Fading ESC

Andy Hoffer · December 19, 2012, 05:50:00 PM

Here's an interesting experience. I recently acquired a used 63" wingspan Starmax P51 Dago Red. It runs on 4S and draws about 40 amps peak with the stock motor and a 15x6 four-blade prop. After the first couple of successful flights, I started to encounter gradual "engine failures", starting about 1-2 minutes after takeoff (usually just enough time to get caught with a tailwind on the downwind). These flights were with fully charged, relatively new LiPos. I had initially suspected the batteries, but an instrumented bench test of the power set-up revealed the ESC was at fault. Minimum voltage under full load was about 3.3 volts per cell. After one to two minutes at full thottle (ESC, batteries and motor remained cool), rpm started to decrease; current draw also decreased and cell voltage rebounded. Rpm eventually decreased to zero, at which point the motor could not be restarted until the motor/ESC had beeped for about a minute. Then the same cycle repeated. I did discover that if I throttled back to zero briefly after the fade had occurred, but before complete shutdown, I could recover full power until the next fadeout, again after one to two minutes.

Has anyone experienced this sort of problem? Short of managing the problem by briefly throttling back to zero every 30 seconds of flight time, my next step will be to replace the ESC (a cheapo TopFire 70-amp model made in China). (http://www.rcuniverse.com/market/item.cfm?itemId=828634)

Comments and sagely wise cracks welcome!

Andy

Ededge2002 · December 19, 2012, 06:19:58 PM

It sounds like its low voltage limiting. Some esc's have there low voltage cut off set at 3.3v per cell. As the cells recover the voltage climbs back up. Another possibility is that as the motor heats up the current required to run it also climbs pulling the pack voltage down even more. Is the origional ESC programable?
Two minutes at full throttle is quite a long time to push the batteries in my opinion. Although "scale" I would ditch the 4 blade prop and try it with some throttle management. But a scale guy I am not!

pmackenzie · December 19, 2012, 07:25:41 PM

Also could be ESC temperature cut off.

Andy Hoffer · December 19, 2012, 08:47:46 PM

Quote from: Ededge2002 on December 19, 2012, 06:19:58 PM
It sounds like its low voltage limiting. Some esc's have there low voltage cut off set at 3.3v per cell. As the cells recover the voltage climbs back up. Another possibility is that as the motor heats up the current required to run it also climbs pulling the pack voltage down even more. Is the origional ESC programable?
Two minutes at full throttle is quite a long time to push the batteries in my opinion. Although "scale" I would ditch the 4 blade prop and try it with some throttle management. But a scale guy I am not!

Quote from: pmackenzie on December 19, 2012, 07:25:41 PM
Also could be ESC temperature cut off.

I thought about low voltage limiting, but it's fading well before any reasonable drawdown, and well above 3.3 volts/cell. I also ran tests at half throttle (10-15 amps), and it did the same thing, albeit after a marginally longer run. Motor never even got warm as tests were run with motor open to the slipstream (firewall clamped in a bench vice.). ESC, also in the slipstream, also remained cool. Batteries (Turnigy Rhinos, rated for 20C continuous, running at 10C max) only got slightly warm, and that only after reaching the end of a series of tests, bringing the resting voltage down to 3.7 volts per cell. I don't know if the ESC is programmable. The instruction, poor at best, is silent on this issue. It's an inexpensive ESC so I would guess not, but I did find a short thread on this with a link to Hobby King downloads so I will give it a try, even though the Brand Name is SUPPO, not TopFire. (http://www.hobbyking.com/hobbyking/store/uploads/635937000X507186X22.pdf).

I have to stay with the 4-blade prop to maintain ground clearance, but for fun I also tried running with just two blades and got the same result.

Thank you both for your input.

Andy

Andy Hoffer · December 19, 2012, 10:00:50 PM

So tonight I tried it again. No fade, even after drawing down to 3.0 volts per cell under load. Go figure!

Well, at least I'll have my Plan B (aggressive throttle management) ready to go.

