Cogging – Agricultural Wind Generators
The cogging torque of the permanent magnet (PM) wind generators is generated from the interaction of permanent magnet MMF harmonics and air-gap harmonics due to slotted structure of stator core. It manifests itself by the tendency of a rotor to align in a number of stable positions even when the machine is unexcited, and results in a pulsating torque, which does not contribute to net effective torque. These torque ripples result in undesirable mechanical vibration and noise. For this reason, the cogging torque should be kept small in the design process for the purpose of constant speed operation and high precision position control.
In direct-driven wind generators supplied in Polaris turbines, the rotor rpm is low, thereby requiring large number of poles to operate these turbines. Generally, rotational speed of a direct-driven electric generator for a wind turbine is lower than conventional electric machines. Due to large number of poles in Polaris PM direct-driven wind generators, the width of the magnet pole is very short. Thus the rotor has almost uniform air gap. With the magnetic pole very short or nonexistent – cogging is virtually eliminated.