Check out the characteristics of four drive motors for electric vehicles

New energy vehicles have three advantages: environmental protection, economy, and simplicity. This is particularly evident in pure electric vehicles: electric motors replace fuel engines, and are driven by electric motors without the need for automatic transmissions. Compared with automatic transmissions, electric motors have simple structures, mature technologies, and reliable operation. They are even considered one of the promising areas for China to achieve "overtaking on a curve" in the new energy vehicle industry. New energy electric vehicles are mainly composed of three parts: the motor drive system, the battery system, and the vehicle control system. The motor drive system is the part that directly converts electrical energy into mechanical energy, and determines the performance indicators of electric vehicles. Therefore, the selection of the drive motor is particularly important.

The drive motor of an electric vehicle requires the following characteristics:

lWide constant power range to meet the vehicle's speed change performance

lLarge starting torque and strong speed regulation capability

l High efficiency and wide efficient area

lLarge instantaneous power and strong overload capacity

lHigh power density, small size and light weight

lHigh environmental adaptability, suitable for harsh environments

lHigh energy feedback efficiency

According to the driving principle, the drive motors of electric vehicles can be divided into the following four types:

1. DC motor

In the early days of electric vehicles, many electric vehicles adopted DC motor solutions. This was mainly because DC motors are mature products, easy to control, and have excellent speed regulation. However, the DC motor itself has very prominent shortcomings. Its complex mechanical structure (brushes and mechanical commutators, etc.) restricts its instantaneous overload capacity and further increase in motor speed. Moreover, under long-term operation, the motor's mechanical structure will produce losses, increasing maintenance costs. In addition, the brush sparks during motor operation will cause the rotor to heat up, waste energy, make heat dissipation difficult, and cause high-frequency electromagnetic interference. These factors will affect the specific performance of the vehicle.

Because the disadvantages of DC motors are very prominent, current electric vehicles have eliminated DC motors.

2. AC asynchronous motor

AC asynchronous motor is a kind of motor widely used in industry at present. Its characteristics are that the stator and rotor are made of laminated silicon steel sheets, and the two ends are sealed with aluminum covers. There are no mechanical parts that contact each other between the stator and rotor. It has a simple structure, reliable and durable operation, and convenient maintenance. Compared with DC motors of the same power, AC asynchronous motors are more efficient and about half lighter. If vector control is adopted, controllability comparable to that of DC motors and a wider speed regulation range can be obtained. Due to its advantages of high efficiency, large specific power, and suitability for high-speed operation, AC asynchronous motors are currently the most widely used motors in high-power electric vehicles.

However, when running at high speed, the rotor of the motor will heat up seriously, and the motor must be kept cool during operation. At the same time, the drive and control system of the asynchronous motor is very complicated, and the cost of the motor itself is relatively high. In addition, the frequency converter is required to provide additional reactive power to establish a magnetic field during operation. Therefore, compared with permanent magnet motors and switched reluctance motors, the efficiency and power density of asynchronous motors are relatively low, and they are not the choice for optimizing energy efficiency.

Asynchronous motors are widely used in the United States, which is also artificially related to road conditions. In the United States, highways have reached a certain scale. Except for large cities, cars generally run continuously at a certain speed, so asynchronous motors that can run at high speeds and have higher efficiency at high speeds are widely used.

3. AC asynchronous motor

Permanent magnet motors can be divided into two types according to the different current waveforms of the stator windings. One is a brushless DC motor, which has a rectangular pulse wave current; the other is a permanent magnet synchronous motor, which has a sine wave current. The two motors are roughly the same in structure and working principle. The rotors are permanent magnets, which reduces the loss caused by excitation. The stator is equipped with windings to generate torque through alternating current, so cooling is relatively easy. Since this type of motor does not require the installation of brushes and mechanical commutation structures, no commutation sparks will be generated during operation. It is safe and reliable in operation, easy to maintain, and has a high energy utilization rate.

The control system of permanent magnet motor is simpler than that of AC asynchronous motor. However, due to the limitation of permanent magnet material itself, the permanent magnet of the rotor will be demagnetized under high temperature, vibration and overcurrent conditions. Therefore, permanent magnet motor is easy to be damaged under relatively complex working conditions. Therefore, this area needs further development and improvement.

Moreover, the price of permanent magnet materials is relatively high, so the entire motor and its control system cost is relatively high. Currently, only China, which is rich in rare earth resources, tends to use permanent magnet motors for electric vehicle drive solutions. Countries like Japan and Europe either use permanent magnet materials made of light rare earths to make permanent magnet motors, or directly switch to switched reluctance motors that do not require rare earth materials but have higher requirements for controller design.

4. Switched reluctance motor

As a new type of motor, the switched reluctance motor has the simplest structure compared to other types of drive motors. Both the stator and rotor are double-salient pole structures made of laminated ordinary silicon steel sheets. There is no winding on the rotor, and the stator is equipped with a simple concentrated winding. It has many advantages such as simple and strong structure, high reliability, light weight, low cost, high efficiency, low temperature rise, and easy maintenance. In addition, it has the excellent characteristics of good controllability of the DC speed regulation system and is suitable for harsh environments, making it very suitable for use as a drive motor for electric vehicles.

However, the switched reluctance motor has the disadvantages of large torque fluctuation, need for position detector, nonlinear system characteristics, jumpy rotation of magnetic field, complex control system, and large pulse current generated for DC power supply. In addition, the switched reluctance motor has a double salient pole structure, which inevitably has torque fluctuation. Noise is the main disadvantage of the switched reluctance motor.

However, research in recent years has shown that the noise of switched reluctance motors can be well suppressed by adopting reasonable design, manufacturing and control technologies. At present, Japan has conducted in-depth research on switched reluctance motors, and Nidec's switched reluctance motors are also widely used in various industries such as electric vehicles and home appliances. At present, some manufacturers in China are gradually paying attention to the future development direction of electric vehicle drive motors.

Create efficient new energy vehicle drive motors

Based on its in-depth exploration and long-term accumulation in the motor and electric vehicle industries, ZHIYUAN Electronics has successfully integrated special test items for new energy vehicles - MAP diagram and regenerative energy feedback test - into the MPT series motor test system, providing excellent testing solutions for electric vehicle motor designers.

1. MAP

According to GB/T 18488-2015 Electric Vehicle Drive Motor System Test Standard, it is necessary to conduct MAP test on the new energy vehicle drive motor to obtain the efficiency characteristics and high efficiency area distribution of the motor. The actual test results of the MAP are as follows:

In the figure, the horizontal axis is the speed, the vertical axis is the torque, and the color represents the efficiency. It represents the distribution of the efficiency characteristics of the motor in different working areas (speed, torque), where the orange-red part is the high-efficiency area of ​​the motor. The wider the distribution of the high-efficiency area, the more energy-saving the motor is when running under various working conditions.

ZHIYUAN Electronics' MPT motor test system has a built-in MAP automated test function. It can automatically control the load and the motor under test to perform corresponding working condition loading according to the loading conditions pre-set by the user, obtain the efficiency under different working conditions, and finally integrate the massive test data into a MAP diagram to intuitively analyze the efficiency characteristics and high-efficiency area distribution of the motor for the user.

2. Regenerative energy feedback test

According to GB/T 18488-2015 test standard for electric vehicle drive motor system, it is also necessary to conduct regenerative energy feedback test on new energy vehicle drive motor. The purpose of this test is to consider whether the drive motor can normally realize electric energy feedback when braking, that is, when running in generator state, and to evaluate the actual energy consumption of the motor.