Modern Special Motor Designs
Introduction
Special motor designs are built for specific purposes. From the way they are designed to their characteristics show how this motors are used. Before designing a specialized motor, one should ask himself a few questions; what is the purpose of the motor? How much am I willing to spend on this motor? What characteristics am I looking to achieve from this motor. And finally what is the efficiency of this motor.
Some of these special motors include;
Conventional brushless motors; come in a variety of configurations. The most widely used look much like brush-type motors. But brushless motors have a wound stator that surrounds a permanent-magnet rotor, an inverse arrangement from that for brush motors. And stator windings are commutated electronically rather than through a conventional commutator and brushes.
Brushless motors generally contain a three-phase winding, although some operate four phase. Brushless motors powering small fans and other constant-speed equipment are often two phase.
Power for brushless motors generally is a trapezoidal ac wave form, but some of the motors operate with sine waves. Trapezoidal-powered motors develop about 10% more torque than those on sine-wave power. Sinusoidal-powered motors, however, exhibit less torque ripple and operate smoother at low speed. Thus, sinusoidal-powered motors are often used for machining, grinding, coating, and other operations calling for fine surface finishes.
Because they have no commutator, brushless motors are more efficient, need less maintenance, and can operate at higher speeds than conventional dc motors. High efficiency and small size are especially important for military, aircraft, and automotive applications, and for portable instruments and communications equipment.
The cost of both brush-type and brushless motors likely will rise because of increasing costs for iron, steel, copper, aluminum, and magnets. The increases will be partly offset by use of neodymium-iron-boron magnets. The magnets are often more powerful than samarium-cobalt magnets and promise to be much less expensive.
Amplifiers for brushless motors are more complex than brush types and generally call for two additional solid-state power switches. These switches account for most of the cost differential between the two types. But switch cost continues to drop, in part because of increased use of MOSFET and insulated-gate type switches. Costs also are dropping for ICs used in commutation, feedback interpretation, and PWM circuits. The lower costs reduce but do not eliminate the price differential between amplifiers for the two types of systems.
Linear DC Motors
A linear dc motor, like a rotating dc motor, generates mechanical force by the interaction of current in conductors and magnetic flux provided by permanent rare-earth magnets. It is constructed of a stator assembly and a slider. The stator assembly serves as the body and contains a laminated steel structure with conductors wound in transverse slots. The slider contains one or more sets of magnets, commutation components, a bearing surface, and its body completes the magnetic flux path between the magnets.
The brush-type slider carriers two sets of brushes. One set picks up the power from a pair of copper rails, and the second set transfers power into the conductors located under the slider through commutator segments. Two of three phases are energized at any one time.
The brushless slider contains an additional set of magnets which activate Hall-effect sensors and solid-state switches to commutate the motor windings. A dc linear motor positioning system is extremely stiff, fast, and efficient. It is capable of precision accuracy to 0.1 micron and does not deteriorate with wear. It can drive loads directly, obviating the need for gears and lead screws. Its typical range of thrust and travel is 2.5 to 2,500 lb and a few inches to about 4 ft.
Coreless DC Motors
The development of moving-coil or coreless motors dates back to the middle 1930s. But it wasn't until the early 1960s that they were produced economically enough to gain wide acceptance.
Major advantages of coreless motors include very low inertia, low mechanical time constant, and high efficiency. Because the core is ironless, its low mass allows more rapid acceleration and deceleration than any other class of dc motor.
Other benefits gained by eliminating the iron core include the absence of magnetic fields acting on the laminations. This interaction in conventional motors appears as torque ripple or cogging plus a resisting torque that decreases motor efficiency. The absence of iron eliminates cogging and the coreless motor operates smoothly, even at low speeds.
Elimination of the iron core dramatically diminishes rotor inductance and resultant arcing. Commutator arcing in conventional motors is caused primarily by the release of stored energy in the armature inductance upon commutation. Excessive arcing produces electrical noise and reduces the life of brushes.
Coreless motors are classified by rotor shapes as cylindrical or disc. Cylindrical rotors are further divided into those containing inside fields or outside fields. The disc types have pancake, printed, or three-coil rotors.
The cylindrical outside-field motor has the smallest mechanical time constant. The stator is a cylindrical permanent magnet surrounded by a mild steel housing. The rotor is a hollow cylindrical coil wound of copper wire and located in the center of the stator. A mechanical time constant of 1 msec is not unusual for this type of motor.
Rotors are typically wound in a skewed or honeycomb pattern (also known as Faulhaber winding) to ensure that all of the core helps produce torque and smooth operation. The flux lines extend radially outward from the permanent-magnet stator through the air gap. The soft iron housing is the flux return path which allows the air gap to be extremely small, producing a high flux density.
The cylindrical inside-field motor is a similar design, but the permanent-magnet stator is located inside the hollow rotor. The motor also features a low moment of inertia, but the mechanical time constant is typically higher than the outside-field motor because of smaller stator magnets.
Coreless motor commutator and brushes are typically small, primarily because they are made of precious metals -- gold, silver, platinum, or palladium. In addition, a smaller commutator has lower peripheral speed, less wear, and accounts for a smaller motor.
Outside-field motors are usually selected for high acceleration. Because of this, the rotor coils must handle a large load torque and dissipate high heat produced by peak currents. To handle the torque, manufacturers strengthen the rotor with glass epoxy. Since the rotor does not have an iron core to act as a heat sink, the housing has ports for forced air cooling.
Recent advances in coreless motor design include the replacement of AlNiCo with samarium-cobalt stator magnets. Also, the mechanical time constant is reduced by as much as two times with aluminum rotor wire instead of copper.
Typical accelerations for these improved motors are 150,000 rad/sec2, but rates up to 1 million rad/sec2 are possible. This greatly exceeds the 30,000 to 50,000 rad/sec2 rates available with iron-core rotor servomotors.
Limited-angle torque motors
A special type of brushless motor is called a limited-angle torque (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 torque 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 torques 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 spotless 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 non varying 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 torque 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 torques 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 torques is 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 peaks. 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 peaks.
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 torqueses generally are controlled through single-phase servo amplifiers. 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 linear to these low power levels. But in this range, linear amplifiers generally are simpler and less costly than PWM types
Reference
- http://www.electricmotors.machinedesign.com/BDEList.aspx
- http://www.faulhaber-group.com/n378492/n.html
- http://www.faulhaber-group.com/n378421/n.html
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