Permanent Magnets in Machines by the Numbers

About this book

Permanent magnets are now in most machines, often where nobody notices them: in the rotors of servo motors, in the couplings of sealless pumps, in holding brakes, door catches and detents, in the separators that catch stray metal before it reaches a product, and in the sensors that tell a controller where a cylinder or a door is. They work silently for years, until a motor runs short of torque on a hot afternoon, a pump stops pumping with its motor still turning, a separator lets a fragment through, or a guard switch is defeated with a fridge magnet.

This book explains how permanent magnets behave in machines and how to design, specify and check them with numbers. It starts with what a permanent magnet is and how to read its demagnetisation curve, chooses between the main magnet materials, and finds a magnet’s working point from the load line of its circuit. It then deals with temperature and demagnetisation, which cause most magnet failures, before working through the main applications: magnetic couplings, eddy currents and brakes, servo motors, tramp-metal separation and sensing. The last chapter covers handling, storage, shipping and incoming inspection.

It is for machine designers, maintenance and reliability engineers, automation engineers and technicians. It assumes school physics and algebra. Each chapter works the running case with real numbers, includes one plausible mistake and how it was found, links the KEVOS® tool that does the chapter’s calculation where one exists, and ends with an exercise and its worked answer.

The running case

Every chapter follows one fictional production line. This is an illustration. A snack-food plant in Ballarat, Victoria, runs a seasoning and packing line, Line 4, with these magnetic parts:

• Clean-in-place pump: a sealless magnetic-drive centrifugal pump supplying 1.5% caustic solution at 75 °C to the line’s tanks and spray devices, driven by a 4 kW, two-pole motor.
• Metal separation: a grate magnet in the seasoning hopper and a plate magnet over the product conveyor before the weigher.
• Bagger: a vertical form-fill-seal bagger whose cross-seal jaws are driven by a permanent-magnet servo motor, with a holding brake on its vertical film-reel lift.
• Guards and sensors: magnetic guard interlock switches, a magnetic catch on the hopper lid, and magnetic position sensors on pneumatic cylinders.

Field strengths, torques, temperatures, prices and rates in the book are invented for the case or calculated from stated assumptions. They do not describe KEVOS® work or any real business, product or manufacturer’s rating.

How to use the book

Read Chapters 1 to 4 first: they set out the curve, the materials, the load line and temperature, which every application chapter uses. Chapters 5 to 9 can then be read in any order. Chapter 10 applies to every magnet in the plant.

Every calculation in the book was checked by running it. The models are simple, and each chapter says where they stop being reliable. A maker’s data for a particular magnet, motor, coupling or sensor always overrides an estimate from this book.

Source limits and professional responsibility

This book is educational. Its models explain behaviour and show which changes matter; they do not replace makers’ data, application engineering or testing of the real product. Machine guarding and interlocking, food safety controls, pressure equipment and electrical work are regulated, and decisions about them belong to competent people working to the model WHS laws, the relevant Australian and international standards and the makers’ instructions. Strong permanent magnets can crush fingers, shatter and affect pacemakers and other active implants; Chapter 10 explains the precautions, and people with implants should get medical advice before handling them.