how to find out which 1 is gone

[zoaman]

I have a 50a esc running on 4s with a 650kv outrunner
this is what I have from specs and my findings.but some thing don't add up
16.8v battery fully charged
0.053 ohms on the 3 wires (correct as per spec)
1.8a per wire at 10.8volts from esc (correct as per spec)no load
now on no load motor should rpm at 650x10.8 =7020
I have 10000 no load
then when I add prop I only get 4750rpm
I have reset esc to factory, battery reduces equally per cell
the problem I have is, it seems as though I only got 2/3 power.
this happened in mid flight from fully working to 2/3 power in a instant
just like a low volt lipo.but was still well above low volts
any help is apreciated

[sosidge]

I don't understand how a 4s LiPo is giving 10.8 volts running a motor under no load. And that you would say this was within spec.

Surely a fully charged 4s (nominal voltage 14.8V) should run a motor at no load at around 14.8V?

[zoaman]

if you look at this link it may explain.but I have done the same check on a 7.4vdc lipo and 2 cell esc it comes out at 5.8v.ac and nothing wrong with that motor or esc.
http://www.dynam-rc.cn/Products_view_213.html

[Dr Fly]

I think you are getting things mixed up. the data sheet says it will pull 1.7a @ 10v. Basically this means it will draw 1.7 amps when you supply it with a 10 volt source. Your 4 cell battery (14.8v) is higher than 10 volts so i would expect the motor to draw more current.

I would expect your no load RPM to be 650 x 14.8 = 9620, which is closer to what you have measured.

[zoaman]

hi Tom.yes missed that but that don't explain the reason of only 4750rpm with a load. yes I know load but half seems to be low. just feels likle only 2/3 of power. this happened in mid flight. like a low volts cut off. but alls well with battery and plenty of power left in battery.
Why only 10.7 out from esc
have done a test on a 7.4 setup n get 5.8 from esc and that runs fine.

[Great_Thark]

What are you measuring the esc voltage with? The esc output isnt dc. The peak voltage output will Be same as the battery voltage but thats not what the meter reads. The peak to peak v wont change, its the frequency and pulse width that changes. Using ac meter will interpret this as voltage change but it's not really whats going on. To really see whats happening you need to put a scope on it. See if all the waveforms are equal and maybe a current shunt and or current clamp on scope as well.
Problem might be the voltage cut off if you have it set for reduce power. Try disabling it.

[zoaman]

yes I have read the esc output with ac.across the 3 wires is equal 10.7v ac drawing 1.7amp no load.i have reset the esc and made sure of low volts cut off and I have adjusted the timing mode (3 different ones).low is 9500 and high being 9800.jus when theres a load it runs a 4750.should it be 14.8 nominal from esc to motor,and ive only got 10.7. one thing I haven't checked is the capacitors.if one has gone would this give a lower volts to motor from esc.?

[Great_Thark]

The capacitor would likely make less difference at full throttle than low throttle as the peak current draw is higher at low throttle, just a lot lot shorter. This is why multimeters don't tell you what is going on.

There are 2 things that FETs need to handle peak current, usually at low rpm and total power at high rpm, less current but for longer. this is why you always use an esc with a rated current higher than you need or measure and they often quote peak current higher than flc, this is not for brief overload but to cope with low rpm current peaks.

You can measure current at the input because the caps smooth it out, but at the output you are only measuring mean voltage across 2/3 of the windings at any one time.

As long as your not overloading the motor / esc its likely an esc problem, for what a 50A esc costs from hobbyking id just replace it maybe go to a 60A

What size prop do you have on it?

This shows what is happening and explains it

http://forums.offshoreelectrics.com/showthread.php?18391-Waveform-of-a-Brushless-Motor-ESC

Have a read of this

[zoaman]

http://www.dynam-rc.cn/Products_view_213.html
this is the spec for the motor.
running a 13x6.i know it says 9/12 but this came stock.
also found this on a site
EMGC-3720 Brushless Motor 650KV 1000W
3720
KV (RPM/V)
650
Operating Power
1000W
Operating Voltage
3S ~ 6S (6V-23V)
Operating Current
50A - 70A
Weight
150g
Recommended ESC
50A - 70A
Recommended propeller
12x6, 14x7, 15x5, 16x6, 18x4, 18x6
Size:
37mm (dia) x 39mm
Shaft Size:
Propeller adaptor (6mm dia
http://static.rcgroups.net/forums/images/beta/statusicon/icon-status-offline.png http://static.rcgroups.net/forums/images/beta/buttons3/icon-pm.png (http://www.rcgroups.com/forums/private.php?do=newpm&userid=472889&quotepost=27194694) http://static.rcgroups.net/forums/images/beta/buttons3/icon-search.png (http://www.rcgroups.com/forums/search.php?do=finduser&userid=472889)

