Magnetics of Motor Drive System for Electrical Vehicle

Abstract

Magnetic Material for Electrical VehicleRecent trends of magnetic mate- rial research about electrical motor drive system for electrical vehicle are shown in this lecture for magnetic material and electrical researcher and engineers. Motor drive system consists of electrical motor and power elec- tronics circuit, and magnetic material are used and an important role in both equipment. The purpose of the electrical motor is to make an electromag- netic torque to drive a vehicle based on Maxwell stress equations. The power electronics circuit such as inverter or DC-DC converter is an electrical power source in controlling the variable voltage and/or variable frequency based on a switching operation of power semiconductors [1]. Soft magnetic mate- rial is used in the stator core and the rotor core in the electrical motor, and permanent magnet is used in the field part of the motor. The electromagnetic torque is almost proportional to square of magnetic flux density. So higher magnetic flux density is required to obtain higher electromagnetic torque. Soft magnetic material is much useful to obtain a higher magnetic flux density in smaller electrical current, which is supplied by the power elec- tronics circuit.Magnetic Material for MotorMost of the electrical motor uses an electrical steel with a few % silicon due to high performance of magnetic characteristics with low price. Mechanical stress in manufacturing the motor core, magnetic non-linearity, and magnetic anisotropy, magnetic hysteresis, magnetic flux distribution base on grain shape and orientation, magnetic vector property of magnetic field and magnetic flux density, higher harmonics components derived from the slot shape of the stator core as well as the power electronics circuit base on the switching operation are main properties of the electrical steel to be discussed and researched. Mutual interaction between the power electronics circuit and the magnetic material or motor are often observed [2, 3]. To reduce the motor core loss, low iron loss magnetic materials such as amorphous and nano-crystal material are reported to be applied to the motor core [4, 5]. The details are shown in the presentation. Hard magnetic material is used in the field of the motor due to no electrical current supply and no slip ring. So most of electrical vehicle are driven by permanent magnetic synchronous motor, because the higher efficiency and the downsizing of the electrical motor can be realized. Due to almost the same electrical conductivity of the NdFeB permanent magnet as the stainless steel, the temperature of the permanent magnet increases and the demagnetization phenomena occur. To avoid it, rare earth problem generates such as Dy or so. Weakening field control scheme is used in the PM motor for the high speed rotation. So higher coercive force is required for the permanent magnet.Magnetic Material for Power ElectronicsIn the electrical vehicle, variable speed operation is required to move from a starting point to a stopping point. So variable speed control is required for electrical motor. Variable voltage and variable frequency are required for the power source of the electrical PM motor, due to the electromotive force theory and the synchronous condition of the mechanical rotational speed and the magnetic rotation speed. The power electronics technology is possible to be realized by the switching operation of the power semiconductors. So the higher frequency operation can be realized by the high performance power semiconductor such as IGBT or GaN-FET or SiC-FET devices because it realizes the downsizing of the power electronics circuits. However, the bottle neck technology for the realization is a soft magnetic material in high frequency and high power operation. Nowadays, the hindrance for the reali- zation of power electronics technology might be said to be a high frequency magnetic material in a point of view of the downsizing, high efficiency and the low cost [6-8]. Ferrite is mainly used in the high frequency magnetic material up to 100 MHz or so. However, it has the low saturation magneti- zation such as 0.4 or 0.5 T, the thermal runaway phenomena of self-heating in high temperature, and the weakening of mechanical vibration which often occurs in automobile car. Metal material such as iron or steel is possible to solve the problems. Iron powder has good performance as the high satu- ration magnetization, no thermal runaway phenomena and no problem for the mechanical vibration. However, magnetic permeability is relative small due to the demagnetization in a small particle. So, as far as high frequency transformer which is quite useful for electrical circuit insulation, leakage magnetic flux may increase and then the low loss transformer might be diffi- cult. Thin plate shape of the steel or iron has no demagnetization phenomena, and then high magnetic permeability can be realized. Micro-meter order steel is trial manufactured by sputtering method, rolling or so [9]. The details will be shown in the presentation at that time. **