use a hinge constraint instead of incrementing cframes.
create a base part that you want to attach your rotating part to.
go to the model tab, under the constraints menu select the drop down and click hinge.
place the first attachment on the middle top of the base part, and the second attachment on the middle bottom of the part you want to rotate.
align the objects together so that the attachments are perfectly alligned.
select the newly created hinge constraint, and search for a property called ActuatorType, and set it to Motor.
a new properties tab should open up when you change it to a motor. In this menu, you can change the angularvelocity to match with whatever speed you want.
if the rotational axis are not properly aligned, you can mess around with the attachment orientations until they are properly alligned (there is a little menu if you click on the hinge constraint, this will display a warning.)
i told you to align the attachments, otherwise it wont work. you need to center both of the attachments and then place them together, otherwise the hinge will fail.