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The previous blog in the series gave a snapshot overview of the different elements that make up a variable speed drive (VSD). The block diagram is expanded in more detail in Fig. 1 below, where the different parts of the drive as well as the various external interfaces to the motor, encoder, and external networks are shown, as well as the power and data flows between the different elements. Every drive architecture is slightly different, particularly in terms of how electrical isolation between the high voltage zone and the Safety Extra Low Voltage (SELV) zone is managed, but the architecture shown in Fig.1 is quite common.
Fig. 1: Detailed Variable Speed Drive Architecture
The block under more detailed consideration in this blog is the AC input stage. As shown in Fig.1, the AC input stage is the main power path from the AC grid supply to the DC/AC inverter stage. This blog discusses the different functions performed by this block, namely:
The main purpose of the rectifier is to convert the mains AC input (1-phase or 3-phase) to a DC bus supply which acts as the input to the DC/AC inverter stage. This is generally achieved through a six-diode bridge rectifier, followed by a large electrolytic capacitor bank across the DC bus, which provides smoothing for the rectified AC signal, and renders it more DC-like, effectively acting as a large low-pass filter. The amount of “ripple” present on the DC bus is directly proportional to the size of the DC bus capacitor bank, which must also be sized to provide holdup of the drive during short AC mains dropout events. Some variable speed drives implement fully controlled AC/DC converters, that use power switches rather than power diodes – these allow for full control of the input AC current, to manage low-frequency harmonics. However, these are much more expensive and are not required in the majority of applications.
The AC/DC rectifier stage is also important in that it helps ensure that the drive complies with conducted EMI regulations such as EN61000-3-2, which is published by the International Electrotechnical Commission (IEC). The DC/AC inverter is a switching converter with many fast transient voltages and currents which result in significant differential and common mode high-frequency currents through parasitic inductances and capacitances, and these must be prevented from polluting the AC network and impacting other equipment. Input EMI filtering is typically implemented using a mix of differential and common mode chokes, and a mix of high-frequency bypass capacitors between the AC lines (known as X-capacitors) and between the AC lines and earth (known as Y-capacitors). These components present a high series impedance to high-frequency currents and a low impedance to earth for these high-frequency currents.
Fig.2: AC Input Stage Circuits
Inrush protection is another important function carried out by the input AC/DC stage. On startup, the DC bus capacitor bank is completely discharged, and the charging current from the AC line voltage to the DC bus is limited only by the resistive impedance of the EMI filter chokes and bridge diodes. Without some additional current limiting, a very high transient charging current will flow from the AC line to the DC bus capacitor bank, and this can result in overload in the filter components and/or the recToes. To counteract this, a series thermistor is sometimes inserted in the charging path – this has high resistance when it is cold, and this limits the charging current to a reasonable level. Once it heats up, the resistance reduces dramatically, limiting the power losses. The one issue with this style of inrush protection is related to multiple restarts in close proximity – in this case, the thermistor will not have cooled down enough in between restarts and damaging over-currents can occur. A better solution is to use a standard current limiting resistor, with a relay or solid-state switch in parallel, that can be turned on once the inrush period is over, as illustrated in Fig. 2.
Last, but not least, transient voltage protection is a critical function performed by the input AC/DC stage. Transient voltage spikes and surges can be present on the AC grid for mcans, and these have the capacity to permanently damage a motor drive. In order to counteract this, components such as Transient Voltage Suppressors (TVS) diodes or Metal Oxide Varistors (MOV) are generally inserted in parallel with the AC line phases. These act to clamp any voltage spikes and absorb the energy associated with them, and in so doing protect the rest of the electronics.
These various elements of the AC input stage are shown in Fig.3 inside a Rockwell motor drive. Even though they are relatively simple components, electronically speaking, they take up a large area within a motor drive and have a large impact on the overall reliability, protection, and lifetime of the drive.
Fig.3: AC Input Stage Elements in a Variable Speed Drive
The next blog in the series will look at the various power supplies needed across the VSD and some of the factors that go into their design.
