The present invention relates to an interior permanent magnet, synchronous machine or IPMSM, with concentrated windings, capable of field weakening operation.
Interior magnet motors have been reported in the literature, and their torque generation characteristics are well understood, an example of this kind of machine can be found in EP 0 823 771 and a technical description of the method of torque generation in Chapter 6 of “Brushless Permanent-Magnet and Reluctance Motor Drives” by T. J. Miller ISBN 0-19-859369-4.
The general advantage of this kind of motor is its ability to generate both a synchronous torque and a reluctance torque which, given a suitable power amplifier, can be independently controlled allowing it to run at speeds far higher than the natural synchronous speed of the motor, and thus to deliver torque and power over a very wide speed range.
A further advantage is the efficiency of the winding method which allows maximum fill-factor while elimination the end-winding losses of a distributed winding construction. The wide speed range capability of a IPMSM is a widely used feature of the standard induction motor, a feature which is not available to any useful extent with a standard synchronous permanent magnet motor. Synchronous motors are however more dynamic than equivalently sized induction motors, i.e. in rotational motors the ratio of torque to rotor inertia can be greater for the synchronous machine, and they would be useful in machine tool applications such as spindle drives were it not for the maximum speed limitation. The IPMSM in conjunction with a suitable active power source overcomes this limitation.
The problem with currently available motors are that their operating speed range is too narrow, and their secondary side (rotor) construction tends to complicated by a requirement for special cut-outs or unusual forms or shapes in order to generate the required difference between the direct and quadrature axis inductance. It is unusual to find an IPMSM which has a speed range of much more than 5:1 i.e. where the ratio of the maximum allowable speed to base speed is greater than 5:1. As the base speed of a synchronous motor tends to be around 3000 rpm, this makes it difficult for such motors to be used in applications where speeds of more than about 15,000 rpm are required. Many spindle drives run at 25,000 rpm or more which requires a field-weakening range of around 9:1 which is not possible with any currently available design.