Efficency Determination of a Brushless DC Motor Under Field Weakening Operation

This paper deals with the calculation of the efficiency for a Brushless DC Motor when used for traction applications in order to determine the optimum operating conditions which will meet EV requirements, particularly high speed efficiency. This usually involves the assessment of the drive losses: the inverter and the motor losses. In practice, the copper losses can be predominant at low speed, whereas iron as eddy current losses increase at high speed. In this analysis, evaluation of the core iron losses resulting from eddy current and hysteresis in surface-mounted permanent magnet is investigated using finite element method. Variation of the flux density in the motor is determined. The iron losses are then derived and evaluated for no-load and on load operations. These losses are incorporated into a motor drive model for an overall efficiency determination. INTRODUCTION Technological developments in the fields of magnetic materials and power devices have improved the performance of the brushless DC motor (BDCM). Its improved performance over brushed DC motors of a similar volume envelope makes the BDCM an attractive alternative in electro-mechanical drive applications. Owing to their low inertia, high power to weight ratio, fast dynamic response and their ability to provide constant torque operation below base speed and constant power over a wide speed range, there is currently great interest in the utilisation of the BDCM in electric vehicles (EV) , where such characteristics are desirable[1-2]. In fact, constant torque operation zone is usually confined to starting and transient overload conditions. Constant power, however, where the field is weakened, corresponds to a large operating zone, especially transport [2]. Therefore, an optimisation of the drive system is required to increase the efficiency, which is a paramount factor for the battery energy consumption in order to improve the travelling range. This paper investigates the performance of the trapezoidal BDCM when used for traction applications in order to determine the system efficiency at optimum operating conditions which will meet EV requirements, particularly high speed operation. However, in this range, the output power capability of BCDM is strongly limited by the increasing of iron losses. The latter act to reduce the power capability at high speed and ultimately limits the speed. Therefore, the need for assessment of the iron losses is required for accurate efficiency prediction. For that purpose, machine magnetic characteristics are first determined using finite element method. This allows the determination of iron losses. For the system efficiency determination, an analytical approach in time domain is used [3], which combines the motor and the inverter circuit electrical equations. In addition, this model incorporates not only the motor characteristics, but also the overall losses. In the second phase, a field weakening is investigated for high speed constant power operation, by adjusting commutation angle advance. The overall efficiency is assessed under no-load and rated load conditions. DESCRIPTION OF THE SYSTEM The system under study comprises a motor (BDCM) which is fed from a voltage source inverter with a constant dc supply as shown in Figure 1. The machine uses permanent magnets made from samarium cobalt material mounted on the surface of the rotor. The stator winding is a conventional three-phase star connected winding, with a floating neutral point. The laminations used in the stator and the rotor are made the magnetic material, namely the steel Transil 330-35. Details of the machine are given in Table 1.