Thanks again,

Andy

Gregor77 · December 20, 2012, 08:58:45 AM

I had a case of this on the GWS 30Amp ESC for the old C47's. At about 2 mins into the flight, I got about 30% throttle.. but that ended up being I had not enough airflow and the ESC was getting too hot and went into a fail safe. At least I could still land it... lol!

Wingnutz · December 20, 2012, 06:56:42 PM

Have the same Mustang model in a different colour scheme and haven't had any grief with it yet....however, have another Starmax (badged as HK) model supplied with a Topfire 45A (I think the Mustang is supplied with a Topfire ESC) which gave me grief right out of the box....
With this Starmax
Topfire 45A ESC with two aileron servos on one channel-> Rx brownout,
Topfire 45A ESC with two aileron servos on separate channels-> no brownout,
PLUSH 60A ESC with two aileron servos on one channel-> no brownout. The model is flying without issues with this ESC.
HK gave me a credit for the Topfire ESC...maybe they know something.
Bill

Bobmic · December 20, 2012, 10:10:33 PM

Hi Andy,
I guess the first thing to do is order a new ESC, if you want to keep it at low cost by the HK red brick i have it in a couple of planes for quite a while and they are quite good - if you want to beon the safe side due to retracts just add a cheap HK BEC.
As for the voltage drop on the batteries - you should never go under 3.3v this is the limit before damaging the batteries. I alway fly with telemetry and sometimes I used Eagletree logger to get better insight and have never got down to 3.3v.
To realy be sure if the problem is with the ESC's cut off voltage you will need a logger or a scope - a watt meter showing 3.3V for a very short while is very accurate and you might be dropping even lower ( either due to faulty ESC or bad battery).
On a different note when you decide you had enough with the 4 blade prop go to an APC 2 blade and 6s.
I am flying my 1.4m P47 with two 2200 3s in series and it flies realy well.

Bobby

Andy Hoffer · December 20, 2012, 11:11:02 PM

Quote from: Wingnutz on December 20, 2012, 06:56:42 PM
Have the same Mustang model in a different colour scheme and haven't had any grief with it yet....however, have another Starmax (badged as HK) model supplied with a Topfire 45A (I think the Mustang is supplied with a Topfire ESC) which gave me grief right out of the box....
With this Starmax
Topfire 45A ESC with two aileron servos on one channel-> Rx brownout,
Topfire 45A ESC with two aileron servos on separate channels-> no brownout,
PLUSH 60A ESC with two aileron servos on one channel-> no brownout. The model is flying without issues with this ESC.
HK gave me a credit for the Topfire ESC...maybe they know something.
Bill

Quote from: Bobmic on December 20, 2012, 10:10:33 PM
Hi Andy,
I guess the first thing to do is order a new ESC, if you want to keep it at low cost by the HK red brick i have it in a couple of planes for quite a while and they are quite good - if you want to beon the safe side due to retracts just add a cheap HK BEC.
As for the voltage drop on the batteries - you should never go under 3.3v this is the limit before damaging the batteries. I alway fly with telemetry and sometimes I used Eagletree logger to get better insight and have never got down to 3.3v.
To realy be sure if the problem is with the ESC's cut off voltage you will need a logger or a scope - a watt meter showing 3.3V for a very short while is very accurate and you might be dropping even lower ( either due to faulty ESC or bad battery).
On a different note when you decide you had enough with the 4 blade prop go to an APC 2 blade and 6s.
I am flying my 1.4m P47 with two 2200 3s in series and it flies realy well.

Bobby

Thanks guys. The brief test down to 3.0 volts was on an old battery which I only use for bench tests. I was trying to determine the ESC low voltage cutoff. I agree that 3.3 volts is the safe limit for the battery.

The ESC is giving continuous runs of up to 5 minutes at half throttle. A brief throttle off restores power if it starts to fade, as long as you get it before total cutoff. Total cutoff by the ESC requires several seconds before the ESC will restart.

