Power measurement devices and how they work

The measurement of electrical power has not changed much over the years. A volt is still a volt and a watt is still a watt. However, the devices used to measure these electrical parameters have changed a great deal. What was once mechanical is now electronic; what was once analog is now digital.

By Jack Smith, Senior Editor, Plant Engineering Magazine January 15, 2002

Types of power measurements

Devices that measure power

How power measurement devices work

Sections: Types of power measurements Induction watthour meter Digital power meter

The measurement of electrical power has not changed much over the years. A volt is still a volt and a watt is still a watt. However, the devices used to measure these electrical parameters have changed a great deal. What was once mechanical is now electronic; what was once analog is now digital.

Types of power measurements

The following list is a sampling of measurements applicable to industrial plants.

Apparent kvah —the square root of the sum of the squares of the active and reactive powers during a one hour period. Generally, this does not apply to nonsinusoidal quantities.

Current —the measure of electrical current flow through an electrical conductor, measured in amperes.

Demand —active, real, or true power. The power that is actually consumed by the load. This measurement takes the power factor into account.

Frequency —refers to the number of complete cycles of ac voltage or current occurring during one second, measured in Hz.

Harmonics —voltages or current with frequencies that are integer multiples of the fundamental power frequency. They are common and sometimes dangerous in nonlinear loads.

kv —the phase-to-phase voltage of a circuit. When phase-to-ground voltage measurement is required, it should be so noted.

kvar —the practical unit of reactive power, equal to the product of the root-mean-square (rms) voltage in kV, the rms current in amps, and the sine of the angle between them. Kvar is reactive power, or the power that is actually “borrowed” from the load and returned to the power source each cycle.

kW —the instantaneous power requirement.

kWh —1 kW for the duration of 1 h.

Phase difference —the difference in phase between two sinusoidal functions, such as voltage or current, having the same periods. The time relationship between current and voltage in ac circuits.

Phasor kvah —A complex number, associated with sinusoidally varying electrical quantities, such that the absolute value of the complex number corresponds to either the peak amplitude or rms value of the quantity (kvah in this case) and the phase to the phase angle at zero time.

Power factor —the ratio of true power (in W) to apparent power (in VA). Expressed in decimal form, for example, 0.97.

Reactance —the opposition to current flow in an ac circuit introduced through inductance or capacitance.

Quadergy —the integral of reactive power with respect to time. It is delivered by an electric circuit during a time interval when the voltages and currents are periodic, the product of the reactive power and the time interval, provided the time interval is quite long in comparison with the time of one period.

Voltage —the greatest rms (effective) electrical difference of potential between any two conductors of a circuit. Some systems, such as 3-phase 4-wire, single-phase 3-wire, and 3-wire dc, may have various circuits of various voltages.

Voltage spread —the difference between maximum and minimum voltages.

Voltampere —the unit of apparent power in the International System of Units (SI). The apparent power at the points of entry of a single-phase, two-wire system when the product of the rms value in amperes of the current and the rms voltage is equal to one.

Watt —the power required to do work at the rate of one joule per second. W=EI, where W=watts, E=volts, and I=amps.

Watthour —the power required to do work at the rate of one joule per second for a duration of one hour.

Induction watthour meter

Figure 1. Whether residential or industrial, the basic power measurement device is the watthour meter. Voltage and current are converted to proportional magnetic fluxes. Magnetic flux is focused on, and interacts with, the disk of the rotor assembly. Time and spatial relationships, as well as the magnitude of flux determine the driving torque effect on the disk. A permanent magnet, which serves as an energy reference, controls the disk’s speed of rotation. This ensures the rotation is proportional to watts. A worm gear meshed with the disk shaft drives a mechanical register which displays accumulated energy in kWh. A shutter assembly converts disk rotations into pulses feeding an electronic register.

Fig 2. The rotor is an aluminum disk with a shaft, worm gear, bearing surfaces, anticreep holes, and a timing mark. It is the part of the watthour meter that is directly driven by electromagnetic force.

Fig 3. The stator consists of a potential coil and a current coil. The potential coil is connected line-to-line to the electrical service. It produces a horizontal flux field proportional to the applied voltage. The current coil is connected in series with one power leg. It produces a vertical flux field proportional to the current flow through the load. A stator may include more than one current circuit. The combined magnetic circuit is so arranged that the joint effect, when energized, is to exert a driving torque on the rotor.

Fig 4. The motor is the combined rotor and stator. With the disk suspended magnetically, it has induced eddy currents, which interact with the flux fields in the stator to produce a torque on the disk proportional to true power. The speed of the disk’s rotation is proportional to true power.

Fig 5. The watthour meter block diagram above illustrates how its components work together to register consumed electrical power.

Digital power meter

Fig 6. Kilowatt hours is only one parameter a digital power meter can measure. It connects to the line, measures instantaneous voltage and current, and then derives other measurements by calculating these data. Some of the quantities calculated are:

Energy (kWh)

Quadergy (kvarh)

Phasor kVAh

Apparent kVAh

Demand values

Current phase angle for each phase

Distortion power factor for each element

Distortion Vah for all phases

I2h, fundamental and harmonics for each phase

I2nh, (imputed neutral current squared hours)

V2h, fundamental and harmonics for each phase

VAh for all phases


Line-to-neutral volts, fundamental for all phases

Voltage phase angle

Watthours all phases

Watthours, fundamental for each phase

Watthours, fundamental and harmonics for each phase.

Some digital electric power meters have the ability to communicate their measurements and calculations. This is done through serial ports, Ethernet connections, and/or embedded web browsers with associated software.

Fig 7. The operation of a typical digital electric power meter can be explained with a block diagram. Usually, a digital power meter consists of voltage and current sensors, clock and voltage reference circuits, a current-to-voltage converter, voltage and current analog-to-digital (A/D) converters, digital multipliers, accumulator, and output registers that process energy pulses.

Fig 8. Analog voltage and current inputs are simultaneously sampled. The samples are converted to digital values by the A/D converter. Digital circuitry processes the data to calculate the desired quantities.

PLANT ENGINEERING magazine extends its appreciation to General Electric Meter and Power Measurement, Inc. for the use of their materials in the preparation of this article.