Motion system design is rapidly changing as smart motors take over more electronic and mechanical functions.
Electric motors, particularly small ones, are playing an increasingly important role in motion system design. Once relegated to essentially a single task — converting electricity to electromotive power — they’re rapidly taking over more functions, including data acquisition and control, network communications, diagnostics, and protection. The catalyst behind this transformation is the marriage between motors and electronics.
Like many products, motors are becoming smarter and more flexible through the integration of computer technology. And with each new hardware and software improvement, designers must rethink not only the role of motors, but also that of related motion components.
Simple but complex
Perhaps the best place to start when analyzing the impact of electronics on motors is the selection and application process. For comparison, consider the steps involved in selecting a conventional motor.
The first step is to size up the torque requirements of the application. Typically, a general-purpose ac motor will suffice, but some applications may require special high-torque motors that could be either ac or dc.
If the application calls for an ac motor, the next step is to match base speed and output torque to the load. This means analyzing horsepower ratings at a specific speed, say 3,600 rpm. Choosing a suitable motor is just the start, however.
Next on the list is selecting ancillary devices such as contactors and thermal overload relays. Each decision is a time consuming process, involving a fair amount of thought. Contactors, for example, come in many types, such as single direction and reversing, and must be carefully selected to meet application demands. Many applications also require some sort of mechanical speed converter because conventional motors rarely produce the required rpm.
Such complexities are further compounded when one considers inventory and costs. One problem is the accumulation of cash resources in inventory kept on-hand to meet lead time requirements. Dealing with multiple vendors also increases costs, as does maintaining a stockpile of spare parts to ensure timely product support.
Extending their reach
As motors evolve, however, the selection and application process is becoming simpler and more efficient. Small wonder. Today’s new breed of intelligent motors — those that incorporate electronic circuits in conventional NEMA packages — provide a wide range of features and functions.
Such motors, for example, offer multiple set point speeds and are easily programmed to run at any one speed setting. And depending on the type of motor, starting torque may be adjusted from around 30% to 300% or more. Users, therefore, needn’t be as concerned about motor speed and, in some cases, related issues such as gear ratios and pulley diameters.
Intelligent motors also incorporate their own protection and control functions, minimizing the need for ancillary components. Software embedded in the motor’s onboard computer, for example, replaces thermal overload relays with multiple, adjustable set points. The same is true for soft start controllers and reversing mechanisms. Smart motors also do away with contactors, responding directly to low-voltage switch closures and PLC outputs.
Optional plug-in cards expand functionality even further. A low-cost card can transform a basic motor into a simple adjustable speed drive or a two-speed motor substitute. A more sophisticated card, on the other hand, could emulate a fully programmable, variable-frequency drive. And by processing input/output signals, plug-in cards can also provide an efficient interface to actuators, sensors, and controllers on the same network as the motor.
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Combining brains and brawn
Almost any motor can perform more intelligently with the addition of electronics, but switched reluctance (SR) motors are a special case because, without highly integrated electronics, they would be nearly impossible to operate and control.
Switched reluctance motors work like electronically commutated dc motors. But unlike their dc counterparts, which employ wound or permanent-magnet rotors, SR motors incorporate rotors made entirely of laminated iron. Electromotive force — produced by sequencing the phase windings with electronically controlled magnetic stator fields — propels the rotor, generating high starting torque and better acceleration/ deceleration rates than comparably sized ac motors.
To get dc-like performance from a rugged, ac-like design, SR motors employ the basic physics behind conventional solenoids. In essence, a SR motor is nothing more than a solenoid wrapped in a circle. The reluctance of the electromagnetic circuit, which includes the stator coils and rotor poles, pulls the rotor into alignment with the rotating magnetic field. This process produces radial magnetic forces, which are about ten times stronger than the circumferential forces that power induction motors.
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Tom Dale is Product Manager for the U.S. Electrical Motors Div. of Emerson Electric Co., St. Louis, Mo.