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 Vehicle Motor Control Unit
While many other types of electric motors exist, we will focus on the AC and BLDC/PMSM DC motors commonly seen in 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, the system pushes three currents into each of the three motor windings, and the currents vary smoothly and sinusoidally as the motor rotates. The encoders or resolvers achieve precise rotor position measurement 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 for AC motors, vector control (FOC) is today’s most advanced method. The rotor and stator fields influence an electric motor’s torque, 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. Torque depends on motor speed and rotor position, while the stator generates magnetic flux.
Field-oriented management uses two techniques to manage torque and flux separately:
- Direct FOC: From flux estimation or measurement, we directly determine the rotor flux angle.
- Indirect FOC: We use the slip calculations and the available speed to deduce the rotor flux angle.
>When using a dynamic model of an AC induction or BLDC motor, we can compute vector control with intricate algorithms and knowledge of the terminal current and voltages. However, this method requires a lot of processing power to use well.
Vector-based control allows uniform management of AC, PM-AC, and BLDC motors. 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 us to decrease the motor speed to almost 0 rpm.
Figure 1 is an example diagram for a three-phase BLDC motor control. Six power transistors operate, one for each of the three phases, or three half-bridges altogether. Engineers use three PWM signals, timed by an MCU or driver IC, for activation. 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 require rotor position sensing and commutation control. Closed-loop speed control also needs motor speed/current measurement and PWM regulation.
BLDC motors achieve absolute position sensing by detecting the rotor position with specific position sensors. Consequently, costs increase and the number of cables increases. Sensor-free BLDC control estimates rotor position using back-EMF, eliminating 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: We require digital processing to provide the best resolution.
Usually, high-precision motor control applications in the industrial and automotive sectors use resolvers and encoders.
Vehicle Motor Control Unit
The Motor Control Unit (MCU) links AC or BLDC motors to DC battery power. The main function regulates the EV’s acceleration when someone presses the throttle.
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. During braking, the motor acts as a generator as its back-EMF exceeds the MCU’s DC supply voltage. 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.
Motor controllers, inverters, and sensors form the foundation of EV propulsion. The intricate relationships between these components ensure optimal efficacy, security, and performance when the car is driving. Advancing motor control algorithms enhance acceleration, regenerative braking, and the EV driving experience.