Back EMF: Definition and Concept

In summary, back-emf or counter-emf is a voltage that opposes the source voltage in an electrical winding, generated by Faraday's Law. It can arise in various situations, such as in variable-speed drives, permanent magnet brushless DC motors, and transformers. It is important as it limits the current in these devices when they are lightly loaded.
  • #1
Vishalatbatu
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Will u tell me what is back emf?
 
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Vishalatbatu said:
Will u tell me what is back emf?

Welcome to the PF.

What can you tell us about it? When does it arise? Where have you heard about it?
 
  • #3
Back-emf or Counter-emf is a voltage developed in an electrical winding by Faraday’s Law that opposes the source voltage, thus limiting the current in the winding. For example,
- Often the current regulator used invariable-speed drives is unable to track the commanded current because of insufficient voltage difference between the motor back EMF and the supply.
- A permanent magnet brushless DC motor with concentrated stator phase windings. The concentrated windings create a square wave flux distribution across the air gap and a trapezoidal shaped back-EMF.
- A representation of a machine in which the stator voltage equations are modeled as a voltage source in series with a reactance (and typically a resistance). The voltage source represents the back emf present on the stator windings due to the coupling between the stator and rotor circuits.

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  • #4
When a current passes through a coil wound on an iron core, it generates magnetic flux in the iron core, especially if the iron core passes through the centre of the winding and back up to the other end around the outside of the coil, without any air gaps.

This magnetic flux is now in the centre of a coil of wire and it then generates a voltage across it. It does not matter if the coil is the same one that caused the magnetic flux or another coil.

So, if it is the same coil, it will generate a voltage that opposes the original voltage. This is back EMF.
If it is a different coil, a new voltage will be generated across it. This is a transformer secondary.

Back EMF happens in some motors, too.
A motor is rotating because of a voltage across it and there is a magnetic field.
Now, you have a rotating set of coils in a magnetic field. This will cause these windings to generate a voltage which will oppose the voltage which caused the motor to rotate.
This, too, is back EMF.

In both cases, back EMF is very important as it limits the current which will flow in the transformer primary or the motor windings when the device is lightly loaded.
 

FAQ: Back EMF: Definition and Concept

What is back EMF?

Back EMF, or back electromotive force, is an electrical phenomenon that occurs when the magnetic field around a conductor changes, causing a voltage to be induced in the conductor that opposes the change in current flow.

How is back EMF measured?

Back EMF can be measured using a voltage sensor or an oscilloscope. The voltage induced by the changing magnetic field can be measured and compared to the applied voltage to determine the amount of back EMF present.

What is the purpose of back EMF?

The main purpose of back EMF is to prevent damage to electronic components. By opposing the change in current flow, back EMF can regulate the amount of current flowing through a circuit and prevent excessive voltage spikes.

What causes back EMF?

Back EMF is caused by the changing magnetic field around a conductor, which can occur when the current flowing through the conductor is changed or when the conductor is moved in a magnetic field.

How does back EMF affect motor performance?

In motors, back EMF plays a crucial role in controlling the speed and torque of the motor. As the motor spins, back EMF is induced in the coils, which counteracts the applied voltage and regulates the speed and torque of the motor. This helps to prevent the motor from overheating and burning out.

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