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General cargo ship deck machinery electric drives

Electric motors on vulnerable deck areas may be protected against ingress of water by being totally enclosed in a watertight casing. Vents are provided on some winches, which must be opened when the motor is operating in port.

The direct current (d.c.) motor, although it is relatively costly and requires regular brush gear maintenance, is still used for deck machinery because it has a full speed range with good torque at any speed. The control of d.c. motors by contactor-switched armature resistances, common in the days when ships' electrical supplies were d.c., has long been replaced by a variety of Ward-Leonard type systems which give a better, more positive regulation particularly for controlled lowering of loads. The Ward-Leonard generator is normally driven by an a.c. motor.

An important feature of the d.c. drive is its efficiency, particularly in comparison with a.c. drives, when operating at speeds in the lower portion of the working range. The d.c. motor is the only electric drive at present in production which can be designed to operate in a stalled condition continuously against its full rate torque and this feature is used for automatic mooring winches of the 'live motor' type. The majority of d.c. winch motors develop full output at speeds of the order of 500 rev/min and where necessary are arranged to run up to two to four times this speed for light line duties. Windlass motors on the other hand do not normally operate with a run up in excess of 2 : 1 and usually have a full load working speed of the order of 1000 rev/min.

Direct current motors may also be controlled by static thyristor converters which convert the a.c. supply into a variable d.c. voltage of the required magnitude for any required armature speed. These converters must be of a type capable of controlled rectification and inversion with bi-directional current flow if full control is to be obtained (Figure 13.1).

Load/speed characteristic of Ward-Leonard thyhstor controlled winch
Figure 13.1 Load/speed characteristic of Ward-Leonard thyhstor controlled winch. See Figure 13.2 for conversions to m/sec (Clarke, Chapman Ltd)

Alternating current induction motors, of either the wound rotor or of the cage type are also in common use. With these the speed may be changed by means of pole changing connections and in the case of the wound rotor induction motor, also by changing the value of the outside resistance connected in the rotor circuit. The pole change method involves the switching of high currents at medium voltage in several lines simultaneously, requiring the use of multi-pole contactors.

The pole change speed control method offers a choice of perhaps three discrete speeds such as 0.65, 0.325 and 0.1025 m/s corresponding to 4, 8 and 24 pole operation. The wound rotor motor is flexible when hoisting a load, because the starting resistances can be reintroduced into the rotor circuit and the load will cause the motor to slip.

The slip gives a range between the speeds dictated by the pole arrangement. As with resistance controlled d.c. motors, difficulty is experienced when providing speed control of an overhauling load, i.e. lowering a suspended load. The disadvantages must be balanced against lower cost, particularly of the cage type induction motor, in comparison with the more flexible d.c. motor. Typical performance curves are shown in Figure 13.2.

Performance curves of a 3 tonne winch. AC pole-changing'cage' motor. (Clarke, Chapman Ltd)
Figure : Performance curves of a 3 tonne winch. AC pole-changing'cage' motor. (Clarke, Chapman Ltd)

Another form of induction motor control system is based on the relationship between output torque and applied voltage, the torque being proportional to the voltage squared. The controller takes the form of a three-phase series regulator with an arm in each supply line to the motor.

A stable drive system can only be achieved by this means if a closed loop servo control system is used in conjunction with a very fast acting regulator which automatically adjusts the output torque to suit the load demand at the set speed. Control of an overhauling load is made possible by using injection braking techniques. A combined system employing both these control principles can provide full control requirements for all deck machinery.

The a.c. drives described operate at the supply frequency and consequently rapid heating of the motor will occur if the drive is stalled when energized. The majority of a.c. motors on deck machinery run at a maximum speed corresponding to the 4 pole synchronous speed of 1800 rev/min on a 60 Hz supply.

These speeds are similar to the maximum speeds used with d.c. drives and the bearings and shaft details tend to be much the same. The motor bearings are normally grease lubricated. However, where the motor is flange mounted on an oil bath gearcase, the driving end bearing is open to the gearcase oil and grease lubrication is not required.

Summarized below some of the basic operation of deck machinery and maintenance guide :
  1. Powering deck machinery -Systems and components

  2. Pump and motor systems are used for powering deck machinery such as winches and windlasses. Pump and actuating cylinders are normally employed for hatch covers. One or more pumps will be used to supply the volume of fluid at the pressure required to operate one or more motors. ......

  3. Mooring equipment for general cargo vessel

  4. The operation of mooring a vessel has traditionally required the attendance of a large number of deck crew fore and aft. Supervision of the moorings was also necessary to maintain correct tension through changes due to the tides and the loading or unloading of cargo. ......

  5. Hydraulic systems for deck machinery and cargo equipment

  6. The three essential components for a hydraulic circuit, are the hydraulic fluid held in a reservoir tank, a pump to force the liquid through the system and a motor or cylinder actuator to convert the energy of the moving liquid into a working rotary or linear mechanical force. Valves to control liquid flow and pressure are required by some systems. ......

  7. General cargo ship deck machinery electric drives

  8. Electric motors on vulnerable deck areas may be protected against ingress of water by being totally enclosed in a watertight casing. Vents are provided on some winches, which must be opened when the motor is operating in port. ......

  9. Handling deck machinery- Anchor windlasses,Anchor capstan & mooring winches

  10. The windlass cablelifter brakes must be able to control the running anchor and cable when the cablelifter is disconnected from the gearing when letting go'. Average cable speeds vary between 5 and 7 m/s during this operation. ......

  11. General cargo ship deck deck crane

  12. A large number of ships are fitted with deck cranes. These require less time to prepare for working cargo than derricks and have the advantage of being able to accurately place (or spot) cargo in the hold. On container ships using ports without special container handling facilities, cranes with special container handling gear are essential. ......

  13. Mechanically operated steel hatch covers

  14. Hatch cover equipment like the other deck machinery, has to exist in a very hostile environment and the importance of regular maintenance cannot be over-emphasized. Drive boxes and electrical enclosures should be checked regularly for water-tightness. ......

  15. Derricks and cargo winches -Ship cargo handling gears

  16. The duty of a deck winch is to lift and lower a load by means of a fixed rope on a barrel, or by means of whipping the load on the warp ends, to top or luff the derricks, and to warp the ship. ......

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