How DTC Works (1)

Figure 1, below, shows the complete block diagram for Direct Torque Control (DTC).

Walk around the block

Figure 1: DTC comprises two key blocks: Speed Control and Torque Control

The block diagram shows that DTC has two fundamental sections: the Torque Control Loop and the Speed Control Loop. Now we will walk around the blocks exploring each stage and showing how they integrate together.
Let’s start with DTC’s Torque Control Loop.

Torque Control Loop

Step 1 Voltage and current measurements
In normal operation, two motor phase currents and the DC bus voltage are simply measured, together with the inverter’s switch positions.

Step 2 Adaptive Motor Model
The measured information from the motor is fed to the Adaptive Motor Model. The sophistication of this Motor Model allows precise data about the motor to be calculated. Before operating the DTC drive, the Motor Model is fed information about the motor, which is collected during a motor identification run. This is called auto-tuning and data such as stator resistance, mutual inductance and saturation coefficients are determined along with the motor’s inertia. The identification of motor model parameters can be done without rotating the motor shaft. This makes it easy to apply DTC technology also in the retrofits. The extremely fine tuning of motor model is achieved when the identification run also includes running the motor shaft for some seconds.
There is no need to feed back any shaft speed or position with tachometers or encoders if the static speed accuracy requirement is over 0.5%, as it is for most industrial applications. This is a significant advance over all other AC drive technology. The Motor Model is, in fact, key to DTC’s unrivalled low speed performance.
The Motor Model outputs control signals which directly represent actual motor torque and actual stator flux. Also shaft speed is calculated within the Motor Model.

Step 3 Torque Comparator and Flux Comparator
The information to control power switches is produced in the Torque and Flux Comparator. Both actual torque and actual flux are fed to the comparators where they are compared, every 25 microseconds, to a torque and flux reference value. Torque and flux status signals are calculated using a two level hysteresis control method. These signals are then fed to the Optimum Pulse Selector.

Step 4 Optimum Pulse Selector

Within the Optimum Pulse Selector is the latest 40MHz digital signal processor (DSP) together with ASIC hardware to determine the switching logic of the inverter. Furthermore, all control signals are transmitted via optical links for high speed data transmission.
This configuration brings immense processing speed such that every 25 microseconds the inverter’s semiconductor switching devices are supplied with an optimum pulse for reaching, or maintaining, an accurate motor torque.
The correct switch combination is determined every control cycle. There is no predetermined switching pattern. DTC has been referred to as “just-in-time” switching, because, unlike traditional PWM drives where up to 30% of all switch changes are unnecessary, with DTC each and every switching is needed and used.
This high speed of switching is fundamental to the success of DTC. The main motor control parameters are updated 40,000 times a second. This allows extremely rapid response on the shaft and is necessary so that the Motor Model (see Step 2) can update this information.
It is this processing speed that brings the high performance figures including a static speed control accuracy, without encoder, of ±0.5% and the torque response of less than 2ms.


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