Beau,
Let’s do some quick math. We need to calculate the watts and HP and see what’s what. For single phase AC and DC we use:
Watts = volts x amps
For example, we have 120 vac and 20 amp (common breaker size).
120 x 20 = 2,400 watts = 2.4 kW
Now we can convert that to HP:
2.4 x 1.341 = 3.2 HP
To size the wire, use this resource:
https://www.omnicalculator.com/physics/wire-size
Filling in the voltage of 120, amps of 20, loss of 3% (just take the default) and distance of 100 ft, we would need 8 gauge wire.
If we use 3 phase, we need to introduce power factor and the square root of 3. Power factor is like efficiency and the square root of 3 is because of how three phase works.
Watts = volts x amps x power_factor x sqrt(3)
Let’s assume 240 vac and 20 amp:
240 x 20 x 0.85 x 1.732 = 7,067 watts = 7.1 kW
Now we can convert that to HP:
7.1 x 1.341 = 9.5 HP
For the same 100 ft wire run, the wire size for each phase is now 12 gauge because of the higher voltage.
Now we have the relationship between HP, watts, volts, amps, and wire size based on distance.
I have some rhetorical questions:
1) What kind of wood are you milling?
2) What size of wood?
3) How fast must you mill?
4) Do you have time to mill it slowly?
If you have time to go slowly, and the wood isn’t too large or too hard, you may be able to use a band saw with a small motor. Why use a band saw? Because the thinner the blade, the less width of fibers you’ll be cutting through, so the less power it will take.
If we wanted to design a band saw, the speed of the blade would need to be around 200 FPM from:
https://www.sawblade.com/band-saw-blade-speed-and-feed-chart.cfm
If we assume an 18” diameter wheel, then we can calculate the RPM of the wheel like this:
200 x 12 / PI / 18 = 42.44 RPM
Where 12 is just 12”/ft (need to convert everything to like labels. Inches, ft, etc.).
Standard motors run around 1,750 or 1,800 RPM depending on the manufacturer. Let’s use 1,800. We’ll also use our 3.2 HP from before for a single phase (or DC) system. Horsepower through a system is constant, but torque can change based on mechanical advantage. Torque (ft lbs) is governed by this equation:
https://calculator.academy/hp-to-torque-calculator/
Torque = HP / RPM x 5,252
So plugging in our numbers, we get:
3.2 / 1800 x 5252 = 9.34 ft-lbs
This would be the torque of the motor shaft presumably. The actual torque provided would be part of the motor specs. We now know our motor RPM and our wheel RPM, so we can calculate the mechanical advantage.
1800 / 42.44 x 9.34 = 396.5 ft-lbs
Not that the “reducer” is 42.45:1 (1800 / 42.44). This is normally done through a set of v-belt pulleys but can also be done through a gear reducer.
The drive wheel is 18” diameter, so 9” radius. We convert the radius from inches to feet to get 0.75 ft. Now we can calculate the “force” being applied to the wood.
396.5 ft-lbs / 0.75 ft = 529 lbs.
Is 529 lbs a lot of force to pull a blade through wood? I would think it’s quite light. Keeping that blade razor sharp might go a long way! Remember, this was only 3.2 HP! Since nothing has a squared function anywhere, if you double the HP of the motor, you double the force on the wood.
It’s also obvious that a thinner blade would be much more efficient than a thicker one. The less width of wood fibers the saw had to cut, the more efficient.
But your original post was about converting solar power to 3-phase power and using that run a mill.
If you insist on using 3 phase AC, just know that about 40% of the energy consumed goes into powering up the windings. This is why many folks use VFDs (Variable Frequency Drives). A VFD cuts way back on the energy consumed by spinning the motor at only the speed needed to do the job, whatevery it might be. A great, great video explaining how a VFD works can be seen here:
https://www.youtube.com/watch?v=yEPe7RDtkgo
IF you have adequate time and IF you’re not milling anything overly large, I would suggest something different.
DC motors are unique creatures. They can run on practically any voltage from nameplate +10% down to zero. Their speed and torque will vary as the volts and amps vary. You can get DC brushed, permanent magnet motors from Worldwide.
https://worldwideelectric.com/product/wpmdc2-18-180v-145tcb/
This is a 180 vdc motor spec’d at 9.8 amps. It’s not going to light the world on fire, but you can run this directly off of solar panels with nothing in between except a disconnect switch. Some call this “daylight drive.” If you have 30v panels at 8 amps each, wire 6 of them in series and you’ll generate 180 vdc @ 8 amps in full sun. If you wire a second set of six and wire the two groups in parallel, you’ll generate 180 vdc @ 16 amps.
If you have 40v panels, wire four or five in series and you’ll get 160 or 200 vdc respectively. This works for basically any brushed motor. I’ve done it for corded power tools.
https://www.facebook.com/1463805713/videos/3120196448244247/
To learn more about daylight drive, check out the videos from Living Energy Farm.
https://www.youtube.com/@livingenergyfarm1912/videos
I hope this helps at least somewhat.
