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A new design technique produces corrosion-proof and waterproof stepper motors, extending motor life in harsh outdoor conditions.
Electric motors used outdoors are susceptible to temperature extremes and moisture buildup, leading to internal corrosion that quickly shortens motor life. However, a new approach to preventing corrosion and early failure uses a system of rugged corrosion proof coatings that are applied to all metallic surfaces both inside and outside the motor, protecting them from moisture and corrosion.
The traditional solution to corrosion problems was to seal the motor in order to keep out moisture that leads to internal corrosion. Shaft seals and end cap/housing seals will delay - but not stop - the penetration of moisture into the motor. Temperature variations will eventually help moisture find its way through any seal. Because motors repeatedly go through cycles of heating and cooling, the air inside the motor expands and contracts; as the motor cools and the air inside of it contracts, a small amount of moisture is sucked inside the motor. This eventually leads to internal corrosion, to which hybrid step motors are particularly susceptible. Once the moisture is inside the motor, the seals act as a barrier to removal of the moisture. In addition, shaft seals typically cause a loss of 3 to 15% of shaft torque due to rubbing friction.
Previous motor coatings were simply not rugged enough to protect the soft-iron magnetic and metallic materials inside a step motor over its design life. The new technique applies the coating to every surface and structure inside and outside the motor, including the rotor, stator, and housing. Thus, every surface that could potentially corrode is covered with this coating, and each coating is tailored for each motor component. For example, the permanent magnets, aluminum endbells, and steel laminations use coatings, and upgraded base materials as needed to protect the parts from corrosion, while maintaining motor performance.
Corrosion can be a problem for all types of motors. Moisture and rust can lead to electrical shorts, bearing failures and overheating. In addition to the problems other motor types have with moisture and corrosion, step motors are particularly susceptible to having the rotor lock up. This is due to the extremely small air-gaps required for these motors to produce good torque. Even a small amount of corrosion can bridge the gap between the rotor and stator, and lock the parts together so the motor no longer rotates.
A hybrid step motor is a type of synchronous, brushless, permanent magnet motor that uses laminated steel or soft iron pole pieces. They incorporate fine teeth distributed around the rotor and stator to create a multi-pole machine (usually 50 or 100 poles). The large number of magnetic poles acts as a torque multiplier and simultaneously a speed limiter. To further increase torque, the air gap between the rotor and stator is extremely small, the smallest of any electric motor, with a typical gap of .002-in. or 0.05mm. This smallest of mechanical clearances between the stator and moving rotor makes the unprotected hybrid step motor particularly susceptible to the effects of moisture-induced corrosion.
Hybrid step motors operate best at speeds below 1,000 rpm, and have more continuous torque than equivalent sized conventional brushless motors. The motor moves in precise increments, called steps. For instance, a 50-pole, 2-phase, hybrid step motor achieves a 1.8-degree full step-by-step movement, while the 100-pole motor achieves a 0.9-degree full step-by-step movement.
http://www.designworldonline.com
http://www.moons.com.cn/moonsweb/english/index.htm
http://www.stoegra.de/pdf/sm_sonderversion_e.pdf