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-   -   2 brushless motor wiring? (http://www.rcuniverse.com/forum/e-flight-power-sources-126/9306000-2-brushless-motor-wiring.html)

HurkeyBird 12-05-2009 02:42 PM

2 brushless motor wiring?
I have a C160 Transall that I'm converting to electric. I plan on using 2 Hoffman Magnetics HMI-I 15 Outrunner 950kV motors. They call for a 45-60 amp ESC. I hate to pull that much current (90-120) out of one battery AND if I put a seperate battery on each ESC I stand the more risk of one motor shutting down before the other if the batteries are not perfectly matched. Can I put the ESC inputs in series to two batteries? In other words the Battery leads of each ESC in series. With 2 ea. 3S2P LiPo's in Series this should (?) give 11.1 volts to each motor. Or a single 6S2P would also work. Either way the wiring would only pull a max of 60amps where with everything in parallel the wiring would have to pull 120 amps max which I would rather not do. Am I correct in my thinking? What is the best way to wire them up. On my C130 (15 years ago) with brushed motors I ran a 2 sets 2 parallel in series with each other and that worked fine.

speedy72vega 12-05-2009 07:13 PM

RE: 2 brushless motor wiring?
Hurkey, i'm going to try to help you. Your post is a little confusing, but I think I know what you're asking.
First off, I want to clear one thing up. The ESC doesn't determine the current draw from the battery, the motor/prop combo does. You choose a speed controller with an amperage rating that will support your motor/prop current requirements.
Here's an example: Let's say you have a motor/prop combo that draws 15 Amps max static at full throttle. You can run an ESC that has a rating of 100 Amps if you really want to, but that motor/prop combo is still only going to draw 15 Amps. The load determines current draw, not the controller. Now, let's look at it the other way around. You have a motor/prop combo that will draw 50 Amps at full throttle. If you run a 30 Amp ESC, the ESC will most likely burn up because it can't handle 50 Amps, understand? The Speed controller only supplies current to the motor, but doesn't determine the amount drawn. Changing props will affect current draw, changing to a higher or lower Kv rated motor will affect current draw. As a general rule of thumb, the higher the Kv rating, the smaller or lower pitch prop you will need, and the lower the Kv rating, the larger or higher pitch prop you can run. The same with increasing or decreasing the number of cells in your batteries. The Kv rating means rpm per volt, so for every volt you feed the motor, it's going to try to turn that many rpm's. The more voltage, the higher the rpm's. The lower the voltage, the lower the rpm's.

Next, let's look at your question about the batteries. First, running batteries in Series compounds the Voltage, so if you ran two 3S packs in series, you would actually be running 22.2 Volts, not 11.1. If you want to stay with 11.1 Volts, you will need to run the packs Parallel. Running them Parallel will increase capacity, but not Voltage.
What you need to do if you want to run 11.1 V is to run the ESC battery leads in Parallel to 1 battery. If you run a 3S 5000Ma pack with a 30C discharge rating, that pack can deliver 150 Amps, should be more than enough.

You really should get a Wattmeter also, will help you tremendously in setting up your system properly.

Hope this helps some.

HurkeyBird 12-05-2009 08:14 PM

RE: 2 brushless motor wiring?
Thanks for the reply Speedy,
I understand everything you are saying. In my C130 with brushed motors having 2 motors in series the 22 volts was split between the motors. In other words 11 volts to each motor. Of course with brushed only one esc is used. This way I ran more cells (twice the voltage) with half the current draw than if I had run an 11 volts battery with motors in parallel. That way there was less loss to heat in the wiring. Just trying to may come up with the similar thing brushless. I would assume that a 5000 ma battery would have a larger wire size than say a 2500 ma. so it would probably be OK.

speedy72vega 12-05-2009 09:57 PM

RE: 2 brushless motor wiring?
Yes, the 5000ma packs do come with thicker wires, enough to handle their discharge rating. One thing to remember though, in high current setups, you will want to extend the wires between the ESC and motors when you install the power system, not the battery to ESC wires.

You can't run the motors in series the way you did the brushed motors, because brushless motors are 3 phase motors, and the ESC's have to time the pulses to the motors separately in order for them to turn properly. That's why the need for each motor to have it's own ESC. Not sure about feeding the battery wires to the ESC's in Series as you described, you get into ESC cell sensing and LVC cutoff voltage problems, it would seem. Best to just run the ESC battery wires parallel right at the battery connector, should be fine. I don't think you will come anywhere near the current neccessary to overload the battery wires, most 5 amp (5000ma) batteries usually come with 10 or 12ga wire.

Depening on what props you choose, I don't think you would be drawing much more than 40 amps or so from each combo, well within the wire capacity for a 25 or 30C pack IMHO.

What do you anticipate the AUW of this plane to be? Just curious.

HurkeyBird 12-05-2009 10:17 PM

RE: 2 brushless motor wiring?
The specs on the plane is 6 to 6 1/2 lbs glow fuel and with the LiPo's etc I can't see it weighing much more if any. 581 sq in wing area. the motors are rated for 3 to 6 lbs ea. 3 or 4 cells, 950 Kv, prop range 10x7 -14x6. Not room on the plane for much more than an 11 in pro. possiably a 12. The reason I was looking at 3 cells is I already have some but do you think 4 cells would be better with a smaller prop? It says 45-60 amp ESC. Of course 45s would be cheaper if they would do.

speedy72vega 12-06-2009 10:48 AM

RE: 2 brushless motor wiring?
For a plane that weight, I would shoot for between 400-500 watts each on the motors to give the plane good performance. I don't imagine you'll be doing anything more than scale type flight with it, and 100 watts/lb would probably be just fine, but I always tend to overpower just a bit. Easier to cut back on throttle than to have to rebuild the power system.

