ESC

[Barrie Dav 2]

I've just been reading an old posting by Timbo (May. 2009) wherein he says that, provided you have the space in the model a larger than needed ESC can always be fitted.

 

Now,  I wanted to purchase a 40A or a 60A esc for a 400 watt motor with the (mistaken) belief that I could gain height with a model and throttle back the motor as per an i.c. engine for a gentle chunter around.  (The large esc would suffice for bigger motors in the future).  However, I was told by a model shop owner that you can't just throttle back a motor like that because the esc would get too hot and that motors/esc's have to be matched.

 

I know that amps, volts etc come into the equation and I know that the esc should have some margin over what is required by the motor but my query is:-

 

Can you or can you not throttle an electric motor like an i.c. one without damaging, ecs, battery or the motor?

 

I've looked on the forum for info but did not find what I was looking for.

[ken anderson.]

hello barrie-timbo is the lad to answer this with authority..but i would say what you have been told is wrong......as far as i understand -yes it's harder work for the ESC...ie it can get hotter.....but i would def say (yes) you can throttle /control the electric motor same as an ic..if not there are going to be some very short run's-model's oos-and super hot batt's.....

 

    ken anderson........

 

ps-while ago since timbo and i suppose me were lad's....... 'ss..electric had just been invented....

[Tim Mackey]

Yes of course you can....this type of nonsense talk is typical of some model shop staff who have embarrisingly little knowledge of the sparky side.  Providing the ESC is of decent quality, and correctly programmed to match the motor in use, then throttling of an electric motor is generally far superior to most IC engines!! After all, most IC motors are not exactly linear in response, and how many do you know that will happily stop turning completely whilst in flight, yet restart immmediately and reliably every time the stick is merely raised a notch.

[Tim Mackey]

To add to your plan about the size of chosen ESC... think of the car engine analogy - ( this applies to both the motor and the ESC TBH )

 

Your car is capable of 70 MPH flat out.

You want to cruise at 70MPH on the motorway all day - which it will do - however the wear and tear, and general strain being placed on it will take its toll, and the ride will be pretty noisy, and everything is being pushed to the limit. Dont expect a long life from it .

 

Now, your mate wants to do the same thing - but he has a car thats capable of 100 MPH flat out. Nuff said

 

Having a larger cpapcity than really necessary on the ESC gives the same headroom, and although its true that they are working a little harder at lower revs, it will still be well within tolerance.

[Barrie Dav 2]

Thanks Ken and Timbo.  That's cleared that up.  Much obliged.

[Steve Hargreaves - Moderator]

It is true that an ESC run at part throttle can get hotter than when run at full chat simply because it switches the current on & off more frequently to make the motor turn slower. No switch is 100% efficient so the inefficiency shows up as heat.....more switches equals more heat. In the real, practical world however this is rarely an issue & it is certainly better to have a larger than needed item than run something flat out all the time as Timbos analagy above demontrates.

 

Personally I always try to have around 20% or so headroom in my ESC...ie a 40A ESC for a 30A current.....

[Barrie Dav 2]

Thanks for your contributionSteve.

[Ed Anderson]

Interesting topic. I have never heard of anyone suggesting that running your electric motor at part throttle put any undue strain on the ESC or that it would cause the ESC to heat up in any way.   And I have never seen it documented in any of the ESC that I use.

 

Is this documented somewhere, that an ESC at partial throttle will heat up more than at full throttle, or is that just an opinion?

 

And, are we talking brushed or brushless ESC here?

 

 

[Steve Hargreaves - Moderator]

Both......!!!

 

All ESCs are basically high speed solid state switches & switching a current on & off creates heat.

 

In the real world though this just isn't an issue (provided your ESC is of reasonable quality). It might occur if you were to run an ESC for a significant amount of time (ie 30-40 mins or so) at part throttle. I recall Nigel Hawes had that problem with his channel crossing Tucano.....after about 40 mins at half throttle the ESC overtemped & cut out....a special ESC with extra heatsinks was supplied by Mtronics & the problem went away.

 

 As you say its not something you often hear about so you can draw your own conclusions from that.... 

[Simon Chaddock]

I flew my Endurance Wing Dragon for 1hr 36 minutes at about 1/3 full power. It was drawing just under 4A but the 20A ESC never flinched.

