The Advantages of Variable Frequency Drives (VFDs)

Are you tired of the constant battle to keep your older overhead crane system up and running? Does it feel like you are constantly replacing worn-out parts and tearing apart gearboxes? Are you constantly taking your equipment offline to replace your crane’s worn down brakes? Hoosier Crane has an answer. We know a lot about overhead crane upgrades and modernization. If your crane seems to break down constantly or needs frequent repair, you may be able to extend its service life with a smarter and more efficient drive control system. We're talking about Variable Frequency Drives (VFDs).

VFDs were introduced to the material handling industry about 30 years ago. They were designed to allow greater precision and control of single-speed and 2-speed motors. Initially, they were only used on the bridge and trolleys of high-speed Class D and Class E cranes. Today, they’re used to control just about every motorized motion on a crane.

What are the advantages?

The biggest advantage is adaptability. Variable frequency drives allow operators or maintenance personnel to modify drive parameters, reset faults remotely, and troubleshoot from a remote location using fault logs, alarms, and drive monitoring. Lower-end systems designed for modular cranes come standard with 30-40 programmable control parameters. On cranes with single-speed or 2-speed motors, a variable frequency drive can be programmed to provide smoother acceleration and deceleration, and also can be programmed to add in additional speed points. For example, on a 2-speed motor with speeds of 40 fpm and 120 fpm, an additional control could be added to allow for a median speed of 80 fpm, or you could slow it down even more by adding in a 10 fpm control. On high-end VFD systems, there may be as many as 200-300 programmable control parameters used to dial in the controls for the bridge, trolley, hoist, and hook. A VFD can provide performance similar to DC controls, but with a single-speed motor. Variable frequency drives allow for acceleration and deceleration to be programmed similar to the feel of a Soft Start device. But, because the VFD controls both the voltage and frequency sent to the motor, the risk of overheating is eliminated for Class D, Class, E, and Class F high-use process cranes. On high-speed crane systems, the speed controls can be programmed to run the bridge and hoist down the runway at 300 fpm and with the flip of a switch, reduce that speed down to 50 fpm in a controlled manner, as the bridge approaches the end of the runway or a pick point.

Reduced Wear on the Brakes If you talk to any maintenance manager he or she will tell you the single greatest benefit of a variable frequency drive is that it eliminates or reduces brake wear. On cranes without VFDs, every time you let go of the button, mechanical brakes kick in to slow and stop the motion of the crane. This wears down the brakes over time, and in high-use crane systems, the brakes may need to be replaced every few months, or even every few weeks, due to mechanical wear. With a VFD system, once the button that controls the forward or reverse motion is released, the VFD controls the deceleration of the crane and slows and stops the crane in a controlled manner. Now, the brakes are primarily being used as a parking brake to hold the crane in place when it’s not in motion. This greatly enhances the service life of the brakes and reduces costly equipment downtime.

Adjustable Acceleration and Deceleration Times. On crane systems not equipped with VFDs, the stops and starts of the bridge and trolley are much more abrupt. Over time, these abrupt movements can cause excessive stress and wear to the following components: Drivetrain—including couplings, keyways, and gearboxes Trolley, end trucks, and wheels Runway beams and structural supports Wire rope or chain used to support the load. A VFD allows you to fine-tune the amount of time it takes for the bridge or trolley to get up to desired speed and the amount of time it takes to slow down to a complete stop—greatly reducing stresses on the crane components and also helping to prevent load swing.

Precise Load Positioning With a variable frequency drive, the bridge, trolley, and hoist can be programmed anywhere within a 40:1 speed ratio. For delicate loads, or loads that need to be very precisely positioned, the operator can nudge the bridge or hook along its path of travel in a very slow and controlled manner. The operator can run the load down the runway at higher speeds, and as it approaches the end of its travel, he can flip a switch that’s programmed to slow the crane down to a predetermined speed—say 10% of the top operating speed, for example—as he fine-tunes the final positioning of the load.

Load Limiting The drive system can be programmed to detect hoist overload conditions—halting the upward lifting motion if it detects that the hook load is reaching or exceeding a predetermined load limit. These can replace load cells in most applications. Torque limiting can also be used to prevent the crane from performing a lift that could overload the motor and cause mechanical fatigue.

Fault Codes and Diagnostics Software and hardware is available to allow operators, maintenance/production personnel, and even remote third-parties to program, monitor, and troubleshoot the drive systems. One crane or multiple cranes can be set up for monitoring and diagnostics related to the drive parameters and drive status. In the event of a problem, an alert with diagnostic information can be sent to a remote technician or even directly to the operator’s transmitter. In most cases, maintenance workers can diagnose and reset the fault without having to climb up on scaffolding or get into a lift to access the crane. If they do have to get up in the air to work on the crane, they at least have an idea of what the problem is and can be prepared with the right tools and parts to perform the repair or adjustment. Other diagnostics information that can be provided include: number of drives, number of cycles, and routine or preventative maintenance alerts based on the usage of the crane.