Homemade electric chainsaw

[2004LB7]

So I built this electric chainsaw a while ago. It had a starter motor to drive the chain and a bank of lifepo4 batteries pulled form some Makita battery packs. Worked fine for a short time before the motor let out the magic smoke. Was running it on 24 volts.

The photo is how it sits now after tearing it apart to start over but lost interest. Been thinking about it again but with a different approach.

What I am thinking of doing now is convering an alternator into a three phase motor. And either driving it with an ESC from the hobby market or directly from my trucks alternator

The ESC will cost more money as I will have to find one capable of handling the current and purchase some more batteries for make another pack for it. This way would certainly be more universal and can use it just about anywhere

The truck alternator powered method, I was thinking I could tap into the 2nd alternator windings and run a cable out. This could connect to the windings of the one on the chainsaw. My thinking is that it would be phase locked and run at the same speed of the truck alternator less some slip. I am familiar with how the alternator to motor conversation works but have never done it before and also directly driving it from another alternator windings is also somewhat new to me.

If I went this route, I wouldn't have to purchase muck of anything but would be limited to using it within a short distance from the truck. I am fine with this as most of its use will likely be removing fallen trees and limbs that block a back road when out camping. So 30 or so feet of cable will probably do. The wire gauge on the other hand may be a limiting factor as the current it may need to carry can be quite large

So, my question is, what route should I take, and if I try out the alternator method, any of you have experience with this or see any problems

 

[2004LB7]

Connection would be like this

Wouldn't even need to heavily modify any of the alternators. The one acting as a motor will, since the spare ones I have, likely has a bad regulator, will just have the diodes connect straight to the excitor windings, maybe through a resistor without any regulator.

 

[malibu795]

 

[DAVe3283]

The connection would also need a DC 12V signal to create the magnetic field in the rotor. Without that, it won't spin, since it isn't a permanent magnet motor.

I wonder if you varied the strength of the field if you could vary the power output? Might cause issues since it will need to stay phase locked to the truck. Hmm, probably would need to be on/off. But it might make for an easier switch than trying to switch all 3 phases at once.

Wiring it to your 2nd alternator would certainly be cheaper than buying an ESC to run it. Interesting idea.

Edit: or you could use the diode pack to create your excitor field. Seems you were ahead of me there, and I skimmed right over it. D'oh!

 

[2004LB7]

Yeah the diode pack on the "motor" can be used to provide the current needed in the rotor. It will to some degree run even without the rotor current but it will act more like an induction motor with little starting current and will stall easy. Best to feed it, for its use requires high torque.

I wouldn't really be able to use it to control the on-off of the saw even it it did stop running with no rotor current as the current through the main windings would still continue and possibly go up. I think I would have to run a contactor to cut at least two of the phases/lines.

It would be nice if I could modify the alternator in the truck for higher voltage to boost the saws performance but then I would have to isolate it from the regular charging circuit of the truck. Keeping it at stock voltage would allow me to keep everything the same except for a pigtail under the hood or somewhere else convenient where I could plug into it.

I could in theory also plug into it a step up transformer, such as one salvaged from a ups and step it up to 120 volts. Could literally have a three phase 120/240 volts right at the truck

 

[Jacy_dzlguy]

Wow cool project! :cool2:

 

[lts1ow]

This is in an nutshell how we motorize gens for vibration testing.

I can tell you that if you do not watch case temps it will burn up if you stall the motor, or not provide adequate cooling. A gen made to a motor is a very very shitty one. Getting it to want to spin can be a challenge too, but I deal with electronics that know where rotor is electrically so not sure it is the same to a brute force method.

If you are going to use AC out of the truck alt, just vary the rotor voltages and alt speed to get the line to line you want. It would technically be disconnected from truck if you are yanking AC out of it anyways right?

Edit: have you done the math to see what the max mechanical RPM will be given the poles/frequency of what you are working with? Jopefully its enough for what you intend to use it for

 

[2004LB7]

You are correct in them being "shitty" motors. But some have used them for go-kart motors and work somewhat well for that so I think it may work alright for this.

I am using the brute force method as it has no sensors. If I was using an ESC I would likely add a sensor in there to help it start and not stall as easy but running straight off the truck I have no practical way of using one.

I haven't done any math yet to see what the alternator spins at while the engine is idling. Anyone know what the pully size is for the drive and alternator on an LMM? Otherwise I'll just measure them when I get home

I didn't plan on electrically disconnecting the alternator from the truck but if I had to it wouldn't be too hard to do

 

[Chevy1925]

How many watts you think that thing needs? You can get fairly decent 2-3000watt Chinese inverters for pretty cheap. I have one in my tracker. Ran a blender and a 4.5” grinder at the same time without issue. Never ran the math on that and I’m sure it’s no where near 2k watts though

When youre in Mexico, you make the best of a broke situation

 

[2004LB7]

I got a cheap inverter too, I think 1500 watts. Runs my store bought electric chainsaw just fine but what fun is that? Plus it only has an 8" bar and takes a special thin chain

As for KWs. I hope to get at least 2 HP. So 746 x 2 = 1492 watts

1492 / 14 volts = 106.5 / 1.73 (square root of 3 for 3 phases) = 61.6 / power factor of 0.8 = 77 amps per phase at 14 volts. Should work but I dont know if I can get the motor alternator to behave properly. May draw more, may draw less. Wont know until I plug it in

 

[2004LB7]

Just put this together

 

[SmokeShow]

Y'all are the coolest nerds I know. :woott:

 

[jlawles2]

Why not just find a 2 alternator bracket from a truck and mount a second alternator for when you want to run the saw. That way you can do whatever you want to the dedicate power supply.

 

[2004LB7]

I already have a second alternator and its in use currently. May hook up a switch or relay do isolate it when needed

 

[gmc502]

I like what your doing but have to ask,,what do you plan to cut w/ this?
I'll stick to my husq. for the stuff I cut.
truck was filled w/ 1 of the 2 logs.1/4's were in the 150-200lb range.

 

[2004LB7]

Man, some of you guys are no fun . Where is the adventure and challenge of making your own tools, especially if they are unique if you just buy everything?

This is not about being practical or even worthwhile. Its more about if its possible and will it work well enough to use. Its the fun of creating

 

[DAVe3283]

This is not about being practical or even worthwhile. Its more about if its possible and will it work well enough to use. Its the fun of creating

X2

I build a bunch of stuff I could easily buy, just to see if I can, and to learn. Even a failed idea can be a learning experience. And if you have the time and money, it is often more fun to learn by trying than to learn from a book or a class.

 

[jlawles2]

Mazda single rotor engine with a saw blade on it...:roflmao:
Probably outrun the pro's in HOTSAW competition.
Now that would be a build. If I had the tools needed to build my own stuff, I would probably be featured on "Hoarders".

 

[2004LB7]

As fun as that sounds, I am determined to do it electrically.

If I fail on this one, or get different inspiration I may try a model car nitro engine. I have a larger one that needs a little carburator work but is otherwise fine. I think it is rated at something like 5 HP at 18k rpm

 

[NC-smokinlmm]

Just do a brushless lipo combo...