A belt-tensioning device includes an electric machine having a drive belt pulley, which is driven about a drive axis, and a further drive belt pulley, where an endless belt wraps around the drive belt pulleys. The device has a housing in which a first and a second spring arm are mounted, in each of which spring arms a tension roller having axes of rotation which are parallel to the drive axis is rotatably mounted. The spring arms are supported against one another and press the endless belt together via the tension rollers in the region of the drive belt pulley. A first and a second further spring arm are provided adjacent to the spring arms which are adjustable by an adjustment element such that the spring forces of the further spring arms either do or do not support the spring forces of the spring arms acting on the endless belt.