Английский язык для судовых электромехаников - [58]

The purpose of this system of vessels is to minimize the heat-flow from ambient to the liquid-helium vessels and coils at 4.5°I\.

The complete assembly is located within the rotor on a fixed support that passes through the bore of the large bearing at the non-drive end and is connected to a steady bearing on a stub extension of the output shaft.

The armature rotor group assembly comprises a drum carrying the slip rings, and rotor conductors interconnecting pairs of slip rings; a non-drive-end section; a drive-end section; and the output shaft.

The armature stator group consists of brushgear and stator conductors, mounted on a structure able to withstand the motor torque reaction on the stator conductors. The stator also includes the bearing and the base frame. For a propulsion application the drive-end bearing could include the main thrust block.

The torque of any motor is the product of the armature current and magnetic flux. In a superconducting motor the use of a pair of slip rings for each armature conductor permits a high armature current together with a high flux output, thus giving- high torque. With a constant field, speed varies linearly with voltage and torque varies linearly with current; thus for a propeller drive obeying the cube law, voltage varies with speed, current varies with speed squared and power varies with speed cubed.

The superconducting marine propulsion motors have the advantages of no practical upper power limit; very large torques; and excellent speed control.

A possible disadvantage is the need to provide a helium refrigeration system, but the main consequence of this is an economic cut-off below a certain rating, probably about 60 kW per rev/min.

An important feature of the motor design is the ability to operate the two halves of the field system independently so that, in the event of one half being inoperative, the motor can work at half voltage and full current, giving half power.

Other benefits are an integrated power plant with the use of more efficient propellers, manoeuvrability and better employment of space due to installation flexibility.

An integrated power plant means a “Central power station" concept with the minimum of auxiliary prime movers, which can lead to a significant reduction in the total installed power, together with general use of a single fuel-grade. It is also clear that a “Total energy” concept should be adopted, witli heal, recovery where possible.

The most efficient propeller combination can be used due to the freedom of choice of speed and torque.

Examples of ship types particularly suitable lor application of superconductive drive motors are icebreakers, large tugs and tankers.

В Starting Control from ECR Navigation

By shifting the telegraph lever from FULL AHEAD to NAVIGATION, a gradual acceleration program is activated; the program is adjustable and enables the engine to reach navigation RPM’s in the desired lapse of time. If any fault occurs during gradual load take-over the engine load increase is interrupted; the engine stabilizes at the value reached at the moment in which the fault occurred and remains in that condition until the fault disappears.

Automatic resumption of power increase takes place according to the preset gradient.

If one of the stand-by causes persists beyond a given time the device automatically starts decreasing the load, beginning from stand-by position and persists until its determining cause disappears. The decrease may occur according to two adjustable speeds which are automatically selected by the programme as a function of the seriousness of the fault.

The following are some, of the anomalous conditions which determine the limiting and safety functions described above: fresh cooling water high temperature at ttie outlet of each cylinder; cylinder exhaust gas high temperature versus average value; cylinder exhaust gas mean temperature;

thrust bearing, journal bearing, stern tube bearing, crosshead bearing, and big end bearing high temperatures;

cylinder average high temperature, piston cooling fluid low flow rate;

engine luboil high temperature;

fuel injectors cooling fluid flow failure.

If any of the above variables, in particular bearing temperature and luboil pressure, reach exceptionally critical values, RPM reduction is converted to an engine lockout.

Emergency

Shield push-buttons are provided both on Bridge and ECR. By means of these push-buttons some safety and protection facilities may be excluded (RPM reduction, torque limitation, acceleration gradient, engine power manual limitation).

If a “crash stop” is affected in this condition, the admission of starting air takes place at a number of RPM’s higher than required for normal reversal and the RPM set-point, as well as positioning limit of the fuel oil lever are automatically brought to a third emergency level.

If said emergency starting RPM’s are not reached within a certain limit, the air valves close and a starting failure alarm is displayed.

To repeat the “crash, stop” operation, the operator must reset the engine telegraph on STOP position and then set the desired number of RPM’s.