This blog is focused on the basic working of the variable speed drive - a critical component in enabling precise and accurate motor control in applications such as conveyors, winding, printing, extrusion machinery, and many other industrial applications. It is estimated that only between 20% and 30% of all deployed motors in the industry are inverter-driven or connected to a variable-speed drive. If each motor is driven in the most efficient way possible - that is with proper load matching and with an appropriate motor drive, it will have the potential to reduce global energy usage by up to 10%. The previous blog in this series focused on the evolution of motor control.
A variable speed drive (VSD) controls the speed and torque of an AC motor by converting fixed frequency and voltage input to a variable frequency and voltage output to achieve precise control of motor speed and torque. It helps in optimizing energy consumption, increasing productivity, extending equipment lifetime, and enhancing the overall efficiency of motor-driven systems. Motor drives are primarily located between the AC grid and the motor. They take their main power source from the AC grid and convert the AC supply voltage to DC using a rectifier and a capacitive filter. The filter capacitor bank helps to reduce the AC ripple on the DC bus. After smoothing, the DC voltage is applied to the inverter stage.
Figure 1 Block diagram
The inverter stage is the most important in the variable speed drive as it converts the DC input into a variable-frequency output. It consists of power transistors, isolated gate drivers, voltage and current feedback circuits, as well as other protection and monitoring circuits. The power transistors convert the high voltage DC bus to variable frequency, variable voltage three-phase AC output for the motor. In the inverter stage, gate drivers turn on and off the power transistors using PWM signals from the motor controller to vary the AC output (voltage and frequency) required by the motor, depending on its load. The gate drivers efficiently turn on and off the power transistors and regulate the output voltage and frequency while also protecting them from any fault conditions. The voltage and current sense circuitry play a crucial role in measuring the current and voltage in the motor and feeding these values back to the current control loop. The motor current is a direct function of the motor torque and the bandwidth of the current control loop determines the overall torque response of the variable speed system. Precise and robust measurement is important to ensure maximum torque per ampere of current and optimize motor efficiency. This in turn saves energy and enhances sustainability.
The inverter is directly connected to the motor itself. In many systems, the motor will have an encoder device attached to it, that measures the position of the motor rotor and feeds this back to the motor controller. The motor controller is the processing unit that takes input from the encoder, the inverter, and any external connectivity interfaces, and transmits output-optimized pulse width modulation signals back to the inverter to run the motor efficiently. The choice of the processor depends on the application and compute power requirement, it can be a microcontroller, an FPGA, a high-performance microprocessor, or a combination of these. The secure connect block is where the user and network interface interact with the motor drive. Connectivity and security are becoming critical elements of the motor drive to enable communication with the outside world. Since motor drives operate in harsh noisy environments for reliable operation, it is essential to design connectivity solutions with added noise immunity and robustness. With increased connectivity comes the need for added security at the device level. Manufacturers have to protect production assets from malware attacks, and untrusted commands to ensure secure boot and trusted execution, ensuring secure communication across the network.
The next blog in this series will start to focus on key individual elements of the motor drive architecture.
This blog looks at how motor control architectures and functions are evolving to meet the requirements of Industry 4.0 – from simple grid-connected motors to complex multi-axis servo drive solutions. This evolution has been accelerated by the increasing complexity of automation required to deliver the higher levels of productivity, flexibility, and autonomy required by the rapid digitalization of consumption, supply chains, and smart manufacturing [1]. The demand for agile production and greater access to production data have both contributed to the increasing complexity of motor control systems. The previous blog in this series looked at the importance of making motors more sustainable.
Agile Production
As industries are adapting to keep up with consumer demand and changing buyer behaviors, agile production, based on reconfigurable production lines, is required to deliver more customization and faster turn-around times. Consumer demand is driving a shift away from low-mix, high-volume manufacturing toward high-mix, low-volume manufacturing, which demands greater flexibility on the factory floor. Complex, repetitive, and often dangerous tasks can now be performed by industrial and collaborative robots, leading to higher throughput and increased productivity. This shift in production patterns has resulted in a higher demand for more complex, customizable, autonomous, and intelligent automation hardware, at the heart of which are motor control systems. For example, systems such as conveyors and handling machines are required to be smarter, have more axes of motion, and be highly configurable.
