Vertically elevating loads is a ubiquitous task in engineering, but prevailing over gravity while maintaining safety and accuracy calls for myriad approaches. Leveraging mechatronics for failsafe backups is one trend; trimming consumption and recapturing energy is another. As in many other motion applications, sensors and microprocessors are proliferating. However, this technological renaissance is particularly dramatic with regard to elevator, lift, and hoist designs, due to their initial costs.

A government contractor for satellite simulator equipment used a Serapid scissor-guided platform lift. The lift (5.5 ft wide by 14 ft long) raises 6,000-lb loads over 13 ft and holds position without power for up to a week at a time.

A perfect example is the adoption of variable frequency drives (VFDs) in cranes and elevators: Hoisting speeds have skyrocketed, and for cranes, capacities exceed 1,000 tons. Explains Jeffrey Iacco, president of overhead-crane manufacturer and distributor IWI Inc., Cleveland, “A lot of older cranes have dated dc controls, but during strategic retrofitting, we fit these with new, more reliable motors and VFDs — the latter allowing for more flexibility in setup and speeds.”

Iacco adds that his company has been converting cranes to VFDs for the last decade, because the change allows plants to consolidate operations onto ac power — a far more universal power supply than dc.

Common nonload brake hoists (which leverage motor torque rather than a mechanical lock to lower, hoist, or suspend loads) employ VFDs in either open or closed-loop vector control, depending on the lift's mechanical design and class. Load-brake hoisting is operated in open-loop vector control — which accurately controls torque over a wide speed range, just like dc drive control.

The lack of shaft position feedback necessitates rotor tracking by other means, but eliminating the feedback device, VFD input, and cabling can offset the slight motor-performance loss. This setup requires no dynamic braking (which dissipates energy via the VFD out to resistors) as the load brake controls the rate of decline and absorbs the descending load's potential energy.

On this automated engine paint line, Magnetek, Inc. supplied componenets to boost flexibility and capacity - with distance detection and collision avoidance. A PLC and limit switches, infrared sensors, or lasers execute these function; radio remote control allows communication between individual engine carriers. Meanwhile VFDs power the carriers through load, wash, dry, paint, and cooldown stations.

“We notice cyclical swings in the percentage of retrofit versus new crane builds, and lately, 80% of business has been new build — likely due to the improving economy,” Iacco mentions. New units are used in myriad applications. Currently, IWI is executing an installation for a packaging company that produces salad bowls and trays; they are also fitting a crane for a hydraulic-cylinder manufacturer. “However, new builds are fit with ac drives at the onset,” Iacco concludes.

Elevators are enjoying technological gains as well. “Elevator controls, which for decades were technologically behind the curve, are now pretty close to the curve in turns of their modernity,” explains David Fried, vice president of Van Deusen & Associates (VDA), New York and Livingston, N.J. Originally, elevator drives were either ac or dc rheostatic — or used variable voltage with a motor generator set, what was called the Ward Leonard system. Increasingly, newer elevators and high-speed varieties for high-rise buildings run on ac power instead of dc. “It's true that some large dc machines are driven by semiconductor-controlled rectifiers (SCRs) of various sorts instead of motor generator sets,” says Fried. Relay-logic controls are superseded by solid-state components. “Even so, though SCRs are a fairly recent addition to the lineup of elevator componentry — only appearing within the last 20 or so years — we're already seeing them superseded by Power Factor 1 types of drives that are even more efficient.”

Carefully metering out power to motors with E-Save Technology improves escaltor and lift efficeincy. Here, annual energy and cost saving estimates are based on Nevada Power calculations for 24/7 operation.

Most modern high-speed elevators found in high-rises sport motors powered via variable-voltage/variable-frequency ac (VVVFAC) drives. Says Fried, “Until recently, VVVFAC drives could only drive at about 50 hp at relatively slow speeds of around 350 fpm, but now the newest units can power 200-hp machines moving elevators at up to 2,000 fpm in the highest-rise buildings.”

Mechanical considerations

On cranes, the mechanical gears, wheels, and bearings can last decades — even if the runways or trolleys require replacement. According to Iacco, where cranes are repurposed, their relocation from one bay to another typically requires a resizing of spans — which can require on-site reconfiguration, welding, and so forth. Even so, older trolleys with cast-iron frames, drums, and gear cases can be replaced with modern, compact units, and in many cases, boost crane capacity with trolley-weight reduction alone. Modern precision gearing can also reduce vibration, wear, and noise common in older cranes, while flux vector control boosts functionality. However, Iacco advises, “Plant engineers leading a repurposing or new crane purchase must have a reasonably accurate estimation of what the crane's duty cycle will be. Will the crane be used infrequently, or will it function as part of a production line operating 24 hours a day?”