Direct Torque Control

Introduction

Variable speed drives (VSDs) transformed industrial processes by giving precise control over motor speed and torque, unlocking major energy savings and better process performance. Much of drive technology's evolution has chased one goal: matching the fast torque response and speed accuracy of DC motors while using rugged, cheap, low-maintenance AC motors instead. Direct Torque Control (DTC), developed by ABB after decades of research, is widely regarded as the most advanced AC drive technology available from any manufacturer. This guide covers how DTC works, what it delivers, and where it fits in practice.

How Drive Technology Got Here

DC motor drives were the original choice for variable speed control because they naturally achieve good torque and speed response. A DC motor's field winding creates a magnetic field always at right angles to the armature field — "field orientation" — maintained mechanically by the commutator-brush assembly regardless of rotor position. Once field orientation is in place, torque is controlled simply by varying armature current while holding the magnetising current steady, with torque as the inner control loop and speed as the outer one. That gives accurate, fast, simple torque control — but brushes and commutators wear down, driving up maintenance and motor cost, and encoders are still needed for speed and position feedback.

AC drives using frequency control (PWM) aim to combine DC-like control with the small size, robust construction, and low cost of standard AC motors. Instead of controlling the motor directly, this approach feeds voltage and frequency references into a modulator that builds a simulated sine wave via pulse width modulation (PWM), relying on a diode rectifier and constant intermediate DC voltage. Critically, there's no feedback device reading speed or position from the shaft — an "open-loop" drive. That keeps cost low and the system simple, but field orientation isn't used, rotor status is ignored, and torque can't be controlled with any real accuracy — this is sometimes called scalar control.

AC drives using flux vector control (PWM) get closer to DC-like performance by achieving field orientation electronically rather than mechanically — the drive needs to know the rotor flux's spatial angular position inside the motor. That requires feeding back rotor speed and angular position via a pulse encoder (a "closed-loop" drive), with the motor's electrical characteristics modelled mathematically by microprocessors. The controller generates voltage, current, and frequency as controlling variables and still passes them through a modulator, so torque is controlled indirectly. This gives good torque response, accurate speed control, and full torque at zero speed — performance approaching a DC drive — but needs feedback, a modulator, and costs more.

Direct Torque Control: A Different Approach

DTC achieves field orientation without feedback, calculating motor torque directly from advanced motor theory rather than through modulation. Its controlling variables are motor magnetising flux and motor torque themselves. There's no modulator stage and, in most cases, no tachometer or position encoder — DTC relies instead on the fastest available digital signal processing hardware and a deeper mathematical model of how the motor actually behaves.

Traditional PWM drives — both frequency-control and flux-vector types — use output voltage and frequency as their primary control variables, which must be pulse-width modulated before reaching the motor. That modulator stage adds processing time and caps how fast torque and speed can respond. DTC skips it entirely, using the motor's own torque and stator flux as primary control variables, obtained directly from the motor itself.

Comparing control variables across drive types: DC drives use armature current and magnetising current; PWM AC drives use output voltage and output frequency; DTC uses motor torque and motor magnetising flux directly. Because DTC (like DC drives) controls actual motor parameters rather than derived electrical quantities, dynamic performance is fast and straightforward — and for most applications, no tachometer or encoder is needed at all.

How DTC Works

DTC has two main sections: a torque control loop and a speed control loop.

The torque control loop starts by measuring two motor phase currents, the DC bus voltage, and the inverter's switch positions. This data feeds an adaptive motor model, which is first primed during an auto-tuning identification run that determines stator resistance, mutual inductance, saturation coefficients, and motor inertia — no shaft speed or position feedback is needed if the required static speed accuracy is above 0.5%, which covers most industrial applications. The motor model outputs signals representing actual motor torque and actual stator flux (and calculates shaft speed internally), which are compared every 25 microseconds against torque and flux reference values using a two-level hysteresis method. The resulting status signals go to an optimum pulse selector, built around a 40MHz digital signal processor and ASIC hardware with optical links for high-speed data transmission, which recalculates the ideal inverter switching combination every cycle — no predetermined switching pattern. This "just-in-time" switching means every switch change is actually needed, unlike traditional PWM drives where up to 30% of switching is unnecessary.

