Milton B Snyder

Mechanical adjustable-speed drives for pump applications are generally of the compound adjustable-pitch-sheave and rubber-belt variety, as illustrated in Figures 1 and 2. The integrated drive package converts constant input speed to an output that is steplessly variable within a certain range. The drive packages are usually driven by constant-speed ac induction motors and usually contains built-in gear reducers to obtain low output speeds.

Drive packages may be mounted horizontally, vertically, or on a 450 angle and are available in standard open or totally enclosed designs. Some of the possible mounting arrangements are illustrated by Figure 3. Speed ranges of 10:1 to 2:1 can be obtained with most units. A typical distribution of available output speeds is 4550 to 1.4 rpm, including drives with and without reducer gearing. Increaser gearing (offered as an integrally mounted package) provides speeds to 16,000 rpm.

Alternating-current induction-drive motors used with mechanical adjustable-speed drives usually operate at a speed of 1750 or 1160 rpm. The electric design characteristics of these motors comply with the standards if the National Electrical Manufacturers Association (NEMA). The NENIA design B motor with normal torque and normal slip characteristics is standard. NENIA design C (high torque low starting current) and NEMA design D (high torque, high slip) motors may be used when their specific characteristics are dictated by the application.

The mechanical mounting characteristics of the drive motors for mechanical adjustable-speed drives vary from one manufacturer to another. Motors of round body, footless, NEMA C-face construction are commonly used. Sometimes standard foot-mounted motors are supplied as an option in conjunction with a scoop support by some manufacturers or as standard practice by others until others may supply mechanically special partial motor frames with their drives.

For a given power rating, many manufacturers will supply drive motors with increased service-factor power. This increased power of the drive motor compensates for and overcomes the inherent mechanical losses of the drive. This in turn causes full-rated power to

FIGURE 1 Pump application utilizing typical mechanical adjustable-speed belt drives (Reliance Electric)
FIGURE 2 Cutaway view of typical mechanical adjustable-speed drive (Reliance Electric)

be developed at the output shaft of the mechanical adjustable-speed drive. Drive output power ratings are more fully discussed under "Rating Basis."

The input speed of the mechanical adjustable-speed drive is typically 1750 or 1160 rpm, as defined by the speed of the drive motor. The output speed of the internal adjustable-speed belt section goes above and below the drive motor speed. A maximum speed of 4200 rpm or greater is not uncommon for fractional and small-integral power drives. The need for stages of output gearing to obtain final output speeds that are usable for pump or other applications is therefore readily apparent.

FIGURE 3 Mechanical adjustable-speed drive mounting and special enclosures

Parallel-shaft or right-angle-shaft reducer gearing, at the option of the design engineer or user, may be incorporated as an integral part of the mechanical adjustable-speed drive package. Generally speaking, gear reduction is required when drive maximum output speed must be lower than 1750 rpm or drive minimum output speed lower than 583 rpm.

The American Gear Manufacturers Association (AGMA) does not define standards for reducers used in adjustable-speed applications. However, most all drive manufacturers produce reducers for these drives in accordance with accepted AGMA standards for constant-speed reducers.

Where infinite, or stepless, adjustment over a specific finite speed range is necessary, stepless mechanical adjustable-speed drives are generally most economical for standard pump application requirements. Initial costs are usually lower than for comparable electric or hydraulic systems, and the mechanical systems are easier to operate and maintain.

Reliability and accuracy of speed control are advantages of the mechanical adjustable-speed belt delve package. Construction details, size, and mounting dimensions are not standardized and vary with the manufacturer but all employ dual adjustable-pitch sheaves mounted on parallel shafts at a fixed center distance and a special wide-section rubber V belt to provide a compact assembly. Most of the designs utilize spring-loaded sheaves for control of belt tension.

A high degree of control and flexibility in application is offered by mechanical adjustable-speed belt drive power packages. Capacities range from fractional to 100 hp (75 kW), with maximum speed ratios of 10:1 in the fractional power sizes, decreasing to 6:1 at 30 hp (22 kW) and 3:1 in the largest sizes. Multiple-belt drive arrangements are usually required for capacities over 50 hp (37 kW). Again, depending on capacity, speeds ranging from a maximum of 16,000 to a minimum of 1.4 rpm are possible, although most of the standard units are within the 2 to 5000 rpm range.

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