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POWER MEASUREMENTS

SINGLE PHASE REAL POWER MEASUREMENTS

Low Power Factor Wattmeter

REACTIVE POWER MEASUREMENTS

THREE PHASE REAL POWER MEASUREMENTS

ENERGY MEASUREMENTS

ENERGY TRIANGLE

SINGLE PHASE ENERGY METER

Errors in Energy meters

Calibration

THREE PHASE ENERGY METER

ELECTRONIC ENERGY METERS

POWER MEASUREMENTS

INTRODUCTION

Electric power is the rate of doing work. It is expressed in Watts. The higher units of power used in practice include kilowatts, megawatts, etc. PWatts = VI COS f , i.e., a power of one watt is said to be expended when a source of one volt passes a current of one ampere through a load resistance/ impedance of one ohm at unity power factor.
The power measurements are made with the help of a wattmeter. Wattmeter is an indicating deflecting type of instrument used in laboratories for measurement of power in various ranges. A wattmeter consists of two coils as shown in the schematic representative figure 4.1
 Current coil (CC): connected in series with circuit and carries the load current. It is designed such that it is wound with 2 to 3 turns of thick wire and hence it has a very low resistance.
 Voltage or Pressure or Potential coil (PC): connected across the load circuit and hence carries a current proportional to the load current. The total load voltage appears across the PC. It is designed such that it is
wound with several turns of thin wire and hence it has a very high resistance.

The wattmeter can be a UPF meter or LPF meter depending on the type of the load connected in the measuring circuit. For power measurements in AC circuits, the wattmeter is widely adopted. In principle and construction, it is a combination of those applicable for an ammeter and a voltmeter.
The electrical power can be of three forms:
 Real power or simply, the power is the power consumed by the resistive loads on the system. It is expressed in watts (W). This is also referred as true power, absolute power, average power, or wattage.
 Reactive power is the power consumed by the reactive loads on the system. It is expressed in reactive volt-amperes (VAr).
 Apparent power is the vector sum of the above two power components. It is expressed in voltamperes (VA).

Thus, it is observed from the power triangle shown in figure 4.2, that more is the deviation of power factor from its unity value, more is the deviation of real power from the apparent power. Also, we have