Driving forces

Electric Drives and their Controls
November 10, 1995

It would be a foolish manufacturer of, say, washing machines who completed the detailed mechanical design, including the speed, diameter of the drum, polar moment of inertia and the pulley diameters for the belt drive, and only then sought a suitable electric motor. Yet all too often mechanical engineering undergraduates adopt precisely this approach to design, with a belated visit to the electronics department demanding advice on a suitable motor for their particular application. What results is usually an oversized drive with complex electronic control systems added to solve problems that could have been avoided earlier.

Students of electronic and electrical engineering are equally blind in their faith that mechanical systems can translate any convenient electrical drive into a satisfactory complete design.

What is needed, of course, is an integrated approach to design based on an understanding of the capabilities and limitations of every element of the drive: power electronics, motors, power transmission elements, transducers and electronic controllers. For the student, the problem with integrated design is assembling a substantial knowledge base from diverse sources of information and the inevitable question "Where do I start?". The notion of an iterative approach to design also worries the student.

Resolving this dilemma is the role of Richard Crowder's textbook.It starts with an overview of modern machine tools and robotics and gives a refreshingly clear explanation of the bewildering acronyms that surround the subject. This leads to a consideration of the criteria governing the selection and sizing of a motor-drive system. As elsewhere in the book the author provides a useful summary to the chapter and step-by-step guides to calculation with, for example, intermittent duties. He continues with a chapter dealing with the principles and selection of velocity and position transducers. The main types of control motors: dc servomotors, brushless permanent-magnet motors, induction motors with and without vector control, and stepper motors are considered in some depth together with their controllers. The final chapter on controllers for automation takes a systems viewpoint with continuous or sampled data analysis and assumes at least a rudimentary knowledge of Laplace and z-transforms. A substantial section on the characteristics of a range of modern power semiconductors is relegated to an appendix, raising the question as to whether such material is strictly relevant.

The line drawings are generally uncluttered and easy to follow whereas photographs reproduced from manufacturers' literature are all too often murky images of machines or racks of electronics. Many pictures have lost most of their impact by being reporduced in black and white and overall the presentation is badly dated.

Despite addressing a real need, at £47.50 it is difficult to imagine this book being recommended to the target readership, final-year undergraduates, other than for reference in university libraries.

Peter Marshall is deputy dean of engineering, University of Surrey.

Electric Drives and their Controls

Author - Richard M. Crowder
ISBN - 0 19 859371 6
Publisher - Clarendon Press, Oxford
Price - £47.50
Pages - 238

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