The hottest AC drives are still crucial in the 21s

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AC drives are still crucial in the 21st century

the tradition of variable frequency drives (VFDs) with smaller volume, more intelligence, higher efficiency and lower cost has lasted for 50 years. With the higher and higher requirements for its performance, this development trend will become more obvious

in the past 50 years, you can see that the overall dimensions and weight of variable frequency drives (VFDs) have changed significantly. However, the more significant change under this appearance is the improvement of performance, efficiency and reliability of these motor drivers. The reason for these changes is the improvement of power switching transistors, microprocessors, other hardware and software functions, which reduces the energy required by users for driver application and maintenance

the early AC drives were open-loop operation with limited functions. A major progress was that Felix Blaschke of Siemens developed the field oriented (flux vector) control technology for induction motors in 1971. With the efforts of others, the variable frequency drives finally achieved or exceeded the performance of DC drives in many applications. Speed sensorless vector control (excluding the shaft angle encoder) and other driving algorithms also appeared later. Moreover, these improvements are still accelerating

william L. siner, product manager of Rockwell Automation, noted that two historic changes have affected the power and control of variable frequency drives respectively. Early AC drivers (1980s) used multiple transistors per phase due to the limited voltage and current levels of transistors. Now it has become an integrated package, so the volume of a 10 hp (1 HP = 0.745kw) driver today is smaller than that of a transistor unit in the driver in the past. "In order for manufacturers to develop smaller and more efficient power devices, the performance of the new generation of transistors still needs to be improved," said siner. Insulated gate bipolar transistors (IGBTs) are still the main power devices

in terms of control, analog control was dominant in the initial stage and is now giving way to digital control, although digital control was only based on integrated circuits at first. Siner also mentioned that the microprocessor-based digital driver appeared later and only provided open-loop (v/hz) control at the beginning. With the continuous development of microprocessor unit, a variety of control modes are added to a driver, and the control mode can be changed only by changing software parameters

multiple control types and interconnectivity

multiple control modes represent the current development level in the field of variable frequency drives. The typical low-end driver provides v/hz and speed sensorless vector control mode, while the high-end driver initially adopts flux vector control, and later adds other control modes. For example, Rockwell's PowerFlex 700s provides several control modes through a built-in Logix processor, including servo control. Technology migration between product lines is another trend. Siner takes low-end PowerFlex 70 with vector control and some high-end drives with v/hz control as examples

why do high-end products use open-loop control? According to siner, the reason is that the v/hz conversion can enable one driver to control many different types of motors. One type of drive can be used in many different occasions to reduce spare parts inventory

interconnection is another important feature of current variable frequency drives. All drives of Rockwell company are equipped in this way. According to the company's experience, at present, the number of connected drives has reached 50% of the total, and the use of high-end units is increasing. "In the past three years, the percentage of connected drives has doubled," says siner

tom momberger, product manager of Danfoss drives, said: "the application of microprocessor technology to variable frequency drives is the main reason for the improvement of AC drive performance today." For the change of physical appearance, he compared a typical analog 5-hp AC driver produced in 1968 (its oil cooling unit needs to be adjusted manually in different ways to use the driver) with the current volume and weight of the variable frequency driver (see Danfoss' photos of the past 5-hp VLT driver and the load can no longer rise). Of course, many other features are added to the new AC drives, such as programming via the operator keyboard or computer. "Microprocessors have made this possible," says momberger

according to momberger, flexibility, intelligence and friendly user operating environment are the characteristics of the development of variable frequency drive technology. Flexibility means that only one type of driver can provide open-loop, closed-loop, flux vector and even near servo control to meet a variety of applications. "This capability lowers drive owner costs by reducing the cost of onsite inventory, operator training, and spare parts," he said

microprocessor units and advanced diagnostic capabilities allow users to intelligently configure a drive, thus reducing commissioning costs and downtime. The software functions, such as automatic motor adaptation and software wizard, remove the uncertainty in the process of setting up the driver/motor connection. "The friendly operation interface in the software function shortens the setup process to reduce possible operator errors and simplifies the interaction with the driver," momberger explained. Danfoss' new product fc-302 automatic drive has all these features

pwm, DTC and modularization

in the important mileage of AC driver development, abb witnessed the emergence of industrial PWM based driver and the introduction of direct torque control in 1995

ilkka ikonen mentioned at the abb oy driver market exchange meeting in Finland that the industrial installation and use of ABB's first PWM driver appeared in the 1970s. The application of paper mill and Metro has laid a foundation for the progress and robustness of this important product. "After this technology proved its reliability and competitiveness in these applications, people began to accept AC drives as the dominant control technology to replace DC drives," he said

