Patent ID: 7350883

Claim:
A method of checking for excessive drag in an elevator system having a car moveable in a hoistway, characterized by: initially, establishing baseline currents while the elevator car is operating properly with no drag, by: (a) with the car either empty or carrying a load which is a small fraction of rated load, recording ( 10 ) a motor current at a plurality of predetermined steady motor current conditions while moving the car upwardly, and recording ( 11 ) a motor current at a plurality of predetermined steady motor current conditions while moving the car downwardly; (b) with the car carrying a load which is either 100% or a high fraction of rated load, recording ( 12 ) a motor current at a plurality of predetermined steady motor current conditions while moving the car upwardly and recording ( 13 ) a motor current at a plurality of predetermined steady motor current conditions while moving the car downwardly; then, during normal operation of the elevator car over time, during at least some normal runs of the elevator car, comparing the motor current used to operate the car with motor current predicted to be required to move the car under its present load, direction and position in the hoistway, by: (c) when the doors are closed ( 21 ) at the beginning of a run, recording ( 22 ) an actual car load, a current floor number ( 25 ) and direction of the car ( 26 ), and from that and the currents recorded in steps (a) and (b), predicting ( 28 ) the motor current required at one of said predetermined steady motor current conditions, including said actual car load and said direction of the car, related to said current floor number; (d) recording ( 33 ) an actual motor current at said one of said predetermined steady motor current conditions; and (e) if said actual motor current exceeds a predicted motor current by a tolerance value ( 34 ), shutting down the elevator car ( 39 ) at the next committable floor.