Introduction
Because of the technological advances made possible by electric motors and batteries, power conversion systems and motor control strategies that optimize efficiency and dependability have to be developed. These days, all of these features are included in the traction inverter, commonly referred to as the Vehicle Motor Control Unit (MCU).
The world’s need for electric cars, or EVs, has grown significantly in the last several years. One of the things boosting the popularity of electric cars is the government’s and automakers’ plan to transition from manufacturing internal combustion vehicles to electric and hybrid vehicles in a few years.
The advancement of lithium-ion battery technology, the dependability and accessibility of high-efficiency powertrain inverters, and the advancements made in engine management and electric motors are all responsible for the availability of EV technology and the reduction of range anxiety among consumers. Electric motors and batteries have facilitated technological advancements, necessitating the development of power conversion systems and motor control procedures that maximize reliability and efficiency.
These days, all of these features are included in the traction inverter, commonly referred to as the Vehicle Motor Control Unit (MCU).
DC versus AC motors
Two primary motor types are used in electric cars:
AC-powered engines:
A motor on alternating current (AC) has three phases and 240 volts. AC motors can also be utilized as a generator to recharge an electric vehicle’s batteries because of their regenerative nature. This motor also has the added benefit of faster acceleration and smoother running on uneven ground. Its cost is higher than that of DC motors, which is its primary disadvantage.
The two main types of AC motors are synchronous and asynchronous motors, also called induction motors. Induction motors can be relied upon, are reasonably priced, and require little maintenance, making them simple to use. In contrast, synchronous motors offer some advantages, including strong low-speed torque, small form factor, high power density, high efficiency, and low weight.
BLDC Motors:
These motors have several benefits over AC motors, such as longer speed ranges, noiseless operation, quicker dynamic reaction times, and more. They have an exceptionally high torque ratio relative to size, making them a great fit for EVs, which require high power density yet lightweight and small form factors.
Moreover, BLDC motors require incredibly intricate hardware and software control. AC motors can store energy for braking and release it for accelerating, but they require an appropriate DC-AC inverter. This restores the essential battery juice while enabling you to drive normally. Some DC systems are more costly and sophisticated than others because they can also accomplish that.
BLDC motors require more sophisticated reversing techniques than AC motors, which can be easily reversed by simply flipping the order of the two phases in the inverter.
Both BLDC and AC motors are viable choices for powering EVs, even though AC motors are preferred when performance and long-range are critical criteria. As with many other electronics applications, the key to this selection is to find the optimal balance between cost and performance.
Algorithms for motor control
While there are many other types of electric motors, we will focus on the AC and BLDC/PMSM DC motors that are commonly seen on electric vehicles (EVs). Consequently, the motor control algorithms used in EVs will depend on the kind of motor and control (open or closed loop). The latter requires the presence of sensors that can accurately determine the position of the motor at any given time.
Trapezoidal control is among the simplest types of BLDC motor control. Even though it’s quite popular and reasonably priced, it has a torque ripple problem when driving.
Sinusoidal control is generally considered to be an improvement over trapezoidal control. The main benefits of this control are less noticeable noise, higher torque at lower speeds, and more precise and seamless operation. To accomplish these, three currents are pushed into each of the three motor windings, and these currents vary smoothly and sinusoidally as the motor rotates. Precise rotor position measurement can be achieved using encoders or resolvers to provide smooth sinusoidal modulation of the motor currents while the motor rotates.
While sinusoidal control is very successful at low motor speeds, it becomes limited at high motor speeds due to an increase in the frequency of the sinusoidal signal. At high speeds, torque production decreases and efficiency diminishes.
Originally developed for AC motors, Vector Control, often known as FOC for Field Orientation Control, is the most advanced control method available today. An electric motor’s torque is influenced by the rotor and stator fields, and it is greatest when they are orthogonal.
The FOC approach aims to replicate the orthogonal relationship present in an AC or BLDC motor. A two-orthogonal component, FOC stands for variable frequency control of the stator in a three-phase motor. One component is the torque, which is determined by the motor speed and the location of the rotor; the other is the magnetic flux produced by the stator.
