WATTMETERS

Wattmeters are used to determine DC power or real AC power delivered to the load.

The wattmeter is an instrument which measures DC power or true AC power. The wattmeter uses
fixed coils to indicate current, while the movable coil indicates voltage . Coils LI1 and LI2 are the fixed coils in series with one another and serve as an ammeter. The two I terminals are connected in series with the load. The movable coil Lv, and its multiplier resistor Rs, are used as a voltmeter, with the V terminals connected in parallel with the load. The meter deflection is proportional to the VI, which is power.

Wattmeters are rated in terms of their maximum current, voltage, and power. All of these ratings must be observed to prevent damage to the meter.
Equation (14-15) is the mathematical representation of calculating power in a DC circuit.
P = VI or P = I2R ............... (14-15)
Equation (14-16) is the mathematical representation for calculating power in an AC circuit.
P = VRms IRms x cos phi or P = I2R ............(14-16)

Three-Phase Wattmeter
Total power in a 3f circuit is the sum of the powers of the separate phases. The total power could be measured by placing a wattmeter in each phase (Figure 12); however, this method is not feasible since it is often impossible to break into the phases of a delta load. It also may not be feasible for the Y load, since the neutral point to which the wattmeters must be connected is not always accessible.

Normally, only two wattmeters are used in making 3f power measurements (Figure 13). In balanced 3f systems, with any power factor, total power is calculated by adding the A and B phase powers. Equation (14-17) is the mathematical representation for calculating total power
(PT).

Pt = Wa + Wb ............ (14-17)
where Wa and Wb are the power readings in Phase A and Phase B.

Wattmeter Summary

1. Measures real power delivered to the load.
2. Single-phase AC or DC - voltage component (movable coil) connected in parallel
   with the load and the current component (fixed coil) connected in series with the
   load.
3. Three-phase AC - summation of Phase A and B powers.

Source : DOE FUNDAMENTALS HANDBOOK - ELECTRICAL SCIENCE - Volume 4 of 4