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Old July 5th, 2005, 06:07 AM   #1
billyboot
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motor 3phase to single phase

Hi I have some queries here I know it is not related but maybe you can help.
We have a 3 phase motor, 250w, 230v, 3500rpm, 2 pole clutch motor. There are no available 3 phase supply on the location so we just want to run it on single phase.
Does anybody knows the parameters to calculate for the size of the capacitor to run it single phase 230v supply?

What kind of capacitor must be used?


billy
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Old July 5th, 2005, 06:16 AM   #2
dchartier
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hope the following thread can help:

http://www.plctalk.net/qanda/showthread.php?t=1989

Daniel Chartier
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Old July 5th, 2005, 07:34 AM   #3
OkiePC
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You may be able to connect an ac drive to the single phase supply and let it produce 3phase for the motor. I have done this with AB 160 series AC drives.
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Old July 5th, 2005, 07:37 AM   #4
jstolaruk
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Quote:
Originally Posted by OkiePC
You may be able to connect an ac drive to the single phase supply and let it produce 3phase for the motor. I have done this with AB 160 series AC drives.
Yep, you may find its easier (maybe less expensive) to use a VFD with a single phase supply. I've used the AB Powerflex 4 with high success.
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Old July 5th, 2005, 08:13 AM   #5
elevmike
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This is a small fractional HP motor (250w), It would be much less expensive and troublesome to replace the motor with a single phase motor. If you cannot do that, then get a small drive like this one: http://www.automationdirect.com/adc/...trol)/GS1-20P5
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Old July 5th, 2005, 09:40 AM   #6
BITS N BYTES
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In my experience it is not possible to run a three phase motor with single phase, IT WILL BURNOUT.
Adding a capacitor will not work. A capacitor is ONLY used for single phase motors.

You can run a three phase motor with a VFD since its output IS three phase. In general, most models of VFD will operate with single OR three phase on the input side, remember that all they are doing is converting the AC input power to DC [that's why they are called inverters].

If you run single phase on the VFD input the output power is normally derated since the DC Buss is not charged to its full capacity as with three phase. This derating factor varies between VFD manufacturer and can range between 50-80%.

Some VFD manufacturers offer both single and three phase input models. The single phase input models do not require derating.
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Old July 5th, 2005, 12:01 PM   #7
TConnolly
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A capacitor by itself will not do the job. However, connecting two motors and a capacitor together will do the job. The extra motor cannot have a mechanical load on it and it is called the idler, and it will generate your extra phase (sort of) - this is refered to as a rotary pahse converter, or roto-phase. The third leg will not be a true third phase, its voltage vector will be nearly the same as a third phase, but the current it delivers will be smaller. However it will allow your three phase motor to operate from a single phase source. This solution is not usually suitable for CNC equipement however but works great for general purpose machine tools, pumps and fan loads, etc.

These units are very common, cheap and commercially available, and there are hundreds of manufacturers. Most of the time they will use a special idler motor that has no output shaft. I recommend a commercial unit as the cost advantage of rolling your own is very small, and a commercial unit will safer, have properly sized starters, wires, and capacitors, and most importantly, a warranty.

Heres an interesting article on it: http://www.metalwebnews.com/howto/ph-conv/ph-conv.html



A VFD is a good solution for single small motors and its output is a balanced three phase, but if you have large motors, or a shop full of three phase eqipment then the roto-phase is the next most economical way. If you have CNC equipment then you may just have to pony up the expense of getting a three phase supply installed.

Your problem is not unique, the country is full of small manufacturing shops (and farmer's barns) that do not have three phase power.

Do a google on "rotary phase converters."

http://www.rotophase.com/phaseconver...TOKEN=91051000

http://www.americanrotary.com/

I recently helped one of my neighbors put one of these in his welding shop. He added a lathe and wanted to be able to add a milling machine and other three phase equipement. Because he wanted to support several machines VFDs were out of the question. So we put in a transformer to kick his 208 suupply up to 480, and added a roto-phase unit with a 20hp idler motor and wired its output to a new three phase breaker panel. Now when he wants to add equipment all he need to do is connect it to the new panel. He has to start the roto-phase first, then his equipement, but it was considerably cheaper than running three phase power to the shop.

Last edited by TConnolly; July 5th, 2005 at 12:26 PM.
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Old July 5th, 2005, 03:13 PM   #8
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A three phase motor will burn up running singel phase. To use a capacitor to make it work also requires an inductor to phase shift the power to simulate 3 phase.

There is a way to some transformers and connect them in the OPEN DELTA configuration. It can be used to run 3 phase motors on single phase inputs. Just make sure the transformers are rated to handle the HP.

Most transformer companies show this method. If you have an UGLY'S book, it is in there also.

I have seen PLANTS with single phase power from the utility run 3 phase motors and drives using the OPEN DELTA method of connection.
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Old July 5th, 2005, 03:37 PM   #9
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If I recall correctly, with the open delta tranformer method you should derate the motor to 60% of the nameplate. The rotary phase converter works on the same idea, but here the idler motor is literally a physically rotating transformer, with the third winding physically moving through a magnetic field so create the third phase. Its still not perfect, only a motor-gen-set will get you that, but those are big bucks.
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Old July 5th, 2005, 03:47 PM   #10
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ALaric,

You have to derate the transformers. If you size them correctly, you can get full HP to the motor.

If you size the transformers KVA to the motor HP exactly, then the amperage available to the motor is the 50% to 60% of normal HP you mention.

Of course, the transformers pricing may be more than a single phase input VFD. Gotta watch them $$$.
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Old July 5th, 2005, 03:59 PM   #11
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Thanks, I knew something had to be derated, I just didn't recall which.
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Old July 5th, 2005, 04:01 PM   #12
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I believe some 3 phase motors can be run on single phase with just the use of a capacitor

Google "Steinmetz motor connection"

The use of this is the CHEAP way to get a unit to run and it has limitations including torque losses and whilst works with some motors is not really recommended

See drawing
Attached Images
File Type: bmp steinmetz.bmp (295.4 KB, 179 views)
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Old July 5th, 2005, 04:15 PM   #13
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Your cheapest solution to this problem is get a phase convertor, a company named phase-o-matic makes them, they are relatively inexspensive, using the open-delta will work but will have a very large current inballance on the center leg to other phases.
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Old July 5th, 2005, 04:17 PM   #14
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That looks like the motor winding is being used as the inductor I mentioned. I would think the motor needs to be derated about 50-60% in that configuration.

I do not remember seeing that before. Just shows you can always learn something new.
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Old July 5th, 2005, 04:49 PM   #15
504bloke
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Like i said it looses a lot of torque.


Also you will find that some motors just dont like it while others will run quite happily.

I cannot remember the *bodge* formulae for working out the Cap size but ill ask tommorrow as i know we have it somewhere.

I have also seen that whilst most motors prefer to be delta connected for this operation some will only run like it in a star connection!
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