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Coming back after 50 years: bit lost

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Denis Watkins13/02/2018 20:48:01
2899 forum posts
141 photos
Posted by Andy48 on 13/02/2018 20:16:00:

Almost all my models have extended leads, one or two significantly so. My Puppetter probably has nearly 30cm extensions. Yet to have the slightest problem, and yet to damage an ESC. Many of my ESCs are over 5 years old. Love these doom mongers!

 

Your right Andy, there is often not enough cable for a normal installation.

The manufacturers just don't want failures of their products due to dodgy soldering, or the Back EMF from long cables.

A long cable, and the motor, has a small amount of capacitance as well as resistance, and when the supply is interrupted, by switching off or disconnected

The small amount of current in the cable, and the motor can runaway unregulated, simplified explanation of EMF

And can be reverse polarised going into the ESC

As you found, with good soldering and installation, and modern ESC components, these small amounts are rarely any problem

Edited By Denis Watkins on 13/02/2018 21:03:34

PatMc13/02/2018 21:26:13
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3744 forum posts
479 photos

It's not the capacitance of the cable but the inductance that causes the voltage magnification problem, that's why it's often advised to use extra capacitors with long leads.
There's no reversal of the polarity or back EMF to the ESC, simply high voltage spikes.

Edited By PatMc on 13/02/2018 21:30:00

Martin McIntosh13/02/2018 21:45:56
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2484 forum posts
947 photos

No, I am not a doom monger, just passing on information learned the hard way. When I was building my Mosquito I was advised that all ESC manufacturers say not to do this, but you may get away with it if you add special high value low Z capacitors fitted directly to the ESC which I did since I needed to extend by 30cm or so with very heavy duty cable. Despite vigorous testing one failed at take off on only the second flight and the model was destroyed (12 months work gone). Draw your own conclusions. The offending item did not completely fail but only gave very much reduced power. I would say that given the OP`s stated very short wiring that short extentions should not be a problem, but the way round it is to lengthen the ESC to motor wires if that would help.

911hillclimber13/02/2018 22:17:43
53 forum posts
23 photos

I have tried to minimise the extension and to keep both leads the same length.

I used the extension lead that came with that isolation switch, and the ext lead gauge is the same as the ESC and motor leads.

Is there a test I can do? I have run the motor without a prop fitted to see if the TX was working for several runs but with no load on the system.

On a positive note, I banded on the flying surfaces and undercarriage and the CofG is spot-on.

Obviously, I will join the local Club closer to reasonable weather, go along with the plane and cross my fingers!

Thank you to all as ever taking time to help me on this quest.

Geoff Sleath13/02/2018 23:55:15
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2711 forum posts
199 photos
Posted by Andy48 on 13/02/2018 20:38:39:

Now there's an interesting point. If you must never extend the ESC power leads to the battery, how do you use a wattmeter, because the wattmeter effectively extends the leads?

Just not for a very long time. My wattmeter tests are carried out for just a few seconds - enough time to note the current before shutting off.

The problem is that many high value capacitors tend to have a lot of inductance associated with them because of the construction method. I used to use solid tantalum capacitors for things I designed at work which have an inductance the same as a block of metal the same size (ie not much). I retired over 20 years ago so presumably capacitor design has moved on that time The ones I used were mil spec because I needed them to work at high temperature.

Geoff

Simon Chaddock14/02/2018 00:38:11
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5169 forum posts
2708 photos

Extending the battery to ESC leads does not kill the ESC instantly (unless you make the leads very long indeed) but the resulting extra load on the capacitors shortens their life and when the capacitors fail the ESC dies.

Piers Bowlan14/02/2018 06:44:46
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1306 forum posts
36 photos
Posted by Martin McIntosh on 13/02/2018 21:45:56:

No, I am not a doom monger, just passing on information learned the hard way. When I was building my Mosquito I was advised that all ESC manufacturers say not to do this, but you may get away with it if you add special high value low Z capacitors fitted directly to the ESC which I did since I needed to extend by 30cm or so with very heavy duty cable. Despite vigorous testing one failed at take off on only the second flight and the model was destroyed (12 months work gone). Draw your own conclusions. The offending item did not completely fail but only gave very much reduced power. I would say that given the OP`s stated very short wiring that short extentions should not be a problem, but the way round it is to lengthen the ESC to motor wires if that would help.

So sorry to hear about the demise of your magnificent Mosquito Martin. Can you be sure that it was the extended battery lead that caused the ESC to fail, despite you adding the special capacitors to smooth out the voltage spikes? As we all know, these ESCs are made in huge numbers and can inexplicably fail occasionally. An 'engine' failure with an electric twin would seem most unlikely compared with an IC model but never say never.

Extend motor wires yes +1

Martin McIntosh14/02/2018 09:19:01
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2484 forum posts
947 photos

Hi Piers, I have actually pieced the wreckage back together and it is ready to go apart from a new bomber nose moulding. This time with YEP 100A ESC`s and the motor wires extended. I cannot say for certain of course that the battery lead extentions caused the failure. Pics on my Mossie thread.

911hillclimber14/02/2018 13:07:02
53 forum posts
23 photos

p1060576.jpgp1060575.jpgp1060573.jpgJust read the Haynes Manual on the DH Mosquito, great book.

I thought I would post the Stol as it now stands, ready for the first flight in the late Spring.

p1060571.jpg

 

Edited By 911hillclimber on 14/02/2018 13:07:39

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