Motor control units for EV

The world is driving towards sustainability. Driving electric vehicles (EVs) is one such measure taken in hands. In EVs, motor control systems manage the motors' speed, torque, and direction. Control is necessary for optimal performance and efficiency of the smart motor management system. This article explains dynamic requirements during a vehicle operation to establish developmental collaboration in the field of motion control, power conversion, and embedded electronics, leading to the advent of Motor Controllers, also known as Motor Control Units (MCUs).

1. Latest innovations in motor control technology for electric vehicles

As EVs evolved, there was a simultaneous need to develop motion control mechanisms for better efficiency during the conversion of electric power. Using embedded electronics Motor Control Units (MCU) were developed to act as an interface between the batteries and motor (Figure 1) to control the electric vehicle’s speed and acceleration based on throttle input.

Figure 1: Typical system block diagram of a motor-control unit in a vehicle

MCUs continuously adapt to varying road conditions for enhanced efficiency, range, and overall driving dynamics. These systems are packed into a compact, efficient, lightweight package of power electronics, motor drive systems, and control units. MCUs are made smart by some of the below-mentioned latest technologies included in it.

Advanced power electronics helps to reduce electrical noise and optimize power consumption and voltage regulation.

Multilevel inverters create the output voltage by combining many voltage levels to produce a smoother waveform thus increasing the accuracy of the system.

Direct torque control (DTC) provides fast torque and flux control response.

Regenerative braking optimization reverses electric motors that propel a vehicle and reverse recharges the battery.

Machine learning and AI in MCUs protect data integrity and reduce human errors.

2. How does MCU coordinate with the motor?

The principal function of a motor controller is to regulate the supply of energy to the motor. The Motor controller receives commands from interfaces such as the throttle, brake, or forward/reverse control switches. It processes these commands and precisely controls the motor’s speed, torque, direction, and consequent horsepower of the motor in the vehicle. The differences between performance and Motor control in AC and DC motors are mentioned in (Table 1)

Table 1: Difference between performance and control units in AC and DC motors

3.Design constraints in the motor control Unit for operational efficiency

Control mechanisms in EVs like electric power steering and brake boosters, heat pumps, and cooling fans require specific MCUs. A typical EV consists of many such systems. MCUs can be designed with or without sensors; for example, the position of the rotor can be located with the help of Hall effect sensors, or the back emf can be measured with a sensor less mechanism.

Figure 2: Block diagram of motor control unit

In three-phase BLDC motors and PMSM in EVs, the speed and torque are controlled by varying amplitudes and frequencies of voltages, and currents applied. This can be achieved by a PWM signal, which can be used to generate an analog voltage by passing the signal through a low-pass filter. An MCU, as shown in (Figure 2) consists of a microcontroller or microprocessor and a few other components. The MCU takes inputs from sources such as Electric Power steering, Electric braking system, and Electric propulsion and processes those inputs to generate a control signal. It also provides an external digital interface, such as the Controller Area Network (CAN) for communication. For high and low-voltage motor control applications products like monolithic driver ICs, power MOSFETs, IGBTs, gate drivers, and power MOSFETs are included.

3.1 Controller for BLDC motor

Figure 3: Circuit diagram of three-phase BLDC motor controller with Hall-effect sensors

Half-bridge controller circuit for a BLDC motor is shown in (Figure 3). The stator consists of three-phase windings positioned at 120° to one another. Each winding has a vector depiction of voltage and current applied to the stator. The above circuit configuration has two transistors as switches, one low-side and one high-side. Motors are embedded with Hall sensors. Hall sensors are commonly used to detect the rotor's position relative to the stator. These sensors provide information about the rotor's position, enabling the motor controller to determine when and how to switch the power transistors that energize the motor's coils. MOSFETs and IGBTs are used to control the flow of electrical current through the motor's coils. The motor controller regulates the current flowing through the coils based on the information received from the Hall sensors. By controlling the timing and sequence of the current flowing through the stator coils, the motor controller can precisely control the rotation of the motor.

