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Lead Contents

Electoric Power Monitoring Equipment

Introduction Features
Principles Classifications
Engineering Data Further Information
Explanation of Terms Troubleshooting

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Primary Contents

Electric Power and Power Consumption

Electric power expresses the amount of work that electricity can do and it is the amount of work done by electricity in one second.
Power (watts) = Voltage (volts) x Current (amperes)
W = V x I
The unit is the watt (W).

Power consumption is the amount of energy used, i.e., how much work the electricity did.
It is calculated as follows: Power (W) x Time (h).
The unit is the watt-hour (Wh).
Examples: 1 kWh = 100 V x 10 A x 1 hour
10 kWh = 100 V x 100 A x 1 hour

< Mechanism of Electricity Generation >

Why AC?

(1) It is simple to produce an electrical generator for AC power.
As shown in the above figure, if you rotate a magnet, electricity is generated (Fleming's rule).
In the past, water wheels were used. Nowadays, turbines are used.

(2) It is simple to convert an AC voltage.
Long-distance transmission is easy for AC electric power.
You can change to the desired voltage by changing the number of turns in the transformer.
Transmitting electricity at higher voltage creates lower loss.

(3) Low-maintenance AC motors can be used.


AC (alternating current) means that the voltage is changed alternately with time. Frequency expresses how many cycles of the wave are repeated in one second. The unit is the hertz (Hz).

Three-phase AC

Three-phase AC is AC that combines 3 systems of singlephase AC with phases that are 120° different from each other.


A transformer increases or decreases the voltage.

Inductive Loads

An inductive load is a load in which the wires are wound into a coil. Motors are a typical inductive load.

Apparent Power

Volt-ampere (VA)

VA is calculated by multiplying the voltage (V) and current (A).
Apparent power is the value found by simply multiplying the voltage and the current.

Active Power

Watt (W)

Active power is power that does actual work and is consumed as energy.
The AC electricity normally used in homes has apparent power (VA) and active power (W). The active power is the apparent power multiplied by the power factor.

Reactive Power

Volt-ampere Reactive (Var)

This is the power that goes back and forth between the power supply and the load without doing any work.

Power Factor

There is active power and reactive power. The power factor is the proportion of active power. This is like the efficiency of electricity usage.