Frequency control of crane hoist speed

The voltage of the stator is usually controlled with thyristors. To ensure stable control of the load, the control system requires information about the motor speed. Up to now, tachometers or encoders have been used to measure the motor speed, however, tachometers, with their associated equipment, tend to be mechanically sensitive and are often difficult to install, due to space constraints. The new control concept overcomes the sensitiveness and other constraints by using the frequency of the rotor voltage to determine the speed. It is expected to be widely used for heavy-duty new cranes where encoders are not desired and for retrofitting all kinds of cranes with slip-ring motors.