Electric Drives of Towing Winches

Flexible-link towing is a physical process involving unsteady motion of inertia masses interconnected by a flexible element. Reliability of towing depends on selection of an optimum length of the towline and on minimizing the towline, overloads under various conditions. Non-automatic towing winches with a pull of 120 kN and above may be used with three-speed a-c cage motors controlled by means of magnetic controllers.

Taking account of the optimum speed ratio of the winches for towlines 37.5 to 48.5 mm in diameter use may be made of three-speed motors with a speed ratio of 1:2:4, developing a rated power and a rated pull at the medium speed; the motors are controllers.

Winches with a 56 to 65 mm diameter towline may be driven by cage motors with a speed ratio of 1:3, producing the rated pull at the medium speed. To obtain low rotation speeds in order to limit the load on the towline these motors must be connected in a dual supply circuit.

The motors must be supplied from the supply mains when running at the rated or high speed and from a directly-coupled thyristor converter, when running at low speeds. To ensure the required power two motors may be connected to one shaft.

The maximum power rating of such systems is 140 kW, which enables them to be used for the largest winches with a pull of up to 650 kN.

The power rating of the motor to be used in the winches is selected to suit the pull required to heave in the towline at the rated speed with a medium towline diameter: P = (Qr*Vr)/n, where P — power rating, kW; Qr — rated towline pull, kN; Vr — rated value of towline heaving speed, m/s; n — efficiency of the driven gear.

The design power must be equal to or below the 30-min motor power rating for non-automatic winches and 60-min power rating for automatic winches operating in the impulse mode. The above power must be developed by the motor at the rated speed.