PM DC Motors

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