Ere Jääskeläinen outlines the benefits of direct torque control (DTC) technology.
Variable-speed drives (VSDs) have enabled unprecedented performance in electric motors and delivered dramatic energy savings by matching motor speed and torque to the driven load requirements.
Most VSDs in the market rely on a modulator stage that conditions voltage and frequency inputs to the motor, but causes inherent time delay in processing control signals.
In contrast, premium ABB drives employ innovative direct torque control (DTC) - greatly increasing motor torque response. DTC technology also provides other benefits ranging up to system-level features.
High-performance AC drives from ABB offer an innovative technology called direct torque control (DTC). As the name suggests, the method controls motor flux and torque directly, instead of trying to control motor currents indirectly like AC vector drives and DC drives. This means better accuracy in matching the driven system's load requirements. Originated by one of the founding companies of ABB and patented in the mid-1980s, DTC eliminates the need for an extra modulator stage thus achieving control dynamics close to the theoretical maximum.
When ABB introduced its first direct torque control AC drive to the market in 1995, DTC was already a leading technology. Subsequent improvements in processor computational power, application programming, and communication interfaces have continually enabled higher DTC performance, providing premium motor control for a broad range of applications. DTC's operation and other technology details are shown in Fig.1.
Why use DTC?
Besides superior torque response, DTC offers other customer benefits, including:
- No need for motor speed or position feedback in 95 per cent of applications. Thus, installation of costly encoders or other feedback devices can be avoided.
- DTC control available for different types of motors, including permanent magnet and the new synchronous reluctance motors.
- Accurate torque and speed control down to low speeds, as well as full startup torque down to zero speed.
- Excellent torque linearity.
- High static and dynamic speed accuracy.
- No preset switching frequency. Optimal transistor switching is determined for every control cycle, allowing the drive to more readily match driven load requirements.
DTC was originally developed for AC induction motors because of their popularity in myriad industrial and commercial applications. Indicative of DTC performance with induction motors is torque response time (to a 100 per cent torque reference step) that approaches the motor's electrical time constant limit. Torque repeatability uncertainty under the same reference command is typically as low as 1 per cent of nominal torque across the drive's speed range.
Beyond induction motors
Demand for higher power density and evolving international efficiency regulations have renewed interest in other motor topologies. The good news is that DTC has been extended to work with permanent magnet (PM) synchronous and new synchronous reluctance (SynRM) motors. The main difference for DTC occurs during motor starting, which ABB drives accommodate. Absence of rotor windings and slip-speed in PM synchronous and SynRM motors extends their efficiency gains over a wider torque-speed range compared to induction motors.
High torque/size ratio of these motors may allow a simpler drive train design. For example, a direct-driven low speed PM motor could eliminate a packaging machine gearbox. One drawback of PM synchronous motors is their reliance on so-called rare-earth magnets for best performance. Here is where SynRM motors provide an alternative - because they don't use permanent magnets.
Recent performance validation
In mid-2012, ABB conducted a series of measurements to ensure that continuing improvements made to DTC technology are keeping its AC drives at peak performance. Here are some significant results:
- Torque stability near zero speed. ABB's ACS800 and new ACS880 industrial drives were compared for torque control accuracy in sensorless (open-loop) operating mode. Each drive ran a 15kW, four-pole induction motor at its rated torque reference and with the load machine controlled to make slow speed reversals near zero speed. Fig. 2. shows customer benefits.
- Torque accuracy during ramping. ACS880 drive's sensorless torque control accuracy was compared using a four-pole induction motor and a synchronous reluctance motor of 15kW size (at 50 per cent of rated speed). DTC held torque error to only a small percent of rated torque for both motor types (Fig.3).
- Servo-class dynamic performance. Speed and angular position of a 1.5Nm, 6,000rpm PM synchronous motor were measured during speed reversal between +/-6,000rpm in less than 25 milliseconds (ms). This is very close to the motor's mechanical time constant of 24ms and a theoretical limit (Fig.4).
DTC today, tomorrow
Updates over time have allowed DTC to reach beyond demanding, high-dynamic applications for which it was created. Software improvements and the commonality of high-power microprocessors make DTC drives economically justified for wider implementation. Ability to respond rapidly to changes in process variables, such as pressure, tension, or position using exceptional speed and torque control dynamics makes DTC attractive to more industries.
Moreover, AC drives offer dramatic energy savings potential for large numbers of variable-speed pump/fan applications. Because pump speed versus power has a cubic relationship, a process sequence that allows the pump to run at 50 per cent speed would use only 1/8 of full power.
Direct torque control has shown a continuum of hardware and software developments over its near 30-year lifespan. Based on firm theoretical foundations and DSP technology, DTC has overcome limitations of the early processors. Powerful processors now rapidly execute complex control algorithms, update motor model parameters, and switch the drive's transistors for optimal performance.
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Ere Jääskeläinen is with ABB Oy Drives, Helsinki, Finland. www.abb.com/drives