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    Default POWER FACTOR CAPACITORS

    Hi All,

    Can anyone help? I am in the process of making my own 20Hp Rotary Converter, I have read many discussions online about the use of "Power correction capacitors" to "tune"
    the three phases on the output, this appeals to me as I am running Cnc equipment on the rotary converter and want to keep the output as stable as possible,

    My question is: What is the difference between a "power factor capacitor" and a "start/run" capacitor, or any other capacitor? or are they the same thing?

    Please excuse my ignorance on this subject.....I am always willing to learn!

    Thanks for any help,

    Allen.

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    Default Re: POWER FACTOR CAPACITORS

    This was an option when I purchased my 220 Single Phase AC/DC Tig welding machine.
    The reason given from the manufacturer is that PFC reduces current draw, so you can lower the breaker size and reduce energy consumption.
    Larry



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    Default Re: POWER FACTOR CAPACITORS

    Hi Larry,

    Thanks for your reply,

    So there is a difference in "power factor capacitor" and a "start/run" capacitors?

    Allen



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    Default Re: POWER FACTOR CAPACITORS

    Power factor capacitor adjust phase shift with inductive load. Motors are inductive load. If the load is capacitive, then the correction is done by inductors.
    In general PFC circuits are more complicated.
    It is just capacitor used for PFC



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    Default Re: POWER FACTOR CAPACITORS

    Hi,

    Thanks for your reply,

    That makes sense,

    Thank you,

    Allen



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    Default Re: POWER FACTOR CAPACITORS

    I am not sure, but I think you are building mechanical 1 to 3 phase converter.
    In general this is 1 phase motor who need start/run capacitors and 3 phase generator. It is good to have PFC capacitor there on the output, for each mottor you are runnining. After contactors.
    The supplier of the motors will provide you solutions.



    Attached Thumbnails Attached Thumbnails POWER FACTOR CAPACITORS-pwrfact4le-jpg  
    Last edited by fobi; 03-13-2017 at 02:29 AM.


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    Default Re: POWER FACTOR CAPACITORS

    You need a capacitor from the third leg of the induction motor (used as a rotary phase converter) to both lines, in order to boost the voltage of that third leg up to an acceptable number, sometimes you can use just one capacitor.

    Without any capacitors, you'll get voltages similar to this: 120, 116, 110 or some multiple of those, such as 240,230,220. IIRC i have seen people report hardly 200 volts out of the third leg on a 240v system. if, without capacitors, your rotary phase converter doesn't even generate 80% of the line voltage i would find another motor. it is probably a relatively inefficient, low power factor motor.

    Without any capacitors, the motor will generate some power through its third phase to start your three phase motor(s), but it will provide no significant power into your three phase motor once its up to speed, and as such you have to derate your three phase motor load to just 57% of its nameplate rating so as to avoid exceeding its nameplate temperature rise.

    delta or wye connected capacitors can be added to each of the induction motors you use your converter to drive, so as to cancel out the reactive load the motor demands. if you set this up carefully you can run several machines off of your phase converter without much concern.

    without power factor adjustment for each motor, each additional motor load will consume reactive power from the third generated leg and that will pull the voltage of the third, generated phase back down. but as the voltage is pulled down, the current drawn from it is less. once the line voltage of that phase drops 10% it contributes nothing substantial to the motor.

    For every hp of motor load at 240 volts you need something like 10uF connected in delta to cancel out that the reactive current consumed by the motor at no load.

    Last edited by Eldon_Joh; 03-13-2017 at 04:02 AM.


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    Default Re: POWER FACTOR CAPACITORS

    This is not going to turn out well.

    Cheers
    Roger



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    Default Re: POWER FACTOR CAPACITORS

    Quote Originally Posted by RCaffin View Post
    This is not going to turn out well.

    Roger
    Cheers



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    Default Re: POWER FACTOR CAPACITORS

    Quote Originally Posted by Eldon_Joh View Post

    Without any capacitors, you'll get voltages similar to this: 120, 116, 110 or some multiple of those, such as 240,230,220. IIRC i have seen people report hardly 200 volts out of the third leg on a 240v system. if, without capacitors, your rotary phase converter doesn't even generate 80% of the line voltage i would find another motor. it is probably a relatively inefficient, low power factor motor.

    Without any capacitors, the motor will generate some power through its third phase to start your three phase motor(s), but it will provide no significant power into your three phase motor once its up to speed, and as such you have to derate your three phase motor load to just 57% of its nameplate rating so as to avoid exceeding its nameplate temperature rise.
    Hi,
    I think this apply to static converters. They work with ... these capacitors indeed. In this case he will need separate converter for each 3 phase motor.
    I don't know what type is the converter. I saw some box in the link, I hope there are all capacitors needed for the work of the idler.
    The PFC capacitors apply only for the tool motors.

    Last edited by fobi; 03-13-2017 at 09:21 AM.


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    Default Re: POWER FACTOR CAPACITORS

    Quote Originally Posted by fobi View Post
    Hi,
    I think this apply to static converters. They work with ... these capacitors indeed. In this case he will need separate converter for each 3 phase motor.
    I don't know what type is the converter. I saw some box in the link, I hope there are all capacitors needed for the work of the idler.
    The PFC capacitors apply only for the tool motors.
    No reason to contribute speculation, it does not appear you are familiar with the terminology used.

    static converter means a capacitor and potential relay. you use them to start a three phase motor from single phase, they burn out motors if you load them beyond 50-60% nameplate hp because they contribute nothing to the third phase of the motor once it is running.

    OP said he wants to build a 20hp rotary phase converter. this means he wants to use a 20hp induction motor to generate the third phase for what he wants to run.

    which, I didn't notice this previously: is CNC equipment.

    that's probably why Rcaffin said "this won't end well"

    OP also indicated he doesn't know the difference between a start and run capacitor. do you?

    here's a link that will actually help OP if he does determine that a 20hp rpc is suitable for his cnc equipment.

    3phconv



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    Default Re: POWER FACTOR CAPACITORS

    Not mentioned so far is the house wiring and heat. To get 20 hp 3-phase you would need at least 25 hp 1-phase, and more likely 30 hp 1-phase, owing to efficiency loses.
    For a start, the motor(s) used are going to get very hot. Where you put them to handle the heat is a good question. The insurance company might be concerned.

    But of more interest to the supply authority and the insurance company would be the load on the house wiring. I have lived in the UK, and the house wiring I saw was 'interesting'. How it would cope with a load of 25 hp ... Oh well, the house fuses will probably blow first.

    Fwiiw: I do run 3-phase at home - on a farm, and I have 60A/phase feeders installed at my request. The supply authority did ask why.

    Cheers
    Roger



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