Power Considerations for Electric Mills

      Thoughts on powering an electric mill without affecting the neighbors' power supply or power quality. March 9, 2010

Question
I have a small manual bandsaw mill that I would eventually like to convert to electric to reduce noise and exhaust. I researched using a 10 HP single phase motor (to replace my 16 HP Briggs), but determined that my service was not adequate (would introduce flicker in my neighbor's circuits).

Has anyone used a DC motor and a bank of deep cycle batteries with a heavy duty charger to run a mill?

Since I only saw 500 BF per day in my one man, manual operation, I would think a 5-10 HP DC motor with a bank of batteries would give me the power I need to make the big cuts and the time between cuts would allow the batteries to be recharged. Any sources of used DC motors in that horsepower range?

Forum Responses
(Sawing and Drying Forum)
From contributor C:
The best way to avoid the neighbors getting that flicker is to use a phase converter and run a 3 phase motor. A 3 phase motor will have a better efficiency rating, and have a smoother torque curve. Another advantage of 3 phase is that the motors are a lot smaller and lighter.



From contributor S:
You would have two choices for a phase converter (that is a machine that takes single phase and makes a three phase supply that can run a 3 phase motor). One is to build or purchase a converter that is built from another 3 phase motor, using the principle that generators and motors are nearly the same. You can find info on these by using a search. These converters need a manual start.

The other is to purchase a VFD, which is a power supply that creates the three phase from a single phase input. Probably a good choice for motors 5hp or smaller, after which the supply cost will go up. There are always a lot of these used around. A buddy has a Delta VFD-B 10 hp that can run a 5 hp 3 phase motor with only a single phase input.



From contributor R:
I used a 10hp single phase and used a 50 amp circuit breaker. It worked out real good.


From contributor T:
If the questioner is worrying about browning out the neighbors, a 3 phase conversion will still brown out the neighbors, only better. You can't get something for nothing. If a single phase 5hp motor draws 25A, then a 3 phase motor producing 5hp through a phase converter is going to draw 25A plus the juice needed to run the phase converter, maybe 27 to 30A total single phase load.

Most of the static phase converters are not true phase converters, only a low amp phase shifter, hence the disclaimer on the product that the motor will only develop 2/3 of rated power. Variable frequency drives will offer less brownout than some of the other options only because you can set them up for slow starts, which will only reduce the starting power requirements.

A DC motor will work fine with a battery bank, but is going to be expensive to set up, maintain and control. So that you are aware, DC is physically more dangerous than AC because electric shock causes muscle contraction. With the cycling of AC there is a moment where voltage will be 0 when you might get yourself disconnected from the equipment, but you don’t get that with DC. And don’t forget that because electrons have to move in and out of the wire linearly (in one end, out the other), with DC larger wire gauge and heavier contacts are needed. The above warnings are based on the same working voltage.



From the original questioner:
Thanks. Thinking it over, a DC motor would require a large current (amps). For example, a 48 volt 10 HP DC motor would draw 180 amps (assuming 85% efficiency). That's a lot of current to run through a flexible wire setup for feeding the mill.


From contributor A:
I use a 7.5 HP single phase motor on my mill at the house. It is 500' from the transformer and when I turn it on, the street light dims. In that 500' it has to travel through 2-100 amp breakers, in panels, and the final one is 50 amps. It draws 200 amps in startup. I called the power company and told them if I blow up a transformer, they have to come out and replace it. Not my problem.

Getting back to the motor, 7.5 HP is more than enough. That is equal to a 15 HP gas engine. I have a Cooks’ mill; the blade is 1-1/4”x 168” and runs on 19” all metal wheels. I have 500 hours on it and 1000 hours on my 28 HP gas powered portable mill. The all electric mill runs a little slower, but not much.

Point of information: 7.5 HP motor should draw close to 40 amps at full load. 10 HP should draw 50+ amps at full load. I have found 10 HP motors that draw 40 amps. It is not 10 HP, it is 7.5 HP. Just remember - more amps, more work! If your equipment does not draw these figures, your motor has not been accurately advertised. I get pissed when an equipment dealer tells me that a 1 HP motor draws 9 amps. Wrong! A 1 HP motor draws 13-15 amps. Sorry for going off. This really gets me going. When looking for motors, look at the amp draw at full load. On the tag or stated it will be (FLA) Full Load Amps.



From contributor D:
Have you considered a genset? A lot of generators in the 10kW size are quiet, and could be far enough from the mill to avoid exhaust issues for the sawyer. You could go 1ph or 3ph. Propane generators are about as clean as you can get. I'll probably go that way with my next mill.

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