Flux vector drive overloads8 December 2004
The US Navy has reported that a crane with a frequency-controlled hoist drive was overloaded to double its rated capacity because the drive was set incorrectly. The crane features a user-programmeable AC flux vector drive. “On this crane, the torque (current) limit parameter was set to 250% of full load motor current. This roughly corresponds to a lifted load of 250% of the crane's capacity,” the Navy memorandum said. “Additionally, a load check function that would have prevented the drive from lifting more than a user-set load was not utilized.”
The memorandum continued: “Naval activities should be aware that there are programmable parameters available to prevent an overload condition and/or provide current limits minimizing the risk and damage in the case where an overload occurs.” In particular, the Navy recommended setting overtorque detection levels between 125%-150% of full load motor torque to provide for load acceleration. It also recommended that users activate load check features to make sure that user-programmed load limits are not exceeded.