Thyristor Convertors

The convertors in dc drive systems are designed to control and supply separately-excited dc machines in applications that demand rapid response, high accuracy and great reliability.

The convertors are available with single-convertor arrangement and in double-convertor arrangement.

Operation. The armature supply unit has a closed-loop current control system which ensures that the armature current follows the set current, reference. The system is based on a current controller, the output signal of which can influence the armature current via the control pulse circuits and the thyristor unit.

The current measurine circuits continuallv supply the current controller with information on the actual value of the armature current, and when this is compared with the reference an error signal is obtained which corrects the output voltage of the controller if necessary.

The controller current includes circuits for adaptive control. This means that the pain of the controller increases if the current is discontinuous. This maintains the response time of the current control system. A typical response time for the current control is 10—15 min.

The current control system includes a buffer amplifier for the actual current value. This makes it possible to connect a meter to displav the armature current, for example. By applying a control signal, the output of the buffer amplifier can be converted, so that the output signal changes sign when the direction of current flow changes in a double convertor.

Electric Drives of Anchor-Handling and Mooring Gear

The electric drives of anchor-handling and mooring gear are used to heave in the anchor and perform mooring operations.

Accordingly, they are used in windlasses, anchor and anchor-mooring capstans, mooring capstans, automatic and non-automatic mooring winches. At present most of these machines are equipped with electric drives having special characteristics which suit the operating conditions of such machines.

According to the Register the power rating of an electric drive used in the anchor-handling gear must be such that the anchor chain is heaved in at a minimum speed of 0.17 m/s with the sprocket pull, in Newtons.

Ordering Directions and Standard Equipment

For ordering electric drive equipment for ship's deck machinery consult tables listing electric drives of various applications.

After determining the power rating of servo motors from the formula given in the catalogue select a power drive equipment with the nearest power rating and then make sure that the control properties of the drive indicated on the mechanical characteristics suit the operating conditions of the electric equipment set.

The order for delivery of an electric equipment set or a separate motor must contain the following information:

• name of the machinery (gear), kind of the current and the ship's mains voltage;
• type of the motor and its electrical characteristics as indicated in tables;
• provision of a built-in disk brake and the operating conditions of the brake;
• design version of the motor;
• number of the free ends of the shaft (without a built-in brake); set number according to tables;
• number and type of limit switches in the standard equipment; provision of a spares set;
• provision of a Certificate issued by the Register.

For ordering separate electrical apparatuses use the data given in tables.

The order for delivery of a magnetic controller and a cam controller must contain the following information: application of the magnetic and cam controllers; class protection; rated current; number of startings per hour; type; operating duty; electric wear resistance level; control circuit voltage; voltage; settings of thermal protection relays.

An example of an order for an electric equipment set:
Electric equipment set for a mooring capstan, 380 V A. C. Motor MA 11 421-4/8 A, two-speed, power rating 7.0/5.6 kW, rotation speed 1400/650 rpm, 30/30 min duty.

Built-in brake type TNT 42, braking torque 12 daNm, 60 min duty. Design version-vertically mounted, flange type, without feet, M 302, for building into capstan head. Set No. 8 acc. to Table 15.
One set of spare parts per standard equipment set. With Certificate of the Register.

The spare parts set for the electrical equipment comprises parts and assemblies which can be used for the repairs to be carried out on the ship. The spares sets are packed in sturdy wooden cases adopted for long storage on the ship.

The spares set comprises the following parts and assemblies: braking disks for brakes — 1 set; brake coil — 1 set; brake spring — 1 set; moving and stationary contacts of all types — 1— 3 sets; springs for apparatuses of all types — 1 — 4 sets; apparatus coils — 1 per design version; fuses — 2 sets: rectifiers-—1 set; vitrified resistors — 1 per design version; cam assembly — 1 — 11 per design version.

The list of spare parts supplied with the given equipment is included in the set of technical documentation.

Electric Drive of Boat Winches

Boat winches are intended to handle rescue boats carried by sea-going and river ships. These do not include the winches used to regularly hoist and lower boats and launches in the performance of routine work and finishing operations of the ship.

Under the International Convention for Safety of Life at Sea all ships must be equipped with rescue boats. The same Convention specifies a number of requirements to davit on which the operating conditions of the electric drive depend to some extent.

The boat-handling gear must provide the means for:

• swinging-out of boat booms or davit with a fully fitted-out boat and lowering of the boat with the full number of people and with the ship rolling by 15° port or starboard;
• lifting of a fully fitted-out boat with the full number of people at a list of up to 8° at such an antirol at which the boat moves clear of the ship, and the swinging in of the davit with a fully fitted-out boat and a minimum number of people;
• swinging-out of booms or davits without a boat with the ship stopped even-keel.

Boats are lowered without the electric drive at a maximum speed of 0.5 m/s, obtained with the help of a mechanical brake.

The suspenders can be lowered either by unwinding the cable from the drum manually or by means of an electric drive.

The principle mode of operation of a boat winch drive consists of lifting the boat, once at a time, a limited time period not longer than 5 min. The boat lifting speed is 0.1 to 0.15 m/s. The choice of the motor must be made so as to ensure starting of the motor at the maximum design load. As the boat lifting time must not exceed 5 min the motors may have to sustain heavy overloads as compared with the catalogue data which represent 60 min ratings.

The control systems for the motors of the boat winches are quite simple. Starting and stopping of a-c motors are effected by means of magnetic starters with a local push button control which may be reversible or non-reversible, depending on the construction of the winches.

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.