Machinery drives - a new kid on the servo block

Until now, when it comes to providing high performance speed and position control, machine builders have had only one option – the servodrive. Yet, a new class of AC variable speed drive from ABB is said to have the potential to provide more cost effective performance.

The new ‘machinery drives’ are said to offer machine builders and system integrators exciting possibilities, with small size, high performance, user friendliness and reliability. They are suited to a wide variety of web applications including winding, coating, slitting, calandering, colouring, cleaning, laminating and printing.

While machinery drives have been under development for the past five years, the breakthrough happened when very high performance became available in standard AC drives. This meant that servos and closed loop systems in general could be replaced by lower cost drives in certain applications.

Converting machinery requires accurate speed control and often motion control - hence the need for encoders. With an encoder on a conventional AC drive, enhanced functionality may be achieved that would normally be limited to using only servos. Therefore, for a conventional AC drive to give servo performance, a feedback loop is always required.

The deciding factors are shaft performance: low speed torque performance and dynamic response. The key to this is the advances that have been made in control technology. Direct Torque Control (DTC) features a very fast torque control loop, which takes it into the “pure servo drive arena” where torque control loops as low as 62.5ms are achieved. With DTC, servo-like performance can be achieved, while the standard AC drive technology offers cost benefits over servodrives.

ABB says machinery drives can control any AC motor type, including standard squirrel cage and brushless, and because they use components that are produced in high volumes, they promise the machine builder the best of both worlds – servo performance with AC drive reliability at a lower cost.

Older machines could benefit from the new drives. For applications with no need for high dynamics or with no demanding position requirements, users should consider standard AC drives as an alternative. Functions include basic motion control tasks, communications and diagnostics, all bundled into an “affordable and easy to use” package

Some experts predict that within 10 years, AC drives will replace some two per cent of the applications servodrives currently dominate. The cost difference really begins to favour AC technology at the higher powers (above 5kW).

One application previously almost exclusive to servodrives that AC drives are now taking on is registration - for example, ensuring a rotary knife cuts at the edge of a label, and not in the middle. AC induction motors were seldom considered previously for this because their low speed characteristics are traditionally poor. Yet the current state of the technology, when used in a closed loop, means that AC induction motors give good torque characteristics at a low speed. This is an area where DTC and flux sensorless technology have widened the scope for AC drives.