What is Direct Torque Control (DTC)?
How does the drive respond when the load changes?
Direct Torque Control (DTC) is a drive control method that focuses on electromagnetic torque and stator flux. Torque is the turning effect at the shaft; flux describes part of the motor's magnetic state. DTC estimates these quantities from measurements and selects inverter switching states to bring them towards their reference values.
DTC is a control method; torque mode is an operating choice. A DTC drive can also operate in speed mode. In that case, an outer speed loop, usually using a PI controller, generates a torque reference for the inner DTC loop. In torque control mode, the drive targets a specified torque while speed can change with the load. This is why speed control and torque control are different functions.
From measurements to switching decisions
A drive can calculate the motor's electrical state using motor currents, DC bus voltage and inverter switching states. Depending on the implementation, voltage may be measured directly or reconstructed. A motor model uses these inputs to produce torque and flux estimates. In a conventional voltage-model flux estimator, stator resistance also matters. DTC should therefore not be described as independent of motor parameters.
Classical DTC evaluates torque and flux errors using hysteresis comparators. A typical structure uses a two-level flux comparator and a three-level torque comparator. Their outputs, together with the stator flux sector, feed a switching table. A three-phase, two-level inverter has six active and two zero voltage vectors. The selected vector influences how torque and flux change next. In classical DTC, switching frequency can vary with operating conditions; other DTC variants incorporate modulation.
Understanding the hysteresis band
The illustration uses a 100 Nm reference and an example ±5 Nm band. The moving point follows the illustrated torque estimate. Select Start to run the illustration; Stop and reset returns it to the beginning.
Reduced motion is enabled; a static illustration is shown.
This is a predefined teaching illustration, not a motor simulation or a drive setting. Actual switching decisions also depend on flux error and sector. Sampling and actuation delays can cause brief excursions beyond the hysteresis limits. The ±5 Nm value is not a recommendation for any product.
How it differs from V/f and FOC
V/f control manages the relationship between voltage and frequency. It can be sufficient for straightforward pump and fan duties; demanding torque control during abrupt load changes calls for more advanced methods. A torque boost setting alone does not turn a V/f drive into a DTC drive.
Field Oriented Control (FOC) resolves motor current into components in a rotating reference frame and regulates the flux-producing and torque-producing current components. It generally uses current controllers and PWM modulation. Classical DTC selects switching states from torque and flux errors instead. FOC can also deliver fast, accurate control. Compare actual drive performance and application requirements rather than choosing solely by the method's name.
Motor identification and encoder selection
During commissioning, enter the correct motor type, nameplate voltage, current, frequency and speed. Perform the manufacturer's specified motor identification or auto-tuning procedure, then configure current limiting and motor overload protection for the actual motor. Support for induction, permanent magnet synchronous and synchronous reluctance motors depends on the drive and its software; a DTC label does not guarantee compatibility with every motor.
Many applications can run without an encoder. However, the accuracy needed at very low speed, during extended operation near standstill or for precise positioning requires separate assessment. “DTC makes an encoder unnecessary” is too broad a claim. If low-frequency vibration occurs, investigate motor data, identification, feedback and the mechanical system together.
Practical behaviour and limits
Consider a machine running at 900 rpm whose required load torque rises from 75 Nm to 100 Nm. In speed mode, the speed loop raises the torque demand, and DTC aims to respond quickly. In torque mode, retaining a fixed 75 Nm command cannot be expected to maintain the same speed. This distinction matters when selecting and setting up winders, extruders and crane drives.
Fast torque control does not eliminate braking energy. Regenerative energy produced when the load drives the motor must be returned to the supply through suitable hardware or managed with a solution such as a braking resistor. Motor cooling at low speed, current capacity and the torque-speed curve remain physical constraints.
DTC's strength is dynamic torque control. In classical implementations, torque ripple, variable switching frequency and model accuracy require consideration. Narrowing a hysteresis band can reduce ripple under some conditions, but may increase switching activity and losses. Commercial drives may not expose these bands as user parameters; experimental values from another device should not be copied directly.