CONTROL TECHNOLOGY HAS REVOLUTIONIZED CRANE PERFORMANCE
While technological advancements have significantly improved the structural and mechanical portions of an overhead crane, the process has been more evolutionary than revolutionary. What has changed dramatically, however, is how overhead cranes and hoists are controlled.
Control systems are the most important aspect of an overhead crane or hoist and without them, the machine would not function nor perform in a safe, reliable, efficient manner. Many older cranes rely on contactors to switch regenerative load currents to resistor banks to dissipate energy. Depending upon the duty, costly contactor tips must be replaced frequently, increasing labor expense. Switching is best done with state-of-the-art AC or DC electronic controls, designed to meet current operational needs. Even owners of cranes with early versions of electronic control may have difficulty obtaining parts such as original printed circuit boards, or the manufacturer may no longer support their particular product. The present trend toward precision handling of materials has created a demand for simple, dependable, high-performance controls capable of communicating with computers and various other operator interfaces. Demand is also increasing for crane diagnostic capabilities, which provide remote monitoring of drive performance.
AC CRANE CONTROL MODERNIZATIONS
Perhaps no other technological development in the past 30 years has done more to revolutionize overhead crane design than the AC Variable Frequency Drive (VFD). Today’s Variable Frequency Controls and Flux Vector Motor Controls have become the industry standard for crane control. Recent improvements in drive technology, such as a new generation of Insulated Gate Bipolar Transistors (IGBTs), sensor-less vector control, powerful microprocessors with flash memory, and improved algorithms, have allowed the downsizing of the power platforms and the inclusion of many high-performance features.
AC Variable Frequency Drives provide greater reliability; enhance performance; improve production throughput; prolong equipment life; and reduce maintenance costs, parts, and personnel in a variety of ways:
They reduce the characteristic high starting currents of AC induction motors, thus minimizing the shock effect on both the load and the equipment. Features, such as programmable acceleration and dynamic braking produce softer stops and starts, assure smooth transitions between speed steps, and greatly reduce brake wear and required maintenance.
- Crane specific firmware, drive support tools, and wired/wireless communication with various peripheral devices provide valuable monitoring, status, performance, and diagnostic information. Features, such as Reverse Plug Simulation, Stall Prevention, Inching Control, Micro-Positioning Control, and Static Stepless “Simulation”, are built in and individually selectable, further enhancing the crane’s performance.
No two applications are the same. Variable Frequency Drives provide the crane or hoist user with a variety of operating modes and a wide range of options.
- Most single-speed squirrel cage motors can be controlled by VFDs, including conical-rotor motors used by many European hoist manufacturers. Multiple control modes (two, three, and five step or infinitely variable speed) allow for changing the operation to suit the needs of specific applications or the desires of individual operators. A wider range of selectable speed choices (up to 1500:1) are available to the user compared to the fixed speed ratios provided by typical two-speed motors (2:1, 3:1, or 4:1) or micro-drives (10:1).
- Safety is built into modern Variable Frequency Drives. They include features which reduce the possibility of lifting an overcapacity load, minimize or eliminate dangerous load swing, prevent overheating of the motor, detect and minimize rapid increases in motor torque, and provide safeguards that limit unauthorized modification of drive parameters or programming unsafe parameters.
APPLICATION-SPECIFIC SOFTWARE ENHANCES PERFORMANCE, SAFETY, AND PRODUCTIVITY
Simple, innovative, low-cost, application-specific software options can be added to many VFDs to meet unique application requirements and enhance performance, safety, and production throughput including Sway and Bucket Control, Drive Synchronization, and Footbrake/Static Stepless Simulation.
Sway Control improves the accuracy of load placement and reduces material damage caused by incidental contact of swinging loads without the need for external Programmable Logic Controllers (PLCs) or costly height measurement devices.
Bucket Control lets you control the various motions of a multi-line clamshell or grapple bucket that utilize two independent open/closing line hoists without the need for a PLC or master/slave arrangement. Features such as “Bucket Position Indication”, provide the operator with the open/close status of the bucket while dredging (underwater) or when the bucket just isn’t visible; “Speed Matching” maintains the bucket in the open position while lowering; and “Torque Sharing” keeps a full bucket closed while raising.
Drive Synchronization offers increased safety by precisely controlling motion and preventing the operator from making an uneven lift. It can operate a multiple-host application either independently or synchronized, synchronize a cable reel to a hoist, synchronize multiple trolleys on a single bridge, or synchronize multiple motions between two or more cranes, without the use of a PLC.
Footbrake/Static Stepless Simulation allows the operator to use a footbrake to either augment or completely control the deceleration and/or stopping of the crane while at the same time providing improved “Reverse-Plugging” response; eliminating current spikes and excess mechanical torque/stress on the drive train; and allowing quick, smooth starting or changing of direction; reducing maintenance costs; and easily interfacing with existing induction masters, foot brakes, and motors. “Static Stepless Simulation” provides the best of all worlds, by combining the intellect, judgment, dexterity, and other positive human traits of the crane operator with the latest in crane control technology and safety
CRANE 1 is a Performance Partner of Magnetek and features and stocks drives and control products for overhead cranes.