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Old June 1st, 2008, 06:52 AM   #1
alezi
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choosing capacitor when translate 3 phase AC motor into single phase

Hello colleagues,



How it is go on, I have one question, maybe simple for some experts, I need to know, how calculate which capacitor we are need when transformed 3 phase AC motor in single phase motor, I know one formula C= 70 √P, when is C in metric units micro F(Farads) and P is power from AC motor in W(Watt),

in example: P=100w

when computing we are to receive very strange results approximately 700 micro F, of course Im informed about chart for choose volume capacitors.



Thank you for replay

Aleksandar
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Old June 1st, 2008, 06:01 PM   #2
eebads2391
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Hi Aleksander ,

Your question is not clear

Please explain what you are trying to do

You can run a single phase motor from a 3 phase supply if you have a suitable/safe means of connecting it to a local distribution board

Not sure why you are requiring to connect capacitors unless you need to provide power factor correction

more information please
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Old June 1st, 2008, 07:58 PM   #3
bernie_carlton
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I believe he's trying to do the opposite - run a 3 phase motor from a single phase supply using capacitors in some way.
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Old June 1st, 2008, 09:06 PM   #4
Gil47
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Quote:
Originally Posted by alezi

I know one formula C= 70 √P, when is C in metric units micro F(Farads) and P is power from AC motor in W(Watt),

Aleksandar
The above formula is unknown to me, and when I do a google search I also find no formula like it or even similar.

We need alot more detail if you would like us to assist you.
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Old June 1st, 2008, 09:28 PM   #5
leitmotif
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Quote:
Originally Posted by alezi
Hello colleagues,



How it is go on, I have one question, maybe simple for some experts, I need to know, how calculate which capacitor we are need when transformed 3 phase AC motor in single phase motor, I know one formula C= 70 √P, when is C in metric units micro F(Farads) and P is power from AC motor in W(Watt),

in example: P=100w

when computing we are to receive very strange results approximately 700 micro F, of course Im informed about chart for choose volume capacitors. Thank you for replay Aleksandar
Most confusing.
Are you trying to run a single phase motor off 3 phase and need to know the capacitor value for the motor?
ANSWER Look on the nameplate or call the factory.

Are you trying to run a 3 phase motor on single phase?
ANSWER There have been several methods to do this - you can still do any of them. BUT if motor is
3 HP or less you can get VFD to run off 120 single phase
50 HP or less you can get VFD to run off 230 single phase
WITH much less hassle and LESS or maybe equal cost.

Dan Bentler
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Old June 1st, 2008, 09:38 PM   #6
TConnolly
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You will not achieve satisfactory results trying to run a three phase motor on only single phase with a capacitor. However, if you add a second motor as an idler motor with a small capacitor to get the second motor started, then you can achieve some marginal results. The idler motor will generate your third phase, however you won't have the motor full rated power. This set up is called a rotary phase converter, and if you search google.com for rotary phase converter you can find many websites with detailed instructions on how to set one upand how to size the ilder motor and the capacitor. For general purpose three phase motors and three phase motors on simple machine tools this is an economical and frequently used alternative.

More expensive options are VFDs and motor/generator sets.
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Old June 1st, 2008, 10:39 PM   #7
bernie_carlton
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A couple of links I found regarding this

http://www.home.earthlink.net/~ojmsmith/

http://www.freepatentsonline.com/4792740.html
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Old June 2nd, 2008, 04:33 AM   #8
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You can run a small (personally had good results up to 0.75kW) 3-phase motor on a single phase net by simply adding a running capacitor.

C = 70 x kW (formula for 230/400V 50Hz 3-phase motors to run on a 230V 50Hz single phase supply)

For example a 0.37kW motor would require a 25F cap.

You'll have to keep in mind that starting torque drops dramatically, but for pumps or other low starting torque applications it is a simple and effective solution (so forget about running an 3-phase air compressor).

Hope this helps
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Old June 4th, 2008, 03:58 PM   #9
Lancie1
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Quote:
You can run a small (personally had good results up to 0.75kW) 3-phase motor on a single phase net by simply adding a running capacitor.
Sparkz, where do you connect the capacitor? What do you connect to the remaining motor phase wire?
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Old June 4th, 2008, 05:21 PM   #10
Freak
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Vsd

Why not use a single phase to three phase frequency drive, should simply need to connect the motor in Delta instead of star. I have several of htese onsite and they are valiable from a variety of suppliers I use SEW and AB.
Cheers
Peter
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Old June 4th, 2008, 06:07 PM   #11
504bloke
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Quote:
Originally Posted by Lancie1
Sparkz, where do you connect the capacitor? What do you connect to the remaining motor phase wire?
On this side of the pond, if the motor is 3 phase star 400V connected then you would connect single phase to 2 legs of the winding in delta and put a capacitor across L and the other delta leg. Dont know if the C value is correct ( i have it at work somewhere ) its a really crude method of running a 3 phase motor on single phase, no starting torque and some motors dont like it and overheat, Normally called Steinmetz Connection from memory and i have even seen this (we do motor rewinds) on OEM manufacturer machinery!

Real crappy in my opinion....

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Old June 5th, 2008, 02:35 AM   #12
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Hi Lancie,

Dave beat me to it in answering your question... but also Peter has a good point.

Nowadays the cap solution is only used for economical reasons: A perm cap costs around 5 to 10, while a VFD costs about twenty times as much. On the other hand, you wouldn't have to worry about torque when using a VFD, so you could run any application (up to 3kW I think?) on a single phase.

BTW, the Steinmetz connection has been around for decades, way before there were "fancy" VFD's ;-)

To Dave: We also do motor rewinds (dad started back in the late 60's).
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Old June 5th, 2008, 01:36 PM   #13
alezi
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Hi,

Thanks a lot for respond every body, some people think this is wonder because today we are have many ways for solve this problem like as vfd, problem is money ,one permanent capacitors is much chipper then vfd,I know all about loosing power and moment inertia when three phase motor work like single phase, when motor give us enough power it is better solution, money we are spend on others things.



And again thanks for replay,

Aleksandar
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