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  1. #27
    'So...just curious; was there a problem with these units with higher voltage at the capacitors?
    Lower voltage bothers me a lot more then higher voltage does
    '

    ME: Well, they arent supposed to have higher voltage than the capacitors rating because it decreases the life expectancy of the Capacitor. Of the ones that have blown where i replaced them with 440 v or 480 v ones...measuring the capacitors voltage afterward, indicates that they were beyond the 370 v. rating.

  2. #28
    Join Date
    Apr 2005
    Location
    NE PA
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    698
    Wow, a lot of comments but most are somewhat inaccurate (I do remember these basics from my EE education).

    Peak voltage for a sine wave system (without distortion) is sqrt2*RMS, or 1.414*RMS. For a 240 volt system it is 340 volts PEAK, which is the maximum voltage impressed across the capacitor.

    As for back EMF, which is a real thing, it only occurs in DC motors (all conventional types) and brushed AC motors (series wound). However back EMF, by its name, subtracts from the applied voltage. If it exceeded the applied voltage the polarity would flip, and the motor would become a generator.

    As for the vector impedance of an LC, they are in series and the combined impedance can never be less than the impedance of components.

    The capacitor is used since when it is connected to the inductance of the motor winding, it created a phase lag to provide a rotation torque vector (magnetic).

    Motor windings can act like a transformer and boost the line voltage, when one of the windings is open circuited, or lightly loaded. When the winding is connected to a capacitor, current flows through the winding-cap path and the line voltage applied is divided across the cap and winding, as a function of the impedances of each. Therefore by definition the voltage across the cap can not be greater than the line voltage.

    Voltage across the cap, and its associated rating, are based solely on the peak-to-peak voltage of the incoming line, which is 1.414*RMS

    Sine wave distortion, due to non-linear loads, like fluorescent lights and VF drives can lead to voltage peaks higher than the 1.414 factor. Poor power factor is the result of these distortions, and can cause higher peak voltages to occur, and fail caps.

    paul

  3. #29
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,914
    From the text of "Electricity for Refrigeration, Heating, and Air Conditioning", 4th Edition. Page 167, chapter 8.3(its about potential relays, but it is relevent to this topic)

    When a single-phase motor is operating, a voltage is produced across the start windings above and beyond the voltage being applied to the motor. The start windings actually act as a generator to produce the back electromotive force of a motor. The back electromotive force of a motor corresponds to the motor speed. The potential relay is designed to open, dropping the starting circuit, when the motor reaches a certain back electromotive force that is predetermined by the manufacturer of the motor.
    Similar text about back EMF can be found in about every textbook I have had. My advanced electricity class in college tought this, and the instructors at several training classes where the subject of what causes the higher than applied voltage across the run capacitor have agreed with this.

    There are a lot of things going on electricly in a motor, but I was tought, and had it confirmed repeatedly, that the high voltage across the start winding was an induced voltage, and it has always been refered to as back EMF.

    When you think about what is involved, coils of wire and rotating magnetic fields, it makes sense that a high voltage would be induced in the windings independant of the applied voltage.

    [Edited by mark beiser on 06-30-2005 at 08:51 PM]
    If more government is the answer, then it's a really stupid question.

  4. #30
    Join Date
    May 2004
    Posts
    92
    OK, guess I'm out of line.

  5. #31
    Join Date
    Apr 2005
    Location
    NE PA
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    Mark:

    I would have to respectfully have to disagree with "Electricity for Refrigeration, Heating, and Air Conditioning". What they are referring to is not correct when they call it back emf. In induction motors the voltages referred to are due to the slip angle between the start and run windings. Even if you want to refer to it as back emf, by the very definition it is a voltage that is opposite or counter to the applied voltage, reducing the applied voltage across the relay. As your quote implies, the back emf causes the resulting voltage to DROP and therefore cause the relay to drop out as speed is reached. For a detailed analysis look at: http://dynamo.ecn.purdue.edu/~esac/e...0machines'

    paul

  6. #32
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
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    14,914
    The relay doesn't drop out as speed is reached, the circuit to the start capacitor is what drops out. The coil of the relay is actually energized by the voltage that is induced in the start winding, thus opening the NO contacts of the relay and "droping" the start components out of the circuit. That voltage is directly proportional to the speed of the motor.

    I don't doubt there are things you can see on a scope that don't show up with a volt meter, but what you guys are saying flys in the face of what generations of people have been tought by generations of instructors and many textbooks and other print references.

    There are lots of things going on inside a motor that you can see with the correct instruments. I don't need to know most of it, and I don't have the background to understand some of what you have said.

    Maybe calling it back EMF is not 100% scientifically correct, but it is what I have always heared it called...

    I do know this though. If you are saying that the voltage that you measure across the terminals of a run capacitor, and energizes the coil of a potential relay, is not induced in the start winding, then you are wrong. At least acording to every instructor I have heard speak on the subject, and every text I have read.
    If more government is the answer, then it's a really stupid question.

  7. #33
    Join Date
    Jan 2001
    Posts
    7,748
    I know of an old junker carrier unit that has operated for years and years with a 370 volt capacitor on a 460 volt single phase condenser fan motor.....never have gotten around to changing that cap.....and probably never will

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