Direct torque control: a motor control technique for all seasons

While enabling unprecedented performance in electric motors and delivering dramatic energy savings, most VSDs rely on conditioning voltage and frequency, causing inherent time delays in processing control signals. In contrast, direct torque control (DTC) drives greatly increase motor torque response, among other benefits.

Electric motors are often at the spearhead of modern production systems, whether in metal processing lines, robotic machining cells or conveyor systems. The motors we see today have certainly benefited from advances in improved materials, manufacturing efficiencies and analytical tools. However, their design principles have remained the same for over 100 years in the case of the workhorse synchronous (or induction) AC motor. Rather, the remarkable performance of these motors in today’s applications comes from modern electronic controls — variable-speed drives (VSDs) — and accurate motor models whose sophisticated control algorithms can be rapidly executed by high-performance digital signal processors. Moreover, the development of VSDs has enabled the use of new AC motor technologies such as permanent magnet synchronous motors and synchronous reluctance motors.

Initially, DC motors drew the attention of drive developers. With an even longer history than their AC cousins, DC motors offered inherently simple speed and torque control. However, higher motor cost, more complex construction with a mechanical commutator, and brush maintenance issues were some trade-offs associated with DC motors.

AC induction motors offered simpler, rugged construction and lower cost, and posed fewer maintenance concerns — characteristics that have led to their wide usage with a huge installed base worldwide. On the other hand, control of induction motors proved to be complex. Accurate speed control, and particularly torque control, remained elusive for early AC drives. Naturally the goal of the early designers was to emulate in AC drives the DC drive’s simple control of motor torque by using armature current. Over time, AC drive designs have evolved offering improved dynamic performance.¹

Most high-performance VSDs since the 1980s have relied on pulse-width modulation (PWM). However, one consequence of using a modulator stage is the delay, and a need to filter the measured currents when executing motor control commands — hence slowing down motor torque response.

An alternative approach to high-performance AC motor control is direct torque control (DTC). This method directly controls motor torque instead of trying to control the currents analogously to DC drives. This means better accuracy in matching the load requirements of the driven system. DTC further eliminates the need for an extra modulator stage and thus achieves control dynamics that are close to the theoretical maximum. The first AC industrial drive with direct torque control came to the market in 1995.²

In principle, DTC was already a leading technology back in 1995, but subsequent developments in processor computational power, communication interfaces, application programming etc have enabled higher performance, providing premium motor control for a broad range of applications.

DTC’s core is the torque control loop, where a sophisticated adaptive motor model applies advanced mathematical algorithms to predict motor status. Primary controlled variables — stator flux and motor torque — are accurately estimated by the motor model using inputs of motor phase currents and DC bus voltage measurements, plus the states of the power-switching transistors in the drive. The motor model also calculates shaft speed. Temperature compensation also helps to enhance calculation accuracy without an encoder.

Why use DTC?

Superior torque response is just one feature of DTC. The technology offers further benefits, including:

• No need for motor speed or position feedback in 95% of applications. Thus the installation of costly encoders or other feedback devices can be avoided.
• DTC control is available for different types of motors, including permanent magnet and synchronous reluctance motors.
• Accurate torque and speed control down to low speeds, as well as full start-up 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.

As the name suggests, DTC seeks to control motor flux and torque directly, instead of trying to control these variables indirectly like DC drives and vector-controlled AC drives do. Separate torque and speed control loops make up the full DTC system but work together in an integrated way (see Figure 1).

Additional motor parameters are automatically fed to the adaptive model during a motor identification run when the drive is commissioned. In many cases, appropriate model parameter identification can be done without rotating the motor shaft. For fine-tuning of the motor model, which is only needed for few high-demand applications, the motor has to be run, but then only for a short time and without load.

Figure 1: DTC operation principle. For a larger image click here.

Stator resistance (voltage drop) is the only measurable parameter needed for estimating the motor’s magnetic flux. Motor torque can then be calculated as the cross product of estimated stator flux and stator current vectors. While stator resistance is the main source of estimation error, its influence diminishes with increasing motor speed and voltage. Thus DTC has excellent torque accuracy in a wide speed range. Moreover, DTC includes advanced ways to minimise estimation error at low motor speeds.

