6121 Types of PMSMs

The PMSM has a stator with a set of three-phase sinusoidally distributed copper windings similar to the windings described in Section 5.4 on AC machines. A balanced set of applied three-phase voltages forces a balanced set of sinusoidal currents in the three-phase stator windings, which in turn establishes the constant amplitude rotating mmf in the air gap. Stator currents are regulated using rotor position feedback so that the applied current frequency is always in synchronism with the rotor. Permanent magnets in the rotor are appropriately shaped, and their magnetization directions are controlled such that the rotor flux linkage created is sinusoidal. Electromagnetic torque is produced at the shaft by the interaction of these two stator and rotor magnetic fields.

PMSMs are classified according to the position and shape of the permanent magnets in the rotors. Three common arrangements of the rotors are surface mounted,

(a) (b)

FIGURE 6.2 Permanent magnet machines: (a) surface-mounted; (b) interior.

inset, and interior or buried. The surface-mounted and interior PM machine configurations are shown in Figure 6.2. The difference between surface mounted and inset magnets is that the magnets in the latter are inside the rotor surface but still exposed to the air gap. The surface-mount and inset rotor PMSMs are often collectively called the surface-mount PMSMs. The other type of PMSM is the interior PMSM, so named because of the magnet arrangement in the rotor. In the surface-mounted PMSM, the magnets are epoxy glued or wedge fixed to the cylindrical rotor. Nonmagnetic stainless steel or carbon fiber sleeves are also used to contain the magnets. The manufacturing of this kind of rotor is simple, although the mechanical strength of the rotor is only as good as that of the epoxy glue. The direct and quadratureaxes inductances of the surface-mounted PMSM are approximately equal, because permeability of the path that the flux crosses between the stator and the rotor is equal around the stator circumference. Uniformity in the magnetic path despite the presence of magnets is because the permeability of magnets is approximately equal to that of the air. The space needed to mount the magnets increases the radial distance of the effective air gap, making self-inductance relatively smaller in PMSMs. In the inset PMSM, magnets are put into the rotor surface slots, which makes them more secured. The direct and quadrature axes reluctances are unequal in inset PMSMs, because space is occupied by a magnet in the direct axis and by iron in the quadrature axis. The quadrature axis inductance Lq is larger than the direct axis inductance Ld, because the direct axis flux path has a larger effective air gap and, hence, higher reluctance, although the length of the air gap between the stator and the rotor is the same. The interior PMSM has its magnets buried inside the rotor. The manufacturing process is complicated and expensive for the interior PMSM. The quadrature axis inductance Lq in interior PMSMs can be much larger than the direct axis inductance Ld. The larger difference in the d-and q-axes inductances make the interior PM more suitable for flux weakening operation, delivering a wider constant power region compared to the other types of PMSMs. The extended constant power range capability is extremely important for EV and HEV applications to eliminate the use of multiple gear ratios and to reduce the power inverter volt-ampere rating. Because of the unequal reluctance paths in the direct and quadrature axes, a reluctance torque exists in buried and inset PMSMs.

FIGURE 6.3 The stationary and synchronous frames in PMSM.
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