Following the article, “Which type of A-S drive is best,” that appeared in the April issue, this Part 2 discusses the characteristics and drive requirements for seven of the most frequently encountered applications. Although, these do not cover all the applications for adjustable-speed drives, they do contain requirements found in most installations, so your application will most likely have a lot in common with one or more of these:
• Centrifugal fans and pumps.
• Mixers.
• Extruders.
• Test stands.
• Conveyors.
• Metering pumps.
• Web-handling equipment.

Naturally, the importance of various selection factors varies within the same type of application. For example, the necessity to use an existing motor may dictate drive selection more than, say, required speed regulation. Thus, again proving the adage that judgment replaces guidelines, and the following are just guidelines.

Moreover, for many applications, more than one drive type will fill the requirements, and various drive manufacturers offer different capabilities for the same type of A-S drive.

Centrifugal fans and pumps

A relatively few years ago, throttling devices regulated the output of centrifugal fans and pumps. Today, the common practice is to adjust the speed of the impeller with an adjustable-speed drive to regulate flow. Powering centrifugal fans and pumps now accounts for nearly 50% of the installed ac drives. The force behind the change: tremendous reductions in energy costs and improved reliability.

Centrifugal machinery usually:
• Requires high efficiency and power factor.
• Runs for long periods with few starts and stops.
• Operates at speeds more than 25% of base speed, except during maintenance.
• Does not require accurate speed regulation (typically 6 3% is sufficient) nor rapid speed changes. Changes occurring over several seconds are desired to avoid “thunder” and thumping, which are created when the pressure changes too rapidly in ducts and pipes.

Possible Pitfalls. Some centrifugal units, especially large ones, may:
• Have an impeller with a large inertia.
• Have large stiction to be overcome while starting. For example, a large fan with sleeve bearings left idle for a few days has both large inertia and little oil film. The combination of these two means the drive must supply above-normal starting torque.
• Rotate backwards while starting. If a pump is idle, water in a pipe above a pump often turns the impeller backwards. Also air can drive an unpowered fan backwards. In these situations, it is frequently necessary to start a rotating load without first bringing the motor to a stop.
• Require stopping a large impeller in less than 20 sec. To do this usually means selecting a drive with regenerative capability or large enough snubbers, as discussed in Part 1 of this series.
• Require a bypass starter to maintain operation even if an A-S drive should fail. Such an option is an across-the-line or reduced voltage motor starter connected in parallel to the drive.

Desirable drive. For applications devoid of the pitfalls listed above, standard ac (inverter) drives controlling high-efficiency motors will often be the most economical selection.

However, for those situations with any “unusual” conditions — including the pitfalls mentioned above — the drive must have the ratings and features to handle the conditions. Specifically, if a bypass starter is installed, all wiring and distribution equipment must be sized to handle the larger starting currents drawn when the bypass is used than when operating with the A-S drive.

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