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Magnetomotive Force (MMF)

Last updated date: 24th May 2024
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Introduction to Magnetomotive Force

Before we are going to start about “what is mmf”,magnetomotive force and its unit. First, we must have the knowledge regarding the magnetic circuit and various terms related to it like magnetic reluctance, magnetic flux etc. Therefore first we are going to study magnetic circuits. A magnetic field, shown as lines of magnetic flux, is restricted to a magnetic circuit, which is a closed channel. In contrast to an electric circuit, where electricity flows, a magnetic circuit has no electricity flowing through it. The magnetic field or flux in a ring-shaped electromagnet with a tiny air gap is nearly totally contained to the metal core and the air gap, which together form the magnetic circuit. 

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The magnetic field of an electric motor is restricted mostly to the magnetic pole pieces, the rotor, the air gaps between the rotor and the pole pieces, and the metal frame. Each magnetic field line forms an uninterrupted circle. The total flux is the sum of all the lines. The magnetic circuit is considered parallel when the flux is split so that part of it is limited to one section of the device and the other to another. A series magnetic circuit is formed when all of the flux is contained within a single closed loop, as in a ring-shaped electromagnet. The current, electromotive force (voltage), and resistance are all related by Ohm's equation in a magnetic circuit, just as they are in an electric circuit.

What is MMF?

The electric current is equivalent to the magnetic flux. The magnetomotive force, abbreviated as mmf, is comparable to the electromotive force and may be thought of as the flux-setting factor. The mmf is measured in ampere-turns and is comparable to the number of turns of wire carrying an electric current.The mmf rises as the current through a coil (as in an electromagnet) or the number of turns of wire in the coil is increased, and the magnetic flux increases accordingly if the remainder of the magnetic circuit stays unchanged. A magnetic circuit's reluctance is similar to an electric circuit's resistance. Reluctance is determined by the geometric and material aspects of the circuit that provide resistance to the presence