The Alternating-Current Coal Hoist

The paper describes an alternating-current coal hoist recently installed near Pittsburgh in which dynamic braking is employed. The brakes and clutches are operated by compressed air, which permits of a maximum rate of operation and also makes feasible the use of various safety devices. The electrical equipment of the coal hoist must be capable of continuous operation at the maximum speed and any attempt to insure against motor or brake trouble by providing excess capacity simply results in the motor being operated at that increased capacity. The control in lowering the empty bucket at high speed must be accurate enough so that it can be slowed down quickly without damaging the barges or steamers in landing. With a friction brake the only way to avoid striking too hard in landing is to make a complete stop before reaching the bottom, thereby causing an appreciable loss of time, which is entirely avoided by the use of dynamic braking. In the case of the plant described the control has proved sufficiently accurate and exact so that a complete cycle of the hoist trip due to the overlap of hoisting and racking movements can be made in 40 seconds instead of the estimated time of 50 seconds.