Measuring Devices

Fig. 1 Ammeter

Fig. 2 Voltmeter

Fig. 3 Wattmeter

Fig. 4 Power Factor Meter

Measuring Techniques


    The wattmeter is used for measurement of average power in any electrical network. The wattmeter has a voltage or potential coil (PC) and a current coil (CC) with polarity marking as indicated in Fig. 5. When the wattmeter is properly connected as shown in figure, the meter simply reads the average power.

Fig. 5 Connection diagram for measurement of power, voltage and current

    If both coils (PC & CC) are reversed then the wattmeter reading remains unchanged. If either one of the coils is reversed, the meter will give the negative value of power. Digital meters will indicate the negative values by a minus sign in the display. Analog wattmeter will simply attempt to read down scale, and the user must reverse the potential coil connection and manually insert the negative value in the recorded data.

Measurement of average power flow in three phase circuits can be accomplished by connecting a wattmeter to read the power flowing in each phase. The total three phase power would then be the sum of the three wattmeter readings. However, if the circuit is connected in star, the pressure coils of all three wattmeters should be connected to the neutral. If neutral wire is not available (for example in three phase, three wire system) or load is delta connected then power measured by two wattmeter method as shown in Fig. 6

Fig. 6 Connection diagram for measurement of three phase power using two wattmeter method


For the energy measurements mainly two types of energy meters are used as shown in Fig. 7 –

  1. Static energy meters (Digital)
  2. Rotating disc type energy meters ( Analog / Digital)
The rotating disc type energy meter is also called induction type energy meter and consisting of the following systems as shown in Fig. 8:
  1. Driving System
  2. Moving System
  3. Braking or Damping system and
  4. Registering system

Fig. 7 Static and rotating type energy meters

Fig. 8 Schematic diagram of Induction type rotating energy meter