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  1. #14
    Join Date
    Feb 2004
    Location
    Pacific NW
    Posts
    570
    Never ceases to amaze me how many hvac techs have a total misunderstanding of ac power, and how incorrect opinions are foisted off as wisdom, must be a gross error in some hvac text somewhere? .

    FWIW:
    1. good news - everyone is correct in that the caps should be rated at 440 Vac for good reliability.
    2. bad news - not a single poster seems to understand ac power flow. The 389 or 400 V across the cap IS NOT generated by the start or run winding. The voltage across the cap plus the voltage across the run winding is exactly equal to the line voltage at EVERY specific point in time. When you hook up your digital voltmeter, you are probably reading 1.11 time the average voltage ( any cheap meter, even a Fluke 77 reads 1.1 times average vs true rms). If you hook up a scope (e.g. Fluke 123 or equivalent) across the cap and run winding, you would see that at times the volts of the sine wave across the cap is negative and across the winding is positive and both are higher peaks and rms than line voltage - read up about leading and lagging currents in capacitors and inductors before blathering about back emf, etc. and about resonance and voltage amplification thru resonance, etc. etc.


    [Edited by junkhound on 06-30-2005 at 12:23 AM]

  2. #15
    Join Date
    Jun 2005
    Posts
    347

    Smile

    I found one today on a no a/c call that had blown apart like one of those magic worms you light a capsul and it grows in to an ash worm. Happy forth of july...

  3. #16
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Junkhound, do yourself a favor. Before blasting people here over giving incorrect information, try researching some about how AC motors work, because you are dead wrong.

    Back EMF is real, and the other posters are correct. In an induction motor with a run and start winding, there is an induced voltage potential between the run and start windings that exeeds the input voltage to the motor.
    This is the same voltage that energizes the coil of a potential relay for a start kit, and is the voltage the capacitor must have a higher rating than.

    http://cougar.slvhs.slv.k12.ca.us/~p...s/backemf.html

    http://www.answers.com/topic/back-emf

    http://www.newton.dep.anl.gov/askasc...0/phy00351.htm

    http://www.chilternweb.co.uk/physics/emag/backe.html

    http://www.4qd.co.uk/fea/pmm.html

    http://www.madsci.org/posts/archives...5941.Eg.r.html



    [Edited by mark beiser on 06-30-2005 at 01:16 AM]
    If more government is the answer, then it's a really stupid question.

  4. #17
    The voltage you are reading is generated accross the start winding called back emf

    [Edited by molabearair on 06-30-2005 at 01:58 AM]

  5. #18
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,776
    You posted some nice links Mark.

    Hopefully others go and read them.
    Contractor locator map

    How-to-apply-for-Professional

    How many times must one fix something before it is fixed?

  6. #19
    Join Date
    Feb 2004
    Location
    Pacific NW
    Posts
    570
    how AC motors work......original comment holds.

    1. never said there was no such thing as back emf, but that is not what causes capacitor voltage higher than line voltage. A cap and inductor of any type (not even a motor) can give higher voltages across the cap than line voltage.....

    2. interesting how a bunch of internet links can be re-interpreted to suit preconceived notions.

    Hook up a 4 channel scope to a motor and them come back and make comments.

  7. #20
    Join Date
    Aug 2004
    Location
    North Richland Hills, Texas
    Posts
    14,915
    Junkhound, the links were just random Google stuff. Your origonal comment is still wrong.
    I have plenty of hard copy text here, I'll see if I can find something to link that is more relevent to PSC motors when I get home tonight.
    If more government is the answer, then it's a really stupid question.

  8. #21
    Join Date
    Nov 2000
    Location
    Eastern PA
    Posts
    68,981
    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.
    Government is a disease...
    ...masquerading as its own cure…
    Ecclesiastes 10:2 NIV


  9. #22
    Join Date
    May 2004
    Posts
    92
    Junkhound, I have some test data in front of me that says we have a unit running @ 230 VAC with a capacitor voltage of 336.3 VAC (.5% instruments with tracable calibration). If your theory holds true, then the capacitor voltage should be 325 VAC, which is peak voltage at a measured voltage of 230 VAC RMS.

    Unless I misunderstood something you said, which is certainly possible.

  10. #23
    Join Date
    Feb 2004
    Location
    Pacific NW
    Posts
    570
    ....then the capacitor voltage should be 325 VAC, which is peak voltage at a measured voltage of 230 VAC RMS....

    You misunderstood, the capacitor voltage has nothing directely to do with 1.414 times the rms value of the line voltage. The capacitor voltage is the capacitor reactance times the run circuit current. The run circuit current is the (vector 'sum' of line voltage minus back emf) divided by the run circuit vector impedance. The vector impedance is the vector (NOT arithmetic) sum of the the winding impedance and the capacitor impedance. Since the vector sum of the inductive and capacitive impedances is often LESS than the impedance of either the run winding or the capacitor, the voltage across the capacitor is higher than the line voltage.

    The higher voltage across the cap is normal, what is not good in the original post is that the manufacturer chose to save a few pennies at the expense of reliability and not do a worst case analysis of the run circuit current and capacitor voltage.

    BTW, had a call-back on a Goodman condensor last month, the scroll rubber feet are held by glorified sheet metal screw which came loose, causing low frequency vibration of the compressor, a few more months and a tube could have broken from fatigue.

  11. #24
    Join Date
    Apr 2003
    Location
    Eugene, Oregon
    Posts
    1,209
    Got that Bubba? Sounds like someone brought out the text book however, if thats from memory you should be a teacher. Right or wrong on your theory, it sounds pretty good
    Proud supporter of Springfield Millers and Oregon Ducks.

  12. #25
    Join Date
    Aug 2002
    Location
    Office and warehouse in both Crystal River & New Port Richey ,FL
    Posts
    18,836
    Originally posted by junkhound
    ..

    BTW, had a call-back on a Goodman condensor last month, the scroll rubber feet are held by glorified sheet metal screw which came loose, causing low frequency vibration of the compressor, a few more months and a tube could have broken from fatigue.
    So is their method that different from other brands??

  13. #26
    Join Date
    Jan 2004
    Location
    Midlothian, Virginia
    Posts
    195


    A cap and inductor of any type (not even a motor) can give higher voltages across the cap than line voltage.....

    this my origional point
    yes when you look at voltage on an ocilliscope it peaks higher than 240 line voltage. meters are buit to read what is called root mean voltage. which is lower than the peak of the sine wave.

    for thouse of you that are old enough to remember the old tv show " we are taking controll of your horizontzl we are taking controll of your vertical" the wave you saw was a sine wave hehehehehe.
    Goodbyee stranger it's been nice. Hope you find your paradise! Hey it aint rocket science, "It's a Trade !"

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