Electric Racing Chainsaw Build, Help Please

[Hillhater]

I bet any competition would spec a chain like racing says a tyre compound to stop someone coming along with a diamond/carbide and having an advantage.

Didnt the OP say they use a special custom chain just made for competition ?
Unlikely more teeth will increase the cut rate...
If they are too close, the cut for each tooth cannot be as deep..which is why the skip tooth chains cut faster.
Think about circular saw blades..fine tooth pitch for smooth finish, big open teeth for fast, rough cutting .
Also look up the commercial , hydraulic drive, chain saws used for logging..404 skip tooth chains.at 150 fps.
They can take a standing tree and convert it into a pile of 3m sawn logs in under a minute.
[youtube]nuuPI2hyt6M[/youtube]

 

[Ianhill]

I get you on the tooth with circular saw reference.

I guess the diameter of the cut is much larger on a log so there's still the same amount of teeth engaged as the smaller saws with huge teeth.

Thing got me confused is the finer tooth chains had competition use versions where as all the skiptooth chains had one variant.

Not as obvious as it looks and very limited experience and knowledge of them I have so I'll bow out.

I will say though those who not tryed a electric chainsaw you will be surprised I went out an got one straight away as I had the batterys to run a 36v anyway and have a spare run so it's no brainer for me.

 

[Hillhater]

OK .. a suggestion to chew over as a starting point .
Motor.. failing any suggestions for a high rpm, high power motor....
Emrax 208..75kW, @ 6000rpm, and 140Nm torque== ( 100hp, 100 lbft) weight 10kg...all stock figures.
Use a jackshaft to double the rpm up to 10-12000rpm with 70Nm torque ( matches a 100hp, 50lbft , 2 stroke )
Battery..
34s, 4.4Ah LoneStar “Sleeper” cells to give 125v and 600amps for 25 sec max. Weight 4 kg
Controller ?. ? ..Sevcon 4 ??...not too sure ?
Losses in the jackshaft wont matter at this level of power!
All up a 20kg. (45lb) power pack.
Comments ?

 

[speedmd]

I think it is just best to take the motor or one like it the size you mention Hill and go with the one with the most rpm suitable rotor and wind it 2 or 3 turn. Pump the required volts and amps to it to give you a completely dominant power plant. Run three heavy welding cables to the saw and make history.

Skip tooth chains work mostly by keeping the saw from bogging down. Once you get more than a half dozen cutters in a log at one time, you just run out of power on a sub 80cc saw and it just wants to stop the chain moving every time you allow it to take a full bite with a sharp chain. There is also no place for the wood chips to go in longer cuts to get across larger logs which then just get in the way, drag down the chain speed and greatly increase the power required to slip though the cut slot. Anyone drilling deep holes understands this chip effect. Skip tooth becomes a much more efficient setup at a certain diameter depending on the wood-chip formation.

Sarcassidy, you don't list your general location. There are quite a few members possibly nearby that could be some big help. This is a vid of a larger zero motor in 4 turn config. [youtube]https://www.youtube.com/watch?v=jzPgDbhrHVE&feature=youtu.be [/youtube] Do you think you could handle the weight of it. Looks like a scary amount of power to be holding onto.

 

[Hillhater]

When you rewind for more rpm, what happens to the power/Torque ?
Why get into motor development, when a simple jackshaft will give the required rpm, hp, and torque, on a stock motor ? Modified motors can /will come later once some level of performance is understood
The Zero motors are bigger, heavier, less power/torque and lower rpm than the Emrax ! ..so not such a attractive starting point
Im only guessing, but i suspect the comp organisers would want the whole powerpack in one unit, no separate battery pack. ( possibly a backpack battery ?)

 

[John in CR]

Ok, so the drive teeth on a chainsaw chain are spaced at double the pitch, so he needs about 15krpm at the sprocket.

Looks like a scary amount of power to be holding onto.

It's scary enough that I wouldn't want to touch one, and I can guarantee the gyroscopic effects will make it far scarier. Since the saw must be moved around by hand during high rpm operation, a long small diameter motor with a 90° gearbox (like angle grinders use) may be required just to make it safe enough to operate.

 

[flat tire]

Stop talking about motor torque, my god. The important thing is torque AT THE CUTTING SURFACE which is determined by gearing. When you can regear, POWER is what determines the torque where the action happens.

