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debequem
12-16-2003, 08:18 PM
Now that my mill is coming I need to make 3-Phase power for it.

Originally I had planned to use a VFD with a step pulley J head machine.

However, the mill I bought has the variable speed head, so I don't need VFD. In a way this is better since the motors fan will now be always running at the correct speed.

What are the various methods to get "3-Phase" to my new machine and most important, the advantages and disadvantages of each method?

Marv

Forrest Addy
12-16-2003, 09:03 PM
You might still want a VFD.

A VFD makes clean fully balanced three phase whereas even the best RPC has balance problems. A VFD driven load motor will develop 100% HP and torque whereas the best RPC permits about 80%. Read the "trouble with" posts on RPC's.

Futhermore there's a definite savings on the power bill.

And a VFD is far quieter than an RPC.

I really can't see any reason for one to have an RPC inless he's running several motors simultaneously or electrical tinkering talent and poverty are both characteristics of the shop owner.

JCHannum
12-16-2003, 09:11 PM
We all have that characteristic to one degree or another.
If you have the bits and pieces to make a rotary phase convertor, and can do it for a few dollars, go for it.
If you are going to purchase a commercial unit, don't. The VFD is a much better buy for your money.
Don't even think about a static convertor.

debequem
12-16-2003, 09:15 PM
When selecting a VFD do I need to select one that matches the HP of the motor? Is it best to get a 2 HP VFD for a 2 HP motor os is a 3 HP VFD okay with a lower HP motor?

Marv

SGW
12-16-2003, 09:26 PM
As far as I know, you ought to be able to use a higer-rated VFD with a lower-rated motor (3hp VFD with 2hp motor, in your example) but I can't see any point to doing it unless you happen to be able to get a better price on the larger VFD, which isn't likely.

My milling machine motor is a 2-speed, rated 1/2 and 1 hp, and the 1/2hp speed works just fine with my 1hp-rated VFD, which I need for the 1hp speed of the motor.

wmgeorge
12-16-2003, 09:31 PM
Marv, Running a 3 phase VFD on single phase requires in most cases a de-rating of the HP rating. Example 3 HP drive on single phase input might only be rated for 50% or 1 1/2 HP, like mine. Your brand might be differant.

I'd still go with the VFD on your mill, you'll love the ability to go from 100 rpm to 2000 rpm just by turning a finger tip operated speed control.

wierdscience
12-16-2003, 09:38 PM
VFD-VS-phase converter,well if you don't need variable speed why spend the extra money? Plus with the VFD you can only run one motor at a time and there is some switching involved to forget to run two.Plus they ain't cheap.

But for little money and a little wiring you can build your own phase converter,run as many motors as you want plus in the future you will more than likely buy other tools with three phase motors.

Did you see this?
http://www.homemetalshopclub.org/projects/phconv/phconv.html
Plus there are lots of other designs avalible

mbensema
12-16-2003, 11:16 PM
When sizing VFDs you need to compare the kVA instead of the HP, but in many cases a 2 HP motor will take a 2 HP VFD. To calculate kVA, multiply the motor nameplate voltage and full load amps and divide by 1000. Once you have done this, then you can select the proper size. Always go larger rather then smaller. A larger VFD can be programmed to properly protect the smaller motor, but a smaller VFD cannot provide full power to a bigger motor and will probably cause nuisance trips.

As was already mentioned, if the VFD is rated for 3 phase input instead of single phase, it will need to be derated. Check with the manufacturer for the proper value and also check that it can be run on single phase input, some older units cannot, and some newer ones may not be rated for that either.

happy02
12-17-2003, 11:39 AM
The post on the rotary convertor with a starter switch is the proper way to go. But if you're electronically challanged then consider the easy way. Get yourself a 3hp 3ph motor. Wire 220v single phase to a wall mount box and wire in a 220v switch. Split the 220v single ph comming out of the switch off; one leg is wired to the driven motor switch, your mill[110v to each of 2 poles of the switch]. The other leg goes to the driver motor [your 3phase motor that you will use as a convertor]. Wire each 110v leg to each of L1 &L2 poles in this motor. Take a cablr from L3 of the driver and wire it to the 3rd pole of your switch on the Driven motor [ the mill]. You will now have to manually start the driver [convertor] motor. You can use a cord wrapped around a shiv and pull it by hand or wire up a small 1/3 or 1/2hp single phase motor beside the convertor mounted on a hinged board and use an old belt to start the convertor. Wire youre little starter motor to a switch and your convertor to a switch. To start put your belt around both shives tighten and atart the 1/2hp starter motor. when this gets the 3ph convertor up to speed, switch it on and the starter motor off. then you can switch your mill on and any other eqpt that does not exceed the 3hp of the convertor. This is really much harder to explane than to do.

andy_b
12-17-2003, 02:01 PM
i'd agree with the RPC if you ever plan on running more than a single machine. i built mine with parts i found online and in local ads. if all i had was a mill with pulleys to change speeds, i probably would've looked at a VFD. but as it is i have an old tablesaw, an old B&S mill, an old B&S surface grinder and an old wood shaper. plus, various bits of used 3-phase equipment are always popping up for sale around me. i went low-tech on the converter. i just have a manual switch i flip to turn on the start capacitor, and after a second the idler motor is up to speed and then i switch the start cap off.

