Variable Frequency Drives Reduce Overhead Crane Mechanical Failures
Variable Frequency Drives (VFDs) used on the bridge, trolley and hoisting motions improve the usability and safety of overhead crane and hoisting equipment by enabling low speeds for precision as well as preprogrammed ramped acceleration and deceleration which reduces or eliminates load swing. A swinging load is potentially dangerous to workers, production equipment, and the load itself. Another significant benefit of Variable Frequency Drives is the elimination of many of the mechanical stresses that can put an overhead bridge crane out of service.
Without VFDs, the bridge crane and trolley motions use reversing contactors, which like a light switch, can, in most cases, be immediately powered to full speed. That action places high stress on the motors, couplings, and gearboxes of the trolley or bridge.
Making matters even worse is that the reverse travel direction can then be immediately powered to full speed in the reverse direction (this is called reverse plugging). Imagine the same stress on the bridge or trolley drive train as a car transmission being thrown in reverse while going forward 10 miles per hour. Not a great practice for the longevity of the equipment!
Another damaging practice is when the operator wants the precision placement of a load, they often repetitively push the trolley or bridge direction button to inch the load. This practice places high levels of stress on the gear train as well as the motor and brake.
Decades-old equipment was often sized for these forces, but today’s lighter designed equipment will not hold up to this style of operation. In addition, without variable frequency drives the trolley or bridge disc brakes are fully engaged to stop the crane when the travel motion button disengaged. The immediate stopping forces are again placed on the couplings, drive shafts and gearboxes as well as the brake. Over time the brakes will require adjustments and disc replacement.
Variable Frequency Drives to the Rescue!
Variable frequency drives allow for programming of the acceleration of the load when the travel motion is engaged as well as programmed deceleration when the travel motion is disengaged. If the travel direction is suddenly reversed, the drive will decelerate the load and then ramp back up in the reverse direction. This deceleration and ramping will dramatically reduce the damaging stresses on the bridge and trolley drive train. An added benefit of VFD’s is that they stop the travel motions using dynamic braking so that the mechanical disc brake is relegated to a parking brake. The dynamic braking uses the motor as a generator to slow the load. The energy is released from a dynamic braking resistor in the form of heat. This dynamic braking function eliminates virtually all disc brake wear and tear.
Fortunately for crane and hoist users, the cost of VFD’s used on the bridge and trolley travel motion has reduced to the point that they are standard features in 90% of cranes sold today. This single technology advancement has increased safety and productivity as well as reduced maintenance and repair costs more than any other improvement in the industry.
Users that continue to specify or purchase contactor driven trolley and bridge drives must insure that their operators are trained to not repetitively plug or reverse plug because that could lead to early failure of the motors, brakes, or gearboxes. The good news is that VFDs can be very easily and inexpensively retrofitted to existing hoist trolley and bridge cranes to reap the many benefits.
CRANE 1 features Magnetek VFD control solutions. For additional information or assistance in upgrading your crane, hoist, or trolley to the many benefits of VFD technology, contact your nearest CRANE 1 office.