Conventional AC operated electric overhead travelling (EOT) cranes uses slip ring induction motors whose rotor windings are connected to a power resistance. Speed control is performed by changing the rotor resistance in 3 to 4 steps by power contactors. Reversing is performed by changing the phase sequence of the stator supply through line contactors. Braking is achieved by a plugging operation. A crane control system has been developed using a variable voltage variable frequency drive and a programmable controller which has the advantage of continuous speed control; reversing is achieved by changing the phase sequence through an inverter. The main advantages of thi system are precise positioning, energy saving and increased motor life. This paper focuses on the application of variable voltage variable frequency induction motor drives in crane applications.
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