DTC in a nutshell
Direct torque control, DTC, is a motor control method famous for its superior dynamic properties. Less known is the fact that DTC optimally utilizes the switching frequency to achieve the target: regulation of torque and flux within specified hysteresis band. Thus, DTC contributes to a high inverter efficiency. In this post we provide an introductory description of DTC in a nutshell.
What does DTC mean?
DTC is a method to control torque or speed of inverter-fed motor. Unlike the field oriented control (FOC), the controlled variables are directly motor torque and flux. Voltages and currents are just intermediate variables.
Switching frequency of DTC
The switching frequency is inherently not constant but adapting to the criteria of hysteresis control. Nevertheless, for a steady state operation (stable operation point) the switching frequency just slightly fluctuates around its mean value. Most switching interventions originate from torque control, significantly smaller portion is triggered by the flux control and finally few switchings happen due to other reasons (e.g. balancing of neutral point).
Principle of DTC
The actual values of torque and flux are calculated by the motor model running in a real time inside the VFD firmware. They are compared with the reference values. Torque and flux reference have a tolerance band around. The bandwidth is not constant but can be tuned (shaped) as a function of output frequency or modulation index.
DTC in a nutshell
DTC = Direct Torque Control, a motor control method supporting highest dynamics
Hysteresis control (controlling motor torque and flux within defined bandwidth)
Motor air gap torque and flux (priority on torque control)
Stator voltages and currents
Time variable, adjustable through the hysteresis band, optimized for control criteria (inherently lowest possible switching frequency to fulfil the control target).
Spectrum of signal is spread over wide frequency range. It contains dominant characteristic harmonics plus a noise (“white noise”). Sideband frequencies are not present.
- Excellent dynamics (e.g. torque step response)
- Lower switching frequency than PWM for the same output quality
- Minimized switching losses
- Optimally utilized power hardware
- Robust control algorithms tolerant to certain error in motor parameters
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References
[1] Direct Torque Control, Wikipedia – The Free Ecyclopedia, https://en.wikipedia.org/wiki/Direct_torque_control
[2] Direct Torque Control Principle, ABB library, available online, https://library.e.abb.com/public/e8499dfbbf9679b2c1256d280041e592/Direct_Torque_Control_principle.pdf
[3] VFD control – Introduction, MB Drive Services, November 2020, available online, https://mb-drive-services.com/vfd-control-introduction/
