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  • Run Capacitor Blues

    It's late on Friday. The weekend is about to begin. You are on your last "No Cooling" call of the day. You're way out in BFE. You get there and the condenser fan is running, but there is no audible compressor. Pull the disconnect, get your hand to the compressor shell, and it's hotter than Jenna Jameson. Get a water hose, cool down the compressor, drink a glass of tea, internal overload resets, and you fire it back up. Compressor hums (maybe because it don't know the words) but will not start. You check the run capacitor and determine it to be faulty. It is a 35 MFD @ 440 V single run cap. You go to the van, look in every bin and you don't have such a beast, but you do have a five gallon bucket full of old run caps that you've pulled off of old equipment that was replaced. You keep these with you for emergency purposes. Kinda like right now. Considering the rules of capacitance, you need to pick out the correct combination of capacitors that would get your compressor up and running, and make the customer happy until you can get back out there and install the correct one.

    A. 25 MFD @ 440 V + 10 MFD @ 370 V WIRED IN PARALLEL
    B. 15 MFD @ 440 V + 20 MFD @ 440 V WIRED IN SERIES
    C. 20 MFD @ 440 V + 20 MFD @ 440 V WIRED IN PARALLEL
    D. 30 MFD @ 600 V + 5 MFD @ 440 V WIRED IN PARALLEL

    Winner buys the next round and posts the next question.

  • #2
    Re: Run Capacitor Blues

    b
    sigpic

    Comment


    • #3
      Re: Run Capacitor Blues

      I also agree B. Buy all in one capacitors and you won't have that problem anymore.
      Buy cheap, buy twice.

      Comment


      • #4
        Re: Run Capacitor Blues

        The correct answer is D. Resistors add up in series. Capacitors add up in parallel.

        Comment


        • #5
          Re: Run Capacitor Blues

          I agree with answer D being correct. JONESY hands the thread to the winner. I really think that's IovetheUSA.

          If you were to connect resistors in parallel you need to follow the "Product Over Sum" rule.
          Last edited by Old Grunter; 08-10-2008, 01:14 PM.

          Comment


          • #6
            Re: Run Capacitor Blues

            The answer is D, and yes, the gold medal goes to Ilovetheusa. MFD adds up in parallel, Voltage adds up in series. I like Bud Light, but since you're buyin' - let's make it a Heineken.

            Later, Jonesy

            Comment


            • #7
              Re: Run Capacitor Blues

              D is the best answer but C would also do in a pinch, A would be better than C on a 240 V compressor. B is the only one that would not work at all due to the 8.6 MFD combined capacitance

              Comment


              • #8
                Re: Run Capacitor Blues

                Thanks for the question, learned something new.
                Buy cheap, buy twice.

                Comment


                • #9
                  Re: Run Capacitor Blues

                  Originally posted by JONESY View Post
                  It's late on Friday. The weekend is about to begin. You are on your last "No Cooling" call of the day. You're way out in BFE. You get there and the condenser fan is running, but there is no audible compressor. Pull the disconnect, get your hand to the compressor shell, and it's hotter than Jenna Jameson. Get a water hose, cool down the compressor, drink a glass of tea, internal overload resets, and you fire it back up. Compressor hums (maybe because it don't know the words) but will not start. You check the run capacitor and determine it to be faulty. It is a 35 MFD @ 440 V single run cap. You go to the van, look in every bin and you don't have such a beast, but you do have a five gallon bucket full of old run caps that you've pulled off of old equipment that was replaced. You keep these with you for emergency purposes. Kinda like right now. Considering the rules of capacitance, you need to pick out the correct combination of capacitors that would get your compressor up and running, and make the customer happy until you can get back out there and install the correct one.

                  A. 25 MFD @ 440 V + 10 MFD @ 370 V WIRED IN PARALLEL
                  B. 15 MFD @ 440 V + 20 MFD @ 440 V WIRED IN SERIES
                  C. 20 MFD @ 440 V + 20 MFD @ 440 V WIRED IN PARALLEL
                  D. 30 MFD @ 600 V + 5 MFD @ 440 V WIRED IN PARALLEL

                  Winner buys the next round and posts the next question.
                  Congratulations lovetheusa and thanks to Jonesy for another great question. If you don't mind, I would like to hijack the question with another question:

                  You pulled the 30uf x 440v and 5uf x 440V capacitor out of the truck, but discover one of the terminals is broken on the 5uf cap. You still have (1) 10uf x370V capacitor, (1) 25uf X 440V capacitor, and (2) 20uf x 440v on the truck. You remember from school that it is generally OK to oversize capacitance up to 10%, but prefer to keep the capacitance closer to 35uf. Consequently, you want to use the 10uf x 370v capacitor in parallel with the 25x440 capacitor, for a total of 35uf capacitance. Problem is, the original capacitors are rated at 440V and you are not sure if the 10uf x 370v can be used safely.

                  A. How could you find out if a 370V capacitor rating would be adequate for the application?

                  B. If you installed the 370V capacitor in parallel with the 440V capacitor, what is the rated voltage for both capacitors?
                  Last edited by spodelee; 08-11-2008, 01:32 AM.
                  spodelee

                  Until lions have their own storytellers, stories of the hunt shall always glorify the hunter

                  Comment


                  • #10
                    Re: Run Capacitor Blues

                    A. Determine the voltage of the system. If its 240 volt, a 370 will be adequate. If its 440 a 370 will not be adequate.

                    B. The voltage ratings for the caps will not change. You will still have a 370 and a 440.

                    Ive replaced a 35uf with a 30uf in a pinch late on a Saturday. It ran fine til Monday.

                    Andy

                    Comment


                    • #11
                      Re: Run Capacitor Blues

                      A. peak voltage = √2 * RMS
                      so for a 240 V system the peak voltage is 340V

                      B. Capacitors in parallel can only be applied across the lowest peak voltage rating of the capacitor bank

                      Comment


                      • #12
                        Re: Run Capacitor Blues

                        I'll throw one on here. How would you measure the peak Voltage across the capacitor if you really needed or wanted to?

                        WB is very correct about "B" and this is something we need to watch out for.

                        Comment


                        • #13
                          Re: Run Capacitor Blues

                          I would use my oscilloscope but not a real common tool thats why you use the true RMS voltage (has to say that on the meter specs) and multiply

                          Comment


                          • #14
                            Re: Run Capacitor Blues

                            Time for Wayne to ask a good question.

                            Comment


                            • #15
                              Re: Run Capacitor Blues

                              Originally posted by wbrooks View Post
                              A. peak voltage = √2 * RMS
                              so for a 240 V system the peak voltage is 340V

                              B. Capacitors in parallel can only be applied across the lowest peak voltage rating of the capacitor bank
                              Well that was easy wasn't it? I never knew a formula existed - I was tought to measure between the run and start winding for the running back- emf voltage. Simple, simple me!

                              Your up - next question
                              spodelee

                              Until lions have their own storytellers, stories of the hunt shall always glorify the hunter

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