Direct Torque Control (DTC) is widely used as a method for controlling AC motors in many demanding applications. It is a unique method for controlling AC motors. In pulse-width modulation (PWM) drives, the output frequency and voltage are the primary control reference signals for the power switches, rather than the desired torque in/of the motor shaft. For those who are not familiar with inverter technology, the DTC principle can be illustrated most accurately via this mechanical analogy: the continuous calculation of the best angle at which to rotate a shaft, with a given arm length and the forces available. These electrical “force vectors” are generated with the help of semiconductor switches called Integrated Gate Bipolar Transistors (IGBT).

Mechanical principle analogy for DTC: Continuously calculate the correct angle to rotate the shaft with the forces available.
Testing of rotating machines like gears, engines, and complete cars is a demanding task. High accuracy and dynamic load control — that is, control of torque — is needed both for day-in-day-out testing, but excels, specifically, on those tests with new complex electronic functions, such as ABS, EPS — or electromechanical innovations like dual — clutch transmissions being introduced in the current generation of automobiles. AC motors drive these test rigs. When manufacturing test rigs for engines, transmission, or chassis dynamometers with high-performance requirements, careful consideration must be given to the AC machines and drives used in such applications (control of speed and torque are paramount). The way the AC motor is controlled by the drive has a primary effect on these considerations.

Testing Processes

Torque rise time seen by DTC control, at left, for 10% change (for this motor, approx. 0.4 ms). At right, a standard 1500-rpm AC motor. Torque rise time for nominal load step measured from torque sensor when shaft has been mechanically locked. The damping oscillation in measurement signal is due to mechanical torsion in locked shaft and sensor itself.
Chassis dynamometers are typically used to test the performance of vehicle, exhaust emission, fuel consumption, noise, and fine-tuning of exhaust, catalyst, and motor fuel-injection system. It is well known that dynamometers with AC motor technology offer the best platform to realize high accuracy, dynamics, and energy savings.

Dynamometers should simulate the real highway precisely. This requires that, during acceleration and deceleration, the roll inertia be compensated dynamically to match the mass of the tested vehicle and the real road. To be able to realize such high-dynamic online compensation (real-world, real use), the load torque of the roll motor must be controlled accurately and with extreme precision at every speed point.

Testing gear-shifting-and-synchronization, calibration of automatic transmissions, clutching, and durability — these are typical testing needs. Inherently, these cases require a capability to change load torque very quickly. And, transmission test-stand configuration can include several motors — one simulating the engine, and two or even more for the simulation of the load. This requires mutual coordination of drives operation; the faster and more accurate it is, the better it simulates real-world conditions like differential-gear operation.

Drives with DTC technology can transfer speed/ torque information via ultra-fast optical links to each other. The speed torque signals can be used as reference to follower drives, to assess/react to desired load share (or as additional inputs to the main drive speed/torque reference). Additional complex functions to calculate speed/torque references to individual drives can be achieved within the DTC drive. This is useful in setting up and delivering testing in time-critical operations. The torque is calculated as a cross product between the stator flux and the stator current: T = Ψs × is . The stator flux is estimated from the stator voltage vector and the stator current: (2) Ψs = ∫(Us – Rs is ) dt . Six voltage vectors and two zero-vectors control the stator flux and the torque. The stator-flux amplitude is controlled to be constant.

For engine dynamometers, the dynamic performance is the key issue to ensure that you can simulate real systems dynamically and accurately. DTC drive technology answers this challenge directly. The test system’s overall dynamic performance can be quantified by looking at the delay from reference change to change in AC motor torque.