Practical design considerations for braking problems in overhead crane drives

In crane devices for controlling hoist and travel motions, squirrel cage induction motors with variable frequency can be used. By controlling the inverter, it is feasible to operate the induction motor in generating mode, leading to a regenerative braking action on the load. Starting from the differential equation describing the motion of the drive, the required power ratings of the braking systems are obtained. The influence of the inertia and the eventual driving components of the load torque are discussed. It is found that relying on the exchange of power between the different motions is neither acceptable due to safety reasons, nor technically feasible as the braking of one of the motions does not coincide in time with the use of power of the other motions. Therefore, other possibilities are given for electrically braking the load, i.e., for handling the braking energy. The use of a pulsed resistor has the advantage of being easy, reliable, and cheap; however, the braking energy is lost as it is dissipated in the resistor. Regenerative braking using a controlled rectifier, handling part of all of the drive power, is more expensive, but the surplus in costs is paid back by the avoided energy loss. However, it is not always possible to install a line-commutated controlled rectifier, as the internal impedance of the three-phase supply may be too high to allow natural commutation. A forced commutated rectifier offers the solution, but its price is high. Using two practical examples, the different aspects involved in the choice are illustrated