61 Permanent Magnet Machines

Machines that use magnets to produce air-gap magnetic flux instead of field coils, as in DC commutator machines, or the magnetizing component of stator current, as in induction machines, are permanent magnet (PM) machines. This configuration eliminates rotor copper loss as well as the need for maintenance of the field exciting circuit. PM machines can be broadly classified into two categories:

• Synchronous machines (PMSMs): These machines have a uniformly rotating stator field as in induction machines. Induced waveforms are sinusoidal, and hence, dq transformation and vector control are possible.

• Trapezoidal or square-wave machines: These are also known as brushless DC or electronically commutated machines. Induced voltages are trapezoidal in nature. The stator field is switched in discrete steps with squarewave pulses.

There are several advantages of using permanent magnets for providing excitation in AC machines. Permanent magnets provide a loss-free excitation in a compact way without complications of connections to the external stationary electric circuits. This is especially true for smaller machines, because there is always an excitation penalty associated with providing the rotor field through electrical circuits. Large synchronous machines use rotor conductors to provide the excitation, because the losses in the exciter circuit, referred to as the excitation penalty, are small, especially when compared to the high costs of magnets. For smaller machines, the mmf required is small, and the resistive effects often become comparable and dominating, resulting in lower efficiency. The smaller cross-sectional area of the windings for small power machines further deteriorates the resistive loss effect. Moreover, the cross-sectional area available for winding decreases as the motor size gets smaller. The loss-free excitation of PM in smaller machines with a compact area is a plus, with the only drawback being the high costs of the permanent magnets. Nevertheless, PM machines are a strong contender for EV and HEV drives, despite their larger size. The factors guiding the trend are excellent performance and high power density achievable from PM drives. Cost is not yet the prime consideration, when acceptance of the EV and the HEV by the people is still at a hesitation stage.

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