FEM Analysis of a Vernier Motor and its Operating Behaviour due to Manufacturing Tolerances

Abstract:
Motors for direct drives are characterized by a very high pole number. However, in case of small machines there is a risk that the exact positioning of the magnets becomes difficult, especially if the magnets are glued on and the exact position is not given by a positive-locking construction. Also the magnet dimensions and the magnetic features have tolerances. Due to mounting inaccuracies there are further deviations between the calculated magnet circuit and the final machine. For example, static and/or dynamic eccentricities may occur. By means of the finite-elements-method the effects of several deviations from the ideal condition are examined and evaluated. As an example a small external rotor motor with 22 poles and 50 mm diameter has been chosen. For this machine important parameters, such as efficiency, cogging torque and radial forces are analyzed. Special attention is given to cogging torques. Depending on the selected number of grooves and magnets, high orders for the occurring cogging torques result, which should be negligible also without skewing of the machine. Due to manufacturing tolerances there may also appear torques with lower order.