When discussing electric vehicle technology, we often talk about the "three electric system", which mainly refers to batteries, motors and electronic controls. When it comes to the biggest pain point affecting the popularity of electric vehicles - mileage, everyone will look at the capacity and charging speed of the battery as very important.
However, there is a marginal effect on the increase in battery life caused by continuous stacking batteries. The brainless reactor battery will not only increase the weight of the car, but also the charging time will be correspondingly longer, and the increase in battery life will continue to decrease.
If you want to further increase the range, you have to think of another way. Why not start with the motor?
Compared with the internal combustion engine, the size and structure of the motor are much simpler, so it is too widely used in practice. From vibrators in mobile phones to large motors in wind turbines, motors play an important role.
The principle of the motor is actually very simple, the energized wire is forced in the magnetic field, and the wire cuts the magnetic inductance line to generate electromotive force, so the two major components of the motor are the stator and the rotor, generally composed of permanent magnets (magnets) and winding coils. Different combinations of permanent magnets and winding coils constitute different types of motors.
The output characteristics of the motor and the engine are very different, the engine needs an external starter to start, and as long as a certain current is applied to the motor, the motor can start to rotate and output power. And in the torque output at start-up, the torque of the motor is much greater than that of the engine. Of course, in the high speed range, the torque output of the motor will be lower.
The efficiency of the motor in the operation process is actually very high, the maximum efficiency can reach 95%, which also makes the energy consumption of electric vehicles form a dimensionality reduction blow to fuel vehicles, in the era of fuel vehicles to achieve 1L/100km fuel consumption requires the use of a lot of avant-garde and expensive technology, but now there are a lot of smaller electric vehicles, they can do 1L/100km quite close to the energy consumption performance.
So what kind of motor does an electric car need?
The power of the motor is equal to the torque * speed, in order to enhance the dynamic performance of the motor, it is necessary to increase the size of the motor. But the battery of the electric vehicle also occupies a lot of space, so the space occupied by the motor is not so large. Therefore, the motor required by electric vehicles must take into account several major characteristics such as wide working range, high efficiency, miniaturization, high power density, and low cost. Therefore, the most used motor in electric vehicles is still a permanent magnet synchronous motor.
Compared with the motor used in a relatively fixed working condition such as a fan and a four-wheel drive vehicle, the motor used by the electric vehicle needs to have a very wide speed and power output range, and the working conditions are much more complex, so the motor of the electric vehicle is the same as the engine, and the range of high efficiency is still limited. However, in general, the efficiency of the motor is still much higher than that of the engine. Below is a diagram of the motor efficiency distribution.
Like the engine, the maximum efficiency range of the motor is concentrated near the medium speed. In both the low-speed high-torque and high-speed low-torque ranges, efficiency is slightly reduced. This involves the two main operating losses of the motor – copper loss and iron loss.
Copper loss, as the name suggests, is the loss produced by copper wire. Although the resistance of the copper wire is relatively low, the current through the copper wire is still very large when starting. In this way, the copper wire will heat up, and some of the energy will be lost in the form of heat. Therefore, copper losses are mainly generated in the low speed and high torque range of the motor.
In the high speed range, the motor will produce a large iron loss. During the operation of the motor, the magnetic field of the winding coil is constantly changing, and such changes will generate eddy current and hysteresis losses in the winding core. The generation of eddy currents is eventually dissipated in the form of heat.
The higher the motor speed, the greater the frequency of magnetic field change of the winding coil, and therefore the greater the eddy current and hysteresis losses. Coupled with greater air resistance and rolling resistance at high speeds, electric vehicles will be very power-hungry at this time.
There are many ways to reduce copper and iron losses, and in mainstream motors, the best way is to use a flat wire winding structure. The flat wire winding structure not only has a larger cross-sectional area, reduces the resistance and heat generation per unit length, but also has a larger contact area with the core, which is more conducive to heat dissipation, and the magnetic flux generated when powered on is also larger, which has been gradually used in the power motor of mainstream hybrid vehicles and electric vehicles.
As for the way to reduce iron loss, one is to replace the electromagnetic steel plate used in the iron core with a thin sheet, and the eddy current can be reduced after changing to a thin sheet. In addition, there is a practice of replacing the core with an amorphous alloy, but this will reduce the power density of the motor, and the processing difficulty of the amorphous alloy is also relatively large, which is not suitable for mass production.
The Renault-Nissan Alliance proposed a solution - to replace the permanent magnet of the permanent magnet synchronous motor with a winding coil that can change the current, and to change the magnetic field strength of the rotor by changing the current to control the weak magnetic field, thereby reducing the strength of the eddy current, hysteresis loss, and enhancing the high speed power output of the motor.
However, because it is a magnetic field generated by the winding coil, the motor volume is larger than that of the permanent magnet synchronous motor, and the magnetic field generated by the winding coil also consumes power, so the ultimate efficiency is not as good as the permanent magnet synchronous motor.
Is there a more perfect solution? The switched reluctance motor is one, and its high speed performance is much better than that of the permanent magnet synchronous motor, which is also the focus of future automotive motor research. If torque ripple and noise and vibration problems can be solved, it will be a better solution than permanent magnet synchronous motors.
Because of the huge difference in the output characteristics of the motor and the engine, the transmission system that matches it also has a big difference from the fuel vehicle. Therefore, most of the motors do not need a starter, do not need a clutch, only need a reduction gearbox is enough, but there are still car companies to give different solutions.
Porsche equipped the Taycan with a two-speed transmission with planetary gears. This two-speed transmission automatically adjusts the gear according to the driving mode and speed, which can solve the problem of low efficiency of the motor in the high speed state. Toyota also recently filed a patent on its patent website for a manual transmission that matches an electric vehicle, but it is not yet known how the system works.
In addition, some component suppliers, such as Bosch's CVT4EV, match the motor with a steel belt CVT gearbox to maximize the advantages of the motor in a simple working condition. And under the high-speed cruise conditions of high speed and low torque, the CVT transmission can reduce the speed of the motor, thereby reducing iron loss and improving efficiency.
Of course, the drivetrain for matching the motor is very rich and diverse, and I will continue to introduce it to you with some examples in the next article.