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 m, State how it may be adapted: (16)

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

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

No calculations are required.

Answer 1

D.C. moving coil meter - main features and principle of operation:

Construction: a permanent magnet with soft-iron pole pieces and a cylindrical soft-iron core creates a uniform radial magnetic field in the air gap. A rectangular coil of fine wire is wound on a light aluminium former and pivoted so it can rotate in the air gap. The coil is mounted on jewel bearings. A hairspring provides the controlling (restoring) torque and also carries the current to the coil. A pointer attached to the coil moves over a calibrated scale. A counterweight balances the pointer.
Principle: when current flows through the coil, the coil sides in the magnetic field experience a force (F = B I l) producing a deflecting torque proportional to the current (T = B A N I, where A is the coil area, N the number of turns). This torque is opposed by the spring torque (proportional to the angle of deflection). At equilibrium the deflection is proportional to the current, giving a linear (uniform) scale. The damping is provided by eddy currents induced in the aluminium former.
The meter measures d.c. only (the direction of deflection depends on current direction). It is accurate and sensitive.

Adaptation of a meter giving full-scale deflection with 150 mA (the question states 150 m, i.e. 150 mA):

Part (a)

As an ammeter to read up to 150 A:

A low-resistance shunt is connected in parallel with the meter coil. The shunt carries the bulk of the current (150 A - 150 mA), while only 150 mA passes through the meter. The shunt resistance is chosen so that 150 mA flows through the meter when 150 A flows in the circuit. The shunt is made of a material with a low temperature coefficient (e.g. manganin) and is connected with short, heavy leads. The scale is recalibrated to read up to 150 A.

Part (b)

As a voltmeter to read up to 150 V:

A high resistance (multiplier) is connected in series with the meter coil. The series resistance is chosen so that the full-scale current of 150 mA flows when 150 V is applied across the combination. The meter then reads the voltage. The scale is recalibrated to read up to 150 V.

(No calculations required.)

✓ Verified Model Answer (Text Solution)

As an ammeter to read up to 150 A:

As a voltmeter to read up to 150 V: