Converting power to three-phase

      Things to consider when converting power from single-phase to three. June 24, 2001

Question
We are considering upgrading to a 25 hp Wood Mizer 3 phase electric mill. If we decide to do this, we would also upgrade or convert our edger to a 15 hp 3 phase unit.

As we only have single phase available and it would be cost prohibitive to get 3 phase service to our farm, wed need a rotary phase converter. Ive done considerable study on rotary phase converters and have seen two homemade converters in use (but not on sawmills). So I know what they are and where to get them.

We would like to hear from anyone that has experience running a sawmill and edger, etc. with 3 phase motors of this size range with a rotary phase converter, especially anyone that has switched from gas or diesel to 3 phase electric (converted or not).

My questions are:
Are you pleased with the results?
Comparative cost of operations?
Performance impact?
Maintenance issues?

Forum Responses
This is not directly related to sawmills necessarily, but take a good look at using a variable frequency drive instead of a rotary phase converter. You get much "cleaner" power and eliminate the "wild leg" problem. The motors will run cooler and last longer. The cost may be pretty close.

Of course, running a generator may make sense depending on kWh cost. You get about 10-12 kWh per gallon of oil.



I recently bought a used Wood Mizer LT40HDG35 bandmill that the previous owner converted from the factory Wisconsin 35 hp gas engine to a 25 hp 3 phase. He had set up a rotary phase converter and was sawing faster, quieter and cheaper than with the gas engine. The 25 hp motor was purchased used along with a new rotary converter from Roe Electric in Toledo, OH. The total installation cost was under $1000. As my present needs require portability, I have re-installed the 35 hp gas engine, but I will return to the 25 hp motor in the near future. Also, I've seen another Wood Mizer LT40HDE25 in operation. It too runs off a rotary phase converter. However, the factory 25 hp motor whines like a jet engine and is no quieter than a gas engine.


Don't forget to price out and budget for mag. contactors, disconnect, etc., all weather-resistant, and a weather-resistant electrician to install it. The cost will shock you, if not the wiring.


Three years ago we converted our LT40HD from 24 hp gas to 15 hp electric. We went from over $400 in gas to under $100 in electric each month.

We installed a Ronk 50 hp phase converter, and also operate a 10 hp electric Morgan edger.

We are at the end of our power line and the substation is 9 miles away. We do have weak power, and that is why we only have a 15 hp on the WM. We tried a 25 hp but could not start it. Check with you local power company to make sure there is enough power.



You mentioned home-made converters. They are made by using a 3 phase motor to supply the missing leg. They have a capacitor network to start the motor turning, the single phase is connected for 2 legs of the 3 phase and this rotating motor is used for the 3 leg. The problem with this system is that the legs are not balanced correctly electrically. The leg this motor supplies may have incorrect voltage and phasing. This is what is causing the motor to scream in above posts. The motor is going to have a short life. If you buy a converter, make sure it is designed for a converter and that the outputs are balanced. The voltage should be within + or - 5% of each leg for best operation.


Converting from gas to 3 phase was a great move for us in our stationary mill. It lowered costs and maintenance. It also quieted our mill down some. We can find 3 phase motors pretty cheap. Luckily, putting in 3 phase didn't cost us anything. I'm sold enough on it that I'm converting our grain dryer on the farm from PTO driven to 3 phase powered by a rotary converter.


I converted my LT40HDG to an entirely electric mill over 8 years ago (240volt/3 phase delta). In a nutshell, operating costs will be less generally speaking, especially when life and maintenance is factored in (maintenance is greatly reduced). There is a noticeable difference in cutting speed due to increased torque and faster delivery of it. When your gas or diesel motor receives additional load (like cutting through a large knot in the middle of a cut), the engine has to slightly slow before the governor responds by sending more fuel to the engine and the RPM is restored to the shaft. This all takes time. For practical concerns, there is no lag with an electric motor. The result is near instantaneous torque upon demand--no need to slow down when approaching a large knot. That is great.

As to 3 phase, I think the suggestion about the use of an inverter is excellent. Not only does this solve your 3 phase problem but it also allows you to adjust the motor/blade speed as circumstances require.



A general rule for electrical vs. gas is 1 hp electric for 2 1/2 hp gas. Better yet is torque--an 1800 rpm electric motor can be loaded for 100% service at 3 ft-lbs per hp. The biggest enemy of an electric motor is overheating: 1) overloading, 2) under voltage, 3) dirt insulating motor, 4) over-greasing bearings.

About variable freq drives: 1) I have not seen any in trade magazines that can run a 15 hp motor on single phase. 2) They require a special 3 phase motor. 3) For a 15 hp system you are looking at $3500 minimum.

An electric motor is a great improvement over any engine--basically no service, ready to go at the push of a button. Little fire danger. Put an ammeter to monitor motor current. It's a great help in telling how the saw is cutting.



What electrical service are you starting out with? I have 1 ph 200 amp service to my work area. Is that enough power for the phase converter and 1 or 2 15 hp 3 ph motors?


I'm kind of winging it here, but I'm pretty sure that you have plenty of power for both motors. I recall that I used to have a 5 HP single phase saw that was fused at 30 amp fuses, ergo 15 HP should require 90 amps more or less. The only problem I think you could have is starting current. That is the energy required to start the armature moving, which is usually 2-3 times the full load amps, but this only occurs for a fraction of a second and is usually compensated for in the fuses (time delay fuses--I use type FRN on mine). Check with an electrician just to be safe, though.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Forget about variable frequency or inverter drives. They don't exist in single phase input larger than three or five horsepower. The reason has to do with the practicality of rectifying single phase input to smooth ripple-free d.c. which is done in the first stage of an inverter drive. This can be done fairly well in a small unit but a large hp unit would require a large number of expensive filter capacitors and other associated electronic devices. Large units are expensive and intended for industrial users who already have 3 phase power.



Comment from contributor C:
The comment regarding single phase inverters over 5 hp being available, while technically correct, does not give the full story. As per Hitachi tech support, you can run single phase input into any of their inverters (and presumably other manufacturers as well) so long as you de-rate them to 75% of nameplate HP.

I currently run a 10hp lathe with a 15hp inverter single phase input. This is a new setup for me, so it has not yet stood the test of time. If you can stomach the cost, inverters (also known as Variable Frequency Drives) are absolutely the best phase converters around. I've got 5 in use and would not be without them. I plan to keep them inline even after my eventual heavy up to 3 phase. One important note is that, unlike a static or rotary converter, you can only run your motor on the inverter. All ancillary controls, safety switches, machine work lights, etc. must be powered single phase.



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