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Limited knowledge of electrical circuits

  1. Jun 21, 2005 #1
    I have a limited knowledge of electrical circuits and recently it got put to test. I live in Arizona and the A/C on my house quit. I found that the condensing fan motor had siezed

    This is what was in the unit. 1/5 hp, 825 RPM 208-230 vac, 1.1 amp with a 7.5 ufd capacitor. The local wholesaler only had 1/4 hp motors, so I ended up with a 1/4 hp 825 RPM 208-230 vac, 2.2 amp. They gave me the same capacitor.

    The new motor ran for a while and started tripping on the internal thermal protection. I called the supplier and was told that this motor should have a 5 ufd capacitor.

    I changed to the 5 and the motor would run longer, but was still tripping. I took this motor back to supplier and brought home a new one. I noticed on the box that the capacitor for this motor should be a 5 MFD 370 VAC.

    My question now is, is MFD milifarad or microfarad???
    Could going from a 1/5 hp motor to a 1/4 hp motor change the capacitor requirement by that much???

    If anyone can help, I should would appreciate it.

  2. jcsd
  3. Jun 21, 2005 #2
    Microfarads are denoted by uf, and millifarads are denoted by mf. My guess is you need a millifarad capacitor. Good Luck!
  4. Jun 21, 2005 #3
    Not likely. I've seen alot of motor capacitors and they are marked in MF. But it is actually micro farad.
  5. Jun 22, 2005 #4
    MFD is micro farad..
    how do you know that the motor is being tripped by the internal thermal shutdown..??
    why is it getting so hot??
    are the airpassages clear??
  6. Jun 23, 2005 #5
    Thanks everyone. I have decided that the MFD is in fact micro. That just doesn't make sense that they would use 2 different letters for one unit designation. I work with micro and milli curies all the time and it confused me.

    There is only one contactor for the unit and it was still closed, the compressor was still running and the fan wasn't. The motor is sealed for outside use and I live in Phoenix, AZ. It gets hot!!!

    I took this motor out and went back to the wholesaler to get a different motor. I have had no problems since I swapped the motors. What can you say???? :confused: Made in China!!! :mad:

    Thanks again for your input!
  7. Jun 29, 2005 #6
    The use of MFD for microfarad is an older industrial marking, (usually with industrial paints or inks) probably more common still with motor parts. the Greek mu will be found more often on modern audio and computer parts (silkscreened or electrostatically printed). Part of the reason is that in older printing practises, special characters were rare and more expensive to set up. With modern printing (usually with computer generated graphics and fonts) there is no cost difference, or problems with availability of alphabets.

    Millifarad Caps will be very large capacitors, and not always marked this way, but sometimes in Farads using decimal points. The important thing still with capacitors, even in motor applications is their duty voltage and surge voltage ratings, as well as making sure that the type of capacitor is the one recommended for the kind of job being performed in the circuit. Large caps tend to be electrolytics due to cost, but most electrolytics have fixed polarity and require minimal charging to form the insulating skin inside. A special breed of 'non-polar' electrolytic caps are made for audio (speaker) applications, usually simply by connecting internally a pair of caps in series with one reversed (+ve to +ve). This can allow the caps to handle A.C. signals without failures.

    In motors, current capacity and duty cycle is also important for prevention of failure. But if caps and motors keep blowing, it seems prudent to examine the load and the setup to see what is causing the failures.
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