Английский язык для судовых электромехаников (Кущ, Воловник) - страница 71
22 6
Listen to the text about convertors. Ask each other questions covering the text. Use Fig. 6.
Presentation. 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.
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n,U
1
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M,I
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ft ft
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| I | |||
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convertor
Fig. 6:
и — single convertor; b — double
The convertors are available with single-convertor arrangement and in double-convertor arrangement,
Single convertors. In a single convertor the thyristor's are arranged in a six-pulse two-way circuit (see Fig. 6,a).
A single convertor can supply current in one direction only, but its output voltage can change polarity. This determines the rotation and torque direction characteristics of the machine, as illustrated in Fig. 6,a.
The single convertor can operate in the first and fourth quadrants. The direction of torque can only be changed by reversing the field current of the machine.
Double convertors. In a double convertor there are two sets of thyristors connected in antiparallel (see Fig. 6,b). Both current and voltage can change sign, and the machine can be braked and reversed without additional equipment; this is known as four-quadrant drive.
23 4
- What is wrong with the computer?
An Electronic Computer
As you know, electronic computers can solve intricate mathematical problems, answer complicated questions, translate articles into a foreign language and what not!
One day Field Marshal Montgomery was given a computer. That unexpected help from the War Office greatly pleased him. It took him about 10 minutes to set the problem. It went something like this: — “The enemy has just opened fire from the South. 12 German tanks have just been sighted on top of the hill in front of our lines.. . Two German planes have been shot down...” And finally came the question: — “Should I attack or withdraw?”
At last came the answer: “Yes”. — “Yes, what?!”
Half an hour later came the next answer: — “Yes, Sir”.
24 7
Listen carefully to the recorded texts. Choose a suitable title for each of them. Give reasons for your choice. Speak on some of the details of automatic control that you know and that are provided by the texts.
A
An automatic control system is defined as one “in which the value of a controlled condition is compared with a desired value and corrective motion is taken dependent on the deviation, without the inclusion of a human element in the closed loop formed by the comparing and correcting chains of elements”.