3. Inductors

Same as the coil used in the previous notes. The Inductor is a coiled conductor usually a copper wire. From the circuit below, an Inductor is connected across a DC voltage source. The current flows across the Inductor coil, and a Magnetic field is generated around the Coil.


This generated Magnetic field forces the current from changing in the circuit even when the voltage supply is shorted. This was discovered by “Emil Lenz” and is called Lenz Law. 

The direction of the Magnetic field and Current are as shown below, It is determined by the right Hand Rule where the thumb points the direction of the current and the folded fingers points the direction of the Magnetic Field. So inductors store current in the form on Magnetic Field. 

The Inductor maintains a constant flow of current through it. Generated Magnetic field forces this to happen. If a switch is introduced in the circuit as shown below.

From the circuit even when the Voltage source is OFF the current still flows in the circuit due to the inductor. For an ideal wire with “0” Resistance, the current flows forever in the loop. In a real case the duration of the current flow depends on the resistance of the wire and the inductance of the Inductor. If an LED is introduced in the circuit the LED glows momentarily even if the Voltage source is turned off. In a steady state condition the Inductor acts like a normal short cable.

No matter what the resistance on the cable is, the inductor will force to maintain the current, be it for a very short time. This results in generating very high voltages when the connected Resistor has a very high Resistance. This is the reason why the Inductors are used in high voltage generating circuits like “Boost converters”.

The inductance of an Inductor increases with increase in N- Number of turns or A- Area of the coil or Permeability of the core and decreases with increase in l- Average length of the coil.  

L : Inductance (Henry, H)

N: Number of turns

u:  Permeability of the core

l: Average length of the coil 

A: Area of the coil

As the Inductance (L) increases the Voltage (v) across will increase. The relation between L and v is as below, Voltage is the Inductance (L) times the rate of change of Current (di/dt).

2. Grounding in AC supply

Household equipments power cord usually has 3 terminals Neutral, Line and Ground. For a 3 phase circuit, circuit below shows Line and neutral. 

3 Phase supply is used for a long distance transmission. Houses usually get single phase and a limited number of 2 phase outlets for high demanding Electrical loads. 

Voltage across the two phases(two coils) will be double the voltage across the single phase(Single coil).

The main purpose of the Neutral cable is to send the current difference back to the source, “The Difference”- because two loads are connected at the same time. 

As Kirchhoff Current Law (KCL) says the net current flowing into the Node is equal to the total current coming out of the Node.

When both loads are equal, same current will be drawn i1=i2. In that case we do not require a Neutral wire to send back the difference current back to the source. This condition is called “Balanced Circuit”. This balanced condition doesn’t apply to a household with multiple different loads. The Unbalanced Condition occurs with an unequal loads are present.


The above circuit is already complete with Line and Neutral. 

What does ground do here?

Well, the ground protects from any short or dangers caused when a Line connection is exposed. The current chooses a lower resistance path to travel and ground is made with very low resistance material.

Where is it connected?

the ground connection is connected to the chassis of the load or any other susceptible spots closer to the “Line”. Because when a bad Line wire touches the chassis, the current chooses ground path and saves from danger.

As shown in the circuit above. When the neutral connection is connected to Ground, it reduces the risk of the Line wire touching the Neutral. When Neutral touches Line, the current chooses lower resistant Ground wire and eliminates the risk of shorting Line and Neutral.

1. DC/AC(1 Phase and 3 Phase) Supply

DC Supply

DC is straight forward. In a DC power supply, the current flow only in one direction, between negative Cathode to positive Anode. When any load (like a resistor or an LED) is connected across a battery, the current flows through the load and the load uses the current flow. In case of a resistor heat energy is released. For an LED light is emitted. 

Single Phase AC Supply

Flow of electric current in the wire generate Magnetic field around the wire and vice versa, current is generated in the wire when a magnetic field is introduced. When a magnet is moved in a coiled wire with multiple turns, a current is generated in the coil. Instead of using a permanent magnet, electromagnet can be used with varying magnetic field to generate more current.

When a magnet is rotated closer to a coil, at the point of strongest field a maximum current Is generated, the current gradually decreases to 0 and finally goes to minimum negative current. This cycle resembles a sine wave, and this cycle happens 60 times in a second (Frequency)~ 60Hz AC. It is 60Hz in the US and is 50Hz in Europe. It varies with what each country decides.

A coil can hold more electromagnetic field compared to a single wire. As the number of turns in the coil increases the electromagnetic field increases, this generate more current. This back and forth Alternating flow of current due to Electromagnetic field is called Alternating Current (AC).

This is how a single phase AC works. 

Three Phase AC Supply

Just like Single Phase, Assume an Electromagnet at the center and if this time 2 more coils be placed with the initial coil at 120 degrees with respect to the first coil. It will be 3 phase AC supply.

Why 3 Phase? When 3 loads need power at the same time, instead of using 3 different sets of transmission cables, a 3 phase AC supply is used.

To close the circuit with three phases, a Neutral connection should be connected. But when all three loads are equal, the net current at the node will be “0”. So for a Balanced load condition, there is no Neutral Line.

To be continued…