For those not particularly familiar with electric motors , the first hurdle is often the confusing and overlapping classifications. This article focuses on the common types of electric motors used in electric vehicles, outlining the basic working principles of each. You'll find that while there are many classifications, the fundamental structure is quite simple. Finally, a table is used to clearly illustrate the key characteristics of each motor.
The aim is to strip away the complexities on the surface and start from the basic structure to understand the working principle of electric vehicle motors .
DC motor
Electric vehicles typically use brushless DC motors , but the principle can be explained by starting with brushed DC motors. Once brushed DC motors are clear, brushless motors become clear.
Working principle of brushed DC motor
Brushed DC motor
As shown in the figure, a brushed DC motor consists of main magnetic poles, winding coils, commutator plates, and brushes.
The rotor coil, energized with direct current, is placed within a magnetic field of fixed direction. According to the left-hand rule, the electromagnetic forces acting on the upper and lower edges of the coil are in opposite directions, creating a torque that causes the coil to rotate.
After the coil rotates 90 degrees, if the current maintains its original direction, the coil will experience a torque opposite to its original direction of motion, causing it to stop moving and begin to move in the opposite direction. The coil will then fall into a state of continuous oscillation.
At this point, brushes and commutator segments are added between the coil and the power supply. When the coil rotates 90 degrees, its direction of motion is parallel to the direction of the magnetic field lines, and it reaches a point near the point where the positive and negative torques are balanced, the brushes detach from the original commutator segments and contact another commutator segment, meaning that the direction of the current flowing through the coil reverses.
Due to inertia, the coil passes the torque equilibrium position. At this moment, the direction of the current in the coil also changes. The coil is then subjected to a force that is exactly in the same direction as the motion, and thus it maintains its state and continues to move.
Brushed DC in practical applications
In practical applications, the main magnetic pole can be either a permanent magnet or an electromagnet. Generally, large motors use electromagnets for the main magnetic pole, while small motors use permanent magnets.
brushless DC motor
In brushed DC motors, the brushes and commutator segments are constantly in a state of friction, making them vulnerable components. To avoid the presence of such components, brushless DC motors were developed.
Working principle of brushless DC motor
The stator consists of windings located on the outer ring; the rotor is a permanent magnet located at the center. A controlled square wave current is applied to the stator, and the direction of the excited magnetic field changes with the direction of the excitation current. The permanent magnet rotor experiences torque in the changing magnetic field, causing it to rotate.
In this way, there is no need to power the rotating parts, and the brushes and commutator segments can be eliminated.
asynchronous motor
Asynchronous motors, also known as induction motors, are often called squirrel-cage motors because the "squirrel-cage" rotor is the most widely used rotor structure.
From the perspective of the motor's structure alone, replacing the permanent magnet rotor of a brushless DC motor with a squirrel-cage winding rotor results in the basic structure of an asynchronous motor.
asynchronous motor
In an asynchronous motor, the rotor itself is merely a coil, with no current flowing through it. However, the winding coils are excited by a rotating magnetic field in space, inducing a current within them. This current is then subjected to the Lorentz force, causing it to move. The direction and speed of the coil's motion depend entirely on the direction and rotational speed of the magnetic field.
The rotational speed of the rotating magnetic field is called the synchronous speed of the motor. The rotor speed, however, is always lags behind the synchronous speed by a certain angle because the motion is generated by the force of the induced current. This angle difference increases as the load increases.
In the case of asynchronous motors, it is necessary to understand how the rotating magnetic field is generated.
Rotating magnetic field of asynchronous motor
The curves in the figure represent the excitation current waveform of the stator winding of the asynchronous motor, which is a set of sine waves. The three circles below represent three moments at intervals of ωt=π/2, and the direction of the stator magnetic field at each moment is analyzed.
The excitation current is represented by "x" indicating current flowing inwards from the plane of the paper and "•" indicating current flowing outwards from the plane of the paper. Starting from the first circle from left to right, on the sine wave graph, take ωt = 2π/3. At this point, iu > 0, iv < 0, iw < 0. According to the right-hand rule, a magnetic field line diagram can be drawn, as shown by the dashed line with arrows in the figure. This set of magnetic field lines is equivalent to a pair of magnets, with the upper part being the S pole and the lower part the N pole, mounted on the stator.
The second circle in the middle corresponds to the second point ωt=7π/6 on the sine curve, with a distance of π/2 from the first point; the third circle corresponds to the third point on the sine curve, with a distance of π/2 from the second point. Based on the direction of the current at each moment, the magnetic field lines at that moment can be plotted.
By observing the three circles in succession, it can be found that as the phase angle of the input current changes, the equivalent magnetic poles S and N on the stator rotate counterclockwise by a corresponding angle, which is to say, a rotating magnetic field is formed.
Permanent magnet synchronous motor
Compared to traditional wound synchronous motors, permanent magnet synchronous motors have no excitation windings, which are replaced by permanent magnets. The stator structures of the two are basically the same. This eliminates the need for brushes, excitation windings, and slip rings, greatly simplifying the motor structure and making it simpler and more reliable.
To explain the working principle, let's start with the working principle of a traditional electrically excited synchronous motor.
