Electrical & Electronics Measurements

Power Measurements in DC Circuits

Measurement of electric power is essential in both commercial industrial applications and domestic applications. Prior estimation and subsequent measurements of instantaneous and peak power demands of any installation are mandatory for design, operation and maintenance of the electric power supply network feeding it.

How Power Measurements in DC circuits are done using an Ammeter and Voltmeter

Electric power (P) consumed by a load (R) supplied from a dc power supply (VS) is the product of the voltage across the load (VR) and the current flowing through the load (IR), thus:

P = VR x IR

Therefore, power measurements in a dc circuit can be carried out using a voltmeter (V) and an ammeter (A) using anyone of the arrangements as shown below:

power measurement in a dc circuit
Figure (a) Power measurement in a dc circuit

Figure (b) Power measurement in a dc circuit

Note that, both the voltmeter and the ammeter require power for their own operations. In the first arrangement above i.e. Figure (a), the voltmeter is connected between the load and the ammeter. The ammeter thus, measures the current flowing into the voltmeter, in addition to the current into the load.

Power consumed by the load = Power indicated by the instruments – Power loss in the voltmeter

Thus, power indicated = Power consumed + Power loss in voltmeter.

In the second arrangement above shown in Figure (b), the voltmeter measures the voltage drop across the ammeter in addition to the voltage dropping across the load.

Voltage drop across the ammeter, VA= IR x RA (where RA is the internal resistance of the ammeter)

Power consumed by the load the load = VR x IR = (VS – VA) x IR

= Vs x IR – VA x IR

= Vs x IR – I2R  x RA    (Equation 2)

= Power indicated by instruments – Power loss in the ammeter

Don’t miss out on key updates, join our newsletter  List

Therefore, Power indicated = Power consumed + Power loss in Ammeter

Theoretically, if we consider voltmeter to have infinite internal impedance and ammeter to have zero internal impedance, then from equation 1 and equation 2, you can observe that the power consumed by the respective instruments goes down to zero. Therefore, in ideal cases, both the two arrangements can give correct indication of the power consumed by the load. Under practical conditions, the value of power loss in instruments is quite small, if not totally zero, as compared with the load power, and hence, the error introduced on this account is small.

Power Measurement in DC Circuits using a Wattmeter

Power in dc circuits can also be measured by a wattmeter. Wattmeter can give direct indication of power and there is no need to multiply two readings as shown in the arrangements above where the ammeter and voltmeter are used.

The commonly used wattmeter for power measurement is the dynamometer. It consists of two fixed coils, connected in series, and positioned coaxially with space between them and a moving coil placed between the 2 fixed coils and fitted with a pointer.

The basic construction of dynamometer type wattmeter

It can be shown that the torque produced by the dynamometer is proportional to the product of the current flowing through the fixed coils times that through the moving coil.

The fixed coils (current coils), carry the load current while the moving coil (Voltage coils), carries a current that is proportional, via the multiplier resistor RV, to the voltage across the load R. Hence, the deflection of the moving coil is proportional to the power consumed by the load. In this construction of the wattmeter, the insertion error (as with the previous arrangements with ammeters and voltmeters) still exists. Relative β positioning of the current coil and the voltage coil with respect to the load, introduce similar Vs errors in the measurement of the actual power. By connecting the voltage coil between A and C, the current coils carry the surplus current flowing through the voltage coil conversely, by connecting the moving coil between B and C, this current error can be avoided,  but now the voltage measures the surplus voltage drop across the current coils.

Connection of dynamometer type wattmeter for power measurement in a dc circuit
Share
John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.

View Comments

Recent Posts

What to Expect from PCB Assembly Services in China

The importance of printed circuit board (PCB) technology has escalated throughout the years with the…

23 hours ago

Magneto-Optic Current Sensors for High Voltage, High Power Transmission Lines

One of the key challenges in measuring the electrical current in high voltage, high power…

3 days ago

How the Wiegand Effect is used in Sensing Instruments

The Concept behind Wiegand Effect Based Sensors   The Wiegand effect technology employs the unique…

5 days ago

Piezoelectric Accelerometer: Principle of Operation & Applications

An accelerometer is a sensor that is designed to measure acceleration or rate of change…

6 days ago

The USB-6009 Data Acquisition Card Features

The USB-6009 is a small external data acquisition and control device manufactured by National Instruments…

1 week ago

How X-Y Tables are used in Position Control Applications

X-Y tables are utilized as components in many systems where reprogrammable position control is desired.…

1 week ago