How close to the limit of an ESC rating should you go

[Robin Kearney]

Hi guys,

I've setup my Pulse 25 with a Turnigy Plush 40A ESC and a G25 motor, these are running off a 3S Lipo and swinging a 12x8 prop.

I checked the current draw yesterday and at full throttle it was showing 39A.

My question is, am I running too close to the limit of that 40A ESC? The spec says continuous draw of 40A, peak of 55A. Somewhat stupidly I did this after I flew her, and found that I rarely needed about half throttle, needing only 3/4 to pull her round a loop, so I don't think I'll be belting around at WOT very much.

If I'm too close to the limit, could I prop her down a bit rather than replace the ESC? Would that entail just reducing the pitch, maybe to a 12x7 or 6?

r.

[Frank Skilbeck]

You'll also find that the motor unloads in flight and the amps drop off, my Tucan pulls approx 45A static on a fully charged 3s lipo, but this drops to below 40A in flight according to my telemetry. But if you never need full power then changing to a 12 x 7 will also drop the amps, I did this on another plane, not to reduce the amps but because I never flew above 1/2 throttle and with the larger prop there was a very strong torque roll on take off.

[Bill_B]

Going close to the max limit is ok as like Frank says, props normally unload once flying (square and over square props can behave differently), but the ESC is working harder at part and low throttle so the more you have in hand the better IMHO, space and weight permitting of course.

[Andy48]

Lets put it this way. How much is your model worth? If the ESC lets go it will do so at the worst possible moment, and you may be needing a bin bag. A 60a ESC is just over the £20 mark. Your call.

[Andrew767]

Posted by Martin Phillips on 07/10/2013 19:42:33:

Lets put it this way. How much is your model worth? If the ESC lets go it will do so at the worst possible moment, and you may be needing a bin bag. A 60a ESC is just over the £20 mark. Your call.

+1

Andrew

[Robin Kearney]

Martin/Andrew I understand that point of view. I'm pretty new to electric flight and I guess what I'm trying to gauge is the amount of headroom people normally give an ESC.

So from what you've said if a setup draws 40ish A, is it typical to put a 60A ESC in?

r.

[Bill_B]

I wouldn't say it's typical to have that much headroom, but I always try and go 10A above what is required (weight and space permitting in small models) as there's no need to crack a nut with a sledge hammer.

[Simon Chaddock]

"the ESC is working harder at part and low throttle....."

I have yet to see any real proof that an ESC actually generates more heat at part load - mine certainly don't.

Indeed some ESC have a thermal 'power reduction' mode. There would be little point in this is if the heat output really increased at part load.

A recommendation for a 50% 'headroom' on an ESC may be considered being a bit cautious but it is not that unusual.

[Bill_B]

Posted by Simon Chaddock on 07/10/2013 23:46:51:

I have yet to see any real proof that an ESC actually generates more heat at part load - mine certainly don't.

Indeed some ESC have a thermal 'power reduction' mode. There would be little point in this is if the heat output really increased at part load.

Perhaps read this thread and you may change your opinions.

[Andrew767]

Posted by Robin Kearney on 07/10/2013 22:29:49:

Martin/Andrew I understand that point of view. I'm pretty new to electric flight and I guess what I'm trying to gauge is the amount of headroom people normally give an ESC.

So from what you've said if a setup draws 40ish A, is it typical to put a 60A ESC in?

r.

Robin

I try to leave 25% headroom so up to 45amps on a 60amp esc this is because i like a safety net as you never know how reliable the makers figures are. Also i try never to exceed 50amps, prefering to up the cell count. I have to say i agree with Simon regarding greater heat generation at part load as mine don't either but that is just an observation, i have no proof.

Andrew

[Piers Bowlan]

Robin, I have two or three of these 40A switch mode Turnigy Plush ESCs and very good they are too. Having said that I would use the 60A version if my motor/prop combo was drawing 39A, regardless of how hot the ESC gets. The extra weight of the larger ESC is minimal in the scheme of things especially considering just a few years ago we had to contend with the weight of a NiCd pack! As for cost, how much is your model worth compared to the cost (your cash and your time) of a replacement model?

I have just checked the spec of the G25 motor and I see it says 32A max and 44A for 15sec. So maybe a smaller prop (11X8) might be a good alternative option with your 40A ESC. Alternatively use a bigger motor (and ESC) .