Access to Production Data
Global spending on digital transformation will reach $6.8 trillion globally by 2023 [2]. Variable speed drives can provide access to machinery data in the form of voltages, currents, position, temperature, power, and energy consumption combined with external sensors for monitoring vibration, and other process variables. With a converged information technology/operating technology (IT/OT) Ethernet network motor and machine data and insights are now more accessible and can be analyzed by powerful cloud computing and Artificial Intelligence (AI) to optimize manufacturing flows and monitor the current state of health of the assets across the entire installation. This optimization of manufacturing flows will further reduce energy consumption in smart manufacturing.
Figure 1: Digital transformation enabled by seamless Industrial Ethernet connectivity.
To deliver this, motion control systems are moving towards a converged connectivity scenario, where legacy, mixed Industrial Ethernet communication protocols are being gradually displaced by Time-Sensitive Networking (TSN)-based systems, standardized around the IEEE802.1 network standards [3], as illustrated in Figure 1. This will enable time-synchronized data from end equipment to be available across all areas of the network in real-time.
A Spectrum of Motor Control Solutions
Ultimately, modern production systems will include a mix of simpler and more complex motor control systems, as those highlighted in Figure 2, with the grid-connected and simple inverter drives being replaced more and more by interconnected, sensor-integrated, synchronized systems. Each of these “flavors” of motor control is described briefly here, along with the applications served by them.
Figure 2: The range of motor control solutions used in modern automation
Grid-Connected Motor: These basic motion solutions run at a relatively fixed speed. True constant-speed applications are fairly limited in industry, but these will continue to be used in situations where motor operation is very intermittent, and the cost of a VSD is not justified (e.g. low power, intermittently used blowers, pumps, valves, actuators).
Inverter Driven Motor: The addition of a simple open-loop inverter to the motor control enables significant reductions in energy consumption by running the motor at the optimum speed for the load and application. Pumps, fans, blowers, and cruder control systems, such as platform movers are typical applications.
Variable Speed Drive: For higher-performance motion control applications, a variable speed drive (VSD) enables accurate torque, velocity, and position control. To achieve this, current and position measurements are added to the basic open-loop inverter drive. Conveyors, winding, printing, and extrusion machinery are typical examples of applications needing VSDs.
Servo-Driven System: Synchronized, multi-axis servo-driven systems are used in more complex motion applications. Machine tools and CNC machines require synchronization of multiple axes, with extremely accurate position feedback. Precision machining and additive manufacturing are key applications that utilize multi-axis servo drives.
Industrial Robot/Collaborative Robot/Mobile Robots: Industrial robots require multi-axis servo drives combined with mechanical integration and advanced machine control algorithms to achieve complex 3D spatial positioning. Collaborative robots (cobots) build on industrial robotic solutions by adding power and force limiting (PFL) and safety sensing to deliver functionally safe, multi-axis machine control where an operator can work safely alongside the cobot. Mobile robots add localization sensing and collision avoidance. The applications served by robotics systems are growing, from traditional automotive manufacturing to handling, palletizing, pick-and-place, packing, and logistics.
In the next blog of this series, we will begin to look into the architecture of the motor drive itself and start breaking it down into its subsystems and components.
References
[2] Shawn Fitzgerald, Daniel-Zoe Jimenez, Serge Findling, Yukiharu Yorifuji, Megha Kumar, Lianfeng Wu, Giulia Carosella, Sandra Ng, Robert Parker, Philip Carter, and Meredith Whalen. “IDC FutureScape: Worldwide Digital Transformation 2021 Predictions.” IDC, October 2020.
This blog series will look at motor drives from many different angles - from architectures to power management, from connectivity to cybersecurity, and much more in between. We are starting from the top down and this first blog looks at how making motors more energy efficient can play a significant role in reducing the harmful emissions which are contributing to climate change.
The Paris Agreement in 2015 set out a plan to limit global warming to 1.5°C by 2050. Meeting the 1.5°C target in 2050 requires an approximately 70% reduction in CO2 emissions from 2018 levels. The current trajectory of global warming has the potential to cause major economic, societal, and environmental disruptions. The world has already warmed by 1.1°C and experts say that it is likely to breach 1.5°C in the 2030s.
