Simulation of cogging in a 100 kW permanent magnet octagonal linear generator for ocean wave conversion

Fluctuations in the torque acting on the rotor in a rotating electric generator i.e., cogging, gives unstable rotor motion and can venture reliable operation of the machine. Cogging also gives a similar fluctuation in the output power. This work presents a study of the cogging force in a linear permanent magnet generator. A constant output power is possible in a rotating machine with three phases winding. In a linear generator, due to the reciprocate piston motion, power fluctuations on the slow time scale of the piston bounce period cannot be avoided. Associated with power variations on a time scale shorter than the piston bounce motion is fluctuations on the force (torque) acting on the piston, i.e., cogging. Cogging is not less dangerous in linear generators compared to rotation machines and the problem requires investigation. The paper investigates means to reduce cogging by improving the geometrical design of the stator. In particular, rational number of the slots per pole and phase reduce the cogging by one order of magnitude.