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A question for the electrical experts .


Chris Head
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Is there any reason that I shouldn't have a rheostat or dimmer switch controlling the speed of an electric chainsaw? I ask because I want to mount an 8 inch carving bar on a Stihl electric chainsaw for detail work inside the barn. Somebody had suggested that it wouldn't be too good an idea as they thought that it might damage the motor and I don't want to wreck (yet another)  a saw.

Thanks, Chris

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Generally, AC Electric Motor Speed Controls use SCRs (Silicon Controlled Rectifiers) or Thyristers.

In practice, most speed controls cause the motor to accelerate and de-accelerate through the phase, so they effectively, "jerk".

Personally, I would think this would very dangerous with a chainsaw! It could be liable to stick: and then suddenly kick in. With the expected results.................................................kick-back and no head, for example!

The big problem could also be torque: as the motor jerks, the torque would be variable and liable to make you lose grip, as the teeth cut and then suddenly froze and then suddenly cut, once more. Exciting.

Never heard wood sculpture described as a dangerous sport before. You might start a new craze.

However, if you are determined to carve a new sculpture using you as the raw material, here is some information:

http://www.oddparts.com/acsi/motortut.htm

http://www.epanorama.net/links/motorcontrol.html

Perhaps a petrol chainsaw and judicious use of the throttle might be a safer route to success!

 

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Thankyou both for your replies, I've just about worked out which end of the chainsaw I can hold and which does the cutting, I've been told not to touch the thing that whizzes around or I could be in trouble. I really just want to know if my suggestion will wreck the motor or not?

Salut, Chris

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Yes it will. Use a suitably rated controller (a big light dimmer will work). Yes they do switch the power on and off, but at 100 times/second, so I don't think you will notice any jerking. Given the cost of a dimmer, it's worth trying (as long as you are sensible).

 

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Firstly, your chainsaw will have a quite high power motor - probably around 1600w - and you won't find a domestic dimmer switch which will cope with that sort of power unless you spend quite a lot of money.

Even if you did  find a suitably rated dimmer it won't help you much. If you run the dimmer at a low output it will wreck the motor so I wouldn't recommend it.  The resulting smoke and burning smell might be quite effective though.

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hi chris

            question..... being the novice you are with chainsaws how do you manage to pick your nose( welsh funnies)

             real question ... do these cheap €40 grinders for chain saw blades work ??? at €6 a throw to have them re-sharpened they could work out better for me !!!

        I know you are going to say use a file.... but it`s time......

                       dave

hope you are still busy

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Thankyou Nick & Eslier, that's saved me a few hundred euro and another telling off. Petrol saws used inside are noisy and the fumes are unpleasant.

Use a file Dave with an appropriate file guide. It takes less time and is a much better method of sharpening the chain. Initially it might feel awkward but with practice it becomes easy. Clamp the bar (saws have guide bars that guide the chain, knives have blades) in an engineers vice making sure the chain can run freely and make sure you sharpen each tooth the same as the last so that they're all the same length or you'll end up with bannana cuts.

Chris

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[quote user="Eslier"]Firstly, your chainsaw will have a quite high power motor - probably around 1600w - and you won't find a domestic dimmer switch which will cope with that sort of power unless you spend quite a lot of money.

Even if you did  find a suitably rated dimmer it won't help you much. If you run the dimmer at a low output it will wreck the motor so I wouldn't recommend it.  The resulting smoke and burning smell might be quite effective though.
[/quote]

Eh?

There are 2Kw dimmers available.

And how will a low supply voltage cause burning in the motor? I can't see a synchronous motor being used in a chainsaw, most other motor types are slowable, but I can't see how low power = flames...

 

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Many years ago I used to use a variable speed controller on an electric drill.  It was a small box (6”x 3”x 1” approx) with a knob and a 13amp socket and a trailing lead with a 13 amp plug on.  It was specially made for such devices but was not excessively expensive (hobbyist type thing and price).

