Unit 16. Indicating meters

1. Words to remember.

indicating meters – электроизмерительные приборы

deflection – отклонение

to exert – вызывать

pivoted coil – вращающаяся катушка

to restrict – ограничивать

rectifier – выпрямитель

eddy current – вихревые токи

impedance – полное сопротивление

2. Read the text.

Indicating meters

D.C.METERS

Fundamentally, all meters (except electrostatic type) 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 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 (without affecting it) they are called voltmeters and, of course, under these conditions draw a minimum of current. 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, permanent-magnet type mutual force is exerted upon a movable, pivoted coil which itself becomes a magnet (upon which the pointer is mounted) 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 when the meter scale over which the pointer moves is calibrated, a true indication of current is obtained.

The basic D.C. meter movement uses the pivoted coil and a permanent magnet (PM). The basic construction is sometimes called the Weston meter movement. The electrodynamometer type of instrument, which is less common than the PM type, employs the same basic moving-coil mechanism but uses a fixed electro-magnetic field actuated by current which is related to or proportional to that in the moving coil.

When the current through a circuit is sent through the fixed-magnet 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. Dynamometers are less flexible than the PM type meters, less sensitive, and generally more expensive. Their basic arrangement, however, allows them to be used on either A.C. or D.C when appropriate scale, multiplier, and shunt arrangements are included in their internal circuitry.

A.C.METERS

The Moving coil iron-type is the most common a.c. meter. In it, included eddy currents are used to produce magnetic force on a structure bearing a pivoted pointer and a thin iron element called a vane. The vane has no coil. The stationary magnetic field is produced by a single current coil surrounding both the fixed metal element and the pointer movement. This coil is so arranged that its own field induces a field in the moving vane and in addition generates attractive or repulsive magnetic forces with respect to its own self-produced magnetic field. Deflection is basically proportional to the current through the main coil. Moving iron-vane meters usually have relatively low impedances (without multipliers) and are simple and inexpensive. They measure either voltage or current, but their use must be restricted to the frequency for which they are designed. Rectifier Type Meters utilize permanent magnet D.C. movements actuated by current developed from rectifying the applied a.c. being measured. Rectifier elements mounted within the meter case may be copper oxide, selenium, germanium, or silicon. The developed D.C. is proportional to the applied a.c., while the rectifies and associated circuitry are designed for operation over as wide a band of frequency as possible.