Focus on finding a lightweight motor that puts out the required power and then determine what drive size (gearing) is appropriate.

 

[serious_sam]

Stop talking about motor torque, my god. The important thing is torque AT THE CUTTING SURFACE which is determined by gearing. When you can regear, POWER is what determines the torque where the action happens.

Agreed.

Everything else being equal, more power = faster cutting.

 

[Hillhater]

Focus on finding a lightweight motor that puts out the required power and then determine what drive size (gearing) is appropriate.

Hmm ?.. isnt that what i did with the 75kw Emrax geared up to 12k rpm ?
...and a jackshaft set up would allow easy changes to peak sprocket rpm if needed.

 

[serious_sam]

Hmm ?.. isnt that what i did with the 75kw Emrax geared up to 12k rpm ?
...and a jackshaft set up would allow easy changes to peak sprocket rpm if needed.

The emrax heaps bigger than is required for such a short duration. It could be done with a much smaller motor.

And if an appropriate direct drive option can't be sourced, the smart idea would be to use a higher speed (smaller rotor) motor (speed ~ power) and gear down.

Plus, the emrax is an axial flux motor. If you're going to go through the bother of gearing, might as well run a high rpm inrunner and gear down more.

 

[stan.distortion]

If you use single stage geared reduction (or step-up) and tweak the weight of the output shaft then torque reaction can be totally cancelled out.

 

[flat tire]

The benefit in the high end axial flux motors is they are really really efficient so end up providing most power / kg. Additional advantage of the super high efficiency is you need less battery for a given mechanical output since less of your power is going to waste. The emrax 188 is only 7kg but could probably do 100+ hp for a cutting run. That, a controller, and 15 lbs of the highest discharge lipo would be the ticket if you want to upset things, for only slightly more weight than a gas motor with half the power.

Astroflight makes (or used to make) custom inrunners that may have a form factor advantage, but are a few % less efficient than high end axial flux like emrax, so ultimately provide less power / weight even for 10 seconds and need more battery overhead for a given level of output.

 

[Hillhater]

The emrax heaps bigger than is required for such a short duration. It could be done with a much smaller motor.

And if an appropriate direct drive option can't be sourced, the smart idea would be to use a higher speed (smaller rotor) motor (speed ~ power) and gear down.

OK, Any suggestions for such a high speed,smaller rotor motor, with “enough” power ??
And why risk modded/overpowered motors when a factory stock one will do the job ?
There are smaller Emrax motors,...
..but this is racing, folk are looking for the most power possible !
...until some rules start to impose limits .

 

[speedmd]

A lighter weight internal mag rotor design like the new z force overvolted would be my first choice. Arlo, and Luke have tons of direct experience with them. Would stay away from anything that's not direct drive until I had exhausted motor options. 10000- 12000 rpm should be plenty, but we could tell better with some tac data from existing setups.
Agree John, a lightweight rotor will help with the gyro effects. Softer start on the throttle ramp also will be helpful.

 

[Ianhill]

[youtube]RYNEEc3pgT0[/youtube]

Be honest there's a seriously finessed machine this is no basic chainsaw.

I don't think it can be beat and come in under weight for under 5k there's a massive challenge thats going to need some improvisation not to spend that on just the motor alone then again on the controller and battery.

The ice guys could use a superbike motor
And just make a simple coolant loop to take the 12seconds run time and have 18000rpm running e85 and that's electric tapped out you ain't beating that and picking that up.

The thread gives good basis to mod the makita 36v when the time comes but till then electric will stay low end and gain traction slowly with battery and motor advancements.

 

[Hillhater]

.

I don't think it can be beat and come in under weight for under 5k there's a massive challenge thats going to need some improvisation not to spend that on just the motor alone then again on the controller and battery.

The ice guys could use a superbike motor
And just make a simple coolant loop to take the 12seconds run time and have 18000rpm running e85 and that's electric tapped out you ain't beating that and picking that up.

Well yes, you have to have the budget if you want to be competitive with the top guys,
Most of those “hot saw” tools cost up to $10k for a new build....$5k for the motor alone.
Superbike motor ?? ..not in the Hotsaw class...its restricted to single cylinder only.
...and only one person holding the saw !
There are other classes for bigger motors etc.