The ESC may have had to dissipate a bit more heat but at such low power levels (25W) nothing really got even warm!

[Peter Wood]

Is there a linear relationship between the amount of heat generated by an ESC and the throttle setting? That is does the esc generate more heat at 10% throttle than 90% say because it is chopping the current more often?

Last week I purchased second hand a brand new unflown CMP C160 complete with motors and esc's. A check on current showed both motors pulling 40+amps - ample power I thought. Take off was uneventful and after gaining height I cut power to about 70%. Very quickly thereafter the plane flicked violently to the right into a spin. I recovered and climbed again. A power off stall test showed no untoward behaviour but on opening up to part throttle it flicked again. Eventually the inevitable happened and I could not recover the spin.

On investigating the remains I discovered the esc were only rated at 30 amps. HOWEVER they ran quite happily at full throttle for several minutes pulling 40amps with no signs of distress. I suspect therefore that they were cutting out at 70% throttle because of the extra heat generated.

Is this a feasible explanation?

[Steve Hargreaves - Moderator]

Sorry to hear about the loss of the model Peter....always sad to hear when we lose one to that great hangar in the sky....

 

However I think you might be clutching at straws & ignoring the obvious here....most electrical items are rated at slightly below their absolute maximum & indeed many will go way over but for increasingly short periods of time....your 30A ESC would take 40A but only for a short time...it would probably take 50A but for less time...60A but less again...100A maybe but only for a few milli-seconds......

 

Also the second thing to remember is that the way our ESCs work is by switching the FULL current on but for varying amounts of time (this isn't quite true but will do as a simple explanation) thus at full throttle the full 40A is flowing more of less continuously through the output transistors....at part throttle there is still 40A flowing through them but not continuusly. As noted above the 30A transistors will probably take 40A but for a good deal less time than they will take 30A.

 

At the risk of being brutally honest here it seems you overloaded the ESCs & they burned out......

 

Another point to note here also is that (generally) the cheaper the ESC the closer to the absolute maximum the rating seems to be.....in other words if you buy a cheap ESC try to allow at east 20% extra in the max current rating (ie if you need 30A max use a 40A controller). Anything being flogged to death will expire sooner than something working well within its limits....

[Chris Bott - Moderator]

The heat dissipated by the ESC must come in two parts. One is the heat generated when the current is passing through the MOSFET's, this must increase with current. With modern MOSFET's however this heat is minimal. The other heat is generated in the time while the MOSFET is switching between on and off or off and on. These time periods are extremely small, but for these moments the resistance is significant so heat is generated.

 

For a brushed motor ESC:- 

When the ESC is passing no current (i.e. throttle off) there is no heat generated.

When the ESC is passing full current (i.e. flat out) there is no switching going on, so the switching generated heat is zero.

For any throttle setting in between the switching rate doesn't change, just the ratio between on times and off times.

So there are the same number of switch events (whatever the throttle setting from 1% to 99%). 

So I would have thought that the switching heat generated wouldn't change between 1% and 99%

 

 

[Peter Wood]

Steve and Chris

 

Thanks for the replies. I have been doing more research since last night  and came across an article on the Giantcod site.

 

http://www.giantcod.co.uk/forum/viewtopic.php?f=15&t=689

 

Now this is pretty heavy stuff and I don't claim to understand it all but what I have taken from the paper is that the switching cycles vary with the throttle setting and as Chris says the heat is generated when the FET's are switching.

 

However the most relevant info in the article was the following ---

 

The worst case scenario for an ESC is to run at around 2/3 throttle setting. In this case, the transistors are carrying a heavy load and switching on and off several times per cycle, with no time to rest between cycles. It is during operation in this range that the ESC will get the hottest.

 

This was exactly the condition when the plane flicked to one side and into a spin.

 

Remarkably the plane is repairable and I will be fitting it with 60amp esc's.

 

Peter

[Tim Mackey]

It is not really conclusive that even if the ESC did overheat the BEC unit failed.  Most if not all ESCs these days have thermal protection circuitry, and even if this did activate it simply stops power to the motor, not the radio.

Whereas I agreee that the escs need upgrading to cope with the power of the motors, I am afraid I would still be looking elsewhere for the cause of your "spin".

