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  1. #27
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    Actually, it's not really that far off, and I'm not in a fightin' mood today, because I have to go crawl through a 180 degree jungle of pipes to change out a 60 series B&G that some jackass hung in there 30 years ago...

    But anyway, the capacitor corrects the phase angle of single phase power, to apply more torque. In this case, it doesn't actually "create" a third phase, but allows to single phase power to "straighten" out enough to cause more torque.

    And, Mark, the start winding (And start capacitor) IS deenergized when a motor reaches full load.

    Put your amp clamp on the start terminal of a running motor, then put it on the run and common terminals.

    No, capacitors don't "switch" anything.

  2. #28
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    Originally posted by fat eddy
    Dave,

    You don't think that installing an out of phase current in another winding resembles three phase, I'll let the heavies handle this one. ( any PE's out there disciplined in electrical? )
    It's a GOOD way to describe what occurs, but, no, it doesn't "fool" the motor into thinking that it is being supplied by three phases.

    The benefit of having three phase, is that the phase angle is already correct, whereas with single phase, it's never correct long enough to keep a higher efficiency motor turning without some phase angle correction, which is where the capacitor naturally enters the picture.

  3. #29
    I know what you are saying but it does not condition the run winding it does create another leg in the start winding.

  4. #30
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    Originally posted by bornriding
    Mark is right here !!

    a start-capacitor has higher capacitance than a run capacitor. Is there to produce a strong starting torque, but is not manufactured to dissipate heat and causes an increase in current draw, therefore it cannot stay in the circuit & therefore must be removed. ( at app.75% speed )

    A run capacitor is of smaller capacitance & is oil filled to dissipate heat, which allows it to remain in the circuit, mainly for operating torgue.

    The torque needed for a motor to move from a still position and run is much more than the torque needed to keep it running.

    And just to see if ya'll are paying attention:
    A motor is an inductive device. That means that because of the inductance, the voltage & current are out of phase with each other. This reduces the power that is capable
    from the device. The capacitor brings the phase between the voltage & the current closer together which causes an increase in power for the device.( purely resistive circuit provides maximum power )
    The run capacitor moves the Voltage & current phase slightly closer together, to get a little more power (which is torque)
    The start capacitor moves the phases even closer, and therefore allows the device to generate more power ( torque ). But the motors are not designed to handle the power from a start cap. for a long time, due to the size of wiring in the windings.
    Remember power equals voltage times current. To get more power, with voltage remaining the same, current would have to increase.

    [Edited by bornriding on 02-26-2005 at 12:17 PM]
    I hate to do this to you, after you wrote all that, but then please expalin shaded pole, and repulsion-start-induction-run motors. (No caps- single phase)

  5. #31
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    Originally posted by fat eddy
    Well besides all of the technical stuff ( which I do know very very well ) The caps intention is to make the motor think it has three phase power which has more torque.
    Here let me show you how well you DO NOT know it.

    A start capacitor turns a single phase motor into a TWO phase motor temporarily for additional torque during starting until the back EMF opens the Potential relay, and takes it out of the circuit.



  6. #32
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    Originally posted by fat eddy
    I know what you are saying but it does not condition the run winding it does create another leg in the start winding.
    No, it actually does nothign at all to the motor itelsf, it shifts the phase angle of the incoming power before it reaches the motor. It does nothing but change the phase angle. Honest.

  7. #33
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    Originally posted by madeinusa
    Originally posted by fat eddy
    Well besides all of the technical stuff ( which I do know very very well ) The caps intention is to make the motor think it has three phase power which has more torque.
    Here let me show you how well you DO NOT know it.

    A start capacitor turns a single phase motor into a TWO phase motor temporarily for additional torque during starting until the back EMF opens the Potential relay, and takes it out of the circuit.


    I'm actually one of the few people, probably, on this board that has had the displeasure of working on two-phase systems. I don't wanna go there.

    But, like I said, it simply, in case of single phase, corrects the incoming power.