Edited By Piers Bowlan on 08/10/2013 05:23:38

[Paul Marsh]

I have the original 35A ESC in my Wot 4 Foam e. Gets really hot,but probably depends on the efficiency of the esc and what type it is. Bought a 40A one, as the 35A gets so hot, that it shuts down. Usually put it outside the plane, where it's ok, but the situation wasn't ideal.

[Biggles' Elder Brother - Moderator]

Generally I allow at least 10% headroom. The main reasons being:

1. As an engineer I learnt a long time ago that good big 'un working well within its envelop is always preferable to a good little 'un on the limit!

2. I don't entirely trust the specs manufacturers give out! Cynical old me eh? I believe that they are - how should I say - "putting things in the best possible light"! It only takes one critical component to be a little below par and the outcome can be a system that struggles to deliver consistantly and long term at the top end of it perfromance range. And dare I say it - I think this factor sometimes needs to be taken even more seriously with "budget priced" items which may well have been designed and made down to a price in terms of component selection and "goods inward inspection of component parts.

As some folks have said - you put a lot effort into getting model in the air, and not a little expense, for the realtively small cost of upping the spec a little the peace of mind and additional security seems to be worthwhile to me.

BEB

[andy watson]

I'm pretty sure Timbo always used to recommend 20% lee way on escs.

Personally I just stick an 80A/100A into everything since the weight and cost penalty is pretty much minimal.

[Robin Kearney]

Thanks for all the feedback guys. it seems to me that the consensus is 39A on a 40A continuous rated ESC is too close and I ought to look to get a bigger one.

Thankfully the Pulse managed 3 flights as it is and is safely back home, but I agree with the view that I'd not want to loose it as a result of a cheap component giving up on me so an upgrade it is then.

I can feel an order coming on

r.

[Simon Chaddock]

Bill B

That post correctly describes how the speed control functions but rather ignores the characteristics of electromagnetic circuits to resist changes in state.

A magnetic coil does not draw more current (actually its less) during a short pulse than if the same voltage is applied constantly,

[Shaunie]

There have been several long threads on this forum about ESC operation but the the point that is most pertinent here is that the transistors (MOSFETs in fact) operate in switched mode i.e either fully on fully off.

When switched on the current is high but the voltage across it is low, power is dissipated but is hopefully fairly small.

When the device is off the voltage across it is high but the current is practically zero so no heat is dissipated.

Unfortunately the devices cannot switch from on to off instantaneously and while the device is switching on or off the current and the voltage are both significant, as a result the energy dissipated can be very high.

Due to the way that ESCs work there are more switching events at less than full throttle giving rise to the opinion that ESCs can get hotter at mid throttle settings than full throttle. I intend to do some tests soon to prove or disprove the theory but as always its a matter of time.

I did have an 80A ESC on a setup running at 34A, doing some bench tests the motor slipped out of sync (squealed) probably due to wrong timing setting and it took about a second and a half to smoke up. I did successfully put 7 new MOSFETs in it but it was a real pain. When the motor stops behaving as a motor for any reason all tha energy has to go somewhere and unfortunately the MOSFETs often cop it!

I would personally allow at least 25% headroom on current rating.

For those that are interested:

The 80A ESC (Emax I think) I mentioned has 7 MOSFETs in each arm of the bridge rated at 13A each (91A total), the effective duty cycle could probably be considered as 1:3 so the average current per arm would be 27 amps and the average current per device about 3.8A which is about as close as I would like to go considering the 13A figure is an absolute maximum with the chip at 20 deg C, pretty hard to achieve without a heatsink the size of a house.

Shaunie.

[Shaunie]

BTW Simon don't forget that when magnetically saturated an inductor is no longer inductive, the only thing limiting the current rise is the resistance of a short length of fat winding wire.

[Steve Hargreaves - Moderator]

Totally agree with BEB.....just to add that you often find that the "branded" ESCs will operate at or even slightly over their nominal limit with no sign of distress whilst budget ESCs often give up as soon as their limit is reached.....personaly I allow 10% min & often 20% "headroom" precisely cos of point 1 made in BEBs post.....thrash anything to the limit & it will fail sooner than an item working well within its spec...