Limited-angle torque motors
A special type of brushless motor is called a limited-angle torquer (LAT). LATs are constrained to produce torque through a rotation angle of less than 180°. But they are widely used to operate servovalves, direct laser mirrors, position missile-guidance radar antennas, open shutters for heat-seeking sensors, and power other systems that rotate through small angles.
The rotor in a limited-angle torquer carries field magnets, and the stator supports armature windings (similar to the construction of conventional brushless motors). LATs, however, are wound single phase, unlike conventional brushless types, which are typically wound for two or three-phase operation. Single-phase construction eliminates the need for commutation circuitry.
Conventional brushless motors can also be used for limited-angle service. But when conventional three-phase brushless motors are used as LATs, only two of the three leads are used.
Armature windings in some limited-angle torquers are embedded in slots around the inside periphery of a laminated stator, a construction similar to that used with conventional brushless motors. In another design, the armature is toroidally wound on a slotless stator. Here, some stators are laminated and others are solid.
Slot-wound LATs exhibit higher motor constant Km than corresponding toroidally wound types. The primary reason is that a larger number of conductors can be exposed to the magnetic field.
In slot-wound LATs, heat is more easily conducted from the armature core to the outer housing than in toroidal versions, which can rely only on the mounting tabs for heat conduction. Thus, slot-wound types generally can carry heavier loads than corresponding toroidally wound motors. Slot-wound LATs, however, exhibit more torque ripple (cogging) and generate greater friction and hysteresis losses.
Cogging is essentially zero in toroidally wound LATs, a result of nonvarying reluctance path and relatively large air gap. Toroidally wound armatures, moreover, are typically molded onto the stator, which protects the windings from damage and holds them in place.
Uniform reluctance paths also make toroidally wound LATs suitable for use as limited-angle tachometers. Here, the motors are often used in pairs, one as a torquer and the second as a tachometer. The tachometer provides a reference speed signal for the motor-control circuit.
LATs produce torque through a rotation angle determined by the number of motor poles. Current of one polarity produces clockwise torque, and vice versa.
Manufacturers generally provide a theoretical torque versus shaft-position curve. Typically, the characteristic curve for LATs is represented by the positive lobe of a cosine function; that is, T = Tpcos(θN/2), where θ = angle of rotation and N = number of poles. The general torque characteristic for toroidally wound motors can be represented by a similar curve, but it may also have a flat portion.
The above equation approximates torque values only for the roll-off portions of the curves. Also, the actual torque-position characteristics may vary somewhat from that shown in the curves. In particular, the curves do not reflect the effects of armature reaction, which depends on both armature current level and field magnet.
In any case, the rotational range of a LAT is generally specified in terms of a so-called excursion angle. This angle represents the difference between the rotor position that produces maximum torque and the zero-torque point on the characteristic curve.
Limited-angle torquers are generally specified with a set of factors similar to those used for conventional brushless motors. Motor performance is determined with an identical set of equations.
Limited-angle torquers are currently available in ratings from 2.8 to 1,000 oz-in. The 2.8 oz-in. LAT, a two-pole motor, has a 90° excursion angle. Its stator is 0.7 in. in diameter, weighs 1.7 oz, and is rated for 80 W peak. The 1,000 oz-in. LAT, a 10-pole motor, has an 18° excursion angle. Its stator is 1.64 in. in diameter, weighs 45 oz, and is rated for 437 W peak.
LATs that have much lower and higher torque ratings are feasible. In addition, excursion angles smaller than 18° are possible, while the maximum possible excursion is 180°.
Limited-angle torquers generally are controlled through single-phase servoamplifiers. Single-phase PWM amplifiers are widely used for the application, but LATs rated up to a few hundred watts are more often powered by linear amplifiers.
Thermal limitations restrict linears to these low power levels. But in this range, linear amplifiers generally are simpler and less costly than PWM types.
Brushless motors in some cases have a cup-shaped rotor that rotates around a wound stator. The so-called inside-out motors power spindles in hard-disk drives, in certain high-speed air-conditioning and ventilation systems, and in other equipment calling for a high inertia. The high inertia makes possible very precise speed regulation.
Applications calling for high torque and low speed need brushless motors having a large number of poles -- in some cases up to 64. The arrangement, often known as magnetic gearing, is an alternative to speed reducers in slow equipment and eliminates the friction, stiction, compliance, and backlash that speed-reducing systems normally exhibit.
The high pole-count motors, which are often called ring motors or ring torquers, exhibit very low torque-ripple. Sinusoidal versions of the motors exhibit even less ripple.
Pancake motors, a type of multipole motor, often power robots, transfer machines, and other equipment that needs high torque at moderate speed. The motors contain a large-diameter ring magnet, two windings, and a disk-shaped rotor. The ring magnet contains from 8 to 16 poles. Speed is constant with torque up to about 67% of peak torque, at which point speed falls.
Permanent-magnet, disc-type stepping motors can also perform like dc brushless motors. The devices exhibit high torque and low inertia, resulting in a high power rate. Eddy-current and hysteresis losses are low, permitting operation at high speed. For a given power output, disc motors are appreciably smaller and lighter than conventional types. A back-emf scheme, moreover, can provide a sinusoidal output like that of a resolver.
Permanent-magnet steppers also operate like dc brushless motors. The motors develop more torque than hybrid steppers. At speeds up to about 3,500 rpm, they produce more torque than dc servomotors. The devices contain a high-resolution position resolver that costs little and imposes no size penalty.
Variable-reluctance (VR) or switched-reluctance (SR) motors also can operate as brushless dc motors. VR motors have salient poles on a soft-iron rotor. Motor action results from interaction between the rotor poles and a rotating field set up by the stator windings. The construction yields high torque-to-inertia ratios.
VR motors cost less than corresponding permanent-magnet brushless types. And because VR types call for unidirectional current, amplifiers for them cost less than those for conventional types. The motors are increasingly used in motion-control systems that require very high torque or high horsepower -- levels where the cost of magnets in conventional PM motors become excessive.
A 150-pole version of a variable-reluctance motor is widely used for powering robots and other machines calling for slow speed and precise positioning. The motors exhibit much wider bandwidths -- typically over 80 Hz -- than conventional motors. A 32-bit microprocessor-based adaptive controller adjusts frequency response in real time.
Another motor recently developed is generally classified as variable reluctance. But the products are sometimes thought of as hybrid permanent-magnet motors because they contain two permanent magnets mounted axially on the rotor. The motors are smaller, less costly, and operate at higher speeds than conventional dc brushless motors. They also develop up to 2,000 oz-in. torque and operate up to 5,000 rpm.