I was referring to Samsonite801 reply of shop air compressor picture above, not an air conditioner. Normally there is not a run cap in air pump compressor or water pumps, as their ON duty cycle is smalll. A run cap would improve run time power factor but the main issue with water pumps and air compressors is they have significant mechanical load during startup. They use large mfd. value elecrolytic starter cap with centrifugal switch to control starter cap engagement during the first 0.4-0.5 seconds of spin up.
For this particular unit, the air pump schematic shows both starter cap and run cap. The addition of run cap is likely for EU power factor regulations, not usually used in U.S. air pump, water pumps.
Problem with water pump and air pump is they already have the large mfd. electrolytic starter cap so MIcroAir, SureStart, etc units, are only effectively giving a softstart voltage ramp without the additional cap starter value boost. Just doing voltage ramp decrease startup surge current somewhat but voltage ramp alone also extends the startup time period.
The softstart voltage ramp surge current reduction may drop below a level short time tolerable by inverter but the increased startup time exceeds the length of surge time the inverter can support the reduced, but still significant surge current. Bottleline it does not allow the inverter to start the pump.
What the auxilary (start winding) does is provide effectively a two phase motor to create a rotating field to self start a motor. There are two criteria for accomplishing this. First is providing more current to starting winding and second criteria is the capacitor and aux winding inductance providing a 90 degree current shift to provide the best phasing for the rotating magnetic field and therefore increases full 360 deg rotational torque. The additional start cap value does not often optimize phase shift, only increases start winding current.
The aux/starter winding is typically physically place at 90 degrees to main winding. Since it is only intended to be used momentarily (<0.5 secs) the aux winding is typically wound with smaller gauge wire so have to be careful of pushing too much start winding current that may damage aux/start winding wire. The starter cap effectively provides a second 90 phase offset power source without the complication and expense of an inverter power supply like used in three phase variable speed motors.
This particular air pump has an extended auxilary/start winding with addition of run cap. The schematic SureStart recommended moved the start cap located in the Surestart box from the original lesser turns auxilary starter winding to the extended auxilary winding. This likely reduced the effect of the starter capacitor to increasing rotational torque.
On this air pump, need to know the size of the original stock start cap value, not shown on schematic, and what the substitued start cap within the SureStart unit is. With the voltage ramp, the starter cap value can be larger value than original stock value, something like 150%-200% of original stock start cap value without putting start winding at risk of damage.
For this air pump, I would recommend eliminating the run cap and not use the extended aux/start winding. Make the replacement start cap value in SureStart unit about 150% of the original air pump stock starter capacitor, and connect the SureStart start cap to the orignal starter winding tap position. If this large value cap is too big to fit in SureStart box than place it outside SureStart box.
Whenever changes are made, or Cap boost/softstart units are moved to a different motor, they usually have to be reset to original untrained state so they can re-learn the optimum ramp. Check softstarter manual or call manf. of unit to find out how to reset unit to untrained, factory default state.
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