Experiments in electromagnetism began in the early 19th century, led by scientists like Michael Faraday. The first practical electric motor was invented in 1834 by Thomas Davenport. This DC motor uses stationary electromagnets as the stator to create a fixed magnetic field. The rotor, the moving part of the motor, is also an electromagnet driven by an electric current, transferred through a commutator and brushes. Although the basic operating principle of the brushed DC motor has remained largely unchanged since Davenport's time, significant advancements have been made in technology, materials, and design, resulting in improved efficiency, reliability, and wider applications.
There is a legend that in 1882, Nikola Tesla was strolling and talking with a friend in a city park in Budapest (today's capital of Hungary, but later the Kingdom of Hungary). Suddenly, he was inspired and recited lines from Goethe's *Faust*. As he watched the sunset, he saw a rotating magnetic field and conceived the principle of the asynchronous motor, which he quickly sketched out on a dusty path.
Asynchronous motors have fewer moving parts and do not require brushes, which were an integral part of DC motors at the time. This makes them more robust, less prone to failure, and requires less maintenance. These motors are more efficient at higher speeds, and because they use alternating current (AC), they naturally align with AC systems, which, thanks to Nikola Tesla, became dominant due to their ability to transmit electrical energy over long distances with minimal loss. Today, asynchronous motors remain the most commonly used type of motor in the industry.
Besides these advantages, asynchronous motors also have some disadvantages that DC motors lack. DC motors are capable of generating high torque immediately upon startup, which is useful for applications requiring strong starting torque. Compared to asynchronous AC motors, brushed DC motors have relatively linear characteristics and can respond quickly to changes in control signals, making them suitable for applications requiring rapid response.
Bus Company Profile: A Modern Automotive Technology Miracle
A new type of electric motor combines many advantages of asynchronous AC motors and bush-mounted DC motors: the "permanent magnet synchronous motor" (PMSM). A PMSM is an advanced three-phase motor with robust permanent magnets installed within the rotor. When current flows through the stator (the stationary part of the motor), an electromagnetic field is generated that interacts with the rotor's permanent magnet field, causing the rotor to rotate. The rotor's speed is synchronized with the frequency of the power supply, hence the name "synchronous" motor. This unique structure allows for a significantly higher power density than other available motor types. Although the basic concepts and theories of PMSM motors were already well-known, their widespread use and commercialization began in the late 20th and early 21st centuries with the development of cutting-edge electronics and control technologies. From industrial drives to automobiles, PMSM motors are popular in a wide range of applications. Many modern electric vehicles use PMSM motors because of their efficiency, power density, and wide operating speed range.
Another type to consider is BDH, which stands for Brushless DC Motor. As the name suggests, these motors have no brushes. While they share similarities with PMS (Pulse Mechanics) because both use permanent magnets, their operation differs. PMS operates on sinusoidal current and typically requires more complex control strategies. On the other hand, BDH motors operate with larger step (trapezoidal) voltages and use Haar sensors or other methods to track the rotor's position.
There are also various other types of electric motors, including stepper motors and switched reluctance motors. However, this article will focus primarily on the control of PMSMs.
Automotive control combines various engineering disciplines, which makes it particularly fascinating. These disciplines include power electronics, analog electronics, digital electronics, control theory, digital signal processing, and embedded software engineering. Furthermore, mechanical and thermal engineering play a crucial role in the design and optimization process.
Field motor control: Bridging the gap between DC and PMS motors
A thorough analysis of the DC motor's electrical properties highlights its ability to independently control the stator and rotor excitation. This independent control mechanism ensures torque generation and significant adjustment of magnetic flux.
When PMS operates based on different fundamental principles, they share an important characteristic with DC motors: only the stator current is controlled. In PMS, the interdependence between magnetic flux and torque arises from the utilization of permanent magnets for rotor excitation.
Direct-axis and quadrature-axis current components
FOC, also known as vector control, is a complex control method designed to replicate the unique torque and flux control of a DC motor. This requires orthogonality between the rotor and stator magnetic fields.
Using mathematical transformations, particularly Park and Clark's transformations, FOC successfully decouples torque from flux. To effectively achieve FOC, it must be understood that precise rotor positioning and skillful control of stator winding currents are indispensable.
Mathematical transformations used in FOC
By transforming the three-phase ABC reference frame into a DQ coordinated system, FAC offers a significant leap forward in motor control. This transformation paves the way for nearly uniform magnetic fields on both axes. The result? A system that facilitates independent control of the current in both DQ directions, similar to a speed-maintaining motor synchronized with the magnetic field induced by the stator windings.
The inherent redundancy of a three-phase motor means that the currents between its phases (A, B, C) are interdependent. This interdependence ensures that the third phase is immediately available in the process of deriving value from the second phase, thus reducing the need for a third dimension. The shift from broadcast to data-driven coordinates in mathematical coordinates is analogous to the transition from Cartesian to polar coordinates. This shift aligns our perspective with the rotor's magnetic field, providing the controller with a simple, quasi-stationary representation. The advantages of size reduction in control lie not only in mathematical elegance but also in ease of control. By reducing the size from 3 to 2, the motor controller can manage torque and flux components separately, making overall control more intuitive and effective.
controller
In addition to controlling torque, motor controllers also have the ability to control speed and even rotor position. Position control is closely related to trajectory generation technology and other related methods. However, our main focus in this area will be on controlling the speed and torque of the electric motor.
Due to the inherent relationships between voltage, current, magnetic field, and mechanical dynamics, a PMSM is typically described as a nonlinear system. However, when analyzing a PMSM at a specific operating point or equilibrium point, it can be approximated as a linear system and successfully applied to control technologies such as the 530 controller.
To adjust the speed of the motor, a traditional cascading method can be used, employing two PI controllers.
controller
The speed controller (outer loop) controls the motor speed and operates at a slower speed compared to the inner loop. When a target value, such as a specific motor speed, is set for this loop, its output decreases cascaded down, becoming the reference for the inner loop and representing the desired torque. This hierarchical structure ensures a faster system response and enhances its resistance to disturbances.
Speed controllers for electric motor control can also be fabricated using sophisticated control techniques. Beyond traditional methods, advanced approaches such as model predictive control, adaptive control, neural networks, state-space control, and fuzzy control systems offer innovative avenues for design and optimization. Compared to more complex control techniques, the low computational overhead, simplicity, robustness, and directness of controller tuning offer significant advantages. A well-tuned IP speed controller, coupled with gain scheduling and other compensations, should be sufficient for most applications.
The torque/current controller (inner loop) takes timely action to counteract disturbances, focusing primarily on measurable aspects of the system, such as torque or current. Given the close relationship between torque and current, the main objective of this loop is to make rapid adjustments without over-adjustment. The controller takes a systematic approach. First, it measures the motor current, DC voltage, and rotor angle. Using the Parker-Clark transform, the three-phase current is then converted into a rotating DQ frame, producing two distinct quantities. Subsequently, the torque (proportional to the Q-current) is controlled.
Then, the controller determines the stator voltage from the controller output through the inverse Park-Clark transformation and converts these voltages into dual-loop voltages suitable for the three-phase voltage-wave M inverter.
Block diagram of magnetic field torque controller
The diagram above highlights the basic components of the FOC torque controller. Its main input comes from the q-current, which directly corresponds to the torque generated by the motor. In addition to the q-current, the d-current, orthogonal to the q-current, plays a crucial role in the FOC of the PMS motor. The q-current is directly related to the torque, while the d-current controls the magnetic flux in the motor. Under normal operating conditions, the d-current of the PMSM is ideally set to zero. This is because the motor's inherent magnetic field is derived from permanent magnets, and any additional flux caused by a non-zero d-current would reduce efficiency and is generally unnecessary. Keeping the d-current at zero optimizes torque per ampere, ensuring efficient motor operation without generating excessive heat. However, in conditions such as a weakened magnetic field, the d-current is adjusted from this zero value to allow operation above the motor's base speed. Properly managing the d-current, keeping it at zero, and adjusting it as necessary, ensures optimal motor efficiency and prevents demagnetization. While some functional blocks require hardware implementation, the core logic of the controller is primarily executed as software within a microcontroller, DSP, or FPGA. The position sensor facilitates FOC operation of the motor at full speed, especially at lower speeds.
Motor controller hardware
Main HW components of motor controller
To implement a permanent magnet synchronous motor controller, the following basic hardware components are required:
Power supply: The source of power for driving the motor and control circuit. It must meet the voltage and current requirements of the motor and provide stable voltage and current without significant fluctuations to ensure consistent motor operation.
In motor controllers, communication interfaces, feedback sensors, control logic, gate drivers, and auxiliary systems may require current isolation to ensure noise-free operation, prevent voltage spikes, and maintain reliable performance between components. It is important to note that the need for electrical isolation depends on the specific application, environment, and design considerations of the motor controller.
