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Old Mar 03, 2009, 08:48 PM
UH-1 Gunship, AH-1G jock 69-71
makeitworst's Avatar
So Cal ... Kingdom of Taxes
Joined Nov 2008
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Max wire length between motor and ESC?

I've seen a few discussions on this topic, the maximum length that the wire between a brushless motor and ESC can be. I'm looking for a concrete answer, based on REAL electronics engineering fundamentals, not old wives tales (don't tell my wife I said that).

Some model planes, especially Glow to Electric conversions just don't allow for putting the ESC within a standard wire length (as delivered with the motors and ESC), so can distances like 9", 12" 15", 20" be acceptable? Can changing the wire gauge compensate for longer distances?

Would love answers from the Forums Electronics Engineers, state you MS of EE origin or IEEE Certificate :^) Seriously.
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Last edited by makeitworst; Mar 03, 2009 at 09:35 PM.
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Old Mar 03, 2009, 10:51 PM
Space Coast USA
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IMO, this is the best you are going to get.
http://www.rcgroups.com/forums/showthread.php?t=952523
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Old Mar 03, 2009, 11:35 PM
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Quote:
Originally Posted by makeitworst

<snip>
I'm looking for a concrete answer, based on REAL electronics engineering fundamentals...
<snip>

Would love answers from the Forums Electronics Engineers, state you MS of EE origin or IEEE Certificate :^) Seriously.
Hi makeitworst,

I have a nagging question in the back of my mind that I must ask.

If two engineers came on, stated their educational degrees, and/or documents proving their authority, and used all manner of technical jargon and formulae, and advanced theory laced with complicated math equations, yet arrived at different answers in spite of their technical explanations and 'proofs'....

How would you tell whom was correct?

Chuck
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Old Mar 04, 2009, 04:19 AM
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Staffs, UK
Joined Nov 2003
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Quote:
Originally Posted by makeitworst
Would love answers from the Forums Electronics Engineers, state you MS of EE origin or IEEE Certificate :^) Seriously.
Before anyone goes to all that trouble I think you should state your qualfications for understanding the answers. There's no point casting electronic engineering pearls to the uneducated masses .

BTW in general motor to ESC distances don't cause real problems. It's battery to ESC that's more difficult.

Steve
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Old Mar 04, 2009, 07:40 AM
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East Anglia, UK
Joined Sep 2002
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Anyone who has unwound a motor can tell you how much wire is permissible in the leads to the motor
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Old Mar 04, 2009, 11:13 AM
We want... Information!
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Hastings, New Zealand
Joined Jan 2001
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Long motor leads increase three fundamental electronic properties - Resistance, Inductance, and Capacitance.

Resistance causes voltage drop and power loss. Since our motors draw high current, the external wiring should have very low resistance to maintain voltage at the motor, and to avoid unnecessary power loss. However, thick wires are heavy and we don't want to add extra weight, so a balance must be struck between wire size and losses. Running a higher voltage reduces resistive losses in the wires, because the same power can be delivered at lower current. Therefore, if wire weight is a problem then you should use lower Kv motors and a higher battery voltage.

Inductance slows down current changes, but causes voltages spikes if the current tries to change rapidly (eg. when power to a motor winding is switched off). Our ESCs have re-circulating diodes which maintain current flow in the motor windings during switch-off, so the inductance isn't a problem. In fact it is an asset for reducing current ripple during PWM. The motor windings usually have much greater inductance than the external wiring anyway, due to the high permeability of the motor's iron core.

Inductance is a problem in the battery wires, because the ESC cannot use re-circulating diodes to prevent voltage spikes at its input. Instead, large electrolytic capacitors are used to provide a current reservoir which soaks up voltage variations, but the capacitors also have ESR and internal inductance which limits their abilities. Therefore it is better to have the extra wiring inductance at the motor end, where the ESC can more easily deal with it.

Capacitance is small and generally not a problem at the frequencies our ESC's use (most ESC's already have snubber capacitors of much larger value, to prevent high frequency ringing in the motor wires).

Quote:
Would love answers from the Forums Electronics Engineers, state you MS of EE origin or IEEE Certificate :^) Seriously.
Engineers? Seriously???

Q: How many engineers does it take to change a light bulb?

A: None. "According to my calculations, the problem doesn't exist."
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Old Mar 04, 2009, 02:32 PM
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New Hampshire (not the old one)
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I'm not an old wife so guess it's ok to tell my tale. Also member of IEEE (Cutout thousands of EDN "Design Ideas").

I had power loss and sync problems extending motor wires and nothing but good luck with long battery wires. Experts say this can cause problems and I'm sure it's true if you run high power setups near their limits. In my case never had trouble extending a foot or so running a few amps.

Quote:
Originally Posted by makeitworst
I've seen a few discussions on this topic, the maximum length that the wire between a brushless motor and ESC can be. I'm looking for a concrete answer, based on REAL electronics engineering fundamentals, not old wives tales (don't tell my wife I said that).

Some model planes, especially Glow to Electric conversions just don't allow for putting the ESC within a standard wire length (as delivered with the motors and ESC), so can distances like 9", 12" 15", 20" be acceptable? Can changing the wire gauge compensate for longer distances?

Would love answers from the Forums Electronics Engineers, state you MS of EE origin or IEEE Certificate :^) Seriously.
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