Three-phase electricity primer

      Pros and cons of three-phase electricity and static and rotary phase converters. November 21, 2000

Question
I am trying to find a site to set up an electric sawmill and was wondering if electricity that comes through a phase converter is the same as straight 3-Phase electricity.

Will I have the same power?

Will there be any adverse effects on the motor?

Will it cost more or less to run?

Forum Responses
I have had good service out of the static converters I purchased from Gerhard Werner Motor Werke. They are very eager to help. The information they gave me was straight forward and easy to follow. I don't have experience with the larger rotary units they sell. But I do know if the rotary unit is sized correctly and the hard to start motors on the system balanced with boosting capacitors, everything will be equal with power company service.

The expense is the thing to deal with, not the lack of technology. Their lower priced products may be seen in Grizzly catalogs. They also make custom systems to order.



Static phase converters work fine for motors that start up and operate with little or no load. This is not the case with sawmill motors. Therefore, you should use a rotary phase converter as they are designed to start and run motors under continuous load.

I have a rotary phase convertor that is rated to start a 5 hp motor which is under load, and 15 hp total once each motor is started, i.e. it will run any number of motors up to a total of 15 hp as long as you don't exceed the load of a 5 hp motor on each startup. However, I can easily start and run my 7 1/2 hp tablesaw even though it exceeds the 5 hp startup rating. The reason for this is that a tablesaw motor starts under a very low load and is only taxed for a short duration.

Three phase motors draw higher amperages to start and when under load than when running under little or no load. A rotary phase converter is a good investment if you are running multiple machines or the cost of replacing a 3 phase motor with a 1 phase is prohibitive. The big advantage is that 3 phase motors are cheaper to buy, require less maintanence and are more efficient to run.



The big issue is how much HP you are looking at. If you want you can make your own phase converter. Find a motor rated somewhat above your total HP and turn it up to nameplate rpm with a single phase motor. You hook your single phase to two of the legs on the large motor and pull your third phase off the third leg. Remember you have National Electrical Code to deal with nowadays. But if you are going to be running a large HP mill you should be looking at a genset.

From the original questioner:
I'm looking at a 25-30 HP 3-phase electric mill with possible additions of a 15 HP 3-phase edger, a dust collection system, etc.

What is a genset?



A genset is an engine-generator setup to make elecric power. Like the diesel generators you see carpenters use on-site sometimes, but bigger. If you decide to go the non-converter route, check with the power company and get them to quote you a price for running 3-phase service to your location. Depending on how far from the nearest 3-phase lines you are, it might turn out cheaper than a genset.


I am to the point where I have to make a decision about having three-phase electric (cost $5000) or just get single phase for now and later buy a phase converter. I don't need three phase now but probably will in the future. What should I do?


If you are really into this for the long run, look real hard at putting in 3 phase to start with. If nothing else, find out from the power company if 3 phase is close to the site. If it is have them run 3 phase line onto the mill site. If you do not need it right away they can set a single phase transformer now and then put in a 3 phase tub when you are ready. This will keep you from having to pay through the nose later to have a 3 phase feeder run onto the property. The big thing here is to find a site with a 3 phase line running by it. If they have to run a 3 phase line several miles you will pay dearly.

Another thing about generator sets is that when they go belly up you are all done working until you get it fixed or rent another. Commercial power is quite reliable. I know this can happen as a friend of mine had his genset go south and he was down for just about a month.



I am 1 mile from the nearest 3 phase lines and the electric company wants $22k just to bring me the power. I am setting on a Baker "A" resaw which is parked waiting on power. I have been watching for an appropriately priced generator with a 3 HP capacity but haven't found one yet. The commercial equipment all uses 3 phase power so I am determined to get it, as soon as I can find it.


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

Comment from contributor A:
You should have seen the first phase converter I built. I had pulleys, ropes, pony motors all over the floor, not to mention the switches and wires and capacitors that took up about half my garage, but it worked and I was proud.

It all started when I bought a lathe and was in the same quandry that you guys are in. I met an old man who knew how to make a backyard converter and that got me interested - so much that I sold the lathe with its converter and started building phase converters for all of my friends. As I got better and better, the converters got better and smaller. The biggest one so far was a 40 hp for a man who had a wood shop, and maybe 4 15s and about 12 10s, but as far as the 5hps and 3hps, I lost count how many I have built in the last 15 years.

All I know is that once you get fascinated, it's easy. Now I have the capacitors and stuff shipped UPS and I have a lot of testing meters and tools that make it easy, no guessing.

Can a converter deliver the same exact 3 phase as the power company? No, but if you take your time putting the right amount of capacitors between line 1 and line 3 and also between line 2 and line 3, you can get it so close that your motor won't know the difference. Some converters that you buy will have 3 phases alright, but the voltage will be 240--223--216 and guess what, the motor runs just fine. The only bad thing is when you work the motor hard, it gets hot. My converters 3 phases will read 240--239--241 or so, because I take my time.

How long will a converter last? Forever.

Is it cheaper than 3 phase from the power company? Yes. I won't even mention the cost of having it brought to you, because even if 3 phase was right there, it would be cheaper for you to have a well-tuned phase converter. If you have a dozen large 10 - 15hp machines, that's different, but the people that I build converters for are much better off investing in a lifetime tool that they can take with them.



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