one interesting point that i didn't believe until i saw it is that the phase outputs do seem to get cleaner as you have more motors in the circuit. with just the idler running two of the legs show about a 4.8A draw, and the third is less than an amp. with a second motor turned on (to power a machine) all three phases show about a 2.7A draw. i haven't played around with fine-tuning the run capacitors yet, so i can probably improve on it a bit.

this was a fun project that gave me some more insight in to how 3-phase works and wiring the machines up. and yes, "electrical tinkering and poverty" are also part of my reasons.
:-)

andy b.

kenc
12-17-2003, 02:42 PM
I'm still unconvinced that a VFD is superior to a good static converter starting an idler motor setup. I have a big old 1960's static converter (with switchable taps to improve run balance) connected to an old 1950's 5hp GE idler motor. Both cost basically nothing and provide nice clean power to both my Mill and lathe. The static converter even has an adjustable timer to disconnect the start caps from the idler (neat). VFD's alter the supplied frequency to the 3phase motor to provide the variable speed capability - I worry about Mill/lathe motor longevity, since their magnetics are designed specifially for a single frequency (60Hz USA. It's well known that 60Hz motors don't do well when used in the UK (50Hz), something to do with field saturation at the lower frequencies if I remember correctly.
Ken

debequem
12-17-2003, 02:43 PM
The 3-phase is only required for one machine.

I don't see a future with any more 3-phase requirements:

1) Because I now have all the machines I need. Yeah!!!
2) My shop will not have any more room for additional room to take more machinery.

Of course we all know how well future plans really work out. :-)

Marv

jbucko
12-17-2003, 03:49 PM
I have a rotary converter in use in my shop,wired through a 3 phase electric panel,as a distribution point.Got the panel and breakers cheap!I also have a VFD for my small Millrite mill,which is a step pulley model,and the lowest speed was 250RPM.With the VFD,Ican get down to about 80RPM with the lowest step on the machines pulley.Works pretty well.Contact Dealers Electric,at www.dealers (http://www.dealers) elecric.com.They do advertise in HSM from time to time,and were extremely knowledgeable and helpful. Jim Churchman

mbensema
12-17-2003, 04:54 PM
Motors are designed for a specific magnetic flux, so as long as it remains within the design specs, the motor will work just fine. A 230v motor and 60 hz has a flux of 3.83, if you take that motor and run it in Europe on 220v and 50 hz, it becomes 4.4 which is outside of a motors normal limits of +/- 10%. The VFD keeps the ratio of the voltage to frequency constant throughout the entire speed range so the saturation problem does not exist.

The main problem with VFD motor failures is a breakdown of the insultation in the windings due to the voltage transients caused by the carrier frequency. The same thing can happen with a motor driven by a RPC if the voltage varies too much. Voltage differences between phases causes excessive current and leads to early winding failures due to higher temps in the windings.

VFDs are meant for varying the speed of the motor and not as a phase converter. When used properly, they work very well.

Arcane
12-17-2003, 08:26 PM
Three phase KVA is average line amps times line to line voltage times 1.73 divided by 1000. Single phase KVA is average line amps times line to line voltage divided by 1000.

BFHAMR
12-17-2003, 09:26 PM
Marv, When I purchased my Sheldon shaper I thought my shop had all the three phase machines it would ever have. Boy was I wrong - seven machines later.

Dan

wmgeorge
12-17-2003, 10:24 PM
Just a bit more feedback on VFD vs phase convertor.
I build a static convertor to start my 3 phase idler motor that supplys power to my lathe. Works great and I have the gear box to change speeds and forward and reverse, just like it was on "real" three phase.

I've used a variable speed Bridgeport, works great...(loved it) but my VFD on my Mill-Drill (3 phase motor) works better! I can drill a hole first and then tap using the VFD speed control from 0 to 100 RPM,s and stop / reverse instantly and back out the tap. This is using a standard keyless Rohm drill chuck. I've tapped over 300 holes with the same 1/4-20 tap... try that with a variable speed B-P. I can go anywhere from 0 to 2000 RPM's just by turning a speed control "pot" mounted right next to my forward-reverse drum switch that controls the low voltage switching on the VFD. My cost on this using eBay and used stuff, less than $300. B.G.

mbensema
12-18-2003, 11:15 PM
You're right Arcane, I forgot the 1.73 for 3 phase

BFHAMR
12-19-2003, 11:39 AM
Here is an interesting site I came across while searching for info on a VFD. I have no connection with them - the usual disclaimers apply.

http://www.isdc.ca/vfd.html


Dan

rockrat
12-19-2003, 11:01 PM
I just got a 10 hp motor to make a 3 phase converter out of. I did a little looking around and there are places in Ohio to find used motors. I happened to find one in the back of the plant with 4" of dust on it. And it worked too, bought for scrap value. Here are some links that I have used so far in my 3 phase challange.
http://www.metalwebnews.com/howto/ph-conv/ph-conv.html
and
http://www.metalwebnews.com/howto/phase-converter/phase-converter.html

I am almost ready to make something on my lathe. A little more SO cord and I will be set. Hope this helps.