Edited By Timbo - Administrator on 25/05/2010 22:37:41

[John Privett]

But if the power is being cut to ONE of the two motors?

[Tim Mackey]

Ah thats a good point....

[Chris Bott - Moderator]

Peter some time ago I had a similar incident with a large twin (DH88 Comet) I'd been cruising around at quite low throttle settings. It flicked once when I opened the throttle rapidly. I got away with that and was settling my nerves as people around me were suggesting I was flying too slow. When I throttled up rapidly again it flicked and went in.

 

Now I'll never know the cause,  I could just have been flying too slow. But another possibility is that one ESC started it's motor before the other. Or maybe at that moment an esc cut out on temperature, this one is unlikely as I have run the setup in single engined models, straining the ESC much more wiuth far less cooling. The ESC could have cut out on low voltage. I was using separate batteries.

 

If it ever flies again, and it could, I'll couple the batteries together into a single source. I'll make absolutely sure that the motors start together every time, and I'll make sure the ESC programming is identical. 

[Peter Wood]

Timbo - After recovering the plane we ran the motors at full power on the ground again with the same batteries with no signs of distress. At that time I had not figured out the likely cause so we did not check then out at 2/3 throttle. So the esc's were not burnt out and it was not the BEC's cutting in. We did extensive checks on the radio which was OK.

The spec for the ESC's states that there is a thermal cutout at 105deg which reduces power to 30%. As it was very hot on Saturday and the ESC's were behind the motor ie in its warmed air, the overheating explanation seems the most probable.

 

Chris raises a good point about twin batteries. It had been suggested to me before I flew the model that I should couple the batteries and motors together. I put it off because of the hassle of making two X leads. I don't think it contributed to the crash but will be making them for the future to ensure that both ESC's are seeing the same voltage and thus reducing the chances of one BEC kicking in before the other.

 

I thought electric twins were supposed to be easy!!

 

Peter

[Chris Bott - Moderator]

Peter were you using the BEC in one of the ESC's?

 

Once, I had an ESC cutting on temperature but eventually found that the temperature was coming from the BEC and not the motor part of the ESC. 

[Tim Mackey]

Ok Peter...get ya

Couple of points

1) You are confusing BEC with LVC

2) I have always recommended that any twin uses a single / parallel connected batteries where possible to eliminate the chance of one battery sagging before the other and allowing the LVC to operate on the one ESC. Of course, this only eliminates that particular issue....manufacturing tolerances etc can still mean one ESC behaves slightly different to the other(s). 

 

Having said all that, LVC should only really be an issue when the batteries are getting near the end of discharge, so in "normal" timed flight, this should not be a concern. The fact that your ESC overheated so soon into the flight is a symptom of stress as you have already suggested.

[Ed Anderson]

If I understand this discussion, we are discussing how many faries can dance on the head of a pin.  That is, that an ESC will produce more heat at partial throttle, but the amount should be so small as to be of no concern, as long as you have proper air flow over the ESC.

 

Do I have this right?

[Bert]

Posted by Ed Anderson on 26/05/2010 13:04:15:

If I understand this discussion, we are discussing how many faries can dance on the head of a pin.  That is, that an ESC will produce more heat at partial throttle, but the amount should be so small as to be of no concern, as long as you have proper air flow over the ESC.

 

Do I have this right?

 

No you don't, all ESCs brushed or brushless are less efficient (produce more heat) at part-throttle settings. This doesn't matter so much at small throttle settings but is more significant at 2/3 - 3/4 settings.

 

It is even worse for brushless ESCs because they have to commutate (switch) between the windings at all times.

 

Basically the ESC needs to be properly sized for the load (bigger is better) and adequately cooled.

 

Bert

 

[Chris Bott - Moderator]

Ed I think you have a point, if we aren't overloading our ESC's then there may be a little heat generated but it shouldn't be of any concern. Where heat does become an issue, manufacturers fit proper heat sinks. 

 

With any type, a gentle movement of air over it should suffice. If it needs any more, shouldn't we have bought a bigger ESC?

 

If the BEC is a linear type (i.e. not switch mode) then heat will definitely be produced. This will go up with cell count as the BEC has to dissipate more and more energy. 

[Tim Mackey]

Yep, I 'm with Chris on this summary.