As somebody else commented it’s speed was not particularly smooth.  The slower the speed the less even it was.  I’ve not seen such devices around as for electric drills tend to have variable speed built-in these days (so I guess there is less demand).

The one I had was being subjected to “hobby level” use and I remember burning a couple out but not the drill.  However, this is limited experience and sample size of one (from 20 plus years ago).

 

(It was actually being used on an electric drill driving a small lathe – where the drill is mounted horizontally to become the headstock – a bit like a horizontal “vertical drill”.  Not very good but in those days reasonable price when funds were limited.  Hence the need to reduce the drill speed).

 

Ian

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Dave's reference to angle grinders is on the right lines.  The Arbotech carving system will do all that you want and is very controllable.  There is also a "mini" attachment for really intricate work.  B&Q (Screwfix) used to do a cheaper version of the big Arbotech complete with storage box etc.  Not sure if they still do but the pukka Arbotech is not all that expensive and you get an angle grinder thrown in.

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[quote user="Chris Head"]

Thankyou both for your replies, I've just about worked out which end of the chainsaw I can hold and which does the cutting, I've been told not to touch the thing that whizzes around or I could be in trouble. I really just want to know if my suggestion will wreck the motor or not?

Salut, Chris

[/quote]

Very droll, Chris, very droll. I had worked out that you recognise the nasty sharp bits: looked at your site, after reading your other thread on poison emails. Very impressive (the site, not the email!). Sculpture is great!

Hard to really answer your question: it would all depend on what type of motor the chainsaw uses.

I would think that the design requirements are for even torque, delivered from the get go: torque needed, rather than power, to overcome the initial guide friction: and even more for the cut to be started. And that's the core benefit of electric motors; as against internal combustion engines: both the torque and power curves of electric motors are flat, once the reach their design rotational speed.

What concerns me, is that as the speed is reduced (by imposing a different set of electrical parameters on the motor, from its design performance), the RPM will drop (obviously) and thus the power output will also drop: but worse, depending on the type of motor, the torque could drop, accentuating the freeze effect I mentioned earlier.

Most semi-industrial motors of this type, are designed to deliver the required power at the set constant speed and torque; with significant starting torque in order to overcome inertia - which includes the inherent friction and accelerating the mass of the moving parts. Now, the starting method is all important here: several different types of starting, for A.C. Induction motors.

Now, the last thing to do with a chainsaw (as I understand it), is to leave it in a cut, turn it off and then restart, without firstly removing the blade from the cut? However, that is almost what you would be doing with a speed controller: stalling the motor in the middle of a cut: and then restarting; even as Nick points out, at a high rate of stall per second.

In order to control speed (and thus power) and most essentially torque, you would need a pretty effective circuit: MOSFET based + micro-processor, ideally. One is detailed below. Even with this approach, momentarily, the current drawn by the motor, through the effective stalls, will leap up through the roof, as it recovers, thus you would need a pretty beefy circuit. (All A.C. motors draw their maxium current when starting from stationary.)

Lamp Dimmer? Not sure: these cannot be used for capacitive start motors, for example. Most dimmers have different circuits for incandescent loads and fluorescent/quartz/halogen, whilst some, now can be used for both.

More web refs.

http://www.curtisinst.com/index.cfm?fuseaction=cDatasheets.dspListDS&CatID=1

http://en.wikipedia.org/wiki/Electric_motor

http://en.wikipedia.org/wiki/Adjustable_Speed_Drive

 

 

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Ian: They have come on quite a bit, now.

As you comment, what is called, "Soft Start" is pretty effective; and as you say, most corded power tools have effective speed controllers these days.

The old circuits were simple Thyristor circuits: and pretty useless, too![+o(]

 

 

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I'm not saying that (i,m here in France) a speed controller is the ideal solution for slowing a chainsaw - a gearbox is. But then again, I'm an electronics engineer, not a mechanical one...