I'm not sure of the specs for your motors, but based on the size of them, they should be capable of the power required. You have to remember one thing when you look at the manufacturers recommendations on props. Generally, the smaller end of the range is usually the recommendation for the higher voltage, and vice versa. In other words, the 10X7 would be better suited for the 4S pack, and the 14X6 would be better suited for the 3S pack. This isn't set in stone, but a rule of thumb I normally go by.

Do you by any chance have a Wattmeter? If not, I strongly suggest you invest in one. It will be the most valuable tool in your box when it comes to setting up electric power systems. With a meter, you will be able to experiment with different props to see where you get the power you want.
Since that is really not a fast plane, you would probably be better off with more thrust, meaning larger prop/less pitch, so 3S might be the better choice. You will just have to experiment to see what the draw is.

The Watt meter I use is the E-flite meter, it handles quite a bit of current, measures under-load voltage, watts, amperage draw, and milliamps used. I think I paid right around $40.00 for it, and it is WELL used!!!!

speedy72vega 12-06-2009 11:08 AM

RE: 2 brushless motor wiring?
I just went to Tower Hobbies and looked at the Hoffman motors. I think the motors you have picked are going to be too big. At 8 oz each, that's 1lb just in motors.
Looking at the specs, the HM 490 910's would work on 3S, as the rated output is up to 450 watts. Those motors are just about 4 oz each, that would save you 1/2 a pound in AUW.

The HM-I 10 690 or 890kv motors would work too, but still around 5.5 oz each.

Just a thought..............

HurkeyBird 12-06-2009 04:25 PM

RE: 2 brushless motor wiring?
Hey Speedy,
This is a post on RCU from a guy flying the C-160 electric:

Twin Engine C-160 Transall Sucsess!
This is the CMP C-160 from AK-Models.

The Maiden went very well although the landing was a little rough. I will likely shim the LE as it needed all of my available up-trim to stal level. Too much power and props are too big. Will try the four blade 10x6 APC props next. It is fast and lands fast but very smooth and flies just like a single.

Here is a poor quality video featuring some candid banter from the annoying onlookers:

Transall Maiden Voyage Video

Setup is:
Twin AXI 2820/10 motors
Twin APC 11x5.5 props
Four 3S1P 2000mAh packs in paralell
Twin Eflight 40A ESCs
Shadow 1 6ch RX
Four HS-81 servos
AUW = 7lbs 12oz


He tried the 4 blade prob and didn't like it. He is not using APE 9/6 electric props with great success. The AXI motors he is us is the best I can tell about equivilent to the motors I have. I already have them so I'm stuck with them.

speedy72vega 12-06-2009 05:44 PM

RE: 2 brushless motor wiring?
Hurkey, I looked up the specs on those AXI motors, and they're closer to the HM-I 10 than they are to the 15. Your plane is going to be a full 6 oz heavier than the one in the thread you posted.
That poster even stated that it was too powerful with those. It's too bad you already purchased the 15's, I think the HM 490 would have been much better suited. The poster also stated that it landed fast, I would suspect excessive wing loading to be the cause. The lighter motors would help with the wing loading problem. Unfortunately, if my suspicions are correct, and his plane is too heavy and overpowered, you're going to have worse problems with yours due to the motors weighing 8 oz each. The AXI motors weigh in at about 5.4 oz each, and the 490s are only 4 oz each.
Also, having to use all of the trim on the elevator suggests either too nose heavy, or possibly not set correctly to neutral position when built. I would prefer to use either 1 battery, or 2 Parallel at the most, but 4 batteries might also be contributing to his weight concern.
Good luck with your project, and please post updates.

HurkeyBird 12-08-2009 07:57 PM

RE: 2 brushless motor wiring?
Thanks Speedy,
I'm going to return these motors and have ordered from NitroPlanes 2ea.:

Model #: Monster Power 10 (63M50)
Kv: 1100Kv
Input Watts: 375W
Resistance: .04 ohms
Idle Current: 2.10A
Continuous Current: 30A
Max Burst Current: 38A
Cells Ni-Cd/Ni-MH: 6-10
Cells Li-Po: 2-3
Weight: 5.0 oz (140g)
Overall Diameter: 1.4 in (35mm)
Shaft Diameter: .16 in (4mm)
Overall Length: 1.65 in (42mm)

Does this look more in line?

speedy72vega 12-08-2009 10:33 PM

RE: 2 brushless motor wiring?
That sounds like a better choice Hurkey, the weight savings will definitely make a difference. That should give you somewhere around 100w/lb, will still fly decent, and you will still be able to use the 3S batteries you already have.
Good luck, keep us posted.

HurkeyBird 12-08-2009 11:11 PM

RE: 2 brushless motor wiring?
I have a couple of Rimfire 28-26-1300 that I may add to convert it to a C-130 look alike. If I do I'll add a couple of 4 in sections between the wing center section and the outer wing sections. This would bring it out to 80". Not exactly a scale C-130 but "stand way off scale" and could be flown at a big bird event. That little extra wing area should help with wing loading also. Thanks for your help bud. I've been into electrics since I built what was probably the first electric C-130 almost years ago, but this is my first attempt at brushless.

speedy72vega 12-09-2009 09:25 AM

RE: 2 brushless motor wiring?
I want to see pics of that!!! :D
Glad to help.

HurkeyBird 12-09-2009 03:43 PM

RE: 2 brushless motor wiring?
OK Speedy,
Once I have started on it I will post pictures as the process progresses on a new post. I will start a new thread tiled "All Forums >> Electric Aircraft Universe >> Glow to Electric Conversions >> C-160 glow to C-130 Electric Conversion". It may be a week or so before it appears. Keep a look out for it.

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