 

[stan.distortion]

$5k? I think an AC system will be unlikely to cost less than double that once development costs are accounted for and I wouldn't be one bit surprised if it runs well into 6 figures. An unfortunate fact with motors is the more power you want to get out of them, the heavier they need to be and 10 seconds might seem like a short amount of time to us but it's an extremely long time to electricity, more than enough for thermal runaway to turn any motor to scrap.

And the sad part is that once the idea catches on then all those expensive AC parts are almost certainly going to get decimated by DC systems built from a few hundred dollars worth of parts from a junkyard. There are a few of simple reasons for that:
Weight is a limiting factor and with torque cancelling a simple relay is all that's needed for DC, the lightest AC controllers I've come across for those power levels are over 3kg (the same weight as a small saw).
Electromagnets are more powerful than permanent magnets. No doubt AC beats DC hands down for efficiency but 2 electromagnets give more force than an electromagnet and a permanent magnet.

Drag race tuning is a money game, for the most part he who pays wins. It comes down to who can afford to blow up the most engines, the tuner with the budget to test 100 engines to destruction is going to get more power than the tuner who can only afford to blow up 10. Exactly the same thing applies to electric, with an unlimited budget it might be possible to build an efficient and extremely powerful AC system for this job but unlimited budgets are extremely scarce and the guy who's picking up DC motors from the junkyard for $100 a time (starters, hydraulic powerpacks, forklifts, etc.) can do 20x the testing of the guy who's fried a single $2000 AC motor.

It's not as established as engine tuning, an unlimited budget would likely be best spent on the technical aspects of the problem for now (permanent magnets simply aren't going to win out when power to weight is the target imo). AC has every chance of coming out on top at some point because it will ultimately come down to getting rid of waste heat and AC will almost certainly stay top dog for efficiency but with a limited budget and the current state of tech DC will almost certainly come out on top on the leaderboard.

EDIT: It will be interesting to see what happens to chains if this catches on. Engine power drove tyre tech hard with drag racing and the chain is already a major limitation with IC engines, electric can get more outright power than IC for the same weight and can give much better power curves, a power ceiling might well be reached that can only be overcome with better chains.

 

[serious_sam]

The benefit in the high end axial flux motors is they are really really efficient so end up providing most power / kg.

Looks like you are correct.

 

[Ianhill]

One problem with the magnetic field analergy is that magnet rotors spin faster the weaker their field strength is, so using a large current to form an electro magnet in the rotor then needs a much bigger one to oppose it to get high rpm, the upside is the motor will have huge torque but very little rpm and a huge gear multiplier needed to get a crazy chainspeed.

Experiment with an alternator it's the same principle even if the outer field is 3 phase, the weaker the rotor field is the faster the rotor spins but it creates less torque as we add power to the inner field the alternator motors rotor slows but its torque rises adding more power to the outer coil then adds some more speed back can do this juggling act till the thing melts a winding or the core absorbs to much eddys and gets hot.

I used to fit 24v lorry starters for a bit in work experience and I tell u nw the weight of them things they ain't no joke and not got anywhere near 60hp it's flywheel gearing is steep to turn the lorry over at a few hundred rpm till it fires for itself at best 10hp when overdriven I'd say and considering it's mass and size it's not gonna cut it against a 50kw bldc I don't know of any dc motor even come close to what's needed on this occasion needing to be power dense is key.

Good read.
https://www.google.com/amp/s/spectrum.ieee.org/transportation/alternative-transportation/this-insideout-motor-for-evs-is-power-dense-and-finally-practical.amp.html

 

[stan.distortion]

Yeah, I'd done quite of working-out for air core designs, nothing like as much as the researchers in that article put into minimising iron cores but enough for it to be obvious there's plenty of scope for improving power to weight ratios. I know I keep harping on about DC but the truth is I've little love for it, AC is the only option for serious development but I'm pretty sure DC will continue to dominate drag racing type competition for some time to come.

For the starter motors, I was actually quite surprised by the low rpm figures @DogDipstick posted (thanks btw!) but it's kind of obvious they only spin at 2-3k rpm when you think about it. They can handle way more, from seeing them unloaded I'd guess around 6 or 7k and at a guess they'd do 2 or 3 times that and hold together ok (might wire one up to the mig and find out ). There will definitely be a big safety margin from the manufacturer and the same goes for amps, easily 50% more at a guess and I wouldn't be surprised if they can handle 2 or 3 times their normal max amps for a second or two. How much that would contribute to power output is anyone's guess though, 2 or 3 times should be possible just with rpm but it would need to be nearer 5 to 10 times to be worthwhile. If going that route then folks drag racing with DC would definitely be worth talking to imo, it could save going down a lot of blind alleys!

