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An electric motor , as the name suggests, is a device that converts electrical energy into mechanical energy. Any electric motor can function as either a motor or a generator. It does not generate energy itself; it is merely a device for converting electromechanical energy. However, the losses during the conversion process are converted into heat. Therefore, the design of any electric motor includes electromagnetic design, mechanical design, and thermal design. People pay close attention to performance parameters such as electrical power, mechanical power, losses and efficiency, and temperature.
Figure 1 Schematic diagram of electromechanical energy conversion
Depending on their structure and application, there are many types of electric motors. However, the main types used in current automotive drives are permanent magnet synchronous motors, asynchronous motors (induction motors), switched reluctance motors, electrically excited motors, and DC motors. At this point, you might be wondering about the differences between these motors and their respective advantages and disadvantages . Let's provide a brief overview.
DC motor
The DC motor is the oldest invention in the electric motor family. Its inventor is the well-known Faraday. A traditional DC motor mainly consists of an armature winding on the rotor, an excitation winding on the stator, stator and rotor cores, a frame, and brushes and a commutator. The excitation winding provides the excitation magnetic field, and the armature winding provides the current that generates torque.
Figure 2. Faraday and his invention of the DC motor.
As mentioned earlier, a DC motor has a field winding and an armature winding. The magnitude of the magnetic field can be controlled by controlling the current in the field winding, and the torque can be adjusted by controlling the current in the armature winding. Therefore, the biggest advantage of a DC motor is its excellent control performance. The output speed and torque of the motor can be adjusted almost linearly simply by connecting an external variable resistor.
However, due to the presence of brushes, reliability is low, maintenance costs are high, and the additional losses caused by brush contact resistance and external resistance are significant, resulting in relatively low motor efficiency. Currently, newly developed electric vehicles generally no longer use brushed DC motors, typically only for applications such as window operation and windshield wiper control, and there is a trend towards replacing brushed commutators with electronic commutators.
Induction motor
The inventor of the induction motor is another technological giant, Tesla. Generally, its stator core has three-phase AC windings embedded in it, and the rotor consists of an iron core and short-circuited squirrel-cage windings. When three-phase AC current is passed through the stator windings, a composite spatial synchronous rotating magnetic field is generated, which cuts the rotor windings, thereby generating current in the rotor squirrel-cage windings. This current is then subjected to the magnetic field to generate electromagnetic force, driving the rotor to rotate.
Figure 3. Tesla and his invention of the induction motor.
Because it does not require brushes on its rotor, has a simple structure, good reliability, and mature production technology, it is widely used in industrial production. It is currently used in some buses, but due to its lower power density and complex control, it is rarely used in passenger cars. To commemorate this legendary figure, Tesla Motors used copper bar squirrel-cage induction motors in its early products, but because its overall efficiency and power density still cannot compare with rare-earth permanent magnet motors, the latest Model 3 has switched to a permanent magnet synchronous motor as its drive motor.
Traditional synchronous motors and permanent magnet synchronous motors
The stator structure of a synchronous motor is the same as that of an induction motor; both are AC motors. Only when the stator windings are energized with symmetrical alternating current will a certain rotating magnetomotive force be generated in the air gap. However, unlike asynchronous motors, the rotor speed of a synchronous motor is the same as the speed of the rotating magnetic field.
Figure 4 shows a traditional electrically excited synchronous motor, in which the rotor salient poles are wound with pre-wound excitation windings, which are led out through slip rings and brushes on the shaft. That is, its excitation magnetomotive force is provided by an external direct current. Therefore, its control performance is relatively good, and its power factor and efficiency can be relatively high. However, because it requires an external exciter, its size is relatively large, and the brushes and slip rings require regular maintenance. Therefore, this type of motor is mostly used in power plant generators and is relatively rare in automobiles.
Figure 4. Electrically Excited Synchronous Motor
The most commonly used type of motor in new energy vehicles is the permanent magnet synchronous motor. Unlike the previous type, its rotor core has no windings, but only surface-mounted or built-in permanent magnets. Its excitation magnetic field is generated by these magnets, and electromechanical energy is converted through interaction with the rotating magnetic field generated by the stator.
Figure 5. Permanent magnet motors with two different rotors
Because cars need to adjust speed frequently, the motor speed is designed to be relatively high. Therefore, the permanent magnet synchronous motor with built-in magnets on the right has an advantage due to its good mechanical strength. Moreover, this type of motor with built-in magnets has a relatively high magnetic reluctance torque, which is more conducive to saving the amount of magnets used and improving the weak magnetic performance.
Switched reluctance motor
A reluctance motor is a novel type of motor with a rotor that has neither windings nor permanent magnets. Instead, it consists of a solid, salient-pole structure made of stacked silicon steel sheets. Based on the principle of minimum reluctance (magnetic flux always closes along the path of least reluctance), it drives the rotor to rotate by switching the energizing sequence of the windings on the stator salient poles, causing the rotor to continuously move to the position of minimum reluctance.
Figure 6 Switched reluctance motor
Reluctance structures are simple, robust, reliable, and low-cost, and have great development potential. As a result, they have seen rapid development in the field of traction speed regulation in recent years. However, due to their inherent large torque fluctuations and significant vibration and noise, they are currently only used in some passenger cars.
Currently, there are also some new hybrid excitation reluctance motors, which typically insert a certain amount of ferrite permanent magnet material into the rotor reluctance slots. Because a portion of permanent magnet torque is introduced, the motor's performance is higher than that of a reluctance motor, while the cost is not as high as that of a rare earth permanent magnet motor.
Conclusion
This article introduces several familiar types of motors. In summary, DC motors are gradually being phased out due to their poor reliability and mediocre performance; switched reluctance motors have immature control technology, exhibit significant noise and vibration at low speeds, and have relatively low efficiency, making them a potential future option; induction motors have secondary copper losses on the rotor, resulting in severe heat generation, low efficiency, and large size, making them suitable for applications in passenger cars where size requirements are not strict; electrically excited synchronous motor systems are large, and the brushes and slip rings require maintenance and pose reliability risks, so they are currently rarely used except as generators.
Figure 7 Comparison of different motors and their performance
The image above compares the structures and performance of several motors produced by the U.S. Department of Energy and Oak Ridge National Laboratory, for reference. For smaller passenger vehicles, permanent magnet synchronous motors are currently the primary choice, and my country possesses a unique resource advantage in rare earth permanent magnet materials. However, with the explosive growth of new energy vehicles, the enthusiasm for researching new, high-efficiency, low-cost, and reliable motors is also constantly increasing.
