Electrical indicating and test instruments

 6.2.3 Electrodynamic meters

Electrodynamic meters (or dynamometers) can measure both d.c. signals and a.c. signals up to a frequency of 2 kHz. As illustrated in Figure 6.4, the instrument has a moving circular coil that is mounted in the magnetic field produced by two sepa[1]rately wound, series-connected, circular stator coils. The torque is dependent upon the mutual inductance between the coils and is given by:

                                            T = I₁I₂ dM/d0                                 (6.5)

where I₁ and I₂ are the currents flowing in the fixed and moving coils, M is the mutual inductance and 0 represents the angular displacement between the coils.

When used as an ammeter, the measured current is applied to both coils. The torque is thus proportional to current². If the measured current is a.c., the meter is unable to follow the alternating torque values and it displays instead the mean value of current². By suitable drawing of the scale, the position of the pointer shows the squared root of this value, i.e. the r.m.s. current.

Electrodynamic meters are typically expensive but have the advantage of being more accurate than moving-coil and moving-iron instruments. Voltage, current and power can

all be measured if the fixed and moving coils are connected appropriately. When used for voltage measurement, the instrument can typically measure voltages in the range of 0 to 30 volts. However, it can be modified to measure higher voltages by placing a resistance in series with it, as in the case of moving-coil and moving-iron meters. Also, as in the moving-iron meter, a series resistance is particularly beneficial in a.c. signal measurements because it compensates for the effect of coil inductance by reducing the total resistance/inductance ratio, and hence measurement accuracy is improved. This series resistance can either be inside or outside the instrument case, as discussed above for the case of moving-iron meters.

6.2.4 Clamp-on meters

These are used for measuring circuit currents and voltages in a non-invasive manner that avoids having to break the circuit being measured. The meter clamps onto a current-carrying conductor, and the output reading is obtained by transformer action. The principle of operation is illustrated in Figure 6.5, where it can be seen that the clamp-on jaws of the instrument act as a transformer core and the current-carrying conductor acts as a primary winding. Current induced in the secondary winding is rectified and applied to a moving-coil meter. Although it is a very convenient instrument to use, the clamp-on meter has low sensitivity and the minimum current measurable is usually about 1 amp.