Seite - 108 - in Hybrid Electric Vehicles

2. Electrical drive used for ISAB applications 2.1. Electrical machines In the last decade, the development of power electronics (inverter/convertor) made the alter- native current (AC) machines the best solution for ISAB applications, especially due to their high power density. These are synchronous reluctance machine (SynRM), induction machine and permanent magnet synchronous machines (PMSM) in both supplying variants: with sinusoidal and trapezoidal current. The detailed investigation of SRM and induction machine is presented in Refs. [11, 12]. In these studies, the complicated electronics needed for SRM and the difficult control of the induction machine (influence of slip in performance of the machine) are highlighted. In this context, the SynRM and PMSM are the best candidates for ISAB applications. The electrical machines used for conventional ISAB applications are exposed at high tem- peratures generated by ICE. This makes impossible the use of the PMSM in high efficiency and low-cost conditions (only with a special method for cooling or using expensive SmCo magnet). Therefore, the SynRM without permanent magnets is the best solution for the direct connection to the crankshaft of ICE (ISAB) and PMSM machine for BSAB. 2.1.1. PMSM machine for BSAB applications The main advantage of the PMSM compared with other types of electrical machine is their high efficiency due to the absence of the field coil losses. The stator is constructed from three- phase windings and steel sheets (the same as the induction machine), but due to the absence of iron losses, the rotor is built from massive steel and permanent magnets. The position of the permanent magnets can be categorized as surface-mounted type and interior type. This posi- tion can have a significant effect on the mechanical and electrical characteristics, especially on the synchronous inductance [13]. Because the permeability value of rare earth magnet (such as NdFeB) is very close to that of the air, the air gap of the machine with mounted surface PM effectively becomes larger in this case. This makes the machine d-axis inductance value very low, with a significant effect on the ability of overloading the machine and operation at flux weakening. Because the maximum torque is inverse proportional with the d-inductance, this becomes very large. But the low value of d-inductance reduces the possibility to operate at flux weakening. This is caused by the need to use a high value of the demagnetization com- ponent of the stator current in order to decrease the flux value in the air gap. Therefore, the remained current on the q axis will be insufficient to produce torque. In the case of the interior magnets, it is possible to obtain a sinusoidal distribution of the air- gap flux by using simple rectangular magnets. A sinusoidal flux distribution reduces consid- erably the cogging torque, in particular in the case of the machine with a large number of pole pairs and a small number of slots per pole and phase [14]. For these structures, it is also pos- sible to increase the flux density in the air gap beyond the value of the remnant flux density of the magnets by using the flux concentrators. Because in this case the d-inductance is usually higher than with that of the surface magnets topologies, the overload capacity of the machine will be reduced and the performance in flux weakening conditions will be higher. Hybrid Electric Vehicles108