1. E LECTRODYNAMOMETER IN POWER MEASUREMENT It may be used to indicate both ac and dc power for any waveform of voltage and current (not restricted to sinusoidal). To measure the power of single phase, the electrodynamometer is connected as shown in Fig. Current coil Potential coil ic ip i e

2. E LECTRODYNAMOMETER ( CONT.) The field coils are connected in series and carrying the total current i c. The movable coil is connected in series with R (Limiting resistor) and it carries small current i p = e/R p, R p = R + coil resistance. The deflection of the movable coil is proportional to i c and i p, thus the average deflection:

3. E LECTRODYNAMOMETER ( CONT.) i c = i p + I Assuming for the moment that i c = i, Then, By definition, the average power in circuit is:

4. E LECTRODYNAMOMETER ( CONT.) Therefore the average deflection is proportional with the average power, i.e  av  P av If e and i are sinusoidal then: e =E m sin(  t), and i(t) =I m sin(  t ± θ) Then the average angle of deflection θ av will be: Where E and I represent the rms of e and i and θ represents the phase angle between voltage and current.

5. D’A RSONVAL M ETER M OVEMENT In order to measure alternating current with d’Arsonval meter movement we must rectify the alternating current by use of diode rectifier in order to use a dc movement to indicate the value of the rectified ac. This method is very attractive because a dc movement generally has a higher sensitivity than the electrodynamometer. RectifierPMMC ACDC

6. D’A RSONVAL M ETER M OVEMENT The rectifier element usually consists of a germanium or a silicon diode. The rectifier produces a pulsating unidirectional current through the meter movement over the complete cycle of the input voltage. Because the moving coil inertia, the meter will indicate a steady deflection that proportional with the average value of the current.

7. D’A RSONVAL M ETER M OVEMENT Since alternating currents and voltages are usually expressed in root-mean-square (rms) values, the meter is calibrated in terms of rms value of a sinusoidal wave form using what is called the form factor which is defined by: