Question

With the aid of sketch describe the main features and principle of operation of a D.C. moving coil meter.

If such a meter is designed to give full scale deflection with 150 mA, State how it may be adapted:

(i) As an ammeter to read up to 150 A.

(ii) As a voltmeter to read up to 150 V.

No calculations are required.

Answer.

D.C. Moving coil meter / Electro Dynamotor instrument

Current supplied to a conductor lying in and at right angles to a magnetic field will set up a magnetic field around the conductor which will react with the main flux and tend to move the conductor out of the field.

The operation of a moving-coil meter relies on this principle, as does the electric motor.

The field of a moving-coil meter is provided by a permanent magnet with the flux being strengthened by a soft iron core fixed in the gap by clamps top and bottom.

A moving coil, wound on a light frame and mounted on a spindle, is fitted so that the coil sides can rotate in the space between the iron core and the poles.

The spindle passes through a hole in the iron core and is supported in bearings at either end.

Current from the supply to be tested passes in and out of the coil via hair springs (non-magnetic) which also control the movement.

The small angle through which the coil moves is proportional to current flow through the coil and a pointer on the spindle indicates the reading on an even scale.

Current in the coil is limited by the small size of the wire to perhaps 69 mA, so that meters intended for use as ammeters measuring current require a low-resistance shunt to bypass the greater part.

Shunt size is fixed in meters for switchboards and starter boxes.

Test ammeters have several built-in shunts arranged with a switch for changing the meter range, and some have external replaceable shunts.

A shunt must be capable of carrying heavy current without overheating and its resistance must not change appreciably with temperature.

False readings are likely if the ammeter is connected to an external shunt with leads of different resistance to those supplied.

ii)

Resistance in series with the moving coil is inserted when an instrument is intended for use as a voltmeter.

Usually the resistance is mounted inside the instrument.

A multi-range meter has a tapped resistance with a selector switch to change the operating range.

The resistance, sometimes called a voltage multiplier or multiplier resistor, prevents large current flow through the instrument, which has only a fine winding on the moving coil.

Thus a voltmeter can be connected across the terminals of a 220 volt d.c. generator and the large series resistance will reduce current flow to the level suitable for the meter.

Voltage variations will vary the amount of current passing through the resistance and the moving-coil meter will register the changes on a scale marked in volts.