Experts say the single most important advance in PM motor technology was the introduction of rare-earth magnetic materials in the mid-1970s. Samarium-cobalt and, to a lesser extent, neodymium has replaced AlNiCo and ferrite materials in many motors. Though rare-earth magnets cost more than other types, their increasingly wide acceptance over the past several years has made prices steadily decline. Maximum-energy product (MEP) is used to indicate the relative strength of permanent magnets. Samarium-cobalt magnets typically run from 18 to 26 MGOe while neodymium ranges from 8 to 35 MGOe. In comparison, the most widely used ferrite magnets only have an MEP of 1.8 to 4.5 MGOe. Rare-earth magnets make for small, lightweight motors with up to 50% more torque than ordinary ones having comparable dimensions. This allows a smaller rare-earth motor to do the job of a more expensive and larger conventional unit. The typical range of torque for both brush-type and brushless dc motors is from 7.0 oz-in. to 4,500 lb-ft. Rare-earth magnets have little rotor inertia because they are light and have small radial dimensions. Flux density is also high. The torque-to-inertia ratio for rare-earth brushless motors exceeds that for conventional brushless motors by 250 to 800%. Similarly, torque-to-weight ratios surpass those of conventional types by 40 to 90%, while power-to-weight ratios are from 50 to 200% greater. Additional improvements have been made to reduce torque ripple. There may be as much as 13 to 17% torque ripple, which causes cogging in switched brushless motors. This effect may be bad for some applications. One alternative is to use a sine-wave drive which has a theoretical ripple of zero (actually 1 to 2%). But sine-wave drivers can cost more than three times as much as switched brushless drivers because they use more complex circuitry. However, experts say sine-wave drivers are
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Rare-earth magnets are permanent magnets made of alloys of rare earth components. Rare-earth magnets have the strongest magnetic field of most magnets.
Industrial magnets are manufactured for industrial use. They include individual magnets, bulk magnet materials, magnetic assemblies, magnetic sweepers, magnetic lifts, magnetic sheet handlers, magnetic retrievers, and permanent / electromagnet combinations.
Brushless motors are synchronous electric motors that have a magnetically (AC induction) or electronically (DC) controlled commutation system instead of a brush-based mechanical commutation system.
DC servomotors are generally small and powerful for their size, and easy to control. Common types of DC servomotors include brushless or gear motors.
DC motors are most commonly used in variable speed and torque applications. They include brushless and gear motors, as well as servomotors.
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