I have a 5 hp Ajax single phase AC motor. When i got it the start and run caps were missing. Is there anyway to determine the proper start and run cap values based on the info on the nameplate? Thank You.


1 Answer 1


The preferable method would be to search the brand and model number to see if you can find detailed information from the manufacturer. You could also contact the manufacturer or a sales outlet for the manufacturer. The web sites of capacitor sellers sometimes have tables of typical values based on power, voltage and frequency ratings. Using a typical value should be adequate, but might have a small effect on motor performance. Check the motor current after replacement and check to see if the motor comes up to speed without too much delay. Check to see that the motor does not get too hot. A problem with the motor could indicate either the wrong capacitor value or a failed motor, more likely a failed motor.

  • \$\begingroup\$ unfortunately the Company that made the motor is no longer in business and im having a lot of trouble finding much info on it. I did get the motor to run using a 30 mfd run cap and no start cap, i just spun it by hand. It got up to speed fast and ran smooth with no load. What size starter cap should i try? Thank you for your help. \$\endgroup\$
    – Bob Samith
    Oct 28, 2016 at 12:37
  • 1
    \$\begingroup\$ It appears that there may be a considerable variation in the cap used depending on the starting difficulty of the load. Many motors of this type are made to be sold with specific equipment. I found a chart with 138-182 mfd at 0.746 kW (1 Hp) at 200-250 volts. The same chart shows 500-580 mfd for 0.746 kW at 110-125 volts. I thought I had seen a site with several charts for run & start caps for different voltages etc., but I can't find it now. For 5 Hp, you could multiply the mfd values by 5. \$\endgroup\$
    – user80875
    Oct 28, 2016 at 14:52

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