Output signals from the motor model — which represent actual stator flux and motor torque — go to a flux comparator and torque comparator respectively. These separate control units compare their inputs to a flux and torque reference value. Already in the mid-1990s the first DTC controlled drives performed these functions every 25 μs using a high-power digital signal processor (DSP). In the latest control generation the interval is reduced down to 12.5 μs, thus further enhancing control performance. Each comparator seeks to hold its respective flux or torque vector magnitude within a narrow hysteresis band around a reference value. DTC’s fast torque response without overshoot comes, in part, from the ability to minimise these vector fluctuations. Exceptional motor response is also due to the DSP control algorithms updating the adaptive motor model at the same high cycle rate.

Flux and torque errors — differences between estimated and reference values — and the angular position (or sector) of the stator flux vector are used to calculate flux and torque status in the hysteresis controllers. Then, these status values become inputs to the optimum pulse selector, where the optimum voltage vector is selected from the look-up table. In this way, the most appropriate signal pulses for each control cycle can be sent to power switches in the inverter to obtain or maintain precise motor torque.

A field-programmable gate array (FPGA) assists the DSP with determining inverter switching logic and other tasks. The FPGA allows for control modifications or drive design updates versus an application-specific integrated circuit (ASIC) which, if used, requires locking in the design.

Performance

DTC provides superior performance features over competing drive methods. Being a ‘sensorless’ control method (speed estimation instead of measurement), costly motor speed or position feedback devices are not needed in most cases. Depending on motor size, static speed accuracy as low as ±0.1% is typically obtained. For higher demand applications, a DTC drive equipped with a standard encoder (1024 pulses/rev) typically achieves ±0.01% speed accuracy.

Dynamic speed accuracy (time integral of the speed deviation under a 100% load impact) is 0.3 to 0.4%-seconds with typical equipment driven by the motor. Using an encoder, speed accuracy typically improves to 0.1%-seconds and matches servo drive accuracy.

Torque response time to a 100% torque reference step is typically 1–5 ms, which approaches the motor’s physical limit. Torque repeatability under the same reference command is typically as low as 1% of nominal torque across the drive’s speed range. As for control at very low motor speeds, DTC provides 100% torque down to zero speed — with or without speed feedback, as well as a position control feature when using an encoder. These performance values refer specifically to induction motor control.

Beyond induction motors

DTC was originally developed for AC induction motors because of their popularity in myriad industrial and commercial applications. No doubt the ‘workhorse role’ of induction motor technology will prevail over the foreseeable future. However, in the quest for higher power density and evolving international efficiency regulations, other motor topologies are drawing renewed interest.

For example, standard IEC 60034 part 30³ defines international efficiency (IE) classes, the highest of which — IE4 (super-premium efficiency) — is becoming difficult to meet for induction motors. An even higher IE5 class has also been proposed.

The good news is that DTC is equally applicable to other motor types, such as permanent magnet (PM) synchronous and synchronous reluctance (SynRM) motors. The main difference occurs during motor starting. Unlike induction motors, PM synchronous motors and SynRM motors require the control system to estimate rotor position at start-up from the location of poles in the rotor, if no position sensor is used.

In these motors, absence of rotor windings and the slip-speed effect inherent to induction motors substantially reduce losses. Moreover, synchronous operation means that excellent speed accuracy is achieved even without a speed or position sensor. Thus, a sensor can be omitted in most cases except in applications such as winches and hoists that require non-zero torque at standstill for long periods.

In a PM motor, permanent magnets are commonly mounted on the rotor’s outer surface. However, a PM synchronous motor variant, the internal PM (IPM) rotor design, embeds the magnets within the rotor structure. An extra reluctance torque component generated in IPM synchronous motors makes them attractive for high-demand applications.

In addition, embedded magnets create very pronounced rotor-pole saliency, which allows accurate speed estimation and enhances DTC’s basic sensorless operating mode. Due to high torque to motor size ratio, a simpler system drive train may be possible when using PM synchronous motors. For example, a direct-driven low-speed PM motor can eliminate the gearbox in packaging machines.

Numerous applications for PM synchronous motors include machine tools, marine propulsion, wind turbines (generators) and cooling tower fans for electric power plants.

One partly economic drawback of PM synchronous motors is their reliance on so-called rare-earth (RE) magnet materials for best performance, such as neodymium-iron-boron. Recent pricing and global supply issues of RE materials have created serious concern for equipment manufacturers that reaches well beyond electric motors.⁴ Synchronous reluctance motors are therefore providing an alternative to PM motors.

Synchronous reluctance motors have a stator structure similar to induction motors. However, the rotor consists of axially stacked steel laminations shaped to provide a cross-section with four poles — with alternating high-permeability (iron) axes and low-permeability (air) axes. Importantly, no magnets are needed in the rotor.