[dodgydiy]

you will probably only get about 70%of the stated peak rpm on load due to efficiency of the motor and speedo and voltage sag from the battery, what capacity and c rating battery are you using, and what sort of current are you drawing with an ammeter? for the battery to sag that much it sounds like the cells cant handle the power output required. also with more cells you may need to go smalller on the prop, again use an ammeter to see how much current you are drawing. if you are drawing more than 30 amp on your 50 amp speedo then consider getting a larger speedo as you never actually see the peaks unless you are datalogging. dont bother measuring voltage after the speedo, it is only possible to do this accurately with an oscilloscope due to the esc's drive frequency, actual voltage will be very close to input

[mpg200]

I agree With Dodgy, Measuring battery voltage in no load conditions is not very informative. You need to know what the battery is doing under load to get an indication of its health. You only need one of the 4 cells to increase internal ressitance to cause the Lipo cut off to operate. See if you can borrow and test another 4 cell lipo pack and see if the problem is resolved.

[Dr Fly]

yep agree with needing an oscilloscope to look at the true output voltage. the ESC regulates the power delivered to each phase of the motor by modulating it at a pretty high frequency. So when each phase is switched on, the esc also switches the phase on and off to regulate the power. This frequency will be different depending on the make and model of the ESC, i have measured 8kHz on a turnigy one. This is a lot higher than the frequency that it is then switching the phases, which will be the motor RPM if its two pole.

I would have thought that this would be pretty hard to measure on an AC voltmeter. My next step would be to buy or borrow a different ESC and or battery and see what eliminates the fault.

[zoaman]

ok so the battery is 2200mah 25c rated.14.8 nominal 4s
draw from battery at full power is 6.86amps
oh and lets not forget that this is all stock. battery esc and motor and prop

[mpg200]

When under full load (6.89A I assume this is average and not peak) try and meaure each individual cell voltage.
This could give you an indication of if it's a battery issue or not. They should all be around the same 3.7V nominal. I had a similar problem with a helicopter.
A 3S pack would measure 4.2V per cell fully charged. But as soon as I gave it a load the middle cell would collapse to 2.6V. This would drop the pack voltage to 11V.
The heli would work with this but not for long, as the good cells would drop to 3.4 volts then the Lipo cut off would activate. Of course running at 2.6V on the middle cell soon killed it so it went to the big recycling center in the sky.

[zoaman]

yes as said battery is ok with 4.2 per cell and after a 5 min run all cells down to 3.75 per cell. if not please advise.
also 6.86a was at full power under load.

[mpg200]

3.75V per cell under full load conditions?

[zoaman]

may be I should have said :yawn:
6.86amp on full throttle for just approx. 10 sec to test the draw...
the battery under flying drops equally to 3.75 per cell on a five minute run in another plane,
so drops equally per cell.
as said before I don't think its the battery. but that's what im asking. how to test if the battery is giving enough or the esc /motor is drawing enough:confused:

[Great_Thark]

that motor & prop should be drawing a whole lot more than 6.86A add 40A. If your windings measure ok and you haven't lost any magnets out of your motor (it happens) it really does look like your esc is duff.

these turnigy dlux are good if a little bulky. worth getting the programme card as you can then access the data log. they are an odd shape so check to make sure it will fit

http://www.hobbyking.com/hobbyking/store/__28710__Turnigy_dlux_55A_SBEC_Brushless_Speed_Con troller_w_Data_Logging_UK_Warehouse_.html

http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=31788

[zoaman]

yes they look good. the reason I wanted to know ,is this is the second set,esc and motor I have had and not sure if they are running on the limits.
as sais this is all stock from dynam pitts python

[Great_Thark]

the motor spec says max 630w. I would say the prop is about right. you could maybe even go a little bigger. The advantage to the above esc is you can see what your peak amps and rpm where when you get it back on the ground.

[dodgydiy]

you should be drawing 30 to 40 amps on a 600w motor if propped correctly, thats pretty close to the 55A limit of that battery, but to be drawing only six amps, if the motor is running smoothly it has to be a speedo problem

[zoaman]

update. I have got my hands on a peak atlas lcr40.well it done a self test and it comes out
15khz and 0.2ohms.so can any body shed any light.is this correct as its quoted a being 0.053ohms. ???

[Great_Thark]

would of course help if you told us what you are testing!

I presume its the motor, is that reading the dc resistance or the reactance at 15khz both are in ohms.

the quoted resistance will be dc. Reactance changes with frequency and changes again in a motor once it is spinning, what you are measuring is static reactance of the winding which is fairly meaningless unless you have a known good motor to compare. The odds are your esc will be switching at 8khz

If you want to be able to diagnose this stuff in the future I suggest you trawl the net for some basic electronics stuff, really need the grounding to understand how it all works and your obviously interested.

In the meantime to get you in the air (winds forecast to die down next week woo hoo)I will stick to my earlier guess of a duff esc. The ones they include in RTF and ARTF tend to be pretty poor.