On the design of high-performance surface-mounted PM motors

This paper considers the major design factors which constrain the maximum acceleration capability of surface-mounted neodymium-iron-boron permanent magnet motors as used in servo drives. Expressions are derived for typical achievable values of the air-gap flux density and the linear current density around the stator periphery. For applications with small values of the duty factor, the stator linear current density is limited by the need to avoid demagnetization. For larger values of duty factor, the constraint is the ability of the cooling system to remove the heat losses within the magnet and insulation temperature limits. The paper derives general approximate expressions for maximum torque and acceleration limits, and shows graphs from which the range of acceleration capability of a permanent magnet motor in a given application can be assessed. The constraints involved in matching the motor design to an inverter with given maximum voltage capability are also discussed