the base plate 55 is substantially in the form of a circular disk with the switching elements 58 59 which are arranged on the rear face not shown of the base plate being arranged on a relatively large segment of more than 200 u00b0 and the actuation circuit 60 being arranged on the remaining relatively small segment in this case the switching elements 58 59 are uniformly distributed over the relatively large segment of the base plate and in this case preferably assigned to a separate group 54 a 54 e of cooling ribs in pairs with a radial orientation the individual cooling ribs 54 are preferably integrally produced with the base plate 55 as a cast part however as an alternative it is also possible to produce the base plate and the cooling ribs or groups of cooling ribs separately and to subsequently connect them to one another the advantageous dimensioning and design of the cooling body 53 ultimately result from a large number of boundary conditions firstly the length of the machine in the axial direction and its weight are fundamentally limited taking into account the respectively required power this results firstly in limiting of the height of the cooling ribs 54 and secondly limiting of the thickness of the base plate in addition it should be possible to manufacture the cooling body as a cast part where possible and for this reason the cooling.