Paired motors could be worth considering too (AC or DC), geared together to eliminate torque reaction with maybe the same-direction motors gear lightened if spinning the chain still causes a lot of kick. Carefully aligned it should be possible to use a single AC controller driving both motors.

 

[speedmd]

The benefit in the high end axial flux motors is they are really really efficient so end up providing most power / kg.

Looks like you are correct.

Besides magnets flying off the rotors at some point, what's to stop this design from running 12000 rpms from just adjusting the windings and supply voltage as on other designs to make it a direct fit. Yes Hill, it will have less torque for the same current that way, but that can be just turned up to what the controller can handle if at all needed. Saw does not need any serious amounts of torque at low (below 4k) rpms and the torque it is currently spec'd at is multiples of the ICE.

 

[Punx0r]

Rotors can be wrapped with carbon filament to help hold things together if things are that bad.

Gearing is a contentious proposition. The "normal" problem with high-power, low weight motors is usually they spin too fast to be of practical use. Now here's an application that almost has the opposite problem. But then, as mentioned, a reduction does give the option to remove the torque reaction, which could potentially be a deal breaker and make the saw uncontrollable at startup.

Similarly, series-wound DC has many advantages: greater strength than permanent magnets, will rev very fast and simplicity & low cost by not needing a controller. The downside is that without a controller equipped with a throttle ramp the thing may kick right out of your hands (again).

Another aspect might be that a controller is needed to limit low or no-load RPM to avoid throwing the chain when not cutting. You're going to want a sufficiently high system voltage to ensure ~50% no load speed when cutting for maximum power, but that might already be close to the limit of the chain and you may not want it buzzing up to ~2x that speed if you miss-time the throttle? Possibly a red-herring that as I'm not sure if the 2-stroke saws are similarly affected.

If the motor and controller have to be mounted on the saw itself then size & weight might also be a deal breaker.

This saw is an interesting idea and while at first look there seems to be several potentially viable approaches, it does illustrate just how many unknowns there are and how much experimenting and testing would be required.

If nothing else, the Perm motor examples show that ~10kW per kg is readily achievable for an off-the-shelf production motor.

Assuming a dual motor & controller setup for torque cancelling and reducing per-unit power levels, what's the biggest/baddest thing currently in the RC world? Yes, they're often not long-term reliable and power ratings based on very short bursts, but that might suffice here. Are any of the 15/18/20 kW ratings I recall bandied about true?

 

[John in CR]

You definitely can't go with DC and no controller (just a switch), because the clip I watched had one of the contestants get on the throttle too quickly and snapped the chain before even getting the saw to the log. If there would be a way use a switch only and have the rpm go from 0 to 100% smoothly and gradually over a period of say 1/8 to 1/4 second, then a series wound DC would be the way to go.

It wouldn't make much sense investing big money in a solution just to win one hot saw competition. Those competitions are sponsored by big company money, so unless those companies are aboard (which is unlikely until batteries make some serious energy density improvements) they'll just change the rules to exclude electrics.

 

[stan.distortion]

You definitely can't go with DC and no controller (just a switch), because the clip I watched had one of the contestants get on the throttle too quickly and snapped the chain before even getting the saw to the log. If there would be a way use a switch only and have the rpm go from 0 to 100% smoothly and gradually over a period of say 1/8 to 1/4 second, then a series wound DC would be the way to go.

It wouldn't make much sense investing big money in a solution just to win one hot saw competition. Those competitions are sponsored by big company money, so unless those companies are aboard (which is unlikely until batteries make some serious energy density improvements) they'll just change the rules to exclude electrics.

Hehe, Edwardian tech to the rescue, multiple relays to bring in packs one by one! And probably some damn big diodes to stop it bursting into a fireball when (not if) a relay sticks! This is getting silly

EDIT: Good point on sponsors and rule changes, I'm betting that's the real reason Stihl discontinued their high tooth chains. It would be nice to see some kind of standardisation coming in with electric competition, something like kw/hr or maximum weight classes could do a hell of a lot for efficiency improvement development.

 

[speedmd]

Avid is showing a 12000 rpm model. AF125 But its 22KG, Ouch! https://avidtp.com/product/evo-motors/