Abstract: An increase of the magnetization current can be prevented during demagnetization and magnetization, and a variable speed operation can be achieved at a high power output over a wide range of from a low speed to a high speed. A rotor (1) is configured by a rotor core (2), a variable magnetic force magnet (3) and a fixed magnetic force magnet (4). A variable magnetic force magnet (3) and a fixed magnetic force magnet (4a) are overlapped in the magnetization direction thereof to form a series of magnets. The series of magnets is located within the rotor core at a position where the magnetization direction is in the direction of a d-axis. On either side of the series of magnets of the variable magnetic force magnet (3) and the fixed magnetic force magnet (4a), fixed magnetic force magnets (4b, 4b) are located at a position where the magnetization direction is in the direction of the d-axis.

Abstract: An image forming-apparatus of the present invention includes a charging device for applying a first potential and a second potential to a photosensitive drum, a keyboard for inputting a magnification of a copy image to be formed on the photosensitive drum, and a controller for controlling the charging device to control the magnitude of the potentials applied to the photosensitive drum from the charging device in accordance with the magnification input by the keyboard, thus increasing a difference between the first and second potentials in accordance with the degree of variation of the magnification in the case where the magnification input by the keyboard is less than 1.00.