  8. #34
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    Yes I agree all a run cap does is bring the sine waves closer, so the motor draws less amperage, and is more efficient.

  9. #35
    An electric motor is basically composed of windings around a magnet. Motors are either multi-phase or single-phase. Multi-phase motors generate starting torque along the various windings by applying out of phase voltages to each winding in a pattern that generates a torque force in the desired direction. Single-phase motors must generate the same starting torque however they have only one phase to work from. This means they have to have a method to generate a shifted version of the single phase voltage to send to one of their windings.

    There are three common methods of creating single-phase electric motors: capacitor start, split-phase, and shaded pole. Each other these motors has some method to provide starting torque to the motor by shifting the voltage given to one of the windings on the motor by some angle. This phase shift corresponds to one winding of the motor having a voltage before another coil. The difference in time between when one coil has a voltage and when a second coil has a voltage causes the torque force and begins the movement of the motor.

    To start to solve why capacitive start motors work we can generalize Ohm's Law, V = IR, and say that V = IZ where Z is a generalized impedance. The impedance is composed of an the inductance, capacitance, and resistance. Inductance will cause the current to lag the voltage, capacitance will cause the current to lead the voltage, and resistance has no effect on the timing between the current and voltage.

    In all motors the windings of a motor are highly inductive so the current always arrives after the voltage. The capacitor changes the relative impedance of the circuit on one winding causing the shift in the relationship between the voltage and current on one winding and the other winding. This difference in the time the electrical energy is dispersed in a winding allows the motor to rotate.

    In summary, the capacitor provides a delay in the energy given to one of the windings. This delay causes the forces of the motor to be unbalanced and the motor then starts. Economically, capacitor start motors are often more costly due to the inclusion of the capacitor however they have the most starting torque This means that you probably have one in your refrigerator, washer, dryer, or other application where you may need a lot of starting force but you won't find them in your electric fan.

  10. #36
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    Right, but the common misconception is that the run and start capacitors are different in their basic use.

    They are the same exact device. The only differences are:
    • Their application.
    • Their construction


    Now, the reason that their constructions are different is because of the application. The start capacitor, being normally rated at a higher capacitance than an average run cap, is being built (today) out of plastic. The reason for this is that they are removed from the circuit within moments of being applied to the circuit. They have time to cool off. The run caps are smaller, but built from metal, with an oil fill, because they are in the circuit the entire time that the motor is running. To make these "cheaper" would cause many failures, due to the inability of the plastic and paper to reject a sufficient amount of heat, generated by the constant charging/discharging that is the true nature of the capacitor.

    Do you remember the old paper-wound start caps that had no outer case? They lasted forever, but you'd get knocked on your kiester if you grabbed one.

    Look at the price difference between a modern run cap and start cap. Notice that they're not too far apart in price range, even though a start capacitor might be rated for 400 microfarads at 440Volts, while a 10 microfarad run cap (At 440 V) isn't that much different in price. This is due to the fact that the run caps need to be made "tougher".

    How amny failed run caps do you see a year compared to failed start caps? The ratio is pretty uneven, with start caps failing more often, due to the abuse that they endure.

  11. #37
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    WOW!!!
    Dave, I hope you're not getting any stupid on you sparing with these er, um, "gentlemen" keep up the good fight.

  12. #38
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    Now, now. The capacitor is confusing. No one here is stupid.

    There has been misinformation on this subject for so long that a lot of people that work with these daily don't fully understand what happens with them, but think they do, because of waht they've been taught over the years.




  13. #39
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    Just to clarify a bit more ont he start WINDING.

    The switching device, whether it's a potential or current relay, or a centrifugal switch, actually opens the start winding, which has the start capacitor in series with the start winding and the "common". No power is applied to the start winding after the switching device opens. There is still a potential present at the start winding, but current no longer flows through the winding itself. There will be some induced current from this winding during motor operation, but the "start" winding is ONLY for starting.





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