Английский язык для судовых электромехаников - [24]

Electric Drives of Towing Winches

4. 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:

2. 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. 3. The power rating of the motor to be used in the winches is selected to suit the pull required to heave in the tow-

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line at the rated speed with a medium towline diameter: P= —— where P — power rating, kW; Q_r — rated towline pull, kN; r_r — rated value of towline heaving speed, m/s; n — efficiency of the driven gear.

4. 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.

Hometask: Ex. 10, 15, 18.

The third class period

19 3

Listen to the sentences and define in each of them the meaning of the word “unit”.

1. The alternator voltage is fed to the AVR unit via fuses and exciter switch. 2. The standard units are those of current, voltage and resistance.

3. So the unit positive charge has been given energy in the amount of volts, by chemical action during its passage through the battery. 4. Apart from general acquaintance with the principle of operation and the design of separate components and the unit as a whole, it is required to analyze thoroughly the arrangement of separate parts in the unit.

20 4

Listen to the recorded text and give it a suitable title.

Each ship is to be provided with at least two main sources of electric power. If these sources are generators, at least one of them is to have its own independent prime mover.

The number and output of the main generators are to be determined with regard to the following operating conditions of the ship: running conditions, manoeuvring, emergency conditions in operation of main sources of electric power, other operating conditions in accordance with the purpose of the ship.

The output of main sources of electric power and of power converters is to be such; that in case of a failure of any one source or converter, the remaining sources would ensure power supply to the systems essential for navigation and safety of the ship under running or emergency operating conditions.

Total output and momentary overload capacity of all a-c generator sets feeding the ship’s mains is to be sufficient for starting the most powerful electric motor with maximum starting current under most severe starting conditions in case of a failure of any of the generators available.

In this case there should be no drop in voltage or frequency that could cause a fall out of synchronism or stop of the generator prime mover as well as spontaneous switching-off the running machines and apparatus.

21 7

Fulfil or explain the following:

1. Diagram the construction and principle of operation of AVR.

2. With your diagram explain in detail the operation of AVR.

3. Explain the formula: P = ■.

22 5

Record your answers on a tape.

1. What is meant by a “frequency measuring circuit”?

2. What circuits are mounted on a printed board of AVR?

3. How can you tell when alternator voltage changes?

4. How do towing winches operate?

Listen to the recorded story and say what was wrong in the woman’s

note.

Cycles on the Porch

When Toronto’s hydroelectric company converted from 25 cycles to 60, considerable advance planning was necessary. Residents, depending on their location in the city, were assigned specific dates when they might expect a call from the power company and were! advised to have someone at home on that day. One woman, unable to postpone her social activities but anxious to co-operate, left this note: “Please, leave the cycles on the porch!”

The fourth class period

26 % 2

In the following sentences insert Infinitives. Compare your results with the text.

1. The regulators permit . .. constant voltage of generators under various amount of load and power factor. 2. This signal is used . .. the alternator voltage if the frequency drops below 95% of nominal frequency.