The influence of control parameters on energy efficiency of switched reluctance generator for vehicle applications

. In the paper, the possibilities of application of switched reluctance generators in modern variable-speed drives were discussed. The process of mechanical energy conversion into electric energy was discussed. Based on simulation and laboratory tests, output power profiles, efficiency profiles and profiles of ratio of output power to phase current were determined in the single-pulse mode. The authors showed that proper selection of control parameters allows the operation of the SRG with maximum power, maximum efficiency or maximum ratio of output power to phase current. Finally, conclusions concerning advantages of using SRGs in variable-speed applications were drawn.


Introduction
Due to the development of variable-speed applications and hence increasing demand for electrical energy, research on efficient, fault tolerant, compact and reliable generation systems are being conducted. In recent years, the intensified development of vehicles with hybrid and electric drives can be seen. Electric machines, which operate depending on the situation, among others, as a generator, are used in both of these types of drives. The development of these types of vehicles is a part of a global tendency to decrease negative impact of combustion engines on the environment. Despite strict emission standards, combustion engines will still have negative impact on the environment. Stricter emissions standards can only decrease but not eliminate a problem of exhaust emissions into the atmosphere. The vehicles with hybrid drives do not solve this problem, but they allow covering short distances only with electric drive. Only vehicles with electric drive alone allow eliminating exhaust emissions into the atmosphere. However, we should be aware that vehicles with hybrid or electric drives are not completely harmless for the surrounding environment. In hybrid vehicles, combustion engine drives electric machine which operates as a generator and charges batteries. In vehicles with electric drive, batteries are charged from the mains. In both electric and hybrid drives, part of accumulated kinetic energy of moving vehicle is converted into electric energy. It is possible when electric machine operates as a generator. Recently, systems for converting kinetic energy into electric energy during vehicle braking are used in many vehicles with combustion engines which are equipped with Start-Stop systems. Such vehicles are very often called semi-hybrid recently.
In vehicles with electric and hybrid drives and with Start-Stop systems, energy recovery during vehicle braking depends largely on the road conditions. During smooth deceleration or braking, energy recovery should occur at as high efficiency of kinetic energy conversion into electric energy as possible. Braking distance is very important while sudden stoppage of the vehicle is necessary. In this case, braking with classic mechanical brakes is very important. However, part of kinetic energy can be converted into electric energy in this case. Emergency braking process should be assisted as much as it is possible by electric machine. It means that converting of kinetic energy into electric energy has to be directed into obtaining as high braking torque as possible.
Various types of electric machines are used in above-mentioned solutions [2][3][4]. However, brushless motors with permanent magnets are currently most widely used. They are characterized by the highest efficiency of energy conversion and very small dimensions. High efficiency can be obtained by using high-energy permanent magnets but this solution has two main disadvantages. The first one is high price of permanent magnets and the second one is the source of origin of permanent magnets. Most of the well-known resources of rare-earth elements which are used in manufacturing of high-energy permanent magnets are in the possession of one of the Asian countries. Therefore, studies on alternative technologies of electric machines for above-mentioned applications should be conducted and switched reluctance machines could be one of them [5][6][7].
In the paper, generating operation of switched reluctance machine is analysed. The process of mechanical energy conversion into electric energy is discussed. Based on simulation and laboratory tests, the authors show that proper selection of control parameters allows operation of SRG with maximum efficiency, maximum power or maximum ratio of output power to phase current (P/I). Finally, conclusions concerning advantages of using SRGs in variable-speed applications are presented.

SRG-based energy conversion
Switched reluctance machines are categorized among machines with electronic commutation where classic commutator is replaced by dedicated power converter with control system. Fig.1a shows a structure of 8/6 switched reluctance machine (Fig.1a). The switched reluctance machines have a rotor without windings; they are concentrated only in a stator. The electromagnetic torque is produced by the tendency of alignment of stator and rotor. The rotor tends to the position where reluctance is the smallest. Energy and Fuels 37 single-pulse mode, both switches are turned-on at the turn-on angle θon, current flows from the dc source and energy is stored in the magnetic field of the machine. Beyond the turn-off angle θoff, both switches are turned-off and the energy provided by the prime mover through a shaft is converted into electric energy. Fig.2 shows idealised waveforms of phase back-EMF eph (Fig.2a), phase electromagnetic torque Teph (Fig.2b), phase current iph (Fig.2c) and phase power Pph (Fig.2d) in the single-pulse mode. The phase BEMF eph changes its direction after aligned position of rotor θa (Fig.2a). The sign of eph is determined by dLph/dθ and its amplitude varies with the rotor speed, so behaviour of phase current iph (Fig.1b) is determined by the relationship between eph and Udc. The change of phase BEMF sign to negative also causes a change of a sign of the generated electromagnetic torque Teph (Fig.1c). Switched reluctance machine starts to operate in a braking mode or in a generating mode (Fig.1c). It can be seen in Fig.1c that when turn-on angle is earlier than aligned position, electromagnetic torque is positive and machine operates in a motoring mode for a while. Output power in the switched reluctance generator is a function of supply voltage Udc, speed n and control variables (θon and θoff).

Results of simulations
In table 1, basic parameters of tested switched reluctance machine are presented. Table 1. Basic parameters of the tested SRM.

Parameter Value
Number of stator phases Ns 8

Number of rotor poles Nr 4
The simulation tests of the SRG were conducted based on nonlinear simulation model built in the Matlab/Simulink environment [8]. The influence of control parameters on properties of switched reluctance generator was determined based on the simulation model.   Fig. 4. The power/current ratio Poutav/ Iphrms vs. turn-off angle θoff at θon=const, Udc=24 V and n=3000 rev/min It can be seen from Fig.3 that selection of control parameters has to be dependent on value of required output power. However, other criteria should be taken into account when control parameters are selected. One of them is criterion of maximum efficiency. The second one is criterion connected with ratio of average output power to phase RMS current Poutav/Irms. Fig.4 shows dependence of ratio Poutav/Irms in the function of control parameters at n=3000 rev/min and Udc=24V. When control parameters are selected to obtain maximum efficiency, output power is significantly limited. However, when control parameters are selected to obtain maximum output power, efficiency is noticeably lower. The selection of control parameters to obtain maximum ratio Poutav/Irms allows obtaining higher output power than for criterion of maximum efficiency. At the same time, efficiency of energy conversion is higher than for criterion of maximum output power. Fig. 5 shows dependence of average output power Poutav in the function of speed n. Control parameters were selected to obtain maximum values of average output power Poutav and ratio Poutav/Irms. 4 Results of laboratory test Fig.6 shows a schematic diagram of a laboratory setup. Yokogawa WT1600 digital power meter was used to measure all electric (current, voltage and power) and mechanical (speed, torque) parameters of the tested machine.  Results of laboratory tests confirmed that control angles at which maximum power was obtained give lower efficiency of energy conversion, so this is not optimal control. Increase of efficiency is connected with decrease of generator output power.

Conclusions
The switched reluctance machines are an alternative technology for brushless machines with permanent magnets. The lack of permanent magnets allows their manufacturing in European market. Despite simple structure, control of SRG compared to other types of machines is more complicated. The control directed to obtain maximum output power causes decrease of energy conversion efficiency. However, this state can be used for emergency braking of vehicle when as high braking torque as possible is required. The control with maximum efficiency or maximum ratio Poutav/Irms can be used when a vehicle