A Segmented Magnet Interior Permanent Magnet Machine with Wide Constant Power Range
A Segmented Magnet Interior Permanent Magnet Machine with Wide Constant Power Range for Traction Application in Hybrid Vehicles
I. INTRODUCTION
The wide constant-power speed range of the propulsion system in a hybrid/electrical vehicle is crucial to meet the harsh operating constraints with minimum power. Recent studies reveal that a highly efficient electric motor with a constant power speed ratio of 1:4 will be ideal choice for the propulsion system of hybrid and electric vehicles .
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In recent years,the Permanent Magnet machine has been adopted for many high performance automotive applications because of its higher efficiency and torque per size ratio. The Interior Permanent Magnet (IPM) machine has the advantage of inverse saliency (q-axis inductance is greater than d-axis inductance) and performs better in constant power region than its counter part surface permanent magnet machine. However, most of the commercially available IPM machines offer only a limited speed range of constant power. The segmented magnet pole IPM rotor presented in offers a very wide constant power range. In this paper an investigation has been made for the suitability of such IPM machine for traction application. Its performance is compared with a prototype IPM machine of the University of New South Wales (UNSW) and a commercially available Kollmorgen IPM machine.
II. SELECTION OF TRACTION MOTOR
In traction operation of hybrid or electric vehicle, the electric motor plays a very important part. The ideal torque and power-speed characteristics of the electrical machine as shown in Fig.1 have two distinctive regions. In the first region, the torque remains constant up to the base speed and known as constant torque region and in the adjoining part, output power remains constant, hence the name constant power region. In the constant power region, the air-gap flux of the electric machine is weakened so that the terminal voltage can be kept constant at rated value. Consequently, it is also known as flux-weakening region of the electric machine. The flux-weakening range of a practical electric motor is limited to a maximum speed at which the output power ultimately becomes zero.
Recent study is showing that power requirement for acceleration in a vehicle reduces with an electric motor that has wider constant power speed range. Reduced power rating means a more cost effective system. Along with constant power range, efficiency of the traction motor is equally important. The over all energy efficiency of the vehicle ultimately determines its fuel economy. Hence, the traction motor needs to be highly efficient in its all operating conditions.
The Induction Motor (IM), Switch Reluctance Motor (SRM) and Interior Permanent Magnet (IPM) motors are the three main contenders of automotive traction applications. All three machines have their own benefits and shortcomings as traction motor of vehicle. The major arguments against use of PM machines are expensive high power magnet material, affect of temperature rise on the performance and severe consequences with short circuit faults. However, it is seen that with better technology, cost of rare earth magnet is in decline and expected to reduce farther in near future. Better protection against Curietemperature-limit and short-circuit fault is feasible with careful designing approach. Weighing these shortcomings against many advantages of IPM machines such as zero rotor copper loss, almost negligible rotor iron loss, higher torque per size and inherent higher efficiency at constant torque region, makes it a very appealing candidate for traction applications.
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