Question

The most of the alternating current and voltage meters are calibrated to read :??
​

Answer 1

Explanation:

AC electromechanical meter movements come in two basic arrangements: those based on DC movement designs, and those engineered specifically for AC use.

Permanent-magnet moving coil (PMMC) meter movements will not work correctly if directly connected to alternating current, because the direction of needle movement will change with each half-cycle of the AC. (Figure below)

Permanent-magnet meter movements, like permanent-magnet motors, are devices whose motion depends on the polarity of the applied voltage (or, you can think of it in terms of the direction of the current).

Passing AC through this D’Arsonval meter movement causes useless flutter of the needle.

Passing AC through this D’Arsonval meter movement causes useless flutter of the needle.

In order to use a DC-style meter movement such as the D’Arsonval design, the alternating current must be rectified into DC.

This is most easily accomplished through the use of devices called diodes. We saw diodes used in an example circuit demonstrating the creation of harmonic frequencies from a distorted (or rectified) sine wave. Without going into elaborate detail over how and why diodes work as they do, just remember that they each act like a one-way valve for current to flow.

The arrowhead in each diode symbol points in the permitted direction of current flow.

Arranged in a bridge, four diodes will serve to steer AC through the meter movement in a constant direction throughout all portions of the AC cycle:

Passing AC through this Rectified AC meter movement will drive it in one direction.

Passing AC through this Rectified AC meter movement will drive it in one direction.

Another strategy for a practical AC meter movement is to redesign the movement without the inherent polarity sensitivity of the DC types.

This means avoiding the use of permanent magnets. Probably the simplest design is to use a nonmagnetized iron vane to move the needle against spring tension, the vane being attracted toward a stationary coil of wire energized by the AC quantity to be measured as in the figure below.

Iron-vane electromechanical meter movement.

Iron-vane electromechanical meter movement.

Electrostatic attraction between two metal plates separated by an air gap is an alternative mechanism for generating a needle-moving force proportional to applied voltage.

This works just as well for AC as it does for DC, or should I say, just as poorly! The forces involved are very small, much smaller than the magnetic attraction between an energized coil and an iron vane, and as such these “electrostatic” meter movements tend to be fragile and easily disturbed by physical movement.

But, for some high-voltage AC applications, the electrostatic movement is an elegant technology.

If nothing else, this technology possesses the advantage of extremely high input impedance, meaning that no current need be drawn from the circuit under test. Also, electrostatic meter movements are capable of measuring very high voltages without need for range resistors or other, external apparatus.

When a sensitive meter movement needs to be re-ranged to function as an AC voltmeter, series-connected “multiplier” resistors and/or resistive voltage dividers may be employed just as in DC meter design: (Figure below)

Answer 2

Answer:

AC ammeters and voltmeters read rms values

Explanation: