# Off the wall question...



## rillip3 (Jun 19, 2009)

bruceme said:


> I'm an EV owner and aircraft pilot and I've built/fly 2 aircraft. I have a new long-term project to build an electric launch sailplane. So while I can't dispute the crackpot claims, I will live.
> 
> A self-launch glider had exactly two speeds... taxi/cruise-sustain (50%) & full (takeoff). I don't really need an elegant brushed aluminum digital controller with fuzzballs, just two speeds (off, low and closed).
> 
> ...


If you only want three speeds, off, half, full, off and full are easy and don't require any work. For half, I think you're over-engineering your options here. Why not just do the wiring such that you can have the pack in full series, or half turned off? A simple switch that moves a wire from touching a lead to the motor in the half position, can then switch it over to touch the next battery terminal in the full position. It will be a little jerky, but you're only doing this once a flight, so it seems like that wouldn't be a problem. I think a transformer or resistor is too much work/a waste of energy. Just installing a lever that will pull a lead from the rest of the pack to a motor terminal should work fine. 

But I have absolutely no credentials, not even EE for dummies, so take that with a grain of salt.


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## Woodsmith (Jun 5, 2008)

How about spliting your pack and then switching the two packs from off, to parallel, to series. That would give you a half voltage setting and keep the pack balanced.


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## dimitri (May 16, 2008)

I see 3 potential problems here:

1. You will murder those cells at 5C contunuous.
2. You have no way of limiting the current. How do you know what the current will be once you apply full pack voltage to a motor? How do you know it won't be 20C or 30C, killing your cells even faster than 5C?
3. You have no way of limiting motor RPM. With full power applied, how do you know when the motor RPM will level off? DC motors will happily self destruct once RPM reaches critical point.


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## dimitri (May 16, 2008)

As for 2 speed setup without the proper controller, you do not want to use resistors, not that you would even find one to dissipate 10kW, nor would you want to waste half your energy, this makes no sense at all.

The only acceptable solution short of proper controller is series/parallel switching of 2 halves of your pack. It would take a few contactors and you'd want to make absolute certain about the switch timing, so you don't get a massive short if one contactor closes sooner then the other one opens.

Contactor rating might be an issue too since you will be interrupting full current on a regular basis, so your contactors may not live too long.

I would not recommend to use half the pack at any time, since you will always have unbalanced pack.


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## bruceme (Dec 10, 2008)

dimitri said:


> I see 3 potential problems here:
> 
> 1. You will murder those cells at 5C contunuous.
> 2. You have no way of limiting the current. How do you know what the current will be once you apply full pack voltage to a motor? How do you know it won't be 20C or 30C, killing your cells even faster than 5C?
> 3. You have no way of limiting motor RPM. With full power applied, how do you know when the motor RPM will level off? DC motors will happily self destruct once RPM reaches critical point.


Hey Dimitri... Great points all and stuff I'm worried about too. I respect your opinion and take it quite seriously. But this isn't a dead short, and the motor doesn't have to grunt through low RPS like a traction motor, it will almost instantly go to full static RPMs. 

I know this works great at smaller scales (RC). Trick is... can LiFePo4's take this kind of abuse. You say no, I say maybe, it's worth researching more. I need to get someone involved who knows more the motors-batteries interaction than me, but my gut is telling me this should work, but it's probably not as simple as I think. 

Thanks! I do appreciate it

-Bruce


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## bruceme (Dec 10, 2008)

Btw.. I screwed up my numbers and I'm too close to spec for comfort.

SkyEnergy says the LiFePo4's are good for 4C continous, 12C max. If I ran 120v-40ah, I'd be right up against all limits for the battery, motor, etc. 

My targets are 19kw for 15 minutes... if you do the math, that's 4C on 40Ah cell. This is not simple stuff, I have weight limits to consider. Just up-sizing is a bad answer.

-Bruce


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## dimitri (May 16, 2008)

I converted a push mower a while ago to electric, recently upgraded it to LiFePo4 pack. Since I had no need for speed control I went without controller, just a contactor.

My calculation of max current started with the motor rating. The motor is rated at 2100W at 48V, so the current was expected to be in 45Amp area. Like you said, it only takes a second to spin up, so initial dead short is not an issue. The size and the mass of the blade determine max RPM without load, this was outside of my control since I used existing blade. The motor is still in one piece, so I guess it worked out well for my mower.

When my motor stalls in tall grass the current shoots up and I already blew 60Amp fuse, which I replaced by 80Amp fuse. Since then I don't let it stall, but I am comfortable knowing that it would not exceed 80Amps , which is 2C for my 40AH cells.

So, in your case, what is the motor power rating at given pack voltage? This would tell you max cruising current. The propeller size will limit RPMs, I guess you might need to experiment in this area to get best RPM for best motor efficiency.

As for cells holding up to specs, there is a video showing CALB cell at 4C and it ain't pretty . Heat buildup inside the case is the main problem, this is what destroys it. The 4C spec probably assumes the user will magically keep the cell cool enough.

Also, remember that max rate and max lifecycle are mutually exclusive, so running at max C rate means minimum lifecycle. Something to consider when figuring long term cost or cost per mile.


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## bruceme (Dec 10, 2008)

I drew a parallel/serial schematic

http://tinyurl.com/outofcontroller


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## piotrsko (Dec 9, 2007)

if you run some of the constant speed props, motor overspeed shouldn't be an issue after the prop is set up. I am guessing you'll need some sort of reduction drive because stuff gets silly when it goes sonic although Les King and IRV Culver says that isn't all that bad.

How far do you need to taxi?? most everyone I know pushes theirs to the end of the runway, sometimes using a ATV.


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## bruceme (Dec 10, 2008)

piotrsko said:


> if you run some of the constant speed props, motor overspeed shouldn't be an issue after the prop is set up.


CS is really only useful if you need power at two very different speeds, like take-off and fast cruise. I won't have a fast cruise, so I'll just setup a fixed pitch climb configuration. But I will likely get a ground adjustable prop, so I can tweak it to match the motor.



piotrsko said:


> I am guessing you'll need some sort of reduction drive because stuff gets silly when it goes sonic although Les King and IRV Culver says that isn't all that bad.


You need gearing when the engine/motor gets power at speeds that are incompatible with the chosen propellor diameter. Diameter = Max rpm. 72" = 2700 RPM, 65" = 3000, 56 = 3500... etc. It's not tough math, tip speed shouldn't exceed 0.8mach.

So find a motor with known power curves and match it to a prop. Adjust blades to achieve the desired static (ground run-up) RPM (typically 200-300 rpm bellow climb or target maximum RPM)



piotrsko said:


> How far do you need to taxi?? most everyone I know pushes theirs to the end of the runway, sometimes using a ATV.


Yeah, I can do that on my Silent-IN self launch. I really wanted it to cruise around on bad gliding days. But given the complexity of this parallel/serial thing, maybe not. It really requires a 200amp DPDT switch, if you happen to have one of those, let me know. I'm not sure I know how good I feel about using single pull solenoids(contactors) for this purpose... what if one doesn't fire or sticks?! cabooom! Thanks again for the input.


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