Surgery

Question

Answer

Geoff Brown, Drive Applications Consultant, ABB Limited

Servo drives are generally used in applications requiring high precision, for example machine tools, and other high-precision machinery in industries such as material handling, packaging, paper, textile and woodworking.

The majority of servo drive solutions utilise a motor with low inertia rotor to provide rapid torque or speed response, and these motors may use permanent magnet or brushless d.c. technology, together with, a feedback device in the great majority of cases to give high accuracy. In addition to the motor the controller or amplifier traditionally has little programmability, and relies on an external controller.

But like many other areas of drive technology, the servo drive is being challenged in its market niche by standard AC drives that are becoming increasingly accurate and provide high performance at a lower cost. There is a new class of machinery drives around that is able to run a standard AC motors as well as specialist servo motors, with or without feedback devices, using a single drive platform.

Historically servo motors have used "square frame" construction, reminiscent of d.c. motors, and a range of "square frame" asynchronous motors matching d.c. motors has also become available. These can readily incorporate fixed speed fans, encoders and brakes.

This gives the designer an opportunity to select the most cost-effective solution amongst a range of options - servo, square frame or standard squirrel-cage motor, with feedback or without, depending on the requirements. The same type of drive can be used throughout the application, regardless of performance requirements.

The biggest leap forward for conventional drives in recent years has been improved torque control as a result of more powerful processors; this has enabled them to be used in the high performance arena. More computer power means quicker execution. This has opened up a.c. drives and motors to applications with a wide speed range, for instance dropping to a dead speed and then rapidly accelerating to high speed.

This all means that the required performance can often be achieved with standard AC motors without feedback devices, and drives with on board functionality, reducing the cost and complexity of the installation. If higher performance is needed, then a feedback device can be added to the standard or square frame a.c. motor. For still higher performance, add a servo motor, with low inertia rotor. For the highest performance achievable, a servo drive will still be needed. But for many applications, the performance needed can be achieved more simply and at a lower cost.

This is a good time to shop around for a high performance drive at a reasonable cost. Don't just instinctively reach for the servo drive option, ask your drive supplier for advice.

Tel: 01925 741 111

Fax: 01925  741 21

Email: mailto:enquiries@gb.abb.com

Web: www.abb.co.uk/energy