Twin Electric Motor Wiring Help

[Tim A]

Need some input and thoughts re a twin electric motor setup intend using a separate Ubec to supply power to receiver and servos (4 control servos & 2 servoless retracts plus 2x 40 esc's). Flight battery 4S & 2S receiver battery.

 

Got the basics sorted but not willing to power up until I get some more input.

 

 

[PatMc]

Just cut the red wires of both ESCs or use opto ESCs & connect the UBEC inline with the battery.

[Ben B]

 Eh? Are you using a 2S receiver battery or a Ubec?  Or is the plan to use the 2S battery to power the receiver but the Ubec to power the servos?

Obviously if you're using one battery and two ESCs you'll need a power Y lead also....

Edited By Ben B on 27/06/2011 10:31:55

[graham kindberg1]

If the esc's have bec's, the red wire in the throttle leads will have to be isolated. The best way to do this is to slip the pin out of the plug, insulate it with 2mm heat shrink. the lead from the ubec plugs into the batt on the receiver. I have not found the need to use a separate battery for the receiver as the main battery will have plenty of reserve to get you home long after the motor/s have cut out on low voltage. I have 5 twins ranging from a 48" running on one bec,(second one isolated) to two 90' warbirds on uebc's, and just the one main battery.

Graham

[Tim A]

Thanks Graham, just what I needed from someone with actual experience with this type of setup, not needing a seperate receiver battery will simplifly things a lot.

Also I have to lengthen battery wires by 12-14 inches as I intend to keep esc's in nacelles for airflow & simplicity, will fit extra capacitors and use 10 SWG wiring which I hope will eliminate or at least minimise any problems. What is your experience good or bad with this? There is a lot of contradictory info on this about but most of it seems to be theoretical rather than actual experience? Also I am using 2.4Ghz as most talk seems to centre around problems with 35 Mhz

[graham kindberg1]

Hi Tim

I regularly extend the battery leads, it is widely known the preference is to extend the wiring from the esc to the motor, than the battery leads. I have for many years extended the battery leads, and correctly done, this will not be an issue. The problem with "interference" is where large fields of magnetic flux is produced by the relative large power (wattage) we use. This is relatively easily controlled by keeping the cables tightly together, a good set of twists throughout the length of the run, with the odd short ring of heat shrink to keep it all in place. The DC cables (batt leads) have a constant magnetic field around them, the positive and negative lead alongside each other will all but cancel each other out. AC leads (esc to Motor) have a making and collapsing magnetic field around them. This too is self canceling as long as the cables are close to each other, again a twisted cable run is needed.

I don't see the need to fit capacitors as these will only flatten out voltage spikes within the circuit, and if you aren't doing anything radical in your setup, shouldn't be needed. They will already be on the output of the speed controller.

 

You are on the right track with the 2.4 radio, it was an overnight gift from God for electric flight! Nailed all those spurious interference issues in one hit.

Regards

Graham

 

 

[PatMc]

Graham, I'm afraid your explanation is entirely wrong.

 

The problem between battery & ESC isn't interference it's voltage magnification (spikes of HV) caused by the inductive impedance of the wire & fast pulsing of the DC. Keeping the wires together & twisting them will make the potential problem worse by increasing the inductance. If they have to be extended extra capacitors across the wires can help.

If the voltage spikes are too high they can cause damage to or entirely burn out components in the ESC.

 

The wires between ESC & motor also carry pulsed DC (not AC) & these can't "self cancel" any magnetic fields either since the current is 120 degrees out of phase between any pair. The edges of the DC pulses carried on the motor wires will be rounded off because of the high inductance of the motor windings (Lentz's law) which will reduce potential RF interference from this source.

 

2.4G has no effect on the voltage spike problem neither has the scale of power used.

[Peter Beeney]

 

 

PatMc, I think to some extent I’m with you on this, but I’m not completely convinced about all of it. I have to say that I’ve not given a great deal of thought to it anyway, but I would certainly consider that power wires are run close together to reduce the inductive effect, i.e. the effect they have on any close adjacent wiring. Again, I’ve not had too much experience of ESCs, but I’d tend to think there is a (relatively) large amount of capacitance on the input to the ESC, as you say, whereas Graham seems to be suggesting this is on the output. If you only increase the cable length a small amount, and the power increase likewise, then you can get away with it, but if you overdo it, again I agree, you might zap the ESC, probably the capacitor first; in some cases they work really hard even as per the standard set up, I know this from an Aveox brushless geared inrunner in a hotliner I use to fly.

Regarding the supply to the motor, AC or DC? - …we’ve had this conversation before, I believe. Rationally, a moment’s contemplation for me says it’s pulsed DC but when a test instrument that’s specifically designed to tell me what I’m looking at says that it’s AC, who am I to argue? I will try an make an effort to have a little pogger on this, and arrive at least at a half-baked conclusion. That will be closer than what I am now!
Also, I thought I’ve always understood that if you extended the motor leads, you twist these together? Sounds like a good idea to me.

I noticed, too, that Graham is also advocating the disconnection of one red wire when using a two BEC configuration. I still regard that as a bit of a hoary old chestnut! That keeps just on circulating, ad infinitum! As I’ve said before, we are ORWISC, Our Red Wire Is Still Connected. This may be a flippant throwaway line, but I just can’t resist throwing it in the ring, and as we’ve never seen anything close to a valid reason why we need to do this, we shall continue so not to do; until there is a genuine argument to change our perspective on this, we will consider that’s it’s not necessary. Just to substantiate our position on this a tiddly tad, there is the tale that I’ve related before, which concerned a battery connected in parallel with a regulator; and when the operator disconnected the battery it caused the model to crash!! Conventional thinking turned completely on it’s head! In some ways it was an unusual case, but very much demonstrating the facts of what I’m saying.

I reckons on how all this new fangled lectric stuff don’t not always ackle like wot it’s s’posed to ackle! Gets me all conn-fused, like, cos I just don’t sees it comin’ or goin’!

 

PB

Edited By Peter Beeney on 29/06/2011 13:20:32

[PatMc]

Peter, when I mentioned "inductive impedance" I should more accurately called it "inductive reactance" . Impedance (Z) being algebraic sum of resistance (R), capacitive reactance (C) & inductive reactance (L) - R+C+L = Z.

I was considering the possibility of damage to the ESC due to voltage spikes which are a proportional to inductance & frequency. Twisting the wires closely together would increase inductance.

I wasn't referring to was any inductive effect on adjacent wires which might cause rf interference.

 

Regarding the supply to the motor it can not be AC since the ESC doesn't alternately reverse the polarity of the supply.

 

I was under the impression that the OP wanted to separate the Rx power supply from the motive power which was why I suggested cutting both reds or using opto ESCs.

Like you I've never seen any convincing argument for not running 2 BECs in parallel.

 

Actually I would be inclined to use the BECs for the Rx & servos but a separate 2s lipo with UBEC or AAA nihm for the retracts.