Design and control of a permanent magnet synchronous motor drive for a hybrid electric vehicle (1997)

The design and control of a 45 kW 6-pole permanent magnet synchronous motor (PMSM) with surface-inset magnets for a hybrid electric vehicle is described. The advanced field weakening strategy takes advantage of the additional reluctance torque. The influence of the saturation effects on the motor torque is considered by computing the inductances Ld and Lq are for each operational point. The accurate prediction of the motor performance is essential for the optimal control of the drive over a wide range of operation. The drive control is implemented on a DSP based, MATLAB/SIMULINK programmable hardware system. INTRODUCTION Hybrid electric vehicles have the advantage to operate with zero emission in the inner city while not being limited in the range for intercity distances. Furthermore, the possibility of regenerating the braking energy is offered. The main parts of the drive equipment are the energy sources, in this study a Diesel generator connected in parallel to a rechargeable NiCd traction battery, the traction motor with converter and controller, and the main controller unit. The drive module is connected to a main controller collecting the status information: acceleration, deceleration, regenerating the braking energy, etc. (Fig. 1). The desired torque Td given by the driver of the vehicle is processed by the main controller. The heart of the drive consists of the permanent magnet excited synchronous machine. [PMSM]. The possibilities of such type of motors for road electric traction was already investigated in the early eighties by Sneyers et al [a,b]. An electric vehicle motor design requires high efficiency, extended field weakening range, high power/weight ratio and high reliability. Water cooling, high energy density permanent magnets and special control strategies (], are applied in order to fulfil these requirements. One of the main design goals for the studied drive system is the continuous operation of the PMSM at high efficiency. Therefore, a rotor geometry with Xq > Xd is chosen to benefit from an additional reluctance torque (Fig.1) (Jahns et al [d],]). The rotor design causes a disadvantage for the motor control: the machine parameters are dependent on the operating point due to saturation effects. As a real time control strategy is essential to assure the continuous operation at highest efficiency, those dependencies have to be known in advance. The control algorithm then directly operates on the pre-calculated set of performance characteristics in form of tables or polynomial functions. The design procedure takes advantage of analytical calculations as well as static Finite Element Analysis (Chang et al [2], Slemon et al [3]) and thermal computations. Apart from the feedforward torque control, several algorithms for flux-weakening have been published recently, including current and voltage limitation and compensation of cogging torque [6,i,j]. A general flux-weakening algorithm is adapted. to the designed motor, simulated and implemented on a DSP based hardware system using a MATLAB/SIMULINK programmable control environment. The optimal negative direct axis current is determined for operating points both in constant torque region as in constant power region. The experimental setup further includes a modified standard inverter for induction motors and a reduced scale prototype motor. MOTOR DESIGN PMSMs are usually classified into surface mounted permanent magnet motors (SPMSM) and interior or buried permanent magnet motors (IPMSM) [d,1]. The first can have sinusoidal back emf, but have limited field weakening capabilities, due to relatively small inductance values. The latter has higher inductances and thus an extended constant power operation range. Moreover, because direct and quadrature inductance are not equal, a reluctance torque contributes to the total motor torque. However, the back emf is no longer sinusoidal. A motor design as shown in figure 2 can combine some advantages of the above mentioned motor types. This motor is referred to as a surface-inset PMSM (Sebastian and Slemon [e]). As in the IPMSM, due to magnetically asymmetrical rotor design, the total torque has an electromagnetic component and a reluctance component. In order to achieve a high efficiency of the PMSM the losses have to be kept low being contrary to a high power/weight ratio. Therefore, the optimal choice of the desired low weight and acceptable iron losses has to be made. In the same way, the contributing losses can be reduced by using more winding copper, yielding in increasing costs and additional weight. The thickness of the iron yoke is reduced by applying a high number of poles. However, the number of poles is limited by the frequency of the inverter, and furthermore, a wide range of field weakening can only be achieved by a relatively small Figure 1 Topology of the hybrid drive concept. number of poles (Hadji Minaglou [5]). Iron losses are further decreased by choosing thin low-loss lamination in the stack of the stator. The power/weight ratio is further improved by applying an indirect water cooling system. A high Ld value extends the field weakening range, however increasing Ld by decreasing the thickness of the permanent magnets has limits. It enables their demagnetisation and reduces the reluctance torque by decreasing the ratio Lq/Ld.