Check for any loose "neutrals" in your electrical system.
Momentary dimming is considered normal,........and as others have said, is due to quite high LRA "Locked Rotor Amperage".
It should only last a second or less.
If your unit is not set-up for hard start with a start capacitor, and relay,........you MAY reduce the dimming by adding a cheap aftermarket hard start kit.
They simply hook across your run capacitor terminals.