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Thread: capacitor and electric motor

  1. #1 capacitor and electric motor 
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    why does adding a capacitor to the input of an electric motor save energy?

    I have seen this advertised somewhere but do not know what to make of it........having seen things like this before and researched I was wondering what the gurus on there think.............

    would it be expected to work better on DC or AC systems, if at all?


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    http://www.aerovox.com/pdf/T2.PDF

    Just off the top of my head it sounds like it can be used to counteract the inductance and provide a better impedance match with the source supply.


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    IS is correct. They are called power factor correction capacitors.
    http://en.wikipedia.org/wiki/Power_f...f_linear_loads

    The basic problem is that to transfer maximum power the voltage should be in phase with the current. That would mean a power factor of 1. If they are out of phase, the power transferred is proportional to the cosine of the angle between the voltage and current and is less than 1. Power can still be transferred, but the current is higher for a given power, thus greater line losses in the transmission line.

    Motors tend to have lots of inductance. That is balanced out by the power factor correction capacitors, which are placed near the loads.
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  5. #4  
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    awesome, thanks.

    I did know about power factor correction and thought it was related. Very simply explained here though so I am much obliged!

    would a battery provide the same effect as a capacitor?

    my hunch is yes, but that an AC output could only be achieved with an inverter...............am I far off in this assumption?
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  6. #5  
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    Power factor only has a meaning when we are talking about a-c. You could certainly charge a battery with a rectifier, and power the motor from the battery with an inverter. That would be an expensive way of doing it and would have its own inefficiencies.
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  7. #6  
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    Quote Originally Posted by Harold14370
    Power factor only has a meaning when we are talking about a-c. You could certainly charge a battery with a rectifier, and power the motor from the battery with an inverter. That would be an expensive way of doing it and would have its own inefficiencies.
    That would be pretty inefficient, not to mention the invert itself would need power factor correction. Higher voltage on three phase (or more) and the use of the capacitors would be a good option. DC would be as well.
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    Electric utilities lose money due to PF<1 (due to requiring extra power generation capacity and also transmission line losses), so they monitor their high-demand customers and charge extra for inefficient PFs.
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    thanks everyone!

    so could I simply add a capacitor to the live wire of a fridge to get the same effect?

    would I need to wire it in parallel or in series?
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  10. #9  
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    Quote Originally Posted by fatman57
    thanks everyone!

    so could I simply add a capacitor to the live wire of a fridge to get the same effect?

    would I need to wire it in parallel or in series?
    It would have to be the right capacitor of the right voltage rating and would vary from application to application. Your likely to have them blow up in your face if you just start playing around. The capacitor would be in parallel. Again, any old capacitor will not do.
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  11. #10  
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    I wouldn't worry about or touch the frig. It doesn't run often or draw much energy, and it may have built-in power factor correction.

    Just enough phase correction would improve it, but too much would make it worse because it would shift the phase angle too much.
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    Forget it. You are only paying for the power that registers on your watt-hour meter, no matter the power factor. The best you could hope to save is some portion (a very very small portion) of the losses in your house wiring, which isn't very much to start with.

    Make sure your coils are kept clean of accumulated dust, and the door seals are in good condition. That will do far more than any capacitor.

    On edit: you might be thinking of one of these, a Nola power factor corrector.
    http://www.allaboutcircuits.com/vol_2/chpt_13/9.html
    This is a bit more complex than just a capacitor. It decreases voltage on the motor during times when the motor is lightly loaded.
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  13. #12  
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    nice link harold, thanks.

    I only mentioned the fridge as an example but seems it was a good one!

    I appreciate that the capacitor has to be of a specific type, I would say I am informed about matching capacitors but not confident yet - I know they can explode just like batteries can, but it is always good to be reminded!

    what i still need to get my head around is the whole farads, microfarad etc thing and how to use it properly.....................anyone willing to explain this in laymans terms - either wiki just isn't helping or I haven't thought about it hard enough.........might get there some day!

    also - can anyone offer any advice or direction on achieving the correct phase correction? How would I measure this or find out if something isn't working as I might have calculated?

    Sorry for all the questions...............!!!!
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  14. #13  
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    IF the CAPACITOR came with the motor it is to tune the impedance, and maximize efficiency


    IF you bought a power saver capacitor array , you are saving on input power but you are losing output power too! and therefor no change in efficiency
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