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Old Oct 13, 2012, 03:10 PM
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why does my motor or ESC stall at 50% max speed

I'm having a problem with my motor stalling, I think the main cause of the problem is me not knowing enough so I was hoping somebody could help me out. When I bring my bldc motor up over 50% max speed while under load it seems to stall and just starts beeping 2 or so times a second. The load being a 10inch prop with a 4.7 pitch. I can bring the motor up to full speed when it's not under load and the currents reads 0.5A.

What I've seen is that at rest I measure 10.6V (8S NiMH battery) across the ESC, but as the BLDC rpm go up the voltage goes down, caused by back EMF I presumed. The current looks to be around 1A and everything seems to cut out when the voltage goes down to 7V. The ESC says it operates from 7.4 11.1v so I'm guessing that the back EMF from the motor is causing the ESC to shut off do to low voltage, as soon as this happens the motor stops and the voltage returns to 10.6 but needs to be re-armed so it just starts beeping.

So what I need to know is how to avoid this situation. Am I missing a component to protect from the back EMF so I can keep the voltage up? Is it just because the prop is too big for the motor? If I add another battery pack in parallel I can get it up to about 65% speed before I get the same result, with this the current goes to 2A, but I can't image this is the correct approach because it would be too heavy. My other thought is that maybe while being under load it cannot spin fast enough to keep up with the PWM and stalls but I don't know enough to know if that makes sense. I also thought it could be because I'm using NiMH instead of a LiPo, would a LiPo battery handle this better for some reason?

Here's what I've got setup. I'm using 8S NiMH battery. My ESC is rated for up to 30A and needs 7.4v 11.1v, because I'm using NiMH and I'm just testing I have it programmed without a cutoff. The motor is 1000Kv rated for 4 10A and I'm using a micro controller to send the PWM to the ESC. Any explanations to help me learn more about this would be appriciated.
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Old Oct 13, 2012, 07:09 PM
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Dr Kiwi's Avatar
Chattanooga, Tennessee, United States
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You are correct in suggesting that you are hitting LVC... the prop may be too big ...but that would apply only if your load current is exceeding ~10A (the motor's limit). More likely the battery is not up to the task and its voltage under load is dropping too far. As you found out using a bigger battery helped the situation. I'm assuming your LVC is set at around 7v = 0.9v/cell. There are decent NiMh packs which should easily maintain >1v/cell under max. load. What is your current draw when the LVC kicks in? What physical size are your NiMh cells?
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Old Oct 13, 2012, 07:51 PM
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Currently I'm using 8 Enercell AA cells connected in series for an individual pack. For the test I did with a pack connected in parallel it was a second pack that was the same. Currently I've got my ESC programmed not to have a cutoff, but according to the specs it needs at least 7.4V to run. When using an individual pack (8 cells), it will draw ~1A before it cuts out, with two packs in parallel I can get double that at ~2A, maybe just a little over and I can push it up to ~65% max before it stops. Each cell is 2500maH, and from what I've read it seemed like they should be able to discharge at 1C, so I was thinking I should be able to get 2.5A out of one pack, and 5A with two in parallel, but I'm not really anywhere near that.

So then it sounds like my battery pack isn't up to the task. If there is a NiMH battery that can do better which would you suggest? Would a C or D cell discharge better? I've stayed away from the larger sizes because putting 8 of them in series to get a good voltage would be pretty heavy. Right now I'd rather not get going with a LiPo battery because my only working space is my living room with plenty of things that wouldn't agree with a LiPo battery issue should I have an accident.
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Old Oct 13, 2012, 09:59 PM
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USA, MO, Ballwin
Joined Mar 2010
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Good call about not charging LiPos inside. You can always charge in an outdoor barbeque grill. I have charged in my stainless kitchen sink when I am nearby.

Your AA cells are just too small. AA batteries are for cameras and toys, your TX, but not high draw uses like motors. I suggest making the move to LiPos.
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Old Oct 13, 2012, 11:27 PM
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Consumer grade AA have a poor output, though they may have plenty of capacity. Think, large bucket, small hole. You need a larger 'hole' (output).

FWIW the higher the capacity in NiMh (comparing similar size) the WORSE the output. 2500mA AA are mostly lousy batteries for about anything. You want good AA get 2000mA Eneloops (still not good for power applications though).
You are better off with something down around 1000-1500 sub-C or there about by guesstimate.

You can get suitable NiMh batteries for about any power application here:
http://www.cheapbatterypacks.com/

Mostly though I agree with the suggestion to go lipo. Get a decent balancing charger and a large clay pot + some kind of lid, or dead pressure cooker from a thrift store to charge in-something like that.
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Old Oct 14, 2012, 08:21 AM
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As flydiver says.. it is time to go Lipo, but if you must use NiMh:


If you need to draw ~10A then these are suitable cells: http://www.cheapbatterypacks.com/pro...-1500-23A.aspx

KAN 700 2/3AA are good cells too, but only good for 7A max.

