Energy conversion by permanent magnet machines and novel development of the single phase synchronous permanent magnet motor. (1998)
AuthorsStrahan, Richard Johnstonshow all
Energy methods are widely used and well understood for determining the torque or force in machines which do not contain permanent magnets. Energy methods are employed to calculate torques or forces of magnetic origin after determination of the energy stored in the electromechanical coupling field. In this thesis, the energy stored in a permanent magnet system is defined, and the energy-coenergy relationship is determined. It is shown how residual magnetism can be incorporated into classical electromechanical coupling theory. It is therefore shown how equations for torques or forces can be derived for permanent magnet systems using energy methods. An analytical method of calculating permanent magnet reluctance torque is developed. The method uses an elementary expression for the magnetic field to obtain the stored energy. This enables an analytical expression for the reluctance torque waveform to be obtained. The method is demonstrated to provide a powerful and fast design tool. The method can be generally applied to reluctance torque problems where the airgap is reasonably smooth. The single phase synchronous permanent magnet motor is used in domestic appliances. It is a motor of very simple construction and high reliability, which is directly connected to an AC mains supply, and runs at synchronous speed. It is becoming increasingly used in preference to the shaded pole induction motor. However, its application is limited by the following characteristics. There is no control over the final direction of rotation, unless a mechanical blocking device is used. There are rotor positions at which only a very small starting torque is available. The characteristic twice electrical frequency torque pulsation yields a speed modulation of the same frequency, which can cause acoustic noise problems. A method of improving torque quality by improving the motor design is proposed to alleviate these limiting characteristics. This is achieved by designing a permanent magnet reluctance torque which cancels out the effect of the backward rotating component of the stator field. In this novel design, the permanent magnet reluctance torque effectively acts as a second balancing phase. An unconventional technique for starting a single phase synchronous permanent magnet motor is demonstrated. This technique uses an inductive reluctance torque, provided by placing a suitably shaped iron lamination on the rotor, to rotate the rotor to a position from which starting can occur.