Figure 2: The synchronous reluctance motor utilises a new rotor design and is optimised for VSD operation. For a larger image click here.

Versions of DTC modified for PM synchronous and SynRM motors have been implemented. In addition to high dynamic motor control, DTC drives — combined with any of the efficient motor technologies mentioned above — offer great energy savings potential for the large number of variable-speed pump and fan applications.

This can be visualised from so-called ‘affinity laws’ associated with pumps and fans that relate variables such as flow volume, pump speed, pressure, power, etc. For example, pump speed versus power has a cubic relationship, meaning that when a process sequence allows the pump to run at half speed, only 1/8 of full power is required. Of course, reduced motor and drive efficiencies at partial loads would lower the system efficiency but overall less energy will be used.

Wider applications

Another aspect of the DTC story is its expansion beyond applications for which the technology was created. Demanding, highly dynamic applications were targeted early on, because they could justify costly initial software developments and available microprocessors. That scenario has changed greatly. Control system software has been amortised over the growing sales volume of AC drives and economically justified to implement in drives for more standard applications. High-performance DSPs also have become common and affordable.

The ability to respond rapidly to changes in process variables such as pressure, tension or position using exceptional speed and torque control dynamics has made DTC attractive to wider industrial and process applications.

DTC can also provide protective functions to connected machinery or the motor itself. Tight torque control can optimise tuning of the speed controller to damp out torsional vibrations.

Minimising overloads and shock loads becomes possible through timely detection of connected system parameter changes and DTC’s fast control response. The concept can be extended to driven-system failure detection. For example, sudden torque loss might indicate a conveyor belt breakage, or higher than normal torque needed to produce some output may indicate binding or abnormal wear in the machine. Drives can therefore be used as part of overall process diagnostics.

DTC has also been applied to reduce harmonic distortion from the drive, hence improving power line quality. Low-frequency harmonics can be mitigated in the line currents by replacing the diode rectifier of an AC drive with a DTC-controlled IGBT supply unit (ISU). The LCL filter of the ISU removes high-frequency harmonics and provides additional filtering for the grid. In many cases even voltage distortion in the grid may be reduced by using a drive with an ISU. Moreover, with an ISU it is possible to feed the braking energy back to the grid. Thus in applications that require frequent deceleration energy cost savings can be achieved.

DTC today and tomorrow

Resting on firm theoretical foundations, direct torque control has shown a continuum of hardware and software improvements over its more than 25-year lifespan. A DSP-based technology from the start, DTC has overcome the limitations of the early processors for speedy calculation of control algorithms. DSP limitations also restricted the drive’s maximum switching frequency in the past, hence its output frequency. DTC relies on rapid switching of the drive’s transistors for optimal performance and timely updating of motor model parameters. Powerful processors are now readily available.

Today, DTC drives have higher output frequency, allowing motors to run faster. This is an important feature for certain applications, such as test benches and machine tools. Drives running induction motors in an industrial application typically provide 2–4 kHz switching frequencies that maximise the efficiency, while machinery drives powering PM synchronous motors typically supply 5–8 kHz switching to run the motors with best possible dynamics.

Software has been another key element behind the success of DTC: improvements and updates include redesigned and optimised code for the whole control system (from user interface to motor shaft) to further enhance drive response time and performance.

Motor models also receive regular updating. Control algorithms are periodically analysed and the resulting improvements are thoroughly verified through laboratory testing with different motors. This can include investigating some new features or control ideas with an existing or modified motor; or looking at some special customer application requirement.

Today, DTC remains a living technology, having built a continuum of advances atop a solid foundation.

References

1. Kazmierkowski MP et al 2011, ‘High-Performance Motor Drives’, IEEE Industrial Electronics Magazine, Sept. 2011, vol. 5, no. 3, pp. 6–26.
2. ‘Direct Torque Control Comes to AC Drives’, Control Engineering, March 1995, vol. 42, no. 3, p. 9.
3. International Electrotechnical Commission, Standard IEC 60034-30, Ed.2: Rotating electrical machines - Part 30: Efficiency classes (IE-code), <http://www.iec.ch>.
4. Bartos FJ 2011,‘Rare-earth magnet supply and cost issues’, Control Engineering, Aug. 2011, <http://www.controleng.com/single-article/rare-earth-magnet-supply-and-cost-issues/7a69ac005081d8fb62ed62fd29633cc5.html>

Image credit: ©stock.adobe.com/YURY MARYUNIN