As you can see, even though Eneloops are superior AA cells, they can only manage 1C (2A) discharge.. your consumer grade 2500mAh cells are probably only good for about an amp. http://www.cheapbatterypacks.com/pro...op-2000AA.aspx
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Old Oct 14, 2012, 08:35 AM
Jack
USA, ME, Ellsworth
Joined May 2008
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The all time safest battery with good power delivery is arguably the A123 (brand name) LiFePO4 (battery type) metal cased 2300 mAH and 1100 mAH cells.

The battery forum has a lot of info on using them, their availability and prices have been up and down a lot over the years. Here are a couple of recent threads about where to get A123 cells on the battery forum:

http://www.rcgroups.com/forums/showthread.php?t=1750495

http://www.rcgroups.com/forums/showthread.php?t=1750102

Those cells are about 40% heavier than LiPO for the same capacity and the charged and rested voltage starts at about 3.3-3.4V per cell. So using packs with one more cell that a LiPO pack would have get you similar starting voltages.

The attached image shows the discharge curves for A123 2300 mAH cells are varous rates. Those are very realistic and reliable numbers and the info comes from one of he guys that was one of first users of them and who has used them for the longest time.

The bottom line for me has always been that if I am flying something that can handle the extra weight and that also has the room for them, I will use A123 cell packs. And in some cases I use A123 and LiPO interchangeably by using one more cell in the A123 packs.

Jack
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Old Oct 14, 2012, 09:19 AM
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So, if you intend to run 'em at 10C you need your LVC set at ~2.7v/cell?
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Old Oct 14, 2012, 12:17 PM
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Thank you for the info, now that I know where my problem is coming from I'll check out these batteries. I like the suggestions of a BBQ or old pressure cooker for LiPo charging, I don't have a balcony/garage I can use at my apartment building but I'm collecting all the LiPo info I can for the future when I'm not so worried about setting fire to my small space and burning down a dozen peoples homes at the same time.
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Old Oct 14, 2012, 12:23 PM
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Lipos do pose a real risk but not an overwhelming one or they wouldn't be so common.
The biggest problem by far is crash damage and people on the end of the charger+battery without knowledge of the devices or screwing up when charging.
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Old Oct 14, 2012, 12:34 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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Quote:
Originally Posted by Dr Kiwi View Post
So, if you intend to run 'em at 10C you need your LVC set at ~2.7v/cell?
The best thing is to have a setting for pack voltage and set the sum of all the cells. On the CC ESCs I can set 10.5V and that gives a 3S LiPO and 4S A123 cutoff that works. I get 3.5 on LiPO and 2.6V on A123 that way.

All things considered, you can turn the LVC off and just fly the A123 pack down until you feel them weaken or to a time duration you have learned.

The image shows how the cells hold voltage at various discharge rates. That sort of goes to answering the OP's question.

Jack
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Old Oct 14, 2012, 12:38 PM
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Quote:
Originally Posted by Dr Kiwi View Post
So, if you intend to run 'em at 10C you need your LVC set at ~2.7v/cell?
One of the hassles with A123 is setting the LVC. Most ESC will only do lipo or NiXX (Castle with the USB programer being one exception). Neither are quite exactly what you want. You can LVC them to 2.0v/cell without problem. Sometimes the best option is NiXX, or like Jack suggests...pay attention and land early.

But....one of the big problems is when they go dry.....they go DRY! Re-arming the ESC may only get you a very weak and short term result.

The 1100mA are just not up to the specs of a decent modern lipo pack for small planes, and the 2300's pretty much are too big until planes get over 3# or so.
I used to use the 1100mA but don't bother anymore, not worth the weight penalty, size restrictions, and LVC hassles. Good batteries though if you can find a place for them.
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Old Oct 14, 2012, 01:08 PM
Jack
USA, ME, Ellsworth
Joined May 2008
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My experience with them has been vastly different than that of flydiver.

I've used 3S 1100 (in lieu of 2S LiPO) on planes down in the 16-18 oz. and up size. Any of my planes that will use a 1500 mAH or larger LiPO can usually fly with a 4S A123. That is, in part, because I tend to over size the power system and not fly at full throttle all the time or most of the time.

The A123 cells hold voltage well and than fall off quicker from 3.0V and down at the end, that can be seen in the graphs above. But you don't have to be very smart to read the signals and/or plan ahead to avoid getting caught in the air with useless batteries.

If you are an ultralight or light weight plane kind of person and apply that to the build and power system, A123 probably will not work for you. But it is not the fault of the battery, it is more the fault of the person that tried to use it in the wrong place.

LiPO is what it is, A123 is what it is. They are apples and oranges to some extent. But the A123 cells can be as useful and are as wonderful as LiPOs in their own way. We are still waiting for the first A123 cell fire and most of us are still waiting for our first A123 cell to die in use. That is worth something.

Jack
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