Compilation / Ma Xiaolei
Edited / Tu Yanping
Design / Zhao Haoran
Source/caranddriver, by DAN EDMUNDS
Car enthusiasts have always been fanatical about engines, but electrification is unstoppable, and some people's knowledge reserves may need to be updated.
The four-stroke cycle engine is best known today, and it is also the source of power for most fuel vehicles. Similar to the four-stroke, two-stroke and Wankel rotor engines of internal combustion engines, electric vehicle motors can be divided into synchronous motors and asynchronous motors according to the differences in rotors, asynchronous motors are also called induction motors, and synchronous motors contain permanent magnet and current excitation motors.
Stator and rotor
All types of electric vehicle motors consist of two main parts: a stator and a rotor.
Stator▼
The stator is the part of the motor that remains stationary, the fixed shell of the motor, and is mounted on the chassis like an engine block. The rotor is the only moving part of the motor, similar to a crankshaft, which transmits torque through the transmission and differential.
The stator is composed of three parts: stator core, stator winding and frame. There are many parallel grooves on the stator body, which are stuffed with interconnected copper windings.
These windings contain neat hairpin-shaped copper inserts that increase the filling density within the groove and the direct contact of the line to the wire. The dense windings increase torque capacity, while the two ends are staggered more neatly, reducing volume and making the overall package smaller.
Stator and rotor▼
The main role of the stator is to generate a rotating magnetic field (RMF), while the main role of the rotor is to be cut by a magnetic field line in the rotating magnetic field to generate (output) current.
The motor uses three-phase alternating current to set the rotating magnetic field, and its frequency and power are controlled by the power electronics of the response accelerator. Batteries are direct current (DC) devices, so the power electronics of an electric vehicle include a DC-AC inverter that provides the stator with the necessary AC current to create the most important variable rotating magnetic field.
But it's worth pointing out that these motors are also generators, which means that the wheels will drive the rotor inside the stator in reverse, inducing a rotating magnetic field in the other direction, sending power back to the battery through an AC-DC converter.
This process is called regenerative braking, which creates drag that slows the vehicle down. Regeneration is not only at the heart of extending the endurance of electric vehicles, but also of efficient hybrid vehicles, as a large amount of regeneration can improve fuel economy. But in the real world, regeneration is not as efficient as "slithering", which avoids energy loss.
Most electric vehicles rely on a single-speed transmission to reduce the speed of rotation between the motor and the wheels. Like internal combustion engines, motors are most efficient at low speeds and high loads.
While an electric car may achieve a decent level of endurance with a single gear, larger pickups and SUVs use multi-speed transmissions to increase range when driving at high speeds.
Multi-gear electric vehicles are not common, and today, only the Audi e-tron GT and Porsche Taycan use two-speed transmissions.
Three motor types
Induction motors were born in the 19th century, and their rotors contain longitudinal layers or strips of conductive material, most commonly copper and sometimes aluminum. The rotating magnetic field of the stator induces an electric current in these sheets, which in turn creates an electromagnetic field (EMF) that begins to rotate within the rotating magnetic field of the stator.
Induction motors are called asynchronous motors because they can only generate induced electromagnetic fields and rotational torques when the rotor speed lags behind the rotating magnetic field. These types of motors are common because they do not require rare earth magnets and are relatively inexpensive to manufacture. However, when the load is continuously high, their heat dissipation ability is worse, and the efficiency itself is lower at low speeds.
A permanent magnet motor, as the name suggests, its rotor has its own magnetism and does not require power to create the rotor's magnetic field. They are more efficient at low speeds. Such a rotor also rotates synchronously with the rotating magnetic field of the stator, so it is called a synchronous motor.
However, there are also problems with simply wrapping the rotor with a magnet. First, this requires a larger magnet, and as the weight increases, it is difficult to keep in sync at high speeds. But the bigger problem is the so-called high-speed "anti-electromagnetic field", which increases resistance, limits the highest-end power, and generates excess heat that can damage the magnet.
To solve this problem, most electric vehicle permanent magnet motors are equipped with permanent magnets (IPM) inside, which slide in pairs into longitudinal V-shaped slots, arranged in multiple splinters under the surface of the rotor's core.
The V-slot keeps the permanent magnet safe at high speeds, but creates a magnetoresistive torque between the magnets. Magnets are either attracted to or repelled by other magnets, but ordinary magnetism attracts lobes of the iron rotor to the rotating magnetic field.
The permanent magnet plays a role at low speeds, while the magnetoresistive torque takes over at high speeds. This is how The Prius adopted this construction.
The last type of current-excited motor is only recently appeared in electric vehicles, both of which belong to brushless motors, and the traditional concept is that brushless motors are the only feasible choice for electric vehicles. BMW has recently been unusually, installing AC synchronous motors with brush current excitation on new i4 and iX models.
This type of motor rotor interacts with the rotating magnetic field of the stator, exactly the same as the permanent magnet rotor, but instead of being equipped with a permanent magnet, it uses six wide copper blades that use the energy of a DC battery to create the necessary electromagnetic field.
This required the installation of slip rings and spring brushes on the rotor shaft, so some people were concerned that the brushes would wear out and accumulate dust and abandon this method. However, the brush array is enclosed in a separate space with a removable lid, and it remains to be seen whether brush wear is a problem.
The absence of permanent magnets avoids rising costs of rare earths and the environmental impact of mining. This scheme also makes it possible to change the magnetic field strength of the rotor for further optimization. Nevertheless, powering the rotor still consumes a certain amount of power, which makes these motors less efficient, especially at low speeds, and the energy required to create a magnetic field accounts for a large proportion of the total consumption.
In the short history of electric vehicles, current-excited AC synchronous motors can be regarded as new things, and there is still a lot of room for new ideas, and there have been major twists, for example, Tesla has shifted from the concept of induction motors to permanent magnet synchronous motors. It's been less than a decade since we entered the era of modern electric vehicles, and it's just getting started.
This article was originally produced by Automotive Business Review
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