Moving Iron Type Instruments

MOVING IRON TYPE

This type of instrument is principally used for the measurement of alternating currents and voltages, though it can also be used for d.c. measurements. There are two types of moving-iron instruments.

(i) Attraction type in which a single soft-iron vane (or moving iron) is mounted on the spindle and is attracted towards the coil when operating current flows through it.

(ii) Repulsion type in which two soft-iron vanes are used; one fixed and attached to the stationary coil while the other is movable (i.e. moving iron) and mounted on the spindle of the instrument. When operating current flows through the coil, the two vanes are magnetised, developing similar polarity at the same ends. Consequently, repulsion takes place between the vanes and the movable vane causes the pointer to move over the scale.

In case the instrument is to be used as an ammeter, the coil has a fewer turns of thick wire so that the ammeter has low resistance-a desirable requirement. In case it is to be used as a voltmeter, the coil has a large number of turns of fine wire so that the voltmeter has high resistance-a desirable requirement.

 Attraction Type Moving Iron Type Instruments

Fig. 4.5 shows the constructional details of an attraction type moving-iron instrument. It consists of a cylindrical coil or solenoid which is kept fixed. An oval-shaped soft-iron is attached to the spindle in such a way that it can move in and out of the coil. A pointer is attached to the spindle so that it is deflected with the motion of the soft-iron piece. The controlling torque is provided by one spiral spring arranged at the top of the moving element. It should be noted that in this instrument, the springs do not carry the current as the same is carried by the stationary coil. The damping device is an aluminum vane attached to the spindle, as shown in Fig4.5, which moves in a closed chamber. In some instruments, damping is provided by the movement of a piston inside the curved chamber [See Fig. 4.6]; the piston being attached to the spindle.

Working

When the instrument is connected in the circuit to measure current or voltage, the operating current flowing through the coil sets up a magnetic field. In other words, the coil behaves like a magnet and therefore it attracts the soft-iron piece towards it. The result is that the pointer attached to the moving system moves from zero position. The pointer will come to rest at a position where deflecting torque is equal to the controlling torque. If current in the coil is reversed, the direction of magnetic field also reverses and so does the magnetism produced in the soft-iron piece. Hence, the direction of the deflecting torque remains unchanged. For this reason, such instruments can be used for both d.c. and a.c. measurements.

Deflecting torque:

The force F pulling the soft-iron piece towards the coil is directly proportional to:

(i) magnetic field strength H produced by the coil

(ii) pole strength m developed in the iron piece

F α mH

α H² (m α H)

Instantaneous deflecting torque a H²

If the permeability of iron is assumed constant, then,

H α i, where i is the instantaneous coil current.

Instantaneous deflecting torque a i²

Average deflecting torque, T_d α mean of i² over a cycle

Since the instrument is spring controlled,

Τ_C α θ

In the steady position of deflection, T_d = T_c

θ α mean of i² over or a cycle

α I² ...for d.c.

α I² r.m.s. ...for a.c.

Since the deflection is proportional to the *square of coil current, the scale of such instruments is non-uniform; being crowded in the beginning and spread out near the finish end of the scale.

Repulsion Type Moving Iron Type Instruments

Fig. 4.7 (i) shows the constructional details of a repulsion type moving iron instrument. It consists of two soft-iron pieces or vanes surrounded by a fixed cylindrical hollow coil which carries the operating current. One of these vanes is fixed and the other is free to move as shown in Fig. 4.7 (ii). The movable vane is of cylindrical shape and is mounted axially on a spindle to which a pointer is attached. The fixed vane, which is wedge-shaped and has a larger radius, is attached to the stationary coil. The controlling torque is provided by one spiral spring at the top of the instrument.

It may be noted that in this instrument, springs do not provide the electrical connections. Damping is provided by air friction due to the motion of a piston in an air chamber.

Working

When current to be measured or current proportional to the voltage to be measured flows through the coil, a magnetic field is set up by the coil. This magnetic field magnetizes the two vanes in the same direction i.e., similar polarities are developed at the same ends of the vanes as shown in Fig. 4.7 (iii). Since the adjacent edges of the vanes are of the same polarity, the two vanes repel each other. As the fixed vane cannot move, the movable vane deflects and causes the pointer to move from zero position.

The pointer will come to rest at a position where deflecting torque is equal to the controlling torque provided by the spring. If the current in the coil is reversed, the direction of deflecting torque remains unchanged. It is because reversal of the field of the coil reverses the magnetization of both iron vanes so that they repel each other regardless of which way the current flows through the coil. For this reason, such instruments can be used for both d.c. and a.c. applications.

Deflecting torque:

The deflecting torque results due to the repulsion between the similarly charged soft-iron pieces or vanes. If the two pieces develop pole strengths of m₁ and m­₂ respectively, then,

Instantaneous deflecting torque a m₁ m₂ α * H²

If the permeability of iron is assumed constant, then,

H α i where i is coil current.

Instantaneous deflecting torque α i²

Average deflecting torque, T_d α mean of i² over a cycle

Since the instrument is spring-controlled,

Τ_C α θ

In the steady position of deflection, T_d = T_c

θ α mean of i² over a cycle

α I²...for d.c.

α I² r.m.s. ... for a.c.

Thus, the deflection is proportional to the square of coil current as is the case with attraction type moving-iron instruments. Therefore, the scale of such instruments is also non-uniform; being crowded in the beginning and spread out near the finish end of the scale. However, the non-linearity of the scale can be corrected to some extent by the accurate shaping (e.g., using tongue-shaped vanes) and positioning of iron vanes in relation to the operating coil.

Comparison of Moving Coil, Dynamometer type and Moving Iron Voltmeters and Ammeters

Table below shows the comparison between permanent magnet moving coil (PMMC), dynamometer type and moving iron voltmeters and ammeters.