Jeff Lovelace
Baldor Electric
Fort Smith, Ark.

Most mechanical linkages for speed adjustment work well and are easy to understand. However, maintenance can become a headache in both labor and replacement costs. Too, if machinery is upgraded to operate more quickly, mechanical solutions must usually be replaced. In contrast, adjustable speed drives (or ASDs) exhibit no mechanical failures, and change speeds and torque with simple parameter entries, on the fly. Also, because processor speed and memory are constantly being increased, drive intelligence is always on the rise.

Belt-drive comparison

One common system used for speed adjustment is a belt drive. Variable-pitch pulley mechanisms on these systems change the speed of the load. In its simplest form, the ratio of an adjustable pulley is opened or closed to regulate speed, and a spring-loaded pulley self-adjusts in the opposite way. Several different manufacturers offer systems designed to specific turn-up or turn-down ratios. More advanced pulley systems are mechanically linked so that as one pulley is opened or closed, the opposing pulley is changed as well. This prevents belt slippage.

Instead of using one 20-hp motor with mechanical links to synchronize motion, four 5-hp drives with motors can be used instead. This eliminates the wear that can cause asynchronous motion.

As with any mechanical gear change, whether it be a true gearbox or rational pulley system, speed and torque are inversely proportional. As the load goes faster, torque drops. Conversely, as load slows, torque increases. This is useful in applications such as cutting, where high torque might be needed on a drill bit, but low torque (and high speeds) is needed for polishing and finishing cuts. Typical ratios are around 3:1, but can be as high as 9:1; horsepower is generally limited to about 50. Because these systems can be a safety hazard, guards must prevent contact with rotating equipment.

More sophisticated arrangements include a pneumatic or hydraulic system to make mechanical changes without manual adjustment. PID controllers make this automatic. However, load torque must be considered anytime this setup is used. If drivers turn faster than the load, available torque on the load goes up. This can pose a problem when a designer must change speed, but maintain available torque. Another issue is wear and tear. Over time, springs lose their effectiveness and belts loosen and begin to slip. If springs are overtightened, then belts tend to create more drag, and further losses occur. So if belt drives are used, much engineering effort must be involved up front for sizing and scheduled future maintenance.