The speed control loop consists of a torque reference controller (limiting the speed control output by torque limits and DC bus voltage, and handling external torque signals), a speed controller (a PID controller plus acceleration compensator comparing external speed reference against the motor model's actual speed), and a flux reference controller (setting the absolute stator flux value, which enables functions like flux optimization and flux braking).

What DTC Delivers

Torque response is typically 1-2ms below 40Hz, versus 10-20ms for flux vector and encoder-equipped DC drives, and well over 100ms for open-loop PWM drives — DTC's response time is essentially at the physical limit given available voltage and current.

Torque control at low frequencies stays accurate down to below 0.5Hz, with full torque available right through to zero speed, without needing an encoder or tachometer.

Torque repeatability without an encoder reaches 1-2% of nominal torque across the speed range — half the error of other open-loop AC drives, matching closed-loop AC and DC drives.

Static speed accuracy is 10% of motor slip — 0.3% for an 11kW motor, 0.1% for a 110kW motor, without an encoder — enough for roughly 95% of industrial applications, compared to 1-3% typical for frequency-controlled PWM drives.

Dynamic speed accuracy in open-loop mode runs 0.3-0.4%sec, around eight times better than other open-loop AC drives (typically ~3%sec); adding an encoder brings it to 0.1%sec, matching servo drive performance.

These translate into practical benefits: fast torque response means less speed drop during load transients and more consistent product quality; torque control at low frequencies with full torque at zero speed suits cranes and elevators needing smooth starting and stopping without feedback hardware; good speed accuracy without a tachometer saves on investment cost while improving reliability; and full torque at zero speed removes the need for a mechanical brake, allowing a smooth transition between drive and brake control.

Other User Benefits

DTC also brings a set of practical operational advantages: rapid control of DC link voltage gives power-loss ride-through, so the drive doesn't trip and cause downtime; automatic (flying) start synchronises to an already-rotating motor without interrupting the process; flux braking provides controlled braking between speed points; flux optimization minimises motor losses and noise, improving efficiency; self-identification/auto-tuning sets the drive up accurately without manual parameter tuning; the absence of a fixed switching pattern keeps noise low and reduces stress on gearboxes, fans, and pumps; and there are no artificial limits on acceleration or deceleration, allowing the fastest possible response.

Standard Applications

Standard applications — fans and pumps in HVAC, water, and food and drink industries, among others — account for around 70% of all VSDs installed across industry. In these settings, DTC helps address harmonics and noise directly: replacing a conventional diode bridge with a DTC-controlled input bridge significantly cuts harmonic distortion, with lower current distortion than a conventional 6-pulse or 12-pulse configuration and power factor as high as 0.99. DTC drives also handle sudden, large load torque changes without tripping on overvoltage or overcurrent, and its 25-microsecond control cycle keeps the DC link voltage from dropping below the 80% control threshold during brief input power loss.

Frequently Asked Questions

Is DTC proven? Yes — hundreds of thousands of installations are in service. One example: a major web machine manufacturer replaced DC drives and flux-vector AC drives with open-loop DTC drives on rewind station centre drives, simplifying construction, improving reliability, and cutting costs (a single tachometer and its wiring can cost as much as a 30kW AC motor).

Does it work with any induction motor? Yes, any asynchronous squirrel-cage motor.

What about multiple motors in parallel? A DTC-controlled inverter driving several parallel motors treats them as one large motor, with no visibility into any individual motor's status. If the number of motors varies, or motor power stays below 1/8 of rated power, scalar control is the better choice.

What are DTC's limits? It covers around 95% of industrial applications. The remaining cases — mostly where extremely precise speed control is required — can be handled by adding a feedback device, though even then it can be simpler than what conventional closed-loop drives need.

Conclusion

Direct Torque Control is the result of decades of variable speed drive evolution. By controlling motor torque and flux directly — without a modulator, and in most cases without feedback hardware — it matches or beats DC drive performance while keeping the robustness, simplicity, and low cost of AC motors. From demanding high-performance applications to everyday pumps and fans, DTC delivers fast torque response, accurate speed control, and real operational benefits: energy savings, less downtime, and lower installation costs.

This article summarises key concepts from ABB Technical Guide No. 1: Direct Torque Control – The World's Most Advanced AC Drive Technology. The full guide is available as a free download from ABB.