Definite and Special-Purpose Motors
Product Announcement from Sinotech
Definite-purpose motors handle specific applications and have well-established NEMA standards. They are produced in high volume, and are low in cost when compared to general-purpose motors with the same ratings.
To apply a definite-purpose motor for a duty other than that for which it was intended must be carefully considered. Modifications can be made easily and inexpensively. Other modifications may require special tooling, increasing cost. The electrical characteristics of the motor must be checked very closely against the load. Testing the motor with the application is recommended.
Instrument motor definitions vary with users and manufacturers of motors. Generally, an instrument motor is a precision motor with fractional or subfractional horsepower ratings. Sizes range from 1/2 to 3/4hp units.
Some motors can be made with toothless armatures, thanks to powerful modern magnets. Coils are wound and assembled outside the motor, then inserted and secured as a unit in the armature.
Toothless construction provides more space for armature coils and provides higher current ratings. Iron losses are cut by 50%, and armature inductance approaches that for cup motors. Toothless motors do not cog at low speed. And because coil insertion is easier, motor diameters can be smaller.
In toothless motors that use ceramic magnets, flux density in the air gap is much lower than in conventional motors. The low density results from the large air gap, an inherent characteristic of toothless construction.
Toothless motors equipped with neodymium magnets, however, operate at high flux densities. These motors contain magnets that are about the same length as the air gap. With this relationship, neodymium magnets operate at flux levels that are close to the magnet's maximum energy product.
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