(Talking about my Graduation Days . .
Those were the starting days of my graduation. I had a phenomenon always roams in my mind,
An inductive coil; Having a rated voltage 230V AC. On connecting the coil with power supply; Coil becomes Electromagnet. But if same inductive coil rolled out & stretched into a straight conductor than a power supply connected to it, A Short circuit will get happened. Wire is same, its diameter & resistance; all is same than why do a short circuit get happen ?
(I remembered another incident . .
I was sitting in a practical viva voce !
The Question been asked; “Why do Induction motor takes high current at the time of starting ?”
I answered, “At the time of starting; rotor do not have any type of momentum that’s Why.”
Examiner stared at me & said, “No !!”
At the end of viva voce, I asked the Examiner about the answer.
He said,” At the time of starting, there is no back Emf !
In this post, We will talk about back EMF which is also known as Counter-EMF or CEMF.
Back EMF, Counter EMF or CEMF
“When a changing power supply get connected across inductive coil, Back EMF is a type of electromotive force which get induced in inductive coil due to induction.”
Back EMF is a type of voltage which oppose the changes in current which has generated it. Since it has a opposing nature; opposing the change in a current of a source which has generated it that’s why it is known as back EMF or counter EMF.
Inductor is composition of number of turns of insulated wires either rounded on silicon steel stamping OR rounded around free space. On getting power supply; Each turn has its own magnetic field which links with magnetic field of another turn. Due to changing current, Due to faraday law of electromagnetic induction, An EMF get generated inside each turn & in total , A bulk emf from all the turns get generated which have opposing nature !
PRACTICALLY OBSERVATION OF BACK EMF
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 fitting inside the ML-3 contactor having the coil voltage rating is 240V AC.
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- Technical specifications of heater
Rated Voltage : 230VAC
Rated Power : 300Watt
Observed Resistance : 176.5 Ω
Calculated Current according to ohm’s law : V/I=230/176.5=1.3Amps
Actual applied voltage : 233.6
Actual Current(Observed through clamp-meter) : 1.25Amps
Calculated current & observed current is almost equal Means there is only resistance which is opposing the line current at which total drop been happening.
- Technical specifications of solenoid coil
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 a 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 & that is Back emf.
TWO ANSWERS WHICH I WAS LOOKING FOR
1. BACK EMF IN SOLENOID COIL
In solenoid coil due to Lenz law; Back emf get generated. As Each turn has its own magnetic field which links with magnetic field of another turns. Due to changing current, Due to faraday law of electromagnetic induction, An EMF get generated inside the solenoid coil.
But if that same solenoid unrolled & stretched into straight conductor than there is no magnetic field linking; no back emf. If a power supply given to that minor resistance straight conductor than very high current will flow; A short circuit will be the result.
2. BACK EMF IN INDUCTION MOTOR
Induction motors are single excited electrical machines in which power need to be connected only at its stator winding. Starting current of Induction motor can go high multiple times of the rated current because:
- There is no back emf at starting.
- There is no momentum at starting; No flywheel effect.
- load connected across the motor at starting.
To prevent that much high initial current STAR-DELTA starters, VFDs are employed for Induction motors to limit the high initial current.
SUM UP
The post reflects the concept of back electromotive force (EMF) and its implications in electrical systems. It discusses the behavior of inductive coils when connected to power, explaining that a short circuit occurs when a coil is unrolled into a straight conductor, as back EMF, which opposes current changes, is lost. The post recalls a viva voce incident where they learned that Induction motors draw high currents at startup due to the absence of back EMF and momentum. Practical observations with resistive and inductive loads illustrate these principles effectively.
Hope, You like the post. If you have any doubt, suggestion or query please do comments !