average value to which the meter is reacting. That is why an ac meter will give an incorrect reading if used to measure dc.">
An additional advantage of damping a meter movement is that the damping systems will act to slow down the coil and help keep the pointer from overshooting its rest position when the current through the meter is removed.
INDICATING ALTERNATING CURRENT
Another problem encountered in measuring ac is that the meter movement reacts to the average value of the ac. The value used when working with ac is the effective value (rms value). Therefore, a different scale is used on an ac meter. The scale is marked with the effective value, even though it is the average value to which the meter is reacting. That is why an ac meter will give an incorrect reading if used to measure dc.
OTHER METER MOVEMENTS
The d'Arsonval meter movement (permanent-magnet moving-coil) is only one type of meter movement. Other types of meter movements can be used for either ac or dc measurement without the use of a rectifier.
When galvanometers were mentioned earlier in this topic, it was stated that they could be either electromagnetic or electrodynamic. Electrodynamic meter movements will be discussed at this point.
ELECTRODYNAMIC METER MOVEMENT
An electrodynamic movement uses the same basic operating principle as the basic moving-coil meter movement, except that the permanent magnet is replaced by fixed coils (fig. 1-15). A moving coil, to which the meter pointer is attached, is suspended between two field coils and connected in series with these coils. The three coils (two field coils and the moving coil) are connected in series across the meter terminals so that the same current flows through each.
Figure 1-15. - Electrodynamic meter movement.
Current flow in either direction through the three coils causes a magnetic field to exist between the field coils. The current in the moving coil causes it to act as a magnet and exert a turning force against a spring.
If the current is reversed, the field polarity and the polarity of the moving coil reverse at the same time, and the turning force continues in the original direction. Since reversing the current direction does not reverse the turning force, this type of meter can be used to measure both ac and dc if the scale is changed. While some voltmeters and ammeters use the electrodynamic principle of operation, the most important application is in the wattmeter. The wattmeter, along with the voltmeter and the ammeter, will be discussed later in this topic.
MOVING-VANE METER MOVEMENTS
The moving-vane meter movement (sometimes called the moving-iron movement) is the most commonly used movement for ac meters. The moving-vane meter operates on the principle of magnetic repulsion between like poles (fig.1-16). The current to be measured flows through a coil, producing a magnetic field which is proportional to the strength of the current. Suspended in this field are two iron vanes. One is in a fixed position, the other, attached to the meter pointer, is movable. The magnetic field magnetizes these iron vanes with the same polarity regardless of the direction of current flow in the coil. Since like poles repel, the movable vane pulls away from the fixed vane, moving the meter pointer. This motion exerts a turning force against the spring. The distance the vane will move against the force of the spring depends on the strength of the magnetic field, which in turn depends on the coil current.
Figure 1-16. - Moving-vane meter movement.
These meters are generally used at 60-hertz ac, but may be used at other ac frequencies. By changing the meter scale to indicate dc values rather than ac rms values, moving-vane meters will measure dc current and dc voltage. This is not recommended due to the residual magnetism left in the vanes, which will result in an error in the instrument.
One of the major disadvantages of this type of meter movement occurs due to the high reluctance of the magnetic circuit. This causes the meter to require much more power than the D'Arsonval meter to produce a full scale deflection, thereby reducing the meters sensitivity.
HOT-WIRE AND THERMOCOUPLE METER MOVEMENTS
Hot-wire and thermocouple meter movements both use the heating effect of current flowing through a resistance to cause meter deflection. Each uses this effect in a different manner. Since their operation depends only on the heating effect of current flow, they may be used to measure both direct current and alternating current of any frequency on a single scale.
The hot-wire meter movement deflection depends on the expansion of a high-resistance wire caused by the heating effect of the wire itself as current flows through it. (See fig. 1-17.) A resistance wire is stretched taut between the two meter terminals, with a thread attached at a right angle to the center of the wire. A spring connected to the opposite end of the thread exerts a constant tension on the resistance wire. Current flow heats the wire, causing it to expand. This motion is transferred to the meter pointer through the thread and a pivot.
Figure 1-17. - Hot-wire meter movement.
The thermocouple meter consists of a resistance wire across the meter terminals, which heats in proportion to the amount of current. (See fig. 1-18.) Attached to this wire is a small thermocouple junction of two unlike metal wires, which connect across a very sensitive dc meter movement (usually a d'Arsonval meter movement). As the current being measured heats the heating resistor, a small current (through the thermocouple wires and the meter movement) is generated by the thermocouple junction. The current being measured flows through only the resistance wire, not through the meter movement itself. The pointer turns in proportion to the amount of heat generated by the resistance wire.
Figure 1-18. - A thermocouple meter.
Q.18 List three meter movements that can measure either ac or dc without the use of a rectifier.
Q.19 What electrical property is used by all the meter movements discussed so far?
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