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The inverter stage is the “muscle” of the drive – a power electronics block that provides the regulated, conditioned power directly to the motor, driving it in the manner required by the end application, providing the amperes needed for torque production, the voltage needed for speed and magnetic flux regulation, and the frequency and phase relationships required for control of the speed and torque in the most efficient manner.
In a previous blog, we finished our consideration of the power supply functions within the variable speed drive (VSD). In this blog, we move into the heart of the drive – the inverter stage. This is highlighted in Figure 1.
Figure 1: Detailed Variable Speed Drive Architecture
The inverter stage fundamentally has two sets of inputs and one set of outputs. The main power input is the DC bus (discussed in the previous blog on the input stage). The main power outputs are the three-phase lines to the motor. The main control inputs are the gate signals to each of the switching power transistors in each leg of the inverter. There are various flavors of an inverter with different numbers of phases, and different power electronics topologies (multi-level, matrix, etc) but the vast majority of inverters on the market are three-phase, two-level inverters as shown in Figure 2.
Figure 2: Inverter Stage Driving Three-Phase Motor
The power transistors in each leg of the inverter are power-switching devices that turn fully on or fully off at a high frequency (usually in the range of 5-20kHz) and a controlled duty cycle or modulation index. They act as quasi-ideal switches that modulate the voltage applied at each motor phase winding and re-create a waveform with low-frequency components (typically sinusoidal) related to the motor velocity (typically in the range DC-1kHz) and instantaneous position and an averaged voltage amplitude related to the motor velocity and rated magnetic flux. This is illustrated in Figure 3. The output inverter phase-to-negative voltage is a pulse width modulated square wave switching between the DC bus voltage and zero. The inherent inductance of the motor windings will filter this signal to result in a motor current at the required low-frequency phase, frequency, and amplitude, with some undesirable switching frequency ripple present.
Figure 3: Inverter phase output voltage and current
The power transistors and associated thermal management (heat sinks, fans) are usually amongst the most expensive components in a VSD and also tend to take up the most space, especially at higher power levels. Power losses in the transistors come from the fact that they are not ideal switches, have a small voltage drop across them when conducting current, and also have non-zero turn-on and turn-off switching times when high voltage and high current within the switch overlap for short periods resulting in switching losses. Historically MOSFETs and IGBTs have dominated the power transistor market, but in recent years, there has been a trend to look to wider band-gap SiC and GaN-based power transistors. These can offer lower switching losses, which can allow for higher switching frequencies or more efficient inverters at the same switching frequency. The incentive for VSD designers to move to SiC and GaN is not quite as compelling as in other application spaces, where moving to higher switching frequencies allows for smaller filters. In VSDs, the filter is the motor winding, so in many applications, a higher switching frequency does not give a huge advantage.
Gate drivers are responsible for converting the logic level signals from the motor controller to signals that have the voltage amplitude and current drive capable of fully controlling the power transistors, and for ensuring that those drive signals are correctly referenced, as illustrated in Figure 4.
Figure 4: Switching function of a gate driver
Other functions provided by the gate drivers can be:
So gate drivers can be very complex devices with full communication interfaces (e.g. ADuM4177), or quite simple devices with only driving functionality (e.g. MAX22700). Whatever features are included in the gate driver, robustness to transients (expressed in specifications such as Common Mode Transient Immunity – CMTI) is an important feature as these devices live in a very noisy domain in the inverter, and need to be able to sit between the quiet, low voltage environment of the motor controller, and the high voltage, high current, noisy domain of the power transistors.
The power inverter is the heart of the VSD and manages the currents and voltages applied to the motor. Safe, robust, efficient switching of the power transistors within the power inverter is an important function of the gate drivers within a VSD. The next blog will consider some of the signals that are measured within the inverter stage to accurately control its operation.
We addressed the requirements and circuits needed within a variable speed drive (VSD) to provide the main control power supply and the power supplies for the inverter electronics – namely gate drivers and current sense circuits in the previous blog in the series.
