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Mar 17, 2009, 08:43 PM
Q' = Q prime
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Mar 17, 2009, 09:08 PM
On the Edge of Space
dLdV's Avatar
In the integral, what property does q represent? In the US we call a term followed by the notation ' prime. W=power, C=capacitance, V=voltage, Q=?.
I am familiar with Q in AC power and a different Q in tuning circuits.
Mar 17, 2009, 09:24 PM
On the Edge of Space
dLdV's Avatar
fh, if you're still there, I've also forgotten what ESR is. Can you jog my memory?
Mar 17, 2009, 09:43 PM
On the Edge of Space
dLdV's Avatar
Never mind, there was a Wikipedia link on the other link that brought Q and ESR into focus. Ha ha, its getting bad when I'm too lazy to follow the links and want to ask somebody instead. I was too lazy to look in my old textbooks because they're covered in airplane boxes. What'll I do when the computer gets covered by airplanes? Thanks for a great thread everyone.
Mar 18, 2009, 07:56 AM
Geaux Saints
Hopalong X's Avatar
Ron van Sommeren

So what is the safe maximum length of battery wires you should use under normal usage with factory ESC?
I realize it may vary with the ESC.

Keep maximum length?
6 inches?
9 inches?
12 inches?
or something different?

Mar 18, 2009, 08:13 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
I don't know of any rules of thumb for ESC-motor length but since there's already a lot of wire in the motor, it will not be very critical. Too long won't harm motor nor ESC. It may cause interferenece (give wires a twist).

Vriendelijke groeten Ron
Mar 18, 2009, 09:01 AM
On the Edge of Space
dLdV's Avatar
I agree with the last post. The motor windings are inductors. There are lots of uses for inductors but in this case we like the fact that they make a magnetic field we can control the strength and polarity of depending on the direction, duration and magnitude of power we put through them. If we measure the strength of the field at the motor windings it will be much much greater than the strength of the field in the transmission line, therefor the inductance of the transmission line is trivial compared to the inductance of the motor load. My longest motor leads are 22" (56 cm) in a pusher A=10. The motor and ESC operate close to max rating and run in the full range of gliding to WOT. I have had no problems with heating, control or reliability. Also, we operate at relatively low frequency so that reflected power, PSWR, is not really a concern.
Mar 18, 2009, 09:30 AM
On the Edge of Space
dLdV's Avatar
Whoops, sorry hopalong, I answered the wrong question. You are concerned with battery wires, not motor wires. Most ESC's come with sufficient input filter capacitors installed to deal with about a foot of battery leads,no problem. If you need to go longer, you should add caps to be safe. The first posting on page 1 of this thread explains how much capacitance to add to the input of your ESC.
Mar 18, 2009, 09:46 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
And I answered the wrong one too

Just follow Ludwich Retzbach's rule of thumb. Too much capacitance is not a problem.

At WOT life is easiest on controller, no PWM switching then, no need for extra caps.
Apr 10, 2009, 09:02 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP

"The main spec you need is low impedance and low ESR(equivalent series resistance). I think the only thing you will find at radio shack will be general purpose caps, not low ESR. The ESR of a cap won't be printed on it, you will have to look up the manufacturers spec sheet. The Rubicon ZL series mentioned in the Schulze instructions is a good one and is available from newark/farnell. The Panasonic FM series is another good low impedance cap and is available from digikey
PS You would want the voltage rating on the caps to be significantly higher than the battery voltage. Same voltage rating is the installed caps. Higher rating is no problem. (thanks jeffs555, from this thread)"

"If you try it with longer wires and no extra low esr caps it may work for a while, but the longer wires put an extra load on the original cap. The extra load shortens the life of the original cap and it will eventually fail, probably catastrophically. (thanks jeffs555, from this thread)"
Apr 10, 2009, 09:26 AM
I ordered my low ESR caps from Mouser Electronics. They will sell you 1 or 100,000. Shipping costs more than the parts for low quantities. They were less than 50 cents a capacitor.

I've seen a capacitor blow up... a small one that we plugged into "megger" (hand cranked meg-ohm meter that could put out 500,000 volts) blowing up a TINY capacitor was impressive.
You don't want the ones on your ESC to blow under max load. (what the cap considers max load...)
Apr 10, 2009, 02:52 PM
Registered User
jooNorway's Avatar
Originally Posted by dLdV
Also, we operate at relatively low frequency so that reflected power, PSWR, is not really a concern.
Try to calculate the lenght when the SWR is as bad as it can be; at 0,5 Lambda "things" happends. I.e. if the lenght from the feeding source to the "passing-device" is 0,5 Lambda everything is sent back As you said, at "our" frequencies those lengths are longer than anybody would be close to.

I`ve seen some caps blown during my 23 years in my present job. Not a pretty sight. Happends in old output stages on HF-equipment etc.
Apr 10, 2009, 09:47 PM
Registered User
So I know what inductance is and I understand exactly what the problem is with long battery wires, but here is my question: the wires between the ESC and the motor have inductance too and if I understand it correctly the motor draws varying current so the magnetic flux will thus vary, so the voltage spikes should also affect the motor. Is the motor more able to stand them than the ESC or am I not understanding something correctly?
Apr 29, 2009, 09:08 AM
Registered User
Hi Ron,
I have a 12" extension between my 11.1 V 3s2100mah lipos and the ESC. Your post says to use one 220uf capcitor for every 4". Some other posts in this thread say a 470uf cap?

If you have to lengthen the battery wires, add extra electrolytic capacitors. As a rule of thumb, for every 4inch/10cm extra length/distance between battery and ESC, add an extra 220uF capacity near the controller (electrolytic condensators, voltage the same as the capacitors already installed, low ESR type) (Ludwich Retzbach, German e-flight author&editor, the 'R' in LRK).

I plan to use three of the 220 uf Panasonic part# EEUFM1E221 that I will order from Digikey, part number P12383-ND. I'll put them in parralell across the ESC/battery leads observing polarity. Does this seem correct?

Thanks in advance. Rich
Apr 29, 2009, 10:25 AM
just Some Useless Geek
TSK06, the difference is that the motor windings are on the switched, or output, side of the control devices. Back EMF is contained and damped by circuitry within the output stage. The long battery leads create EMF pulses on the input side, which is the power supply for the entire unit. There is no built-in dampening for that.

You'll notice that all ESCs have electrolytic capacitors on the input to dampen some of that inductive pulsing. But as has been previously pointed out in this thread, you need to add more capacitance if you add more lead length.

One trick I do is to add caps in the middle of the battery leads if I extend them. Typically the battery leads on an ESC are kinda short. Nonetheless, they offer some inductance. Having the extension wires on the other side of a capacitor turns the whole mess into a kind of "double pi" filter.
Last edited by A Useless Geek; Apr 29, 2009 at 10:34 AM.

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