Figure 4: Three-phase pulse-width modulation (PWM)

Pulse-width modulation (PWM) helps regulate the current injected into the rotor's windings and runs the commutation process more smoothly and efficiently. PWM switching frequency can be altered depending on the applications. (Figure 4) shows a sinusoidal pulse width modulated signal. The frequency should be sufficiently high to prevent power loss. The stator's physical limitations determine the maximum frequency level. However, the specifications are adapted to the design of the MCU.

3.2 Controller for PMSM

Figure 5: Permanent magnet synchronous motor (PMSM) motor control unit

The speed control loop outputs the machine's reference electromagnetic torque. The stator current's reference direct and quadrature (dq) components corresponding to the commanded torque are derived based on the vector control strategy is indicated (Figure 5). The stator current's reference dq components are then used to obtain the needed gate signals for the inverter through a hysteresis-band current controller.

Compared to scalar-controlled drives, the main advantage of this drive is its rapid dynamic response. The inherent coupling effect that happens between the torque and flux in the machine is handled through decoupling (stator flux orientation) Control, which permits independent Control of flux and torque. However, due to its computation complexity, the implementation of this drive requires fast computing processors or DSPs.

4. Applications of MCU in regenerative braking systems (RBS)

Figure 6: (a) vehicle a) acceleration process, b) regenerative braking process

RBS mechanism recovers some of the energy lost during braking and, in return, uses the energy to recharge the battery. Such an action not only improves the efficiency but also extends the range as battery regeneration and storage provide a pathway for introducing larger batteries for extra range without increasing the time needed for charging.

Operation: When the vehicle moves, the electric energy from the batteries is converted into the vehicle's kinetic energy (i.e., the wheels rotate) by means of an electric motor (Figure 6a). On the other hand, when braking, i.e., when the brake is activated, the electric motor changes its direction; that is, it switches from the role of the converter to the role of the electric generator (Figure 6b). Thus, the motor changes the vehicle's kinetic energy through the wheel's rotation, converted into electrical energy. All these switching corresponding to the control signal is handled by the MCU.

Figure 7: Example of a regenerative braking system blending profile during a brake application

The challenge of ensuring appropriate torque distribution to all wheels of a vehicle during regenerative braking is considerable, and intrinsic torque variations between electric motors on either side of the vehicle can present stability issues during braking, requiring sophisticated control strategies as shown in (Figure 7).  Including MCUs for control management helps in resolving these issues.

ST microelectronic’s -L9907 (automotive FET driver for 3 phase BLDC motor),

Figure 8: Gate driver board (L9907) - components and connectors - top view

L9907 is shown in Figure 8. In automotive applications, it is a gate driver unit that regulates six N-channel FETs for brushless DC motors. Numerous electric motor control strategies can be used due to the independent control of each pre-driver channel. With its built-in booster, this fully integrated 3-phase pre-driver can drive any type of Power MOS transistor in 3-phase BLDC motor applications up to 54 V. Through SPI, a full diagnostic is accessible.

Newark has partnered with many different suppliers catering to a wide range of motor control accessories such as Motors & Motor Controls

Conclusion

In EVs, MCUs help to move the vehicle by converting the battery's electrical energy to mechanical energy. AC motors, such as PMSM and Induction motors, provide high power density and efficiency with a wide range of speeds. DC motors, such as BLDCs, are simple in construction and less efficient than present-day AC motors. The control mechanisms for each of these motors may or may not involve sensors (like position sensors). Sensorless control mechanisms are preferred as they eliminate the power needed to operate the sensor circuitry. Vector-based motor control mechanisms help fine-tune the EV motor’s required torque and speed. Using the FOC technique, the speed of the motor can be varied in precise values. MCUs also interface with motors and batteries to reverse charge the EV’s battery in applications like regenerative braking systems.