🤔

Yes much more efficient. The esc might get hot while it is doing it’s job but it’s not doing it simply by “wasting power”Not sure we could run the size of motors we run these days with the old rheostats

[MattyB]

10 hours ago, Philip Lewis 3 said:

 

But going back to my previous post even if you propped the motor to allow it's max power to be generated at 100% throttle how much of an average flight would be flown using 100% throttle (or full power) anyway?

 

Surely even in a properly set up system as you desribe most of the flight is going to be flown at part throttle anyway?

 

Yes, the mean average power will undoubtedly be less than the endpoint limited “100%” throttle, but the median throttle rating in the average sport model probably is still full power. Perhaps that is a factor? I’m sorry, I am at the end of my personal knowledge here - for more on this you’d need to ask an esc designer or manufacturer.
 

In posting I was not trying to categorically prove premature ESC death by part throttle running (I don’t think that is possible). I was merely pointing out the well understood factors that mean an esc is working harder and generating more heat at part throttle, leading to it getting less efficient as a result. As I don’t have any more insights to share that I could back up with data I’ll quietly leave at this point!

 

Edited September 19, 2023 by MattyB

[Futura57]

10 hours ago, Futura57 said:

Hmmm, so are ESCs any better than the good old rheostatic resistor which controlled current to a device by dissipating (wasting) unwanted supply power as heat 🤔

I was being somewhat facetious here 😉 not intending to impugn any opinions. Jeez, I sound like I swallowed a dictionary 😂 and it's 9:30am and I just woke up!

[PatMc]

On 16/09/2023 at 23:31, RottenRow said:

 

This isn't correct I'm afraid.

 

The wattmeter, connected in the battery to ESC leads, actually measures the motor power (watts) plus the power losses in the ESC (that cause it to heat up). This is true at any throttle setting. As the losses in the ESC are comparatively small, this means that the wattmeter will indicate close to the motor power or wattage. (In practice there will also be a small amount of power being fed by the ESC BEC to the receiver and servos, which the wattmeter will also include in its reading).

 

Whilst it is true that the wattmeter is measuring the 'ingoing' battery voltage, it is also measuring the 'ingoing' current to the ESC. This means the wattmeter will read correctly.

 

The actual current in the motor windings will be greater than the DC current flowing from the battery, especially at part throttle settings. As we can't measure the actual motor voltage or current (it's three phase for a start, and constantly varying), the best that we can do is to measure the input voltage and current.

 

The arrangement can be considered to be an electronic transformer....ie  power in = power out.

 

For an older brushed motor and controller, it would be true that the current is the same on both sides (of the ESC), but not for a brushless motor and controller.

 

Brian.

Happily my explanation was spot on 👍 

In fact I did some practical testing with a model yesterday using a wattmeter & tachometer with results that demonstrate what I've explained. I haven't had time to post the results but should manage later tonight or tomorrow.

 

BTW you are completely wrong regarding the current & voltage levels at different points in the circuit. In any series circuit the current is constant throughout but the voltage varies at different points.

 

[RottenRow]

19 minutes ago, PatMc said:

BTW you are completely wrong regarding the current & voltage levels at different points in the circuit. In any series circuit the current is constant throughout but the voltage varies at different points.

I’m sorry but I’m not wrong. I do know all about the current and voltage levels in a series circuit… Kirchoff’s laws… but a brushless motor / ESC / battery isn’t a series circuit. Certainly not a simple DC circuit to which your ‘rule’ applies.

 

Neither am I wrong about the correctness of the readings given by a wattmeter connected in the DC side of the circuit.

 

I’m not going to argue the point but perhaps you would like to look it up on the internet or in a reference book.

 

Here is a good starting point…

 

https://support.maxongroup.com/hc/en-us/articles/360016161594-Motor-controllers-Input-current-versus-Motor-current-Power-Conversion

 

Brian.

[Dickw]

1 hour ago, RottenRow said:

I’m sorry but I’m not wrong. I do know all about the current and voltage levels in a series circuit… Kirchoff’s laws… but a brushless motor / ESC / battery isn’t a series circuit. Certainly not a simple DC circuit to which your ‘rule’ applies.

............................................................

Here is a good starting point…

https://support.maxongroup.com/hc/en-us/articles/360016161594-Motor-controllers-Input-current-versus-Motor-current-Power-Conversion

 

 

I think the difference in our understanding is because the Maxon controllers referred to in your link are a completely different type of controller to the simple devices we use for our motors.

Quote - "Slightly simplified, a PWM power stage is essentially a step-down DC-DC converter or Buck converter." (see their diagram below)

That would explain the concept of energy conversion and the output current being higher than the input current.

 

Our ESCs are not DC-DC converters or Buck Converters, they are simple DC switching devices without the inductor shown. The only inductance in our ESC after the FETs is the motor winding, which is generating the motion and is not an energy store in the same way as the fixed inductor (or "choke") in the Maxon controllers.

 

Dick

[RottenRow]

I agree that the Maxon controllers have many more features and operate with much more precision (they are sensored for a start) than our model ESCs but I used their example as I couldn’t find one specifically for model ESCs.

 

However they do operate on the same principle and they do not have a separate DC:DC converter, it is all done in the output MOSFETs and motor windings. The diagram you have reproduced above is an equivalent circuit and isn’t supposed to represent what is in the controller component for component.

 

The switching sequence of the MOSFETs in a brushless motor controller is such that the applied voltage across any individual winding gets reversed during motor rotation. This, along with the voltage induced into the winding as it passes the magnets, means that the motor voltage and current waveforms are not simple or constant. This is complicated even further by the fact, as the motor windings are (usually) star connected, two windings are being powered simultaneously (in series) by the controller but a fraction of a second later one of those two is replaced by the third winding as a different MOSFET switches on. It is this that creates the rotating magnetic field that the rotor then follows.

 

My main reason for raising this in the first place was to doubt the comments about the use of a wattmeter in the battery leads being accurate at anything other than full throttle. I still believe this statement to be untrue.

 

DC (battery) power in must equal motor power out plus losses.

 

Brian.