EC 217 MEASUREMENTS AND INSTRUMENTATION Chapter #2 Electromechanical Indicating Instrument

C ONSTRUCTION OF D ’ ARSONVAL GALVANOMETER Scale Pointer Permanent magnet Iron core Coil Spring NS

C ONSTRUCTION OF D ’ ARSONVAL GALVANOMETER ( CONT.) It consists of a coil of fine wire suspended in a magnetic field produced by a permanent magnet (horseshoe shaped) with soft iron pole pieces attached to it. Between the pole pieces there is a cylinder of soft iron, which serves to provide a uniform magnetic field in the air gap. Between the pole pieces a coil is wound on a light metal frame and is mounted so that it can rotate freely in the air gap.

C ONSTRUCTION OF D ’ ARSONVAL GALVANOMETER ( CONT.) The pointer attached indicates the angular deflection of the coil and therefore the current through the coil. Two phosphor bronze conductive springs provide the calibrated force opposing the moving coil torque. Current is conducted to and from the coil by the control springs.

T HEORY OF OPERATION ( CONT.): Current from a circuit in which measurements are being made with the meter passes through the windings of the moving coil. Current through the coil causes it to behave as an electromagnet with its own north and south poles. The poles of the electromagnet interact with the poles of the permanent magnet causing the coil to rotate. The pointer deflects up scale whenever current flows in the proper direction in the coil.

T HEORY OF O PERATION For this reason all dc meter movements show polarity marking. The coil will continue to rotate until its electromagnetic torque balances the mechanical counter torque of the spring. This deflection measures the magnitude of the current.

T HEORY OF O PERATION The basic law for electromagnetic Torque is T = B * A * I * N = constant * I where T = Torque (N.m) B = magnetic flux density in the air gap (weber / m 2 ) A = effective coil area (m 2 ) I = measured current (a) N= number of coils turns.

T HEORY OF O PERATION The typical values for the previous parameters are A = 1.75 cm 2 B = 0.2 tesla N = 84 turns Coil resistance = 88 Ω Power dissipation = 88 μw For full scale deflection T= 2.92 * N.m

D YNAMIC B EHAVIOR The dynamic behavior of the galvanometer includes the speed of response, damping and overshoot of the moving element. The dynamic behavior of the galvanometer can be observed by suddenly interrupting the applied current so that the coil swings back from its deflected position towards the zero position. It will be seen as a result of inertia of the moving system, the pointer swings past the zero mark in the opposite direction, and then oscillates back and forth around zero. These oscillations gradually die down as a result of the damping of the moving element, and the pointer will finally come to the rest at zero.

D YNAMIC B EHAVIOR ( CONT.) The motion of a moving coil in a magnetic field is characterized by three quantities: Moment of inertia (J) of the moving coil about its axis of rotation. The opposing torque (S) developed by the coil suspension. The damping constant (D).

D YNAMIC B EHAVIOR ( CONT.) The differential equation that relates these three factors yields three possible solutions, each of which describes the dynamic behavior of the coil in terms of its deflection angle θ.

D YNAMIC B EHAVIOR ( CONT.) The three types of behavior are known as over damping, under damping and critical damping as shown in Fig. Curve I – Over Damping Curve II – Under Damping Curve III – Critical Damping Time (sec) Galvanometer Deflection III I II

D YNAMIC B EHAVIOR ( CONT.) The curve I shows the over damping in which the pointer returns slowly to its rest steady state position. The curve II shows the underdamped case in which the motion of the coil is subject to damped sinusoidal oscillations when the pointer returns to its rest steady state position. The curve III shows the critical damped case in which the pointer returns promptly to its state position without oscillations. For most application we prefer to operate under the conditions of II, III.