At first glance motors appear to be complicated machines, and in truth they are. But the principle of operation, electromagnetism, is relatively simple, understood even by high school students. Differences aside, today’s various motor technologies are at heart quite similar and quite understandable.
The origin of the earliest motors — machines that convert electrical energy to mechanical power — can be traced to designs conceived by Michael Faraday. In 1831, Faraday formulated the fundamental concepts of electromagnetic induction, noting that a current-carrying conductor in a magnetic field sees a force proportional to the strength of the field and the current passing through it.
Electric motor design, both then and now, centers on the placement of conductors in a magnetic field. The conductors, of course, are in the form of windings with many turns of wire, each contributing to the intensity of the electromagnetic action. The more current, Faraday pointed out, the more force (torque) you can expect. Motion, the ultimate goal, is thus the result of two magnetic fields (one on the rotor, the other on the stator) pulling on each other. This concept is the basis of all dc and ac motor designs, and the starting point for modern motion engineering.
ABC’S of ac
Alternating-current (ac) motors are the most widely used motors in the world. They are essentially constantspeed devices, as determined by the number of magnetic poles and input frequency. In general there are two types of ac motors — induction and synchronous.
Induction motors may be viewed as a type of transformer, the primary winding corresponding to the stator and the secondary to the rotor. Applying a voltage to the “primary” does two things: It forces current through the stator while inducing current in the rotor. In other words, it sets up a magnetic field in the stator, while inducing a second field on the rotor. The interaction of these two fields is what makes the rotor move.
The speed of the magnetic field going around the stator determines rotor speed. The rotor will try to follow the stator field, but will “slip,” particularly when a load is applied. Induction motors, therefore, always run slower than the rotating stator field.
The stator in an induction motor consists of steel laminations and turns of copper wire. The rotor, on the other hand, is typically made from stacked laminations with large slots on the periphery. In a “squirrel cage” rotor, the slots are filled with copper or aluminum bars short-circuited by conductive end caps. This “one-piece” casting usually includes integral fan blades to circulate air for cooling.
Standard induction motors operate at a “constant” speed determined by standard line frequency. There are ways, however, to control speed. Microprocessor- based drives using vector control technology, for example, manipulate the magnitude of the magnetic flux in the rotor and stator fields, achieving a sort of variable-slip response. With an appropriate feedback sensor this control method is viable even in positioning applications.
Although very demanding jobs — such as rapid start-stop positioning — would be out of the question, some indexing applications are, nevertheless, feasible. The limiting factor, though, is heat. As motor size goes up to keep temperatures in check — larger motors cool better — the torque-to-inertia ratio become prohibitive to speed.
The advantages of induction motors are well known, including low initial cost, availability of standard sizes, reliability, and quiet, vibration-free operation.
Synchronous motors are similar to induction motors, differing mainly in rotor construction. The rotors are designed to rotate at the same speed as the stator field, hence the name “synchronous.” There are basically two types of synchronous motors: self-excited (like induction motors) and directly excited using permanent magnets.
Self-excited synchronous motors (sometimes called reluctance synchronous motors) employ a rotor with notches, or teeth, on the periphery. The number of notches corresponds to the number of poles in the stator. Often the notches or teeth are called “salient poles,” reflecting the fact that they create an easy path, a handle almost, for the magnetic flux field, thereby letting the rotor lock in and run at the same speed as the rotating field.
Directly excited synchronous motors (sometimes called hysteresis synchronous or ac permanent-magnet synchronous motors) employ a permanentmagnet alloy rotor. The permanent poles are, in effect, the “salient poles” and therefore prevent slip.
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An important consideration for synchronous motors is the “coupling angle,” the small distance by which the rotor lags behind the stator field. This angle increases with load, and if the load goes beyond the motor’s capability, the rotor will bog down, eventually pulling out of synchronism.
Synchronous motors are generally operated open loop, providing absolute constant speed for a given load within the limits of the coupling angle or “pullout” torque. The motors are not selfstarting, however, requiring capacitor or split-phase start windings (or special controls) that gradually increase frequency and voltage to get the rotor moving.
Synchronous motors may be used for speed control with the addition of a feedback device. They also accommodate vector control. In general, though, the rotor is larger than that of an equivalent servomotor, meaning slower response for incrementing applications.
