Direct torque control

Lore & Background

DTC was patented by Manfred Depenbrock in the US and Germany, the latter filed on October 20, 1984, under the term direct self-control (DSC). Isao Takahashi and Toshihiko Noguchi described a similar technique termed DTC in a September 1984 IEEJ paper and a late 1986 IEEE paper; the innovation is usually credited to all three. The only difference between DTC and DSC is the flux vector path shape: quasi-circular for DTC, hexagonal for DSC, with DTC having higher switching frequency and targeting low-to-mid power drives, while DSC is used for higher power drives.

DTC was studied in Baader's 1989 thesis. The first major commercial DTC products were ABB traction applications for German DE502 and DE10023 diesel-electric locomotives in the late 1980s, followed by the 1995 launch of the ACS600 drives family (later replaced by ACS800 and ACS880). DTC has also been applied to three-phase grid side converter control, doubly fed machine control (early 2000s), interior permanent magnet synchronous machines (late 1990s), and synchronous reluctance motors (2010s). ABB's first servo drive family using DTC, the ACSM1, was introduced in 2007.

Reader's Guide

DTC's significance lies in its simplicity and very fast torque and flux control response for high-performance induction motor drives, as noted since its mid-1980s introduction. It requires no PI current controllers, no dynamic coordinate transforms, and no separate modulator; the hysteresis control directly defines switch signals. The switching frequency is not constant but can be kept near a reference by controlling tolerance bands, resulting in torque and current ripple similar to vector-controlled drives at the same frequency. The random switching process avoids peaks in the current spectrum, reducing audible noise. DTC automatically accounts for DC circuit voltage variation, and synchronization to a rotating machine is straightforward.

However, DTC requires a high sampling rate (up to 40 kHz) leading to higher switching loss compared to FOC, a more complex motor model, and inferior torque ripple. At low speeds, stator resistance errors in flux estimation become critical, and zero-frequency operation is not possible without a speed or position sensor. The first industrial application came later than expected due to the need for more sophisticated hardware to match FOC performance. Depenbrock's key DTC patents expired in the mid-2000s, potentially allowing other companies to include similar features.

Did You Know?

• DTC was patented by Manfred Depenbrock as 'direct self-control' (DSC) in Germany on October 20, 1984.
• DTC requires a sampling rate up to 40 kHz, compared to 6–15 kHz for FOC.