These are highlighted once again in graphical form in Figure 1, which shows a generalized view of a typical VSD architecture.
Figure 1: Detailed Variable Speed Drive Architecture
This blog addresses two more important power supply topics within the highlighted block of the VSD in Figure 1:
Protection of the main control power supply (usually 24V) from transients and anomaly conditions is extremely important within a VSD. The reason for this is that there are generally two paths for this supply rail. The first path is that described in the last blog in which the 24V control power supply is derived from the AC mains, or high voltage DC bus via an isolated power conversion stage. However, this 24V rail can also usually be supplied directly from an auxiliary power input connected to the VSD. Most automation environments distribute 24V supplies around the control cabinets for a range of control equipment, and the VSD usually taps into this supply. This auxiliary 24V input is usually diode OR’ed with the converter-derived 24V supply, to ensure that only one supply source is active at a time. The advantage of having this available is that even if the AC mains drops out or trips, usually the auxiliary 24V supply will keep running, meaning the VSD’s control electronics can stay energized, resulting in the capability to quickly restart the VSD and not lose any context information. The disadvantage is that the VSD power supplies are now exposed to all of the voltage transients existing on the factory-wide 24V supply system – these can occur through switching loads that excite voltage ringing on distribution cables, lightning events that propagate through the system, electrostatic discharge from nearby electric fields or human contact, as well as electromagnetic interference picked up on cables. Hence the a need for solid protection on this auxiliary supply input.
The VSD functional safety standard IEC 61800-5-2 Annex A (Section B.3.2) shows a recommended Power Supply (PS) and Voltage Monitor (VM) subsystem for such a 24VDC rail. It consists of protection against a variety of common faults such as reverse polarity, overcurrent, and voltage transients.
Figure 2: Recommended auxiliary supply protection per IEC 61800-5-2
As the transients involved can vary in energy level and duration, designing a discrete circuit can be time-consuming and prone to error, and may have poor long-term reliability. Integrated solutions such as Analog Devices Surge Stopper family integrate all of the IEC 61800 requirements, including input reverse polarity detection and protection, overcurrent, short circuit and inrush current detection and protection, and high voltage transient disconnect.
Figure 3: Analog Devices’ Surge Stopper implementation of IEC 61800-5-2
Figure 3 depicts the use of surge stopper solutions from Analog Devices as a potential implementation of IEC61800-5-2 protection recommendations. Components such as the LT4363 or the integrated MOSFET variant LTC4381 can provide robust and reliable protection solutions for the main control power supply.
Downstream of the main control power supply sit the low voltage (<12V) power rails. These are by and large the point-of-load (POL) supplies that directly power the main controller devices (CPU, FPGA), other digital components (memory, transceivers, interfaces) analog circuitry (ADC, DAC, op-amps, etc), and I/O devices and terminals. The specific design of these power architectures is unique to each VSD, and it is difficult to speak in generalizations, but the typical approaches to these power supplies are illustrated in Figure 4 and Figure 5. In Figure 4, single-stage switching regulators are utilized for some key low voltage rails, with downstream low-dropout (LDO) regulator being added for noise-sensitive analog rails or very low current rails at lower voltage levels.
Figure 4: Single-stage switcher approach
In Figure 5, a single intermediate rail is generated (in this case 5V) and this is further stepped down to individual rails as needed by the POL devices. Also shown here, is the potential need for startup sequencing or tracking of supply rails, which can be important in the complex multi-controller systems used in many VSDs.
Figure 5: Multi-stage switcher approach
POL power supply architecture design is complex and involves many tradeoffs related to overall efficiency, cost, space, and noise. Tools such as LTpowerCAD, LTpowerPlanner, and LTSPICE, from Analog Devices can significantly ease the design challenge.
