2.4.3 Types of crane control systems

Older cab-controlled cranes usually have three phase slip ring type motors controlled by drum controllers, so called because of their upright drum shape. A crank rotates contacts within the controller and switches the amount of external resistance in the rotor circuit. This controls the maximum current generated in the rotor and thereby the torque and speed. The motor is started with high resistance giving maximum torque from zero speed. Resistance is reduced as speed builds. This type of control enables a skilled operator to start and stop the crane motions smoothly and control the speed.

When this type of crane was converted to pendant control, timers were required to control the switching of the resistance. This effectively controlled the rotor current but also quickly took the motor up to synchronous speed. In effect, most of the facility to control speed was lost. Also, it was often found that the speeds were too fast particularly that of the long travel motion.

New cranes designed for pendant control often use squirrel cage motors which quickly reach synchronous speed. Therefore, taking the tension at the start of a lift or positioning the load precisely can require rapid and frequent incremental movements by quickly pushing and releasing the appropriate motion button. This is not good for the motor or the control system.

Dual speed hoists offer a better solution and dual speed motors are sometimes fitted to the travel motions. Some dual speed motors have the same rating on both speeds. These are often controlled by separate push buttons; the slow speed being used for precise positioning and the fast for general movement when precision is not required. However, some dual speed motors have a much reduced rating on the slow speed. Its purpose is to provide a transition between being zero speed and fast speed and should not be used for prolonged periods. These are often controlled by a single dual position push button, a half depression giving the slow speed and full depression giving the full speed.

A more recent development is the inverter control system. The speed of an AC squirrel cage motor is controlled by the frequency of the power supply. Inverter control systems rectify the AC supply into DC then invert it back to AC using electronics which allows the frequency, and hence the motor speed, to be controlled. The result is a continuously variable control of the motor speed and the facility to brake through the motor. This system can be used in cab-controlled cranes and also pendant and remote controlled cranes. It works particularly well with remote controls where the operator station can be worn on a strap around the operators neck and incorporate joystick controls.