Converter: A device that converts power from a DC source to three-phase alternating current (AC) to drive a PMSM (Motor Motor Module). In a three-phase inverter bridge, there are six switches arranged in three pairs. Each pair handles one of three phases. In each phase, one switch manages the positive half-cycle, while the other manages the negative half-cycle. Pulse-width modulation (PWM) signals control these switches. The modulation of these signals determines the magnitude, frequency, and waveform of the AC output voltage for each phase. The inverter receives its DC voltage input from two terminals called DC links. Capacitors are typically included in the DC links to stabilize and filter the DC voltage. As output, the inverter produces three AC waveforms. These waveforms are typically sinusoidal, with a 120-degree phase difference between them. In many inverter designs, each switch, whether IGBT or MOSFET, is paired with a diode in an anti-parallel configuration. This configuration ensures that when the switch is off, the corresponding diode provides a path for current, preventing potential hazards. This arrangement also facilitates the recirculation of inductor current, which is common in motor operation. A common motor operation.
Inverter three-phase MOSFET bridge
The choice between insulated-gate transistors (IGBTs) and metal-oxide-semiconductor (MOSFETs) in motor controllers depends on various factors related to application requirements and the inherent characteristics of these devices. IGBTs are generally best suited for high-voltage applications, often exceeding 600 volts and a switching frequency below 20-30 kHz. IGBTs are more robust to short circuits and offer better thermal stability. MOSFETs are used in low-to-medium power applications where high switching frequencies and low voltage efficiency are problematic.
Gate drivers are a crucial component of inverters, acting as an interface to convert low-voltage control signals to high-voltage levels. They integrate protective elements such as desaturation detection and undervoltage lockout, ensuring rapid switching speeds within the set off time between switch-off intervals to minimize losses and electromagnetic interference. This provides current isolation for safe and noiseless operation, improving the overall efficiency of the inverter by reducing switching and conduction losses. A very important component of the gate driver is the gate resistor, located on the gate of an IGBT or MOSFET. They are essential for the operation and control of power electronic switching devices. They help control the switching speed of the transistor, striking a balance between excessively fast switching that may generate harmful microwave or electromagnetic interference and slow switching that may increase losses. These resistors help dampen oscillating trigger parasitic inductance and capacitance bound to the transistor. By limiting current flow into the gate, the gate resistor provides effective protection against the gate oxide layer, preventing potential damage. Properly selecting the gate resistor value is crucial, as it determines the balance between several performance and protection aspects of the switching device. These resistors help dampen oscillating trigger parasitic inductance and capacitance bound to the transistor. By limiting current flow into the gate, the gate resistor provides effective protection against the gate oxide layer, preventing potential damage. Properly selecting the gate resistor value is crucial as it determines the balance between several performance and protection aspects of the switching device. These resistors help dampen oscillation triggering parasitic inductance and capacitance bound to the transistor. By limiting current flow into the gate, the gate resistor provides effective protection against the gate oxide layer, preventing potential damage. Properly selecting the gate resistor value is crucial as it determines the balance between several performance and protection aspects of the switching device.
Ensuring optimal electrical and mechanical design for inverters is crucial. In this regard, expertise in power electronics, building design, and thermal design is essential.
The control unit, responsible for signal processing and generating voltage-width modulation (PWM) signals for the inverter, is typically a microcontroller, digital signal processor, or programmable gate array (FPGA). This unit also integrates memory, analog interfaces, and digital interfaces. Furthermore, safety logic can be embedded in this central unit to ensure protective measures are in place to prevent malfunctions and potential hazards. Choosing the right microcontroller (MCU) or digital signal processor (DSP) for motor control applications requires consideration of several key parameters and characteristics to ensure optimal performance, reliability, and efficiency. The main factors to consider are: processing power, peripherals and I/O, memory, safety features such as hardware interlocks, error correction code (ECC) memory, monitor, hardware fault detection and protection mechanisms, such as fault detection and protection functions (often referred to as "differential" or "differential point"), power consumption, operating temperature range, development ecosystem, and cost. Some applications may benefit from microcontrollers with FPGA capabilities, enabling custom hardware logic.
Feedback sensor:
• Position sensors: These are typically rotors whose position is tracked by encoders or resolvers.
In addition, there are sensorless observation techniques, primarily based on Kalman filters, which can be used to estimate rotor position without direct measurement. However, these sensorless methods may have some drawbacks, such as reduced accuracy, instability in certain situations, and challenges during startup or at low speeds. These techniques and their specific details will not be included in this paper.
• Existing sensors: Used to measure the phase current of the motor. In practice, IIA + Ib + IC equals zero, but typically only two current sensors are needed for accurate current assessment.
• Voltage sensor: Used to measure the power supply voltage of the drive motor (DC bus).
• Temperature sensor: Temperature sensors in motor controllers are crucial for monitoring and regulating heat to ensure motor protection, optimize efficiency, extend motor life, provide feedback for advanced control algorithms, and manage active cooling systems.
Communication interface: Used for communication with external devices or networks, such as CAN, UART or Ethernet.
Protective components: These include relays, fuses, and protective diodes to ensure the safe operation of the system and to prevent excessive voltage or current.