Bill's point about brownouts suggests that ESC performance could be worse when I add servos to the mix, so a retest with a full
set of active servos is warranted. In any event it looks like a new ESC is a must.

Thank you both for your insights.

Andy

Ededge2002 · December 20, 2012, 11:16:34 PM

4s with a esc's bec can be trouble. This can be more problems with high servo counts

Andy Hoffer · December 21, 2012, 08:07:31 AM

Quote from: Ededge2002 on December 20, 2012, 11:16:34 PM
4s with a esc's bec can be trouble. This can be more problems with high servo counts

Thanks Ed. I will definitely do some more exhaustive testing for this stock TopFire 70-amp ESC. I have used ESC/BEC combinations for years and have never encountered a problem with as many as 6 servos (2 for flaps, 2 for ailerons, 1 each for rudder and elevator, using an E-Flite Power 32 and E-Flite Pro 60 ESC with 4S). The Starmax Dago Red P-51 has 6 servos plus 3 retracts. It's previous owner had used 4S without issue, so maybe the ESC is simply reaching it's end of life, and it trying to take the plane with it.

Andy

piker · December 21, 2012, 09:38:43 AM

I don't have any insight, but i would like to add my understanding / experience on a couple of points.

My understanding is that the SAFE low voltage limit for Lipos is 3.0v. The drop dead limit, the point where damage will occur, is 2.5v per cell. Of course, keeping a margin of error for cheap lipos is probably a good idea.

And I use a BEC on anything from 2 cells to 6 cells all the time. Never a problem (knock on wood), but perhaps I'm using better (more $$$) quality ESCs.

Good luck with your investigation, Andy.

Robert

Bobmic · December 21, 2012, 12:37:55 PM

Hi Rob,

You are correct about the cell voltage - I missed a not in my statement ..
"a watt meter showing 3.3V for a very short while is NOT! very accurate and you might be dropping even lower ( either due to faulty ESC or bad battery)."

Lipo's are operational between 2.7V to 4.25V but since things are fluctuating quite a bit with in our toys and I guess most of us are not planning to send our measuring tool for calibration and re certification every 6 months or so going bellow 3.3V will probably end up with a lot of spare partially good batteries

As for "better (more $$$)" - I thought this only relates to women and teenagers clothing and accessories, not for our "cheap" electric stuff.
But if you want a REALLY good EDF unit I have something for you

Bobby

Skyking · December 22, 2012, 05:25:22 PM

Hi Andy,
I have the same plane.
Before I maidened it I cut a 1 inch hole in the bottom of the cowl angling back. (I used a sanding drum in a hand drill with the abrasive sticking 1/2 way off the drum. Neat).
I mounted the ESC where it would benefit from this opening.
I leave the cooler door open at the bottom of the fuse just aft of the wing.

I have not had any problems except the landing gear ripping off.
In the spring we should have some awesome formations with all these Mustangs.

Kind regards,

Ken

Andy Hoffer · December 23, 2012, 12:09:40 AM

Quote from: Skyking on December 22, 2012, 05:25:22 PM

Hi Andy,
I have the same plane.
Before I maidened it I cut a 1 inch hole in the bottom of the cowl angling back. (I used a sanding drum in a hand drill with the abrasive sticking 1/2 way off the drum. Neat).
I mounted the ESC where it would benefit from this opening.
I leave the cooler door open at the bottom of the fuse just aft of the wing.

I have not had any problems except the landing gear ripping off.
In the spring we should have some awesome formations with all these Mustangs.

Kind regards,

Ken

Hi Ken:

I think I've got plenty of cooling in the airframe. Previous owner made similar mods to yours. Bench testing has shown the ESC fades at 3.0 volts/cell under load (abouot 5/8 throttle), and shuts off motor power completely if the fade is allowed to continue for 10-15 seconds. Temperature is not an issue as the ESC remains cool. I recover full power if I briefly throttle off before complete motor shutoff, otherwise receovery takes several seconds. Bottomo line: careful throttle management until I replace the ESC.

Many thanks for your insights.

Cheers!

Andy