1. Introduction
By matching motor speed and torque to the demands of the driven load, variable speed drives (VSDs) have provided unprecedented performance for motors, saving significant amounts of energy. Most VSDs on the market rely on a modulation stage. While it can regulate the voltage and frequency input to the motor, it introduces an inherent delay in process control signals. In contrast, superior ABB drives employ innovative Direct Torque Control (DTC), greatly enhancing the motor's torque response. Other benefits offered by DTC technology include system-level functionality.
ABB's high-performance AC drives offer a technology called Direct Torque Control (DTC). As the name suggests, this method directly controls the motor's flux and torque, rather than attempting to indirectly control the motor current as AC vector drives and DC drives do. This means a higher degree of matching with the load demands of the driven system. DTC originated from one of ABB's founding companies and was patented in the mid-1980s. It eliminates the need for an additional modulation stage, thus enabling control power close to the theoretical maximum.
When ABB launched its first Direct Torque Control (DTC) AC drive in 1995, DTC was already a leading technology. Subsequent improvements in processor computing power, application programming, and communication interfaces have continuously enhanced DTC performance, providing high-quality motor control for a wide range of applications.
Figure 1
2. Why choose DTC?
In addition to superior torque response, other customer benefits offered by DTC include:
In 95% of applications, motor speed or position feedback is not required. Therefore, the installation of expensive encoders or other feedback devices can be avoided.
§DTC control can be used for different types of motors, including permanent magnet motors and newer synchronous reluctance motors.
§ Reduce speed to low speed through precise torque and speed control, and reduce speed to zero speed through full starting torque.
§ Excellent torque linearity.
§High static and dynamic speed accuracy.
§No preset switching frequency. The optimal transistor switching scheme is determined for each control cycle, making it easier to match the drive and driven load requirements.
3. Overdrive induction motor
Due to the widespread use of AC induction motors in numerous industrial and commercial applications, DTC (Distance Torque Control) was originally developed for AC induction motors. The torque response time (achieving 100% torque given step) is an indicator of the DTC performance of an induction motor, closely approximating the motor's electrical time constant limits. The uncertainty in torque repeatability under the same given command is typically as low as 1% across the entire speed range of the drive.
The pursuit of higher power density, along with the ever-evolving international efficiency standards, has reignited interest in other motor topologies. The good news is that DTC has been extended to work with permanent magnet (PM) synchronous motors and the new synchronous reluctance (SynRM) motors. A key difference with DTC lies in the starting of motors controlled by ABB drives. Compared to induction motors, efficiency gains are extended across a wider torque-speed range because PM and SynRM motors lack rotor windings and slip speed.
The high torque-to-size ratio of these motors allows for simpler drivetrain designs. For example, direct-drive low-speed PM motors avoid the use of packaging machine gears. One drawback of PM synchronous motors is their reliance on so-called rare-earth magnets for optimal performance. This is why SynRMs are used as an alternative – they do not use permanent magnets.
4. Performance Testing
In mid-2012, ABB conducted a series of measurements to ensure the continuous improvement of DTC technology in order to maintain peak performance of AC drives. The following are some of the notable results.
Torque stability is close to zero speed
The torque control accuracy of ABB's ACS800 drive and the new ACS880 industrial drive was compared in sensorless (open circuit) operation mode. Each drive operated a 15kW four-pole induction motor at rated torque and controlled the load machine to reverse at low speed near zero speed.
Figure 2
In the near-zero speed range, both drives demonstrated outstanding sensorless control capabilities over extended periods of operation, but the new ACS880 exhibits a smaller deviation from the torque setpoint, thus providing better motor control performance.
Torque accuracy during uphill climbing
The sensorless torque control accuracy of the ACS880 drive was compared (at 50% of rated speed) using a four-pole induction motor and a 15kW synchronous reluctance motor. For both motor types, DTC kept the torque error within a small percentage of the rated torque.
Figure 3
The maximum torque error of a synchronous reluctance motor is slightly lower than that of an induction motor.
servo-level dynamic performance
The speed and angular position of a 1.5 Nm, 6,000 rpm PM synchronous motor were measured during speed reversals within ±6,000 rpm and over time periods shorter than 25 milliseconds (ms). This is very close to the motor's mechanical time constant of 24 ms and theoretical limits.
Figure 4
Although the ACS880 is not a servo drive, the DTC can perform fast and accurate motor acceleration simultaneously in both closed-loop and sensorless control modes. By comparing the measured acceleration time and motor mechanical time, a method for measuring torque accuracy during extremely fast acceleration is provided.
5. DTC Today and Tomorrow
With updates to DTC, its performance capabilities have surpassed the high-demand, high-dynamic applications that were its original design goals. Improvements in software and the commonality of high-power microprocessors make wider implementation of DTC drives economically viable. The ability to rapidly respond to changes in process variables such as pressure, tension, or position using superior speed and torque control makes DTC attractive to a wider range of industries.
Furthermore, AC drives offer tremendous energy-saving potential for a wide range of variable-speed pump/fan applications. Because pump speed and power have a cubic relationship, if a process allows the pump to operate at 50% speed, only 1/8 of its full power can be used.
Over its nearly 30-year lifespan, Direct Torque Control (DTC) has undergone continuous hardware and software development. Built upon a solid theoretical foundation and DSP technology, DTC has overcome the limitations of earlier processors. Currently, powerful processors can quickly execute complex control algorithms, update motor model parameters, and switch drive transistors to achieve optimal performance.
ABB's respected tradition of drivetrain engineering and its vast resources have been invested in the development of Direct Torque Control (DTC). DTC remains a dynamic technology. By integrating intelligent user interfaces, drivetrain diagnostic features, and system-level software, it has evolved into a brand that transcends simply "torque control."
Looking to the future, ABB will follow the same path it has taken with enduring DTC technology. ABB Drives customers can be confident that the benefits they are currently reaping from their investment in direct torque control will continue for a long time to come.
