Calculating required motor size

[flyingsparks]

I have been asked to look at a job at a saw mill. They have an old saw which use to be run with an engine. Now they want to put a motor on it instead. 3 phase is available. I am yet to go and have a look at this job, but if we know the HP of the engine, I guess we could use a motor of the same size? But there again there is calculating the gearing differences between pulleys etc

Anyone have much experience with this kind of thing? Cheers all.

 

[Jackrae]

An electric motor for a saw will generally be much less HP demanding than an engine driven unit. Engine HP for driving farm and industrial machinery were generally way over-rated with respect to HP on the basis that they never run flat out (the speed at which their HP rating is specified).

So you might be able to use a motor of only 1/3 the rating of the existing engine - maybe even less.

What size of blade is the saw and is it circular, band or reciprocating.

As to reduction belting etc, assuming the old saw worked and worked well, what speed did it run at. You now gear the motor (using flat or V-belts) down to that speed. Generally 3 phase motors run at just under a factor of mains synchronous frequency so 3000, 1500 or 750RPM

 

[crystal ball]

I would consult my local rewind firm, pretty good with tech advice, they would like the trade obviously

 

[Porque223]

Compare the speed/torque of the engine with the speed/torque curve of your candidate motor.

Using this
http://www.simetric.co.uk/si_cc2hp.htm
I tried to come up with a single formula for all cases but it seems to be impractical. The actual/predicted value in some cases is too far off.

E.g., for a known application (leaf blower),

cc hp ratio cc/hp
27 0.95 28
27 0.95 28
27 0.95 28
27 1.1 25
27 1.1 25
and these points can be fitted to a straight line and a formula for that line can be calculated using, for example,
http://www.shodor.org/chemviz/tools/regressionjava/
so intermediate values of hp or cc can be calculated.

Worst case, you can measure the cylinder displacement of the existing engine.
Since you know your application you almost certainly can come up with a motor that meets or slightly exceeds the engine specs.

Another possibly easier option:
Sawmill blades seem to be within 2' and 5' diameter. Using this search
http://www.google.com/search?client...+diameters+"three+phase"+hp&ie=UTF-8&oe=UTF-8
post at least three blade diameters and hps from commercially available setups, with blade diameters above and below your application size.

This was done, more or less, on a thread where the OP had a 1450w motor to be used as a table saw.

 

[flyingsparks]

Thanks for all your very useful posts. At least now I have bit of an idea as to what I'm looking at and so won't sound so much like I have no idea wha I'm talking about! I will certainly take another look at those links. Will see what happens tommorow at the site visits. Cheers.