how does the power electronic drives employed in cranes?
Most large cranes run electrical motors. Depending on what power is available at the site, the crane may run from a generator or be powered by the grid. The power requirements depend on the size of the crane. Larger cranes may use a six-pole, three-phase, one-wye, 440-volt circut. Smaller cranes could use a three-phase 220-volt circuit.
The primary function of an electronic drive is to provide for operation at various speeds. Slow speed operation is used when moving a load into position. Faster speeds are used for moving loads over longer distances. When the operating speed is steplessly adjustable, ramped acceleration and deceleration control is easily provided as well. Controlled acceleration provides smoother operation because acceleration at a constant rate means accelerating with a constant force or torque. Electronic drives control torque and current during starting. They perform a function similar to a reduced voltage motor starter, but they do it steplessly, more efficiently, with more complete control of applied torque. They provide more torque per ampere of current than any other starting method. This is very beneficial for the frequent starting and stopping often required for cranes. Electromechanical motor controls for cranes require additional contactors, relays and other components to perform functions such as reversing, motor overload protection and braking. These functions are incorporated into the electronic controls with electronic drives. Electronic braking can return energy to the power supply and bring the load to nearly a complete stop before a friction brake is applied. For holding the load in position, an electromechanical brake is still required. Electronic drives can be used on bridge, trolley and hoist applications.