There are many types of dc motors — brush and brushless, steppers, shunt wound and series wound — but all have one thing in common: easy speed control. This makes them a natural fit for servo positioning and speed-control applications.
Shunt-wound motors have parallel rotor and stator windings. The stator may connect to the same power supply as the rotor, or it may be excited individually. With separate supplies, rotor voltage can be varied — with respect to constant stator voltage — to adjust speed.
A parallel or shunt-wound connection between the rotor and stator provides a relatively flat speed-torque curve with good speed regulation over wide load ranges. Because of demagnetization, however, shunt-wound dc motors lack the starting torque of other dc winding types.
Series-wound motors place rotor and stator windings in series. This generates two strong fields, producing high starting torque. Typical applications include cranes and hoists; applications to avoid are those where the motor may lose the load and “run away.”
Compund-wound motors, in contrast, employ both parallel and series connections. The ratio between the rotor and stator fields determines the shape of the speed-torque curve. In general, small compound motors have a strong shunt field and a weak series field, translating to high starting torque and a relatively flat speed-torque response. Reversing applications are somewhat impractical because the polarity of both windings must be switched, requiring large power circuits.
Get into positioning
Positioning applications call for special motors. Stepper, ac vector, dc servo, and brushless dc and ac are among the most common choices.
Step motors are electromechanical devices that convert digital inputs (with the help of a controller) to analog motion. Though there are many types — solenoid activated, variable reluctance, permanent magnet, and synchronous induction — all index in fixed angular increments when energized in a programmed manner. In other words, rather than continuous motion, a step motor delivers a series of discrete angular movements of uniform magnitude.
Step motors are particularly well suited to applications where the control signals appear as pulse trains. One pulse causes the motor to increment one angle of motion; ten pulses equate to ten increments, and so on.
Most step motors are used open loop, which unfortunately invites oscillations. The cure normally requires a complex “loop-closing” circuit or a feedback device. Even with that, step motors are limited to about 1 hp and 2,000 rpm.
The workings of an open-loop system are best illustrated with an example. Suppose a step motor is used in a bin sorting application. Everything proceeds as expected as long as the motor can click off one increment per pulse. But if a mechanism jams and the stepper can’t move, the controller may not be aware of the problem and will continue sending pulses that are essentially ignored. It’s not that difficult to imagine that eventually, if the system loses too many steps, it could be placing items in one bin that actually are meant for another.
Permanent-magnet dc (PMDC) motors are a popular choice for incrementing (start-stop) applications. And with appropriate feedback they are quite effective in closed-loop servo control.
The magnetic engine of a PMDC motor consists of a stator field coming off permanent magnets and a rotor field induced by current passing through a commutator, or switching circuit, into the rotor assembly. The stator field is stationary, while the rotor field moves. Each time the two fields are nearly aligned, the commutator switches the rotor current. As long as the rotor field doesn’t catch up to the stator field, the rotor will keep moving. Rotational speed, how fast the rotor turns, depends on the strength of the rotor field; the more voltage, the faster the rotor turns.
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PMDC motors have linear speed-torque curves with relatively high starting (acceleration) torque. The linearity owes to the permanent magnets; the torque-generating flux in the stator field remains constant at all speeds. Pound for pound PMDC motors are quite powerful, suited for rapid positioning applications.
Brushless servos can be dc or ac depending on the feedback device and control scheme. With Hall sensors, for example, a three-phase brushless motor typically energizes two of the three motor windings at a time. To make one mechanical revolution, the controller must sequence through six commutation sections, applying a dc voltage to each. The dc magnitude is directly proportional to operating speed, thus the term “dc brushless.”
Encoder feedback is used in applications that require position data. Some encoders are available with Hall outputs, which are used for commutation.
Resolver feedback also provides position data, but the control method is different. Here a sinusoidal waveform is applied to the motor windings, giving rise to the term “ac brushless.” The advantage over dc brushless is that for the same torque an ac brushless motor will draw less current. The controller, therefore, tends to be smaller and less expensive. This is to be expected when a three-phase winding is powered by a three-phase sinusoidal current.
Brushless motors are fast, produce a lot of torque in a small package, and have little inertia, translating to faster acceleration. They’re also good at low speeds (down to zero speed) and provide long, reliable maintenance-free life in demanding applications.
John Mazurkiewicz is director of motor development for Baldor Electric Co., Ft. Smith, Ark.