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Direct Torque Control (DTC)
What if the drive skipped the current controller and the PWM stage and switched the inverter straight from torque and flux errors?
Direct Torque Control is a high-performance way to run an AC induction motor. The drive estimates the motor’s stator flux and torque from measured currents and the DC bus voltage, compares them with the wanted values, and picks the inverter switching state that fixes the error right away. The result is a very fast torque response, at the cost of more torque ripple and a variable switching frequency. This page explains the idea, lets you move the flux vector and see which voltage vector the drive picks, and compares DTC with field-oriented control (FOC). Details differ between drive makers.
Field-oriented control (FOC) works through current controllers and a modulator (PWM) and so steers the torque indirectly. DTC removes those two stages: it controls torque and flux directly with simple comparators and a lookup table. Fewer stages is why the response is fast, and the comparators are why the ripple is higher.
| Vector | Angle | Effect on flux now | Effect on torque now |
|---|
The inverter has six active voltage vectors, 60° apart, and two zero vectors (V0 and V7, all outputs to one rail). The part of a vector pointing along the flux changes the flux size: along it raises the flux, against it lowers it. The part at right angles to the flux, ahead of it, speeds up the flux rotation and raises torque; behind it, it lowers the torque. So which vector is best depends on where the flux is. That is why the table works with six sectors. Here V1 is at 0°, and the flux is in sector k when it is within 30° of Vk.
The classic table (Takahashi) for a two-level flux comparator and a two-level torque comparator: for flux in sector k, flux up and torque up gives V(k+1); flux up and torque down gives V(k−1); flux down and torque up gives V(k+2); flux down and torque down gives V(k−2), counting round the six vectors. Many versions add a third torque level, using a zero vector (V0 or V7) when the torque error is inside the band, which lowers switching and ripple.
A simplified torque model: each inverter choice pushes the torque up or down at a fixed rate, and the controller checks the error once per sample time. A narrow band gives low ripple but many switchings (more inverter losses and noise). A wide band does the opposite. A slow sample time adds extra ripple on top of the band, because the torque keeps moving between checks. This is the basic trade-off of conventional DTC.
Illustrative step to rated torque. DTC pushes the torque up at the maximum rate the inverter allows, then holds it in the band; FOC follows a smooth rise set by its current controller. Typical torque response times are about 1 to 2 ms for DTC and 5 to 10 ms for FOC; actual figures depend on the drive and motor.
| Feature | DTC | FOC (vector control) |
|---|
Advantages
- Very fast torque response
- Simple structure, with no current controllers or PWM modulator
- Good dynamic performance
- Not sensitive to rotor parameters
- Suits high-inertia and high-torque loads
- Can run without a speed sensor
Limitations
- More torque and flux ripple than FOC
- Variable switching frequency (basic form)
- Can be noisier acoustically
- Needs a good flux estimate: sensitive to stator resistance and current offsets at low speed
- Less smooth torque for precision positioning
- Needs a fast processor, because the table is evaluated at tens of microseconds
Variants such as DTC with space-vector modulation (DTC-SVM) keep the fast torque loop but add a modulator to get a fixed switching frequency and less ripple.
| Application | Why DTC fits |
|---|
DTC is a family of methods, and commercial drives add their own refinements. Response times and ripple are typical and drive dependent. Always follow the drive manual for set-up and motor identification.