Introduction
Electric motors and batteries have produced technological results that have necessitated the development of power conversion systems and motor control strategies that maximize efficiency and dependability. These days, the traction inverter, also known as the Motor Control Unit (MCU), incorporates all these functions.
The demand for electric vehicles, or EVs, has increased dramatically in the world recently. The government and automakers’ decision to switch from producing internal combustion vehicles to electric and hybrid vehicles within a few years is one of the factors contributing to the increasing popularity of electric automobiles. The development of lithium-ion battery technology, the accessibility and dependability of high-efficiency powertrain inverters, and the advancements made in engine control and electric motors have all contributed to the accessibility of EV technology and the reduction of range anxiety among users. Electric motors and batteries have produced technological results that have necessitated the development of power conversion systems and motor control strategies that maximize efficiency and dependability.
These days, the traction inverter, also known as the Motor Control Unit (MCU), incorporates all these functions.
DC versus AC motors
In electric cars, two main types of motors are utilized:
Motors AC. An AC motor is a three-phase motor that runs at 240 V and is powered by alternating current. AC motors can also be utilized as a generator to replenish an EV’s battery because of their regenerative characteristic. This motor also has the benefit of greater acceleration and smoother operation on uneven surfaces. The primary disadvantage is that it is more expensive than DC motors.
Asynchronous motors, often known as induction motors, and synchronous motors are the two primary kinds of AC motors. Induction motors are dependable, inexpensive, and easy to operate (they don’t require a lot of maintenance). Conversely, synchronous motors have several benefits, such as low weight, small form factor, high power density, high efficiency, and strong low-speed torque.
BLDC Drives. Compared to AC motors, these motors provide several advantages, including faster dynamic reaction times, noiseless operation, greater speed ranges, and more. They are an excellent fit for EVs, which need high power density but lightweight and tiny form factors, because of their extremely high torque ratio about size.
Moreover, BLDC motors need highly sophisticated hardware and software control. AC motors can recover energy from braking and release it during acceleration, although they do require an appropriate DC-AC inverter. This makes it possible to restore necessary battery power while driving normally. Certain DC systems are more costly and sophisticated because they can also accomplish that.
In contrast to AC motors, which can be easily reversed by simply switching the order of the two phases in the inverter, BLDC motors require more complex reversing techniques.
Although AC motors are favored when performance and long-range are severe requirements, BLDC and AC motors are good options for powering EVs. Finding the ideal balance between cost and performance is crucial in this decision, just like it is in many other electronics applications.
Algorithms for motor control
While there are many various kinds of electric motors, we will concentrate on the ones that are typically seen on electric vehicles (EVs), such as AC and BLDC/PMSM DC motors. Therefore, the type of motor and control (open or closed loop) will determine the motor control algorithms utilized in EVs. For the latter to be possible, sensors that can precisely determine the motor’s position at any given time must be present.
One of the most basic forms of BLDC motor control is trapezoidal control. Despite being highly affordable and well-liked, it has a torque ripple issue when commuting.
Typically, sinusoidal control is regarded as the advancement from trapezoidal control. Reduced audible noise, increased torque at lower speeds, and more accurate and smoother functioning are the primary advantages of this control. The three motor windings are driven with three currents that fluctuate smoothly and sinusoidally as the motor turns to achieve these outcomes. Encoders or resolvers can be used to measure the rotor position accurately, which is necessary to provide smooth sinusoidal modulation of the motor currents as the motor rotates.
Because the sinusoidal signal frequency increases at high motor speeds, sinusoidal control, while highly effective at low motor speeds, becomes limited. Efficiency declines and torque production is reduced at high speeds.
Vector Control (abbreviated FOC, for Field Orientation Control) was first created for AC motors and is currently the most sophisticated control technique on the market. The torque in an electric motor is dependent on the stator and rotor fields, and it reaches its maximum when they are orthogonal.
Replicating the orthogonal relationship found in an AC or BLDC motor is the goal of the FOC technique. FOC, or variable frequency control of the stator of a three-phase motor, is a two-orthogonal component. The torque, which is based on the motor speed and the rotor’s position, is one component and the magnetic flux generated by the stator is the other.
Two methods are used by Field Oriented Control to control torque and flux independently:
Direct FOC: The rotor flux angle is computed directly from flux estimation or measurement.
Indirect FOC: The slip calculations and available speed are used to indirectly determine the rotor flux angle.
