Iron-Vane Voltmeters and Ammeters

Author: E.E. Kimberly

In the most popular and most rugged type of portable iron-vane ammeter, the elemental parts of which are shown in Fig. 16-6 (a), two iron vanes are mounted coaxially in a coil. Current passing through the coil produces a magnetic flux, which is parallel to the axis of the two vanes. One vane is fixed, and the other is free to rotate against the restraining torque of a spiral spring. When flux appears, the two vanes are magnetized with like poles. They then repel each other, and the action produces a deflection of a pointer attached to the movable vane. The torque is proportional to the square of the flux and, hence, to the square of the current in the coil. It is also a function of the angle turned through. The scale therefore is not uniform, and either end, or both ends, of it may be too badly crowded for accurate reading. Readings in the lowest 20 per cent of the scale are not reliable.

Fig. 16-6. Magnetic-Vane Voltmeters and Ammeters

The iron-vane voltmeter is similar to the ammeter, but many turns of small wire are used in the coil instead of a few turns of large wire.

Another type of iron-vane instrument, called the inclined-coil attraction-type voltmeter and ammeter, is shown in Fig. 16-6 (b). In this instrument the axis of the coil is inclined 45° in one direction while the iron vane is inclined 45° in the same direction. When flux appears in the coil, the vane attempts to align itself with the flux against the action of a restrain ing spiral spring. By this ingenious arrangement a scale with a span of almost 180° is possible. Because of the crowding of the scale at the ends, the instrument is usually designed to be used in only the most open part of the possible scale.

Voltmeters and ammeters of the iron-vane and dynamometer types, when calibrated at a frequency of 60 cycles, will have a degree of accuracy ordinarily acceptable up to about 130 cycles. At frequencies above 130 cycles the accuracy becomes noticeably poorer with increase in frequency, and so meters for high frequencies should be recalibrated at the frequencies at which they are to be used.

Iron-vane instruments are suitable for both alternating-current and direct-current measurements. However, when such a meter is used in direct-current measurement, readings should be taken with reversed connections also, to eliminate the effect of possible stray fields and of magnetic retentivity of the iron vane. The true reading is then the average of that taken with normal connections and that taken with reversal of connections. This type of meter measures effective values.