There is no minimum speed limit on motorways. However, only motor vehicles designed for a speed of at least 40 km/h* may use motorways.
This does not mean that you may always drive at 40 km/h on motorways. You must maintain an appropriate speed that is adapted to the traffic situation and which does not exceed the speed limit, just like on any other type of road.
* Exception: Mopeds class 1 are not allowed to use motorways, even though they are designed for a maximum speed of 45 km/h.