Using complex algorithms and knowledge of the terminal current and voltages, vector control can be computed when utilizing a dynamic model of an AC induction or BLDC motor. Nevertheless, the application of this technique necessitates a high level of processing resources.
One inherent advantage of vector-based motor control is the capability to control many AC, PM-AC, or BLDC motors using the same scheme. Brushless motors that use the FOC approach can achieve up to 95% efficiency, which is efficient within their maximum speed range. Additionally, because of its accuracy and tight control, the motor speed may be lowered to nearly 0 rpm.
Figure 1 depicts a typical diagram for a three-phase BLDC motor control. Six power transistors total—three half-bridges for each of the three phases. Three PWM signals, whose timing and order are managed by an MCU or an integrated driver IC, are typically responsible for activating these. The microcontroller receives position feedback from three Hall sensors.
An analogous figure illustrating the control of an AC induction motor can be found in Figure 2. Here, a precise encoder interface provides the position feedback.
Motor Control Unit: sensor versus sensorless
Because BLDC motors are not self-commutating, they are more difficult to handle and require knowledge of the rotor position and mechanism to commutate the motor. Two more requirements for closed-loop speed control are the measurement of the motor speed and/or motor current and a PWM signal to regulate the motor speed and power.
Absolute position sensing is achieved by BLDC motors by using specialized position sensors to detect the rotor position. Costs increase and there are more wires as a result. By estimating the rotor position using the motor’s back-emf (electromotive force), sensorless BLDC control does away with the requirement for position sensors. For low-cost variable speed applications like fans and pumps, sensorless control is crucial. Sensorless control is also necessary for compressors used in air conditioning units and refrigerators that use BLDC motors.
Position sensors come in three primary varieties:
Hall-effect sensors are inexpensive and simplify designs. But when measured against other sensors, their resolution is poor.
Encoders: they need digital processing and are costly.
Resolvers: the best resolution is achieved at the expense of digital processing.
Typically, high-precision motor control applications in the automotive and industrial domains employ resolvers and encoders.
Motor control unit
An electrical module called the Motor Control Unit (MCU) serves as an interface between the motor (AC or BLDC) and the batteries, which are DC power sources. Its primary responsibility is to regulate the EV’s acceleration and speed in response to throttle input.
An MCU’s primary responsibilities include the following:
- Regulate the speed and torque of the motor.
- Turn on and off the engine
- Turn the motor backward.
- Brakes that regenerate energy. Because the back-emf produced by the motor is greater than the DC supply voltage to the MCU, the motor functions as a generator when it is braking. Through the Motor Control Unit, current flows from the motor to the battery as a result of this potential difference.
- The Motor Control Unit implements a variety of safety strategies to safeguard EV components, such as:
- Overvoltage: this is the result of the input battery voltage going above its bounds.
- Under voltage: Should the Motor Control Unit function below the lower voltage threshold, it would draw greater currents from the battery, perhaps leading to a thermal runaway that could cause irreversible damage to the cells or a decline in performance.
- Overcurrent: The Motor Control Unit continuously checks the current level and cuts off the battery supply if it detects an overcurrent.
- Overheating: similar to the preceding point, this occurs when the motor controller’s internal temperature rises above a safe level.
Conclusion :
To sum up, this overview of electric vehicle motor control offers a thorough grasp of the essential elements and features that determine how these cars’ electric motors operate. As the automotive sector transitions to more environmentally friendly and sustainable modes of transportation, electric vehicles are essential for lowering carbon emissions and increasing energy efficiency.
The core of electric vehicle propulsion is made up of the motor control systems that have been mentioned, which include inverters, motor controllers, and different types of sensors. When a vehicle is operating, the complex interactions between these parts guarantee maximum effectiveness, safety, and performance. Furthermore, technological developments keep improving motor control algorithms, giving owners of electric vehicles improved acceleration, regenerative braking, and overall driving experiences.
To solve issues like range anxiety, advancements in battery technology, and infrastructural development, the shift to electric transportation calls for continuous research and development. The industry is ready to overcome these obstacles and increase the accessibility and appeal of electric vehicles to a wider range of consumers as motor control systems for electric vehicles advance.
To summarise, comprehension of electric vehicle motor control is essential for researchers, engineers, and enthusiasts alike as we jointly strive towards a more environmentally friendly and sustainable future in the transportation sector. Undoubtedly, the continuous advancements and enhancements in electric vehicle motor control will facilitate the extensive integration of electric vehicles and ultimately lead to a more ecologically conscious and clean automotive landscape.