December 1, 2022

Tyna Woods

Technology does the job

Key Aspects of Motor Control Design

In the technological development of Industry 4.0, motor control plays an important strategic role. The use of energy is a major issue in industrial development. Electricity consumption is increasing significantly, owing in part to the increased electrical demand for industrial electric motors. As a result of these rising demands, developers and component manufacturers alike are looking for efficient solutions in the field of motor control.

Along with energy consumption, design complexity is increasing as a result of stringent control requirements involving many electronic technologies that necessitate considerable effort. Wide-bandgap (WBG) materials are one example.

Motor control has a wide range of applications, ranging from simple fan and pump control to more complex industrial control problems such as robotics and  servo drive mechanisms. We’ll look at the main components of a motor control system in this section.

Motors and gate drivers

DC motors are the most common because they are less expensive. They are made up of a stator (fixed part) — i.e., the permanent magnet — and a moving part (the rotor), which houses the winding connected to the commutator that supplies the current. 

The direct current is regulated to control the speed of the motor. Full-bridge, half-bridge, or step-down converters are used to drive the DC motor in this case, depending on the nature of the application.

An AC (alternating current) motor is essentially a transformer with the primary section connected to the alternating current voltage and the secondary section conducting the induced secondary current. The speed of this motor is controlled by microprocessor-based electronics, an inverter, and signal conditioning.

A controller is a type of electronic device that serves as the “brain” of a control system. The number of controllers used is determined by the number of individual processes that must be controlled. A complex system may have several controllers. Each of these controllers can send commands to the motors while also receiving instructions from the actuators.

Three-phase motors powered by alternating voltage are commonly used in industrial robotic systems (AC). As an illustration, consider Fig. Figure 1 depicts a block diagram of an electronic control circuit in which a dedicated microcontroller (MCU) generates a PWM signal.

As an alternative to MCUs, DSP or FPGA solutions are better suited for implementing complex digital filtering algorithms.

Fig. 1 shows a block diagram of a three-phase induction motor control that is powered by an alternating current (AC).

Trinamic’s TMCM-1637 5-ARMS and TMCM-1638 7-A RMS slot-type modules with two field-oriented controller/drivers that add Hall and ABN encoder functionality for field-oriented control are two examples of controllers for DC motors (or vector control). These modules are compatible with single-phase DC motors, bipolar stepper motors, and three-phase brushless DC (BLDC) motors (Fig. 2).

Fig. 2: Solutions for the TMCM-163x (Source: Trinam

IGBTs

Insulated-gate bipolar transistors (IGBTs) represent a significant advancement in electrical power control electronics. The high switching frequency is the source of innovation in switching solutions. IGBTs are electrical power control devices with basic functionality that are well-suited to solving complex motor control problems.

The most recent solutions have established an excellent relationship between switching speed and behavior stability in particularly extreme use conditions, such as when implementing inverters to drive electric motors in the automotive sector. One example is the 1,200-V IGBT S series  from STMicroelectronics. These IGBTs are designed for use at low frequencies (up to 8 kHz) and have a low Vce (sat). The trench-gate field-stop technology used in the 1,200-V IGBT S series is third generation.

GaN and SiC

WBG Semiconductors, gallium nitride and silicon carbide, on the other hand, are making inroads as a replacement for silicon-based devices in motor control applications. Lower power losses, higher efficiency, higher switching frequencies, more compact size, higher operating temperature (well beyond silicon’s upper limit of 150°C), greater reliability under difficult operating conditions, and high breakdown voltages are the main advantages offered by WBG materials in power electronics.

The higher electron mobility of a gallium nitride (GaN) high-electron–mobility transistor (HEMT), for example, translates into faster switching speed because charges that normally accumulate in the joints can be dispersed more quickly. GaN‘s faster rise times, lower drain-to-source on-resistance (RDS(on)) values, and lower gate and output capacitance all contribute to its low switching losses and ability to operate at switching frequencies up to ten times higher than silicon.

Reducing power losses has additional advantages, such as more efficient power distribution, less heat dissipation, and simpler cooling systems. To operate within the device’s safe operating limits, many motor control applications require a fan to provide forced air cooling. Power dissipation can be reduced and “fan-less” operation enabled by using GaN, which is especially important in low-weight applications like electronic drones.

Electronic designers in industrial power applications can benefit from using SiC MOSFETs, which offer significant efficiency improvements, smaller heat-sink size, and lower cost than traditional Si-based solutions such as IGBTs. SiC technology has a very low RDS(on) per unit area, high switching frequencies, and negligible energy losses during the reverse-recovery phase, which occurs after the body diode is turned off.

