A customer calls in and asks,
“Why must I have double shoe collectors on my crane? The National Electric Code (NEC) does not call for it.”
CMAA Specifications 70 and 74 require double shoe collectors when variable frequency control (vfc) is used in any motion of a crane. Article 610 of the NEC addresses the installation electrical equipment and wiring on cranes. It does not directly address specific control types, such as contactors or variable frequency.
The Crane Manufacturers’ Association of America (CMAA) publishes two specifications for overhead cranes: CMAA 70 for Top Running and Gantry Multiple Girder Cranes and CMAA 74 for Top and Under Running Single Girder Cranes. In both specifications, Section 5.14.7 reads, “A minimum of two collectors for each runway conductor shall be furnished with inverter use.” Inverter is another name for variable frequency drives or controls.
Inverters have fault protection built into their programming. Monitoring the incoming power is part of this programming. When a collector shoe loses contact with a conductor bar, for any reason, the inverter will go into fault mode and shut down if there is no second shoe to maintain contact. This is to protect the electronics in the inverter and motor(s). In addition, it will most likely register a phase-loss fault code. If this happens, a crane technician will have to troubleshoot and reset the inverter to get the crane back into operation.
There is a variety of reasons why collectors lose contact: normal wear, slight misalignment or adjustment, expansion joints, dirt, etc. Double shoe collectors are necessary to reduce the risk of these types of “nuisance” trips. Crane and trolley traverse motions, plus main and auxiliary hoist motions, can be controlled by an inverter. In addition, inverters can either be open or closed loop. Once any one motion is inverter controlled, double shoe collectors are required.
One final point, the use of double shoes does not replace necessary and required interval inspections and preventative maintenance of the crane and electrification system.
This blog post was written by Larry Lynn, former Product Trainer for Columbus McKinnon Corporation.