OVERHEAD CRANE CONTROLS
Motor controls generally come in two varieties:
across the line contractor controls
variable frequency drive (VFD)
Across the line controls are generally limited to one and two speeds (dependent on motors used), and result in large inrush currents caused by the motor having to instantly run up to full operating speed. This is most noticeable with the hoisting motion. This also causes the movement of the crane to be erratic as the crane tries to switch from stationary to full speed in as short a time as possible. This also causes excessive mechanical and electrical wear and tear to the equipment.
For smaller capacity and light duty cranes the above may not be as important an issue, but for any crane used on a regular basis, VFD controls are always recommended. Since the speed of the crane can be very precisely controlled, most of the mechanical and electrical wear and tear is significantly reduced. The crane motions are electrically brought to a standstill by generator actions rather than mechanical force, and the motor brakes are used ONLY to prevent rotation and to hold the crane in place. As the VFD's are controlled by programming, the speed of any motion can be adjusted to any value at any time. This feature can be used to easily adjust a new crane's speed to an existing crane. As well, acceleration and deceleration can be adjusted to match operational requirements as necessary. Over speeding of the hoist is possible under varying load conditions to allow decreased cycle times. and built in safety features greatly increase the expected life span of the motor being controlled.
An overhead crane pendant provides detailed control over the individual speed and motion of the equipment. The operator of the crane can utilize the pushbuttons on the pendant to control each specific function of the crane from bridge travel, trolley direction, lift speed, etc. The pendant is specifically designed and wired to suit the functions and configuration of the crane that it controls.
The pendant is typically mounted in one of two ways. It can either be mounted to a sliding track system which allows the user of the crane to stay safely back from the load being lifted. The second typical arrangement is mounting the pendant to the electric trolley hoist itself. When the pendant is mounted to the trolley, the user will have to move and operate the equipment while being positioned beside the load.