How can I slow down a 240V AC motor used for an exhaust fan?
The fan most likely has an asynchronous AC motor and this means the speed of the arrangement is bound to be near the synchronous speed (of 3000rpm at 50Hz). You cannot lower it a substantial amount without having substantial losses within the rotor. Which means much additional heat. As the fan motor is cooling itself by rotating the fan, this is double bad.

So the only reasonable way to change the speed of the motor (and the power of the fan arrangement) is lowering the frequency. You would need a VFD for that. For 1.5kVA, it would be quite expensive. Better use a smaller motor/fan arrangement.