Figure 1 outlines a path to the 1.5°C target by reducing CO2 emissions to under 10 Gt CO2, as covered in World Energy Outlook 2019 [1]. In that report, the International Energy Agency (IEA) looks at two scenarios for the trajectory of global emissions. The first is the Stated Policies Scenario, which estimates emissions based on publicly announced government policies. The second is the Sustainable Development Scenario, which looks at additional paths to mitigating emissions. The largest opportunity to reduce CO2 emissions as part of the IEA’s Sustainable Development Scenario is gains in energy efficiency, accounting for 37% of the Sustainable Development Scenario’s reductions when compared to the Stated Policies scenario With 25% of CO2 emissions coming from industry in 2022 [2], accelerating industrial energy efficiency investments will be an important part of the path to net zero emissions in 2050.
Figure 1: Path to CO2 emissions reductions [1]
The global electricity supply in 2022 was 28,642 terawatt-hours, contributing 13.2 Gt of carbon emissions [3]. Industry consumes an estimated 30% of global electricity and within industry, electric motors make up approximately 70% of electricity consumption [1]. Clearly, the efficiency of these components is a potentially critical contribution to the efficiency savings identified in Figure 1. The most basic and lowest efficiency motion solutions are based on direct AC grid-connected 3-phase motors that use switchgear to provide on/off control and basic protection. These motion solutions run at a relatively fixed speed, independent of any load variation. Adjustments in output variables (such as fluid flow in pumps and fans) are implemented with mechanical controls such as throttles, dampers, and valves, whereas significant speed changes are implemented with gears.
Figure 2: The part played by Industrial Motors in global energy usage
The addition of a rectifier, DC bus, and a 3-phase inverter stage, as illustrated in Figure 3(b) creates an inverter with variable frequency and a variable voltage output that is applied to the motor to enable variable speed control. This inverter driven motor significantly reduces system energy consumption by running the motor at the optimum speed for the load and application. Examples include higher efficiency pumps and fans. When added to the existing motor of a pump, fan, or compressor, an inverter can potentially reduce power consumption between 25% and 60%, depending on the motor and application[4,5]. For higher performance motion control applications, a VSD (Figure 3(c)) enables accurate torque, velocity, and position control.
Figure 3: (a) Grid-connected AC motor (b) Inverter fed motor (c) Variable speed motor drive
It is estimated that only about 1-in-6 of all deployed motors in the industry are inverter driven or connected to a VSD[6]. By moving more deployed motion assets from grid-connected motors to inverter-driven or VSDs, it is possible to significantly reduce the energy consumption and CO. These reductions in energy consumption would enable more sustainable manufacturing with reduced CO2 emissions. It has been estimated that if all deployed motor driven systems were operated at maximum efficiency, it would reduce global electricity demand by 10% and remove 2490 Mt of CO2 emission in 2030 [7].
To accelerate the deployment of higher efficiency motor driven systems, the International Electrotechnical Commission (IEC) has contributed to the definition of energy efficient electric motor standards. This includes the IEC 60034-2-1 test standard for electric motors and the IEC 60034-30-1 classification scheme comprised of four levels of motor efficiency (IE1 through IE4), with the IE5 level due to be introduced in the future. The higher-level standards can be achieved either through more efficient motor design or through the addition of an inverter or VSD to a standard motor design. As the efficiency classes become more stringent, meeting them by improved motor design alone is becoming more and more challenging, and costly. Alongside the additional benefits provided by variable speed control in the application, the case for VSD-attach to most industrial motors is becoming more compelling.
The next blog in this series will highlight the other trends in industrial automation – apart from energy efficiency – that have been driving the evolution of motion control away from simpler grid-connected machines to highly controllable multi-axis VSD-based systems.
[1] World Energy Outlook 2019. International Energy Agency, 2019.
[2] World Energy Outlook 2022, International Energy Agency.
[2] Electricity Market Report 2023. International Energy Agency, February 2023.
[4] Achieving the Paris Agreement: The vital role of high-efficiency motors and drives in reducing energy consumption. ABB, 2021.
[5] Applications of variable speed drive (VSD) in electrical motors energy savings. R. Saidur, S. Mekhilef, M.B. Ali, A. Safari, H.A. Mohammed, Renewable and Sustainable Energy Reviews, Jan 2012.