ABB believes that its direct torque control is an advanced technology, which can directly control the torque and speed of the motor without controlling the voltage and frequency respectively. What is worth mentioning is its fast torque response time and control accuracy, "usually 10 times faster than pulse width modulation." Direct torque control can also optimize the motor flux, which can improve the overall energy utilization of the motor and driver. Direct torque control (DTC) does not use a regulator and does not require motor shaft position and speed feedback. "With direct torque control, the torque can reach 100% when the speed is zero, and the micro torque increment can be controlled in less than 1ms at low frequency," ikonen said

today, ABB's AC drives can meet a variety of user needs by "configuring products to order", and its modular design plays an important role. It can meet almost all the needs of buyers, such as delivery time, quality and cost, just like the spot

the reliable switching devices and microprocessors developed by Bosch Rexroth company make the current variable frequency drives smaller, more efficient and more robust. Peter Fischbach, manager of the components department, said: "the design of the power supply and the heat dissipation system has been completely changed based on the thyristor or field effect transistor of the isolated power module and the later insulated gate bipolar transistor, combined with the sinusoidal pulse width modulation control method."

Figure 1: the same series of variable frequency drives of Danfoss company have undergone great changes in more than 30 years of development

vlt 200 includes analog PWM control, while VLT 3000 turns to digital control

igbt, mpu

rexroth released a full range of drive products based on IGBT in 1988. In fact, the company's AC drive business developed as early as 1965, and launched its first high-speed, rack type industrial variable-frequency drive in 1968, which can make the operation speed of induction grinding motor reach 180000rpm

the development and continuous improvement of microprocessor units (the second milestone) enabled Rexroth to produce one of the earliest microprocessor controlled industrial variable frequency drives in 1982. This variable frequency drive is characterized by a dot matrix LCD operator module with keyboard and menu guided setting function, which replaces the setting based on analog potentiometer. In 1989, the updated development led to the emergence of a full series of drives using IGBT and flux vector control. Fischbach said that these variable frequency drives are characterized by maximum starting torque, improved speed control at low speed and feedback control, which reach and exceed the performance of DC drives

jim Thompson, driver design engineer of Emerson control technologies (CT), believes that the early variable frequency drives were limited by the use of silicon controlled rectifier and rather complex six step control. "The power inverter using silicon controlled rectifier is very large and needs to be equipped with complex commutation circuit, including many inductors and capacitors," he said. "The six step output generates high-order harmonics in the motor, resulting in unwanted additional heating, and this scheme can not control the motor current in a fast state" - it needs a driver with higher performance to solve it

in addition to increasing the switching speed of power devices, IGBT also allows rapid adjustment of motor operating voltage. "This makes the field oriented control (vector control) with quite high bandwidth feasible, and can quickly and accurately control the speed profiling and positioning," Thompson explained. The high cost of electronic control also limits the performance of early AC variable frequency drives. "Digital control was not very practical at that time, mainly because it was necessary to add a huge system level circuit (or computer) auxiliary device to the drive module." He added

Figure 2: the progress of variable frequency drive technology is shown in that the new 2HP PowerFlex 4 drive of Rockwell Automation

is 70% smaller than the 1332 AC drive (left) with the same power level produced in 1985

powerflex 4 weighs only 2.2 pounds and can be placed in the palm of the hand, while 1332 weighs 16.8 pounds

emerson CT believes that the fast PWM output is the main feature of the variable frequency drive, because it produces the minimum harmonic current and can dynamically control the motor torque. The feature of rich configuration functions makes modern AC drives more distinctive. Typical optional features include adjusting speed or torque, accepting a variety of analog or digital reference voltages, feedback speed or torque, and controlling synchronous (servo) and induction motors. According to Thompson, relatively inexpensive and scalable selection modules are another popular feature, including the provision of additional i/o ports, feedback or communication functions

the early development direction of DC driver is variable speed motor control. YASKAWA has a long history of research in the field of DC and AC motors. Dr. Tsuneo Kume, American R & D director of Yaskawa, said that the variable voltage/frequency control was realized by using thyristors and gate turn-off power switching devices, which made great progress in the industrial field of AC drives in the 1970s. According to the data of Yaskawa (and other companies), the main industrial breakthrough of variable frequency drives is to deal with them in steel plants

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