A start capacitor is found in the circuit of start windings when the motor is starting.
This capacitor contains a higher capacitance than a run capacitor. It varies, but a start capacitor will measure between 70 and 120 micro Farads.
The start capacitor provides an immediate electrical push to get the motor rotation started. Without a start capacitor when the voltage is applied, the motor would just hum. The start capacitor creates a current to voltage lag in the separate start windings of the motor. The current builds up slowly, and the armature has an opportunity to begin rotating with the field of current.