Synchronous motors come in two structural forms: rotating magnetic pole type and rotating armature type. Due to the advantages of rotating magnetic pole type, such as smaller rotor weight, simpler manufacturing process, and smaller current passing through brushes and slip rings, large and medium capacity synchronous motors mostly adopt the rotating magnetic pole type structure.
Depending on the rotor shape, rotating magnetic pole types can be divided into salient pole and non-salient pole types, as shown in the figure below. Salient pole types are mostly used in applications requiring low speeds.
(a) salient pole type (b) non-salient pole type
Schematic diagram of a synchronous motor
Like other rotating electrical machines, synchronous motors consist of two main parts: the stator and the rotor. The stator of a synchronous motor mainly consists of the frame, the core, and the stator windings.
To reduce hysteresis and eddy current losses, the stator core is made of thin silicon steel sheets stacked together, and the inner surface of the stator core is embedded with three-phase windings that are symmetrical in space.
The rotor mainly consists of a shaft, slip rings, an iron core, and rotor windings. To balance the requirements of magnetic conductivity and mechanical strength, the rotor core is often forged from high-strength alloy steel.
When a synchronous motor is working, three-phase symmetrical current is passed through the three-phase windings of the stator, and direct current is passed through the excitation windings of the rotor.
When a three-phase alternating current is passed through the three-phase symmetrical windings of the stator, a rotating magnetic field will be generated in the air gap. The generation and manner of this rotating magnetic field are exactly the same as those of the asynchronous motor described above.
When a direct current is passed through the rotor excitation winding, a static magnetic field with constant polarity is generated. If the number of pole pairs of the rotor magnetic field is equal to the number of pole pairs of the stator magnetic field, the rotor magnetic field will rotate synchronously with the stator rotating magnetic field due to the magnetic pull of the stator magnetic field. That is, the rotor rotates at the same speed and in the same direction as the rotating magnetic field. This is the basic working principle of a synchronous motor.
The direction of rotation of the stator rotating magnetic field or the rotor is determined by the phase sequence of the three-phase current flowing into the stator windings. Changing the phase sequence can change the direction of rotation of the synchronous motor.
Switched reluctance motor
Switched reluctance motors, also known as reactive synchronous motors, are motors whose rotors themselves have no magnetism. They rely on the principle that movable parts in a magnetic field attempt to minimize the magnetic reluctance of the magnetic circuit. That is, the magnetic flux always closes along the path of least magnetic reluctance. The tangential tension is generated by the twisting of the magnetic field. Therefore, its structural principle requires that the magnetic reluctance of the magnetic circuit should change as much as possible when the rotor rotates.
The torque generated by the difference in magnetic reluctance in the two orthogonal directions of the rotor is called reluctance torque or reaction torque.
The specific working process of the switched reluctance motor is shown in the figure (only one phase is shown).
When the control switches S1 and S2 for phase A winding are closed, current flows through phase A winding, generating a tangential force that attracts the rotor to rotate in the direction where the rotor tooth axis and stator pole axis of phase A winding coincide, i.e., attracting the rotor to rotate counterclockwise. When the rotor is aligned with the stator of phase B, phase A is turned off and phase B is turned on. The resulting tangential force continues to attract the rotor to rotate counterclockwise, and so on, continuing to turn on phases C, A, and B, causing the rotor to rotate continuously counterclockwise and output mechanical energy. Conversely, if the conducting phase changes from phase A to phase C, and then from phase C to phase B, and this phase sequence is repeated, the rotor will rotate clockwise. Therefore, the rotor direction of rotation is independent of the direction of current in the phase windings, and depends only on the sequence of phase winding energization.
When the main switching devices S1 and S2 are simultaneously turned on, the A-phase winding absorbs electrical energy from the DC power supply U; when S1 and S2 are simultaneously turned off, the winding current continues to flow through the two freewheeling diodes, feeding the remaining electrical energy back to the power supply U. Therefore, the switched reluctance motor has the ability to regenerate electrical energy, resulting in relatively low energy loss and high system efficiency.
Comparison of six types of motors
When working with motors, I often experience a sense of memory loss. Even when I clearly understood and memorized something, I don't know where to begin when I want to explain it to someone else.
After some trial and error, I found that using tables to compare the basic characteristics of different types of motors makes them easier to understand and remember.
Note: The relative positions of the stator and rotor also exist in brushless DC motors with external rotors, such as ceiling fans, but they are not used in electric vehicles.
It's easy to see from the summary table that, superficially, the motor is really not complicated.
The basic structure of an electric motor consists of a stator and a rotor.
The basic form of a stator consists of coil windings and permanent magnets.
The basic forms of rotors include coil windings, permanent magnets, and the special magnetic conductor rotor of switched reluctance motors;
The principle of electric motor rotation is that a rotor coil or permanent magnet rotor carrying direct current experiences a force in a changing magnetic field, thus causing it to rotate. A special type is the switched reluctance motor, which uses the minimization of magnetic reluctance as its power source.
In principle, there are only a few basic structural forms of electric motors. However, in each different application scenario, the motor is refined to suit the specific functions and local conditions, along with the corresponding control system. The needs of the control system, in turn, lead to adjustments in the physical design of the motor. This process has gradually given rise to a wide variety of electric motor types.
Treat the basic structure and functions as a framework, and gradually add relevant knowledge such as applicable environment, product cost, speed regulation method, controller characteristics, etc., to form your own motor knowledge base.