Field Oriented management uses two techniques to manage torque and flux separately:
- Direct FOC: From flux estimation or measurement, the rotor flux angle is directly determined.
- Indirect FOC: The slip calculations and the available speed are used to deduce the rotor flux angle.
When using a dynamic model of an AC induction or BLDC motor, vector control can be computed with the use of intricate algorithms and knowledge of the terminal current and voltages. However, this method requires a lot of processing power to use well.
Vector-based motor control has the natural benefit of being able to control multiple AC, PM-AC, or BLDC motors using the same scheme. The FOC technique allows brushless motors to reach up to 95% efficiency, which is efficient within the maximal speed range. Its accuracy and precise control also allow the motor speed to be decreased to almost 0 rpm.
Figure 1 is an example diagram for a three-phase BLDC motor control. Six power transistors are involved, one for each of the three phases, or three half-bridges altogether. Three PWM signals are typically used to activate them, and an MCU or integrated driver IC controls the timing and sequence of these signals. The microcontroller receives positional feedback from three Hall sensors.
An analogous picture that illustrates how to control an AC induction motor is picture 2. Here, an accurate encoder interface provides the position feedback.
Sensor versus Vehicle Motor Control Unit without sensors
BLDC motors are more challenging to operate and require an understanding of the rotor position and mechanism to commutate the motor because they are not self-commutating. Measurement of the motor speed and/or motor current and a PWM signal to control the motor speed and power are two more prerequisites for closed-loop speed control.
BLDC motors achieve absolute position sensing by detecting the rotor position with specific position sensors. Consequently, costs increase and the number of cables increases. By using the motor’s back-emf (electromotive force) to estimate the rotor position, sensor-free BLDC control does away with the requirement for position sensors. For low-cost variable speed applications like fans and pumps, sensor-less control is crucial. Sensor-less control is also necessary for compressors that use BLDC motors, including those in refrigerators and air conditioners.
Position sensors are available in three main types:
Hall-effect sensors reduce design complexity and are cheap. Yet, their resolution is subpar when compared to other sensors.
Encoders are expensive and require digital processing.
Resolvers: Digital processing must be used to obtain the optimal resolution.
Resolvers and encoders are typically used in high-precision motor control applications in the automotive and industrial industries.
Motor control unit
The batteries, which are DC power sources, and the motor (AC or BLDC) are interfaced with each other by an electrical module known as the Motor Control Unit (MCU). Its main duty is to control how fast and how much acceleration the EV experiences when the throttle is applied.
The following are some of an MCU’s primary responsibilities:
Control the motor’s torque and speed.
Switch the engine on and off.
Reverse the motor’s direction.
brakes that produce energy again. When the motor is braking, it acts as a generator since the back-emf it produces is higher than the DC supply voltage to the MCU. This potential difference causes current to flow from the motor to the battery through the Vehicle Motor Control Unit.
The Vehicle Motor Control Unit uses a range of safety measures, including the following, to protect EV components:
- Overvoltage: this occurs when the input battery voltage exceeds permissible limits.
- Under voltage: If the MCU operates below the lower voltage cutoff, it will drain the battery more fully and may result in a thermal runaway that could destroy the cells permanently or result in a decrease in performance.
- Overcurrent: If the Vehicle Motor Control Unit detects an overcurrent, it will cut off the battery supply. It does this continuously.
- Overheating: as with the previous point, this happens when the internal temperature of the motor controller goes above a safe threshold.
Conclusion:
In conclusion, this overview of electric vehicle motor control provides a comprehensive understanding of the key components and characteristics that dictate how the electric motors in these cars function. Electric vehicles are crucial for reducing carbon emissions and raising energy efficiency as the automobile industry shifts to more sustainable and ecologically friendly forms of mobility.
The motor control systems—which comprise inverters, motor controllers, and several sensor types—that have been discussed form the basis of electric vehicle propulsion. The intricate relationships between these components ensure optimal efficacy, security, and performance when the car is driving. Additionally, as technology advances, motor control algorithms get better and better, providing electric car users with better acceleration, regenerative braking, and overall driving experiences.