motor slowly loosing speed/power - RCCanada - Canada Radio Controlled Hobby Forum
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Old 04-11-2012, 03:53 PM   #1
Thunder Chicken
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motor slowly loosing speed/power

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So being new to electrics I do not have the knowledge to troubleshoot this problem so here are the basics:
Turnigy 50-65-275kv motor
Zippy 20c 5000mah lipos (4s and 3s in series for 7s total) Both charged and reading 4.15-4.2v per cell.
70a Turnigy K force ESC
4 blade 15" prop, adjustable pitch.
watt meter and programming card for above mentioned esc.

I mounted everything up to the test stand to play and learn. When I power up I initially get 12-1400 watts, at about 40 amps then it starts to drop slowly, loose RPM and stabilizes at about 5-700 watts and 20-30 amps. The initial throttle setting is advanced rapidly to wide open. Then it 'bogs down' so to speak. Everything feels warm to the touch but not hot (after running it like this for a minute)
I tried changing the timing setting in the esc and no change at all. I also adjusted the low voltage cut-off to the lowest setting and no change. Once I had enough fun, I checked the batteries and they were all about 3.7v per cell. The 420 charger shows them at 50%.
I'm leaning towards the batteries? is the 20c rating (on a 5000mah pack) enough for this?
I'd like to get about 1500 watts out of this set-up, on 6 to 8s. Have I chosen some wrong equipment?! (besides cheaping out with all HK stuff!!)
Thanks for any insight
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Old 04-11-2012, 04:18 PM   #2
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Re: motor slowly loosing speed/power

Are the batteries brand new? Most need a cycle through them (not run to hard). Also what type of connectors are you using? If the are cheap they may be heating up and increasing resistance. The only other thing I would check is the esc as it may be flakey, try another if you have one
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Old 04-11-2012, 06:23 PM   #3
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Re: motor slowly loosing speed/power

Measure the voltage right at the input of the ESC with a voltmeter. I would guess that the voltage is dropping either because the battery can't handle the load, or the resistance of the connectors and wire is increasing when they get warm. Another possibility is that as the motor gets hot the winding resistance increases and it will not continue to put out the power you expect. Try cooling the motor with a fan or even better stick it in the freezer and run a test ..
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Old 04-12-2012, 07:03 AM   #4
I am: Philip A
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Re: motor slowly loosing speed/power

Another possibility is the ESC could be overheating. Most ESC's have a safety feature that reduces power if they overheat.

BTW, you didn't say what make/model Wattmeter you are using. If it's one of the cheaper Hobbyking brands that uses a crude current shunt, I would be very suspect of the readings.

Another thing.... Assuming your last set of readings are accurate:

700W/30A=23.3v or 3.33v/cell

Those lipos are pretty close to fully discharged at that level. And it appears you only ran them for a few minutes. Considering your discharge rates and Lipo size, you should get something around 8min or so of run time at full throttle

I am seriously questioning the accuracy of your wattmeter!

Last edited by philipa_240sx; 04-12-2012 at 07:12 AM.
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Old 04-21-2012, 08:13 PM   #5
Thunder Chicken
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Re: motor slowly loosing speed/power

Thanks for the tips.
I checked the watt meter's calibration and adjusted it although it still does funky things (what do u expect for $8!)
I'm using the HK deans connectors.
This first run was with batteries that I've had here (since new) for a while. I charged them up and ran them.
I'm happy to report that after recharging them the system worked great.
Fully charged I was getting 1200ish watts, with a voltage on the fully charged 7s of about 26 volts. After about 5 min of running the voltage was down to 24v and about 1000 watts. Also should mention the RPM is about 5500-5800 on the 15" 4 blade. It sure feels like there is LOTS of thrust there, It should fly this 15 pound Otter no problem. Now to get it painted............
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