Understanding Back EMF: The Secrets Unveiled

 

What Is Back EMF?

Back EMF is an induced voltage that opposes the change that produces it.

In an inductive circuit, when current changes, the magnetic field associated with the inductor also changes. According to Faraday’s law of electromagnetic induction, this changing magnetic field produces an induced EMF. The polarity of this induced EMF is such that it opposes the change in current, in accordance with Lenz’s law.

This is why it is commonly called back EMF or counter EMF.

The effect becomes particularly noticeable when the current through an inductive load changes rapidly. For example, when the supply to an energized contactor coil or another inductive load is suddenly switched OFF, the magnetic field collapses. The collapsing magnetic field can produce a high transient voltage across the switching contacts.

If the voltage becomes high enough, it can cause an electrical arc across the contacts.

 

Back EMF, Counter EMF and CEMF

You may see three different terms used in electrical literature:

• Back EMF
• Counter EMF
• CEMF (Counter Electromotive Force)

The terminology can vary with the application.

In a motor, back EMF commonly refers to the voltage induced in the motor winding as the motor rotates.

In a stationary inductive coil, such as a contactor or solenoid, the more precise explanation involves self-induced EMF produced when the current changes.

These effects are related to electromagnetic induction, but they should not simply be treated as exactly the same phenomenon in every circuit.

For this, We have two types of loads, One is ‘Heating element’ which is a type of resistive load having the rated voltage 240V AC, 300Watt. Another second load is ‘solenoid coil’ which is a type of inductive load fitted inside the ML-3 contactor having the coil voltage rating is 240V AC.

 

Technical specifications of heater

Calculated current & observed current is almost equal. That means there is only resistance which is opposing the line current at which total drop been happening.

• Rated Voltage : 240VAC
• Observed Resistance : 167.7 Ω
• Calculated Current : V/I= 1.37Amps
• Actual applied Voltage: 233.6
• Actual Current(Observed Current) :  0.038Amps

Calculated current & Actual current have huge difference, Actual current is far lower than calculated current. Which means there is something else another than resistance which is opposing the current flow.

In an AC circuit, Along with resistance there is reactance which is working across the circuit. Which in-turn in along with resistance appear as an impedance. Along with impedance there is one more thing and that is “Back emf”.

The spark behind the switching processes