This second part of the series gives the types of adjustable-speed drives best suited to common applications and how to avoid familiar pitfalls. See April 1994 for Part 1.
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.
• Test stands.
• 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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Available in an almost infinite variety of sizes, shapes and design, mixers do share a few characteristics, whether they are designed for mixing liquids or solids. The rotating action frequently changes the viscosity of the mixture. Some start with highly viscous material, and the mixing action reduces the viscosity. For others, the change is just the reverse. Moreover, almost all mixers give off gas or particles suspended in air.
Possible Pitfalls. Three factors must be carefully considered:
• Magnitude of highest torque requirement, frequently the starting torque.
• Composition of the vapor or dust produced during the mixing action. Many such applications give off flammable, toxic, or corrosive vapors or dust.
• Need to wash down the motor or drive controller.
Desirable drive. A wide range of drives are successfully applied to mixers, including dc drives, brushless dc, standard ac, and vector ac drives. With any of these drive types it is essential to supply a system that can deliver the needed maximum torque. If you are considering an ac drive, and if the starting, or maximum, torque exceeds 100% of the normal full-load running torque, a vector ac drive should be used rather than a standard ac drive.
The enclosures of both the motor and controller must withstand the environment. Many mixing applications require wash down motors and drives.
For explosive environments, motors are available with explosion-proof enclosures. However, drive controllers must be located away from the hazardous environment.
Whether designed for extruding plastic, metal, or spaghetti, extruders operate at full torque at low speeds for long periods. Also, they must typically maintain the set speed within 1% or less.
Possible Pitfalls. Underestimating the required torque, neglecting to consider the abrasion of some extruding compounds, extended speed ranges, and high-ambient motor temperatures are the most common oversights.
Desirable drive. Extruders have been traditionally powered by dc drives. The dc motors are usually equipped with tachometer-generators (tachs) to provide precise speed feedback and separately powered blowers to enable operation over a 2:1 or more speed range.
Recently, brushless dc drives and vector ac drives have gained popularity. The ac drives are often less expensive on extruders requiring drives of 50 hp or less. DC and brushless dc drives are typically more economical on larger extruders.
However, many factors influence the total cost. Such factors include required speed regulation, environment, and maintenance capabilities, in addition to speed, torque, and power requirements.
As with dc motors, blowers are also needed on ac motors that operate over a 2:1 or greater speed range. An alternative to the blower cooled motor is to install an oversized motor, but this is usually an expensive approach.
If a motor is subjected to an ambient temperature above 40 C, separate cooling air may be required.
Testing the operation of fuel pumps for jet engines, alternators for many types of vehicles, as well as quantifying the output from engines requires as test stand. These can power a unit or absorb power produced by a unit. Many characteristics are common to both cases:
• High speeds.
• Accurate speed regulation.
• Speed verification.
• Torque (current) regulation.
• Method to quickly decelerate the driven equipment.
In addition, some test stands require high speeds — above 3,600 rpm and a few reach 10,000 rpm.
Possible Pitfalls. The most common problem is not fully understanding the quantities to be tested, nor the details of the verification.
Also, the nonlinearities of A-S drives can effect test accuracy. Modern drives are accurate, smooth running, and suitable for most applications. However, there is output torque ripple, small instantaneous speed changes, and magnetic field variations. These can excite mechanical resonances in some test systems, and even small amounts can ruin some tests.
Desirable drive. Several drive types are successfully applied on test stands — dc drives, dc brushless, and ac vector drives are selected when torque regulation is needed. If it isn’t, standard ac drives can also be used.
For speeds above about 3,600 rpm, ac motors and drives are usually the best choice. Special motor balancing and other special motor features may be required. The motor manufacturer can provide valuable guidance.
Kenneth F. Deken is marketing manager, ac drives and industrial controls, for Reliance Electric Co., Cleveland.