The measurement of alternating current (or voltage) is the measurement of a quantity that is continuously reversing direction. The permanent-magnet, moving- coil instrument movement cannot be used since the AC field of the moving coil reacting with the unidirectional permanent-magnet field will produce a torque reversing in direction at line frequency.
Because of its inertia, the moving element will be unable to respond to this rapidly reversing torque and the pointer will only vibrate at zero. A different type of meter movement is therefore required.
The moving-iron instrument is specifically designed to operate on AC circuits. This instrument is called the moving-iron type because its moving member is a piece of soft iron in which magnetism induced from a field coil interacts with the magnetic field of a fixed piece of soft iron to produce torque.
Figure 1 - Mechanism of Moving-Iron Instrument
When current flows through the field coil the two pieces are magnetized with the same polarity, since they are both under the influence of the same field and, hence, repel each other, causing the pivoted member to rotate. The angular deflection of the moving unit stops at the point of equilibrium between the actuating torque and the counter torque of the spiral control spring.
The illustration shows that the operating current flows through a stationary winding. Depending upon the use for which it is designed, the coil may be wound with fine or heavy wire, giving this type of instrument a wide range of capacities. The instrument will tolerate overloads with less damage to springs and pointer than will most other types of instruments, since, with excess current, the iron vanes tend to become saturated and limit the torque.
Damping is provided by either a light aluminum vane fixed to the shaft and moving in a closed air chamber, or by a segment of an aluminum disc moving between poles of small permanent magnets.
The bearing system may consist of a pivoted shaft turning in jeweled bearings or may be of the taut-band suspension type where the moving element is supported by two metal ribbons under tension sustained by springs.
Application of Moving-Iron Instrument
Measurement of Current
Since the actuating coil may be wound with a choice of many wire sizes, the instrument may be constructed to measure current from a few milliamperes up to 100 or 200 amperes in self-contained ratings. For measuring currents beyond this range, a 5-ampere instrument may be used with a current transformer.
Current Transformer Field Test Set
A special application of the moving-iron ammeter is the current transformer field test set. The circuit of this instrument is shown in Figure 2. It is used to check current transformer installations in service on the secondary side, for possible defects such as short-circuited primary or secondary turns, high-resistance connections in the secondary circuit, or inadvertent grounds, any of which could cause incorrect metering.
Figure 2 - Circuit of Current Transformer Field Test Set
It is essentially a multi-range, moving-iron-type ammeter with a built-in burden which is normally shunted out, but which can be put in series with the meter by the push button.
In the typical instrument illustrated here, ammeter current ranges of 1.25, 2.5, 5, and 10 amperes are obtained from the tapped primary winding of a small internal current transformer, the secondary winding of which is connected to the ammeter which has corresponding multiple scales. It is thus possible to obtain a reading well up-scale on the ammeter for most load conditions under which the current transformer is operating.
The rotary burden switch permits the addition of 0.25, 0.5, 1, 2, or 4 ohms to the secondary circuit as desired. The imposition of an additional secondary burden on a current transformer having the defects previously mentioned will result in an abnormal decrease in the secondary current. The extent of this decrease and the ohms burden required to effect it depend on the characteristics of the transformer under test.
The check on the current transformer consists of inserting the field test set in series with the current transformer secondary circuit and comparing the ammeter readings under normal operating conditions with the readings after the additional field test set burden is added.
Measurement of Voltage
By the use of an actuating coil of many turns of fine wire in series with a resistor, the moving-iron instrument may be used to measure voltage. Such a voltmeter may have an operating current of around 15 milliamperes with a range up to 750 volts.
External multipliers may be used to extend this range. These voltmeters are used in applications where sensitivities lower than those of the rectifier d’Arsonval instrument are satisfactory. The moving-iron voltmeter may be used on DC with some loss in accuracy. The best accuracy is obtained by using the average of the readings taken before and after reversal of the leads to the instrument terminals.
This instrument will not indicate the polarity of DC.
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