MOVING IRON INSTRUMENT APPLICATIONS IN METERING

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 below.

Circuit of Current Transformer Field Test Set.

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.

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.