CONTROLS
Left and below: Handheld remote control systems from Autec and from Danfoss.
and they do this because of the fundamental revolution in control systems that has come about through variable frequency drives. Digital control systems of all types almost all share one or two basic technologies, and it is worth briefly surveying these to get some understanding of what they do. One of those technologies is the variable frequency drive. If you have an electric motor that runs on DC
(direct current) is easy to control. To make it go faster, you increase the voltage; to slow it down, you decrease the voltage. But if your motor runs on AC (alternating current), as the vast majority do, then increasing the voltage will have no effect. The speed of an AC motor is tied to the frequency of the alternating current, which in the UK and Europe on the mains supply is 50Hz (cycles per second) and in the US is 60Hz. To control the speed of an AC motor, therefore, you need to change the frequency of the alternating current that is supply – and until recently this was far from being easy. Black boxes that achieve this are called Variable Frequency Drives; they first emerged in the 1980s but only in the past couple of decades have they become reliable, widely available and mainstream. Cranes fitted with it have infinitely variable speeds (as opposed to the fast, slow or stop options of the past), and not only variable speeds but precision inching travel which helps deposit the load in exactly the desired position. “Variable speed drives are used to offer speed and
torque control of hoist and travel motors,” says Salter. “They are used a lot more now due to the favourability of their cost, reliability and operation. They offer a more flexible speed control, which can be tailored to suit a cranes operation.” Other advantages follow from that: “Variable speed drives also prevent harsher mechanical actions on components such as gears, wheels due to the softer start and stopping. For
safety reasons, hoist motors require additional speed feedback (usually via encoder) and this is why more basic cranes will still have DOL (direct online) control.”
Maintaining direct control at all times Digital control systems all rely on communication: data, speeds, positions, loads and the like must be passed from sensors to controllers and back to motors, actuators and safety cutouts. This would lead to a
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