Due to the features such as energy savings, size reduction, higher integration, and reliability, the use of SiC devices in motor control and electrical power control applications is a true breakthrough. These characteristics make them ideal for high-reliability industries such as automotive and industrial automation control.

Turn-on and turn-off commutation speeds in industrial gate drivers must be monitored carefully. SiC MOSFET dV/dt can, in fact, reach significantly higher levels than IGBTs. High-commutation dV/dt causes voltage spikes over long motor cables and may generate common- and differential-mode parasitic currents, which can cause winding insulation and motor bearing failures over time if not addressed properly. Despite the fact that faster turn-on/-off improves efficiency, the typical dV/dt in industrial drives is often set at 5 to 10 V/ns for reliability reasons.

STMicroelectronics conducted a comparison on two similar 1.2-kV power transistors — a  SiC MOSFET and a Si-based IGBT — and discovered that the SiC MOSFET device can guarantee significantly less energy loss for both turn-on and turn-off, even under the imposed conditions of 5 V/ns (Fig. 3).

Fig. 3: Inverter-based drive with two levels and three phases (Source: STMicroelectronics)

Thanks to the features such as energy savings, size reduction, integration opportunities, and reliability, the use of SiC devices in motor control and electrical power control applications in general is a true breakthrough. Among other options, it is now possible to use the optimum switching frequency in the inverter circuit for the connected motor, which provides significant benefits in motor design.

Infineon Technologies’ SiC-based CoolSiC MOSFETs with.XT interconnection technology in a 1,200-V optimized D2PAK-7 SMD package, for example, enable passive cooling in power-density–critical motor drive segments such as servo drives, assisting the robotics and automation industry in implementing maintenance-free and fanless motor inverters (Fig. 4).

Fanless solutions in automation open up new design possibilities because they reduce maintenance and material costs and effort. CoolSiC trench MOSFET chip solution from Infineon with.XT interconnect technology provides appealing thermal capabilities in a small form factor, making it well-suited for drive integration in a robotic arm, for example. CoolSiC MOSFET SMD devices have a  3 µs  short-circuit and are rated  from 30 mΩ up to 350 mΩ. This satisfies the servo drive requirements.

Fig. 4: Reduced conduction loss in all operating modes (Source: Infineon Technologies)

Microcontrollers

Hardware and software components make up motor control solutions. The hardware component consists of electronic control devices such as IGBTs, SiC and GaN MOSFETs, power diodes, and so on, whereas the software component addresses hardware control, which is becoming increasingly complex and sophisticated. 

The availability of computing architectures optimized for power device control and management enables developers to achieve performance in the control field that would otherwise be impossible.

NXP Semiconductors and Renesas Electronics are two examples. NXP’s MPC57xx family of 32-bit processors is based on Power Architecture technology and is designed for automotive and industrial powertrain applications, as well as other automotive control and functional management applications. The processors provide AEC-Q100 quality, on-chip security encryption protection for tamper-proofing, and ASIL-D and SIL-1 functional safety (ISO 26262/ IEC 61508) support. They support Ethernet (FEC), dual-channel FlexRay, and up to six SCI/8 DSPI/2 I2C channels for various communication protocols.

Renesas provides the RA6T1 32-bit MCUs, which are based on the Arm Cortex-M4 core and run at 120 MHz, along with a set of peripherals optimized for high performance and precision motor control. A single RA6T1 MCU can control up to two BLDC motors at the same time. Furthermore, the Google TensorFlow Lite Micro framework for TinyML applications improved failure detection in the RA6T1 MCUs, providing customers with an intelligent, simple-to-use, and cost-effective sensorless motor system for predictive maintenance.

Motor requirements vary depending on the application, which may need to be optimized and fine-tuned for a specific use case. The market provides several solutions in terms of IGBTs, WBG semiconductors, and MCUs to meet those requirements. However, new hardware needs to be developed that offloads real-time critical tasks from the processor, while enabling more diagnostics, predictive maintenance and AI, and functional safety systems.

Renesas offers the RA6T1 32-bit MCUs, which are based on the Arm Cortex-M4 core and run at 120 MHz with a set of peripherals optimized for high performance and precision motor control. A single RA6T1 MCU can control two BLDC motors at the same time. Furthermore, the Google TensorFlow Lite Micro framework for TinyML applications enhances failure detection in the RA6T1 MCUs, providing customers with an intelligent, simple-to-use, and cost-effective sensorless motor system for predictive maintenance.