This blog has completed the topic of power supplies within the VSD, underlining the importance of protection circuits for the main control power supply, and touching on the challenges related to POL low voltage supply design. In the next blog post, we will start looking at the main DC/AC inverter stage that implements the power conversion to control the motor itself.
The last blog addressed the AC input stage of the motor drive. This is primarily an AC-DC power conversion stage where the AC input power is converted to a DC level that provides the input to the main three-phase inverter. Several other power domains are required in the drive apart from the high-voltage inverter power domain. Managing these power domains, providing the appropriate ground referencing and isolation, and protecting them from external transients is a critical task of the variable speed drive designer. This blog examines the topic of power management in a variable speed drive and looks at some of the requirements that exist in the different power domains. In the last blog, we introduced a typical architecture diagram, shown here again in Figure 1. This section of the drive under consideration is highlighted.
Figure 1: Detailed Variable Speed Drive Architecture
Firstly, let’s identify the main power supplies within a variable-speed drive. These are:
Each of these power supplies carries out a unique function and has certain requirements associated with it. In this blog, we will look at the first two of these power supply sub-systems.
Main Control Power
The main control power supply is usually a 24V DC bus (although it can sometimes be a 12V bus). This power supply has several functions. It acts as the intermediate DC voltage from which the low-voltage rails are distributed. It can also be used to directly supply the power for any cooling fans that may needed within the drive housing or for other higher voltage loads such as motor brakes, as illustrated in Figure 2.
Figure 2: Main Control Supply
This is mostly derived from the main high-voltage DC bus via an isolated DC-DC converter. In some cases, it can be derived directly from one of the AC input phases. In both of these cases, a flyback converter is a popular choice of power topology. Flybacks are not the nicest power converters to work with! They are quite noisy, and it can be challenging to provide tight control of the output voltage. However, they have some major plusses associated with them in their simplicity (one transistor, one magnetic component) and low cost. In the context of variable speed drives, which can be highly competitive in terms of cost, and which are already huge noise generators, the disadvantages of flyback converters are mostly outweighed by their advantages. An example is shown in Figure 3 of a solution from Analog Devices which has the advantage of providing good control of the output voltage without requiring an optocoupler to provide output voltage feedback to the controller, a “no-opto flyback”. In higher power variable speed drives, a flyback may not be suitable and in this case, half-bridge or full-bridge isolated topologies are generally chosen (e.g. LLC converter).
Figure 3: No-Opto Flyback Converter Example
Inverter Isolated Power Supplies
Isolated power supplies are needed for each of the gate driver units, which are responsible for controlling the inverter power switches. Gate drivers and the inverter will be covered in a later blog but for this blog, the requirement is typically three 12-20V supply rails (sometimes with a negative rail also), referenced to the three motor phase switching nodes and 1 low side supply referenced to DC bus negative rail. The “grounds” for these power supplies are not tied to safety earth, or even to a quiet ground reference, and are rapidly moving over hundreds of volts in the case of the gate driver supplies – hence the need for isolation. Overall this usually translates into four isolated DC-DC converters, normally from the intermediate DC bus to the gate driver supply. These isolated voltage rails are designed to provide the switching voltage levels for the power transistors used in the VSD – whether they are power MOSFETs, IGBTs, or SiC/GaN devices. Any current sensors connected in the motor phases, will also generally require isolated supply voltages – these are often derived directly from the gate driver supplies using an LDO (low dropout) regulator as shown in Figure 4.
Figure 4: Inverter-isolated Supplies
Once again, multi-output flybacks can be a solution for these supplies, although these tend to have quite complex and bulky transformer designs due to the need for so many outputs.
Summary
The power supplies within a variable-speed drive fulfill different functions that are fundamental to the overall operation of the drive. Isolation for reasons of electrical safety, and galvanic separation of voltage reference levels is an important factor in many of these power supplies. The next blog post will continue this topic and will look at the power supplies needed for the various control and output circuitry, as well as the protection needs of the power supply sub-system.
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.
