Английский язык для судовых электромехаников (Кущ, Воловник) - страница 27

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Answer the questions related to the text below.

1. What do they call truly ammeters?

2. What sort of measuring instrument is called a voltmeter?

3. What is the construction of most meters?

4. Which is more common: the electrodynamometer types of instrument

or the permanent magnet (PM) type?

5. Under what condition does the instrument become a wattmeter?

6. How do dynamometers differ from PM instruments?

Fundamentally, all meters measure current and in this sense are ammeters. When they have low internal resistance so as to produce no effect on a circuit’s performance and are actuated through direct connection in series with the main current path, they are truly ammeters. When they have high resistance so that they may be placed in shunt or directly across a voltage source they are called voltmeters. Their indication of voltage is based on the product of a known, calibrated internal resistance in series with the meter movement, and the current flowing through that resistance.

Most meters are constructed to use the attractive and repulsive force characteristics of magnetic structures to produce deflection of a pointer. In the most common PM type mutual force is exerted upon a movable, pivoted coil which itself becomes a magnet when it passes d-c current being measured.- When a movable magnet is mounted in the

field of a fixed permanent magnet and when it is passing current of the correct polarity, a force is exerted upon the movable magnet. In the resultant alignment of the fields, definite and proportional movement is produced against the restraining force of a spring connected to the pivoted coil. Deflection is arranged to be proportional to the amount of current through the coil, so that <5 ^>h when the meter scale over

which the pointer moves Fig. 1. Basic d-c meter movement using pivoted coil is calibrated, a true indi-and permanent magnet cation of current is ob

tained.

The basic d-c meter movement uses the pivoted coil and a PM. (Fig. 1). The electrodynamometer type of instrument, which is less common than the PM type, employs the same basic moving-coil mechanism but uses a fixed electromagnetic field actuated by current xvhich is related to or proportional to that in the moving coil.

When the current through a circuit is sent through the fixed-mag-net coil and the voltage across it applied to the movable coil, this instrument becomes a wattmeter, since deflection is then proportional to the product of voltage and current.