when should a run capacitor be replaced

Discussion in 'General Electronics Chat' started by lyarbrou, Dec 24, 2008.

  1. lyarbrou

    Thread Starter New Member

    Dec 24, 2008
    I recently had a maintenance check of my gas furnace. The technician said the run capacitor for the blower motor on the furnace needed to be replaced, that the capacitor had a 10 microfarad rating and was testing only 8 microfarads. He indicated the cost would be $120 for the capacitor. He also indicated that if I didn't replace it my blower motor would likely burn out. I got the model number (GE 97F 9002, 10 uF, 370 V AC) and did some research on the WEB. I found these capacitors for sale for $15. The technical specs indicated that the 10 uF rating was plus or minus 6% (and the lifetime was approximately 60,000 hours). Thus a new one could show a reading ranging from 9.4 to 10.6. I have found a thread on this site that indicated an 8 uF run capacitor had been replaced with a 5 uF one and that this was not likely to have a significant effect on motor life. I have seen suggestions elsewhere that run capacitors should be replaced if their capacitance drops 10% below their rated value.

    1. When should a capacitor be replaced? What is the threshold? 10%, 20%
    2. Would the continued use of this capacitor likely have damaged my blower motor as the technician indicated?
    3. Is the time dependence of the decrease in capacitance of a capacitor such as this with use an increasing function or a linearly decrease with age (hrs use)?
  2. mik3

    Senior Member

    Feb 4, 2008
    1. These capacitors have tolerances up to 20% or more so it is not strange to have a deviation on its value.

    2. If the run capacitor is bad the motor may be damaged due to overheating.

    3. Electrolytics degrade with time, I am not sure about these used on motors.