Hi.
I have two equal 230VAC squirrel cage fan airhandlers for dwellings central airconditioning and heating. One has a motor with a 3μF run capacitor; the other uses a 7.5μF run capacitor. Both units have 3 speed selection, both are preset for 'high'
What in the motor characteristics/specifications determine the capacitor value fitted ?

 

Usually the capacitor value is determined by the motor power rating. That includes both the running torque and the starting torque, and the running speed. A motor with a lower starting torque will usually take longer to reach the rated speed, which mightbe desired in some air handling applications.

 

The ideal condition is to have a nice 90 degree phase shift at sufficient run winding current to have a nice quiet 2-phase motor condition while running.

I've used this page in the past for values for some random motor to get start and run values. https://inspectapedia.com/electric/Motor_Capacitor_Selection.php

 

Hi.
I have two equal 230VAC squirrel cage fan airhandlers for dwellings central airconditioning and heating. One has a motor with a 3μF run capacitor; the other uses a 7.5μF run capacitor. Both units have 3 speed selection, both are preset for 'high'
What in the motor characteristics/specifications determine the capacitor value fitted ?

As a general rule the 1ph capacitors are aimed at achieving a 90° phase shift of the start winding, WRT the run winding. if this is a multi speed version the centre is aimed at 90° and the other windings either side of it.