Moving Coil Type

MOVING COIL TYPE

There are two types of moving coil

1. Permanent Magnet Moving Coil Type

2. Dynamometer Type Moving Coil Instruments

1. Permanent Magnet Moving Coil Type

The basic principle of operation is that when a current carrying conductor is brought in a magnetic field (they should not be parallel to each other) a torque on the conductor is produced. The instrument consists of a rectangular coil pivoted so that its sides lie in the air gap between the two poles of a permanent magnet and a soft-iron cylinder. The air gap between the magnet poles and iron core is small and the flux density is uniform and is in a radial direction, so that the flux lines are always at right angle to the current carrying conductor and hence when current passes through the coil, a deflecting torque is produced owing to the interaction between the two fluxes, one due to permanent magnet and the other due to the magnetic field of the coil. This is shown in Fig.4.2.

Construction:

Fig. 4.2 shows the various parts of a permanent-magnet moving coil instrument. It consists of a light rectangular coil of many turns of fine wire wound on an aluminum former inside which is an iron core as shown in Fig. 4.2 (a). The coil is delicately pivoted upon jewel bearings and is mounted between the poles of a permanent horse-shoe magnet. Attached to these poles are two soft-iron pole pieces which concentrate the magnetic field. The current is led into and out of the coil by means of two control hair-springs, one above and the other below the coil, as shown in Fig. 4.2(b). These springs also provide the controlling torque. The damping torque is provided by eddy currents induced in the aluminum former as the coil moves from one position to another.

Working:

When the instrument is connected in the circuit to measure current or voltage, the operating current flows through the coil. Since the coil is carrying current and is placed in the magnetic field of the permanent magnet, a mechanical torque acts on it. As a result, the pointer attached to the moving system moves in a clockwise direction over the graduated scale to indicate the value of current or voltage being measured. If the current in the coil is reversed, the deflecting torque will also be reversed since the direction of the field of the permanent magnet is the same. Consequently, the pointer will try to deflect below zero. Deflection in this direction (i.e. reverse direction) is prevented by a spring "stop". Since the deflecting torque reverses with the reversal of current in the coil, such instruments can be used to measure direct currents and voltages only.

Deflecting torque:

The magnetic field in the air gap is radial due to the presence of soft-iron core. This means that conductors of the coil will always move at right angles to the field. When current is passed through the coil, forces act on its both sides which produce the deflecting torque. Referring to Fig.4.3

let,

B = flux density in Wb/m²

l = length or depth of coil in m

b = breadth of coil in m

N = No. of turns in the coil

If a current of I amperes flows in the coil, then force acting on each coil side is given by;

Force on each coil side, F = B I l N newtons

Deflecting torque, Td = Force perpendicular distance = (BIlN) × b

Td = B I N A newton-metre

where A (= b × l) is the area of the coil in m². Since the values of B, N and A are fixed,

Td α I

The instrument is spring-controlled so that T_C α θ.

The pointer will come to rest at a position where

Td = T_C

θ α I

Thus, the deflection is directly proportional to the operating current. Hence, such instruments have uniform scale [See. Fig. 4.2 (i)].

Advantages

(i) Uniform scale i.e., evenly divided scale.

(ii) Very effective eddy current damping because the aluminium former moves in an intense magnetic field of the permanent magnet.

(iii) High efficiency as it requires very little power for its operation.

(iv) No hysteresis loss as the magnetic flux is practically constant.

(v) External stray fields have little effect on the readings as the operating magnetic field is very strong.

(vi) Very accurate and reliable.

Disadvantages

(i) Such instruments cannot be used for a.c. measurements.

(ii) About 50% more expensive than moving-iron instruments because of their accurate design.

(iii) Some errors are caused due to variations (with time or temperature) either in the strength of permanent magnet or in the control springs.

Applications:

Permanent-magnet moving coil instruments are acknowledged to be the best type for all d.c. measurements. They are very sensitive and maintain a high degree of accuracy over long periods. The chief applications of such instruments are:

(i) In the measurement of direct currents and voltages.

(ii) In d.c. galvanometers to detect small currents.

(iii) In ballistic galvanometers used mainly for measuring changes of magnetic flux linkages.

2. Dynamometer Type Moving Coil Instruments

In this instrument the permanent magnet is replaced by one or two fixed coils which carry current to be measured or a current proportional to the voltage to be measured and which are connected either in series or parallel with the moving coil. The coils are usually air cored. The torque of the instrument is dependent upon the magnetic field strengths of the fixed and moving coil i.e. the torque is proportional to square of the current in an ammeter and square of the voltage in a voltmeter. Dynamometer instruments can thus be used in alternating current circuits for which square law is essential. These can be as well used for de circuits also.

When the meter is used as an ammeter Fig. 4. 4 (a) or as a voltmeter Fig. 4.4 (b), the torque of the meter is proportional to the product if the flux of the fixed coil (FC) and the flux of the moving coil. The scale of the instrument is thus a square one as shown in Fig. 4.4 (e).

Since energy must be used to create two magnetic fields, the electrodynamometer movement is less sensitive as compared to that in PMMC instruments.

The control torque is provided by spring torque. The damping is provided by either eddy currents or air piston attached to the pointer.

These instruments are not in common use as

(a) They have low torque/weight ratio

(b) Owing to T_D α I² the scale is non uniform

(c) These are expensive.