The enclosure selected affects cost substantially. Corrosive and hazardous atmospheres encountered dictate the protection needed.

Today's standard motor enclosure for indoor applications is the open, drip-proof enclosure for induction and high-speed synchronous motors. For large motors, open, drip-proof construction is available up to about 2Q,0(>0 hp and is used for squirrel-cage, synchronous, and wound-rotor motors.

For larger motors, the next degree of protection is weather-protected. Type I (WPI), which is an open machine with ventilating passages constructed to minimize the entrance of rain, snow, and airborne particles to the electric parts.

Where a higher degree of protection and longer life is desired, weather-protected, Type II (WP II) motors are recommended. This type of enclosure is used on the motor-driven compressors shown in Chapter 8, Figure 8-15. They are equipped with extensive baffling of the ventilating system so that the air must turn at least three 90° corners before entering the active motor parts. Maximum air-intake velocity is limited to no more than 600 fpm, and blow-through provisions are also provided. In this way, rain, snow, and dirt carried by driving winds will be blown through the motor without entering the active parts. The WP II are the predominant choice for large outdoor motors in petrochemical applications in North America.

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