New generation motor control improves speed and torque accuracy 12 August 2014

ABB's direct torque control (DTC) – used on its variable speed drives since 1996 – has been significantly improved, bringing even higher accuracy motor speed and torque control, according to the firm.

Now in its fourth generation, the new DTC also enables more motor types to be controlled and is said to improve performance in open and closed loop.

Support for high-speed motors up to 500Hz and absolute encoder and resolver support are built in, while other improvements include more choice of set-up runs, allowing the drive to automatically identify the motor from its performance characteristics.

Drives equipped with the new DTC can now respond to changes in the customer speed reference much more quickly, says ABB, with the motor shaft adapting within 2msec, instead of 10msec.

ABB says that almost any motor can now be specified for speed and torque control, with DTC allowing ABB's latest drives, ACS880, to manage squirrel cage, permanent magnet and ABB's advanced synchronous reluctance (SynRM) motors.

ACS880 offers ful scalar control with its own ramps, fixed- and skip- speeds – useful in multi-motor applications and test installations before switching to DTC.

With DTC switched on, field orientation is achieved without feedback, using advanced motor theory to calculate the motor torque.

ABB says the latest iteration of DTC now uses the fastest digital signal processing hardware available "and a more advanced mathematical understanding of how a motor works".

DTC also offers torque control at low (or zero) speeds – making it particularly useful for cranes, hoists, winches and lifts, where the load needs to be started and stopped regularly without jerking, and with load on the cables.

Torque linearity allows an accurate and consistent level of winding, while its dynamic speed accuracy allows a motor to quickly recover to a stable state following a load change.

Another feature is flux optimisation, which provides enhanced energy saving beyond that achievable with standard variable-speed drives. ABB explains that DTC adjusts the motor flux to the optimal value for the load conditions at the time, thus constantly minimising energy draw.

Brian Tinham

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