4. DTC performance test
In mid-2012, ABB conducted a series of measurements to ensure continuous improvement of DTC technology to maintain peak performance of AC drives. Here are some notable results.
Torque stability is close to zero speed
Compare the torque control accuracy of ABB's ACS800 drive and the new ACS880 industrial drive in sensorless (open) mode. Each drive operates a 15kW four-pole induction motor with rated torque and controls the load machine to perform low-speed reversal at near zero speed.
In the near zero speed range, both drives exhibit outstanding sensorless control over long periods of operation, but the new ACS880 has less deviation from torque reference and therefore provides better motor control performance.
Torque accuracy in climbing
A four-pole induction motor and a 15 kW synchronous reluctance motor were used to compare the sensorless torque control accuracy of the ACS880 drive (at 50% rated speed). For both motor types, the DTC maintains the torque error at a small percentage of the rated torque.
The maximum torque error of a synchronous reluctance motor is slightly lower than that of an induction motor.
Servo-level dynamic performance
The speed and angular position of a 1.5 Nm, 6,000 rpm PM synchronous motor were measured during a speed reversal between ± 6,000 rpm and less than 25 milliseconds (ms). This is very close to the mechanical time constant and theoretical limit of the motor 24ms.
Although the ACS880 is not a servo drive, the DTC can perform fast and accurate motor acceleration in both closed loop and sensorless control modes. Comparing the measured acceleration time with the mechanical time of the motor provides a measure of torque accuracy in extremely fast acceleration.
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