[6] U.S. Industrial and Commercial Motor System Market Assessment Report, Volume 1. Lawrence Berkeley National Laboratory, January 2021
[7] Energy-Efficiency Policy Opportunities for Electric Motor-Driven Systems. International Energy Agency 2011.
Guaranteeing the authenticity of an edge device and the data it provides is increasingly critical as the potential for security threats grows. In the fifth blog of our remote IO blog series, we will discuss how cybersecurity is a topic of concern within the industry as connectivity and IP addressability opens up the possibility of hacking and system attacks. With edge-to-cloud connected factories generating larger volumes of data in remote locations across the factory, automation vendors must consider how to overcome several new design challenges which weren’t a problem years ago.
Why is security a problem now?
We all have heard about the connected factory of the future; well, the future is here today with all types of sensors and actuators connected together and sharing information to help the factory run as efficiently and as flexible as possible. Realizing Industry 4.0 involves increased access to edge devices in the factory and accessibility in controlling them. The resulting transparency and data insights can reduce network planning, CapEx and OpEx. Increased bandwidth and optimized machine interworking are also enabled. With that, the famous quote from the Spiderman movie comes to mind, “With great power, there must also come great responsibility.” Similarly, you cannot have all this interconnectivity that provides the power to make smarter decisions without considering the consequences.
Providing a data path from edge to cloud introduces new threat vectors, meaning that cyber security risks to Industrial Control Systems (ICS) must be reassessed. ICS cyber security solutions need to adapt to address the changing risk, and traditional countermeasures applied to the system, such as firewalls and placing a device behind a locked door, are counterintuitive to the goals of Industry 4.0. This means devices will need to be security hardened to enable increased functionality in a secure method. Identity and integrity will be at the core of every device in the field to enable trusted data and secure operation.
System designers for automation vendors, and system integrators may encounter the following issues as they are planning their system’s design (see figures 1 & 2).
Figure 1- Common Design Challenges Parts 1 - 4
Authenticity
Third-party clones or counterfeit components could enter the system and pose safety risks while not meeting the customer’s expectations and specifications, affecting system integrity and reliability.
Figure 2 - Common Design Challenges Parts 5 – 8
IoT security
A variety of security techniques to protect against malware attacking an entire infrastructure that has not been properly secured.
Figure 3 - Moving Security Closer to the Edge where the Data is Born
Is there a solution?
The ultra-low power MAXQ1065 Cryptographic Controller with ChipDNATM is an ideal security coprocessor for embedded devices. It can provide turnkey cryptographic functions for root-of-trust, mutual authentication, data confidentiality and integrity, secure boot, secure firmware update, and secure communications with generic key exchange and bulk encryption or complete TLS support.
Figure 4 - MAXQ1065 Block Diagram
DeepCover® embedded security solutions cloak sensitive data under multiple layers of advanced security including active die shield, encrypted storage of keys using the ChipDNATM PUF technology, and externally callable algorithmic subroutines. Along with this, the MAXQ1065 has 8 KB of secure flash memory that can be used for keys, secrets, certificates, or user data.
The encryption (ECDSA, AES-256, SHA-256) and secure boot ROM features of the MAX32672 MCU also protect against intrusion and support the secure root of trust to ensure no malicious execution.
How does this secure the system?
The root of trust begins with safe key management. Sophisticated invasive attacks are frequently launched in an attempt to obtain cryptographic keys from secure ICs. If obtained, the security provided by the IC is thoroughly compromised. A key derived from a physically unclonable function (PUF) provides an unprecedented level of protection against invasive attacks, since the PUF-based key does not exist in memory or another static state.
Attackers can’t steal a key that isn’t there. ChipDNA provides symmetric key for encryption and decryption, as well as private keys for ECDSA and/or ECDHE. ChipDNA derives a cryptographic key from a precise analog characteristic of the IC. This key is based on a physically unclonable function and is not stored in memory or any other static state where it could be compromised. Each IC’s unique ChipDNA-generated key, since it is based on a physical unclonable function, is repeatable over temperature, voltage, and IC operating life conditions.
With legislative requirements such as the upcoming European Cyber Resilience Act (CRA), security will not only become more important, but mandated, for manufacturers of factory automation equipment/modules.
References