The actual torque of a 1200W B&D chainsaw is pretty low (although electric motors are typically high torque), and is definately a function of speed - there will be a point where the losses in the motor will overcome the drag (of the chain, bearings etc) and it will stall. At this point, the torque is less than the losses. However, there will be a point in the power & torque curves (flat as GS points out) where the motor will still be working at maximum output - this is not (indeed unlikely to be) at the max or design speed of the motor - but the only way to find this is by experimentation.

And no, you don't want to stall the chain in the work, because the torque to restart it is much greater than the torque needed to "run". When considering the stop-start effect, you must remember 2 things; a) an A/C supply turns off 100 times per second (a 50Hz supply starts at zero, goes to (say) +115V, passes through zero, goes to (say) -115V, then back to zero, 50 times per second). As the voltage passes through zero, the motor freewheels until the next curve. This brings me to b), the inertia (flywheel effect) of the motor, gearbox and chain - pretty incalculable by most of us, but a signficant factor to consider!

Modern speed controllers modify the shape (rather than the size) of the A/C supply. Given that energy (and hence torque) is "enclosed" by the curve of the A/C supply, surely the speed is controllable whilst reducing the effect on the torque? (ie. a small square wave would produce more torque than a small sine wave at a lower speed?)

And so it goes on. As I said above, you won't reach a conclusion without experimenting!

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Sorry Gluestick, it was a little droll I guess! I'm about to replace a broken Husky saw for the Stihl electric, I don't need power it's simply for detailing with a specialist bar and chain so I'll buy the smallest one possible, probably around 1400 watts. I really don't know what type of motor it has but I'll find out. I don't want to risk going through another saw though, my wife isn't too pleased with me at the moment!

Tony, the Arbortech is Ok up to a point but I find they greatly lack the precision and control of saws though, but if you're not used to chainsaws they're probably a safer bet.

Thanks all, fascinating info and most of it Chinese to me!

Chris

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OK,Chris, please let me know and I'll respond.

Stihl is one of the best: all made these days by Electrolux, as are most garden products, as I found out when I tried, vainly, to buy a new cutting deck housing for my Rallye ride-on mower, made originally by AYP in the USA, sold retail new in France. In the end, I imported the spare part from a very helpful US dealer and actually had the bit, before Electrolux bothered to answr my query to their Technical Service and Spares division: after I had, with great difficulty, tracked 'em down![Www]

Personally, I am so dubious about chainsaws, my new thermique, remains in its box: still have circa 30 trees to prune and lop this Autumn, so I'll have to pluck up courage: once I've bought my chainmail suit![blink]

 

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Gluestick

Off topic I know but do you recall who you got the AYP deck from? I've just tried to get one sent over from the US and the supplier dropped out at the last minute saying that the part was too large to ship through standard parcels and DHL would charge $300 for shipping.

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As they might say, Alane, "Surely do!"

Originally, after much research, I contacted a chap named Doug Elbon at a company named Brown & Wiser Inc. (The distributors for APC) [email protected]

Doug kindly checked the stock of American Yard Products parts (and my specific part) and put me onto Stuart Owens [email protected] one of their retail dealers.

Stuart had the part and most kindly created one dedicated page on their web site for me! In order that I could place a secure order (SSL site; no problem with using a credit card).

It was sent via DHL. Delivered very promptly thereafter!

Their complete details are:

Farmers Exchange
215 W. Canal Drive
Kennewick, WA  99336
Phone: 509-586-3101
Email: [email protected]
Web: http://farmex.now.tc

I was literally amazed (though on reflection not so, having spent some time in the USA) at the service and speed of responses. Too used to laisez faire England, I suppose![blink]

Hope this helps.

P.S. Thank goodness for PC file architecture and Outlook and archiving emails etc.

P.P.S. Total cost (including shipping) was $184.70, which at that time was £105.00!

 

 

 

 

 

 

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