3 Things You Need to Know About Electrical Power in AC Circuit

Electrical power plays a crucial part in our everyday life from lighting, heating, cooking and much more. It’s not just voltage or just current but the two together – electrical power that is the root factor that drives our modern industry. In this blog post, we will discuss 3 things you need to know about Electrical power in AC circuits.

Electrical power is the rate, per unit time, at which an electric circuit transfers electrical energy and convert to some other forms of energy such as mechanical energy, magnetic field, electric field, light and heat. An excellent example of electrical power is a light bulb. When you connect an incandescent light bulb to electricity, it is consuming electrical power and converts into light and heat. Electrical power can also convert into stored energy such as electric field and magnetic field, like in a motor.

But there is not one, not two but three different types of electrical powers in AC circuits and that’s what we will cover here. Those three powers are

  1. True Power
  2. Reactive Power
  3. Apparent Power

Electrical Power Type one: True Power is Truly Useful

The True power is the actual power being used or dissipated by resistance in the circuit. It is the power that gets converted into other useful power; for example, an AC motor will consume true power and convert it into mechanical power to drive a load.

The best example of the true power that can be seen in our everyday life is the electric heater, which consumes true power to dissipate heat or an incandescent light bulb, which converts true power to produce light and heat.

P = V x I x pf

Where 
P = True Power (W)
V = Supply voltage (V)
I = Line current (A)
pf = Power Factor (no unit)

In calculation, the True power not only considers the supply voltage and current but also another quantity called the Power Factor. Power Factor is quite similar to the efficiency of the system but is a number between 0 and 1. True power is measured in Watts (W)

In the video, we can see the meter in the AC Pracbox is showing

True power = 23.4W
Supply Voltage = 22.1V
The light bulb draws Current = 0.1.054A
Power factor = 0.999

This reading means, 23.4W of true power will 100% convert into light and heat energy because it is a purely resistive load as also indicated by the power factor of 1. 

So the 3 things you need to know about True power

  1. It’s the useful power
  2. It’s measured in Watts or W (kW more often)
  3. It’s the power that generates heat

Electrical Power Type Two: Reactive Power is Not so Useful – But It is Necessary

The electrical power used by reactive components is the Reactive power. All electrical loads can be broken down into a combination of three components – Resistors, Capacitors and Inductors. For example, the light bulb in the previous example is mainly resistive, and a three phase electric motor is a combination of resistive and inductive. Inductive and Capacitive components are considered reactive components

Capacitors store energy in between capacitor plates using electrostatic fields and give back to the circuit. Similarly, inductor also stores energy within itself using magnetic field and returns to the circuit. The reactive power is what these components use to develop the electrostatic and electromagnetic fields.

Then How is Reactive Power Not Useful

A motor would not spin without the magnetic field that the windings generate using the reactive power, then how come we consider it not useful? That’s because the magnetic field is only one part of the solution.

For example, how useful would you consider a motor that spins without anything connected to it? No much. But is it going to use the current from the supply? Of course! It’s the reactive power that will make is spin, but the true power produces useful torque.

Q = V x I x sinθ
Where
Q = Reactive Power (VAR)
V = Supply voltage (V)
I = Line current (A)
θ = Angle between True Power and Apparent power

So the 3 things you need to know about Reactive power

  1. It’s not considered a useful power but is necessary
  2. It’s measured in Volt-Ampere Reactive or VAR (usually kVAR)
  3. It’s the power that is needed by the reactive components like capacitors and inductors to work

Electrical Power Type Three: Apparent Power is Actually Very Apparent

The electrical power that we supply to any equipment is the apparent power. Basically, when you connect something to a power outlet, the voltage and current that it supplies to the equipment is what makes the Apparent power. Put simply, it’s the supplied power.

So, in calculations, the apparent power is the product of the RMS voltage and RMS current taken from the supply. Apparent power is measured in volt-amperes (VA), and it is represented by the letter “S”

S = V x I
Where
S = Apparent Power (VA)
V = Supply voltage (V)
I = Line current (A)

The 3 things you need to know about Apparent power

  1. It’s the supplied power
  2. It is measured in Volt-Amperes or VA (kVA is more appropriate)
  3. It’s the largest power out of the three

How to These Electrical Powers Relate to Each Other

The relationship between the three electrical powers in an AC circuit can be expressed in a right angled triangle. Why right angled triangle? The reactive components absorb some power and return to the circuit later, which cause the phase shift or leading and lagging between the current and voltage, these are at 90 degrees with respect to the resistive components

The angle between the True Power and the Apparent power is of the key importance here. Bigger angle means that the system is drawing more current than needed, only to waste on its reactive parts. We call this low power factor. In the video below, I have shown the effects of three components – resistance, capacitance and inductance by connecting them in the AC pracbox and in the table there are all values for comparison.

Components Voltage Current Power Factor Phase Angle True Power Apparent Power Reactive Power
20 ohms Resistor
11.1 V
0.55 A
1
0 Deg
6.1 W
6.1 VA
0 VAR
80uF Capacitor
12.3 V
0.33 A
0
90 Deg
0 W
4.06 VA
4.06 VAR
680mH Inductor (18 ohms)
12.2 V
0.094 A
0.307
72.12 Deg
0.3 W
1.15 VA
1.11 VAR

Conclusion

The three things that you needed to know about electrical power in AC circuits is that there are three different powers, unlike DC circuits, where there is only one. We also looked at the three basic points to know about each type of electrical power. The quantity that binds these powers together is the power factor, which we alluded to in this post, but it begs a whole post all on its own.

I hope this post gave you some insight into the three electrical powers in AC circuits and their basics.

Thanks for reading.

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About the Author

Try Mao

Try Mao

Try is an electrical engineer with a passion for programmable logic controllers and hardware. He loves taking photos and is a gun with a camera. His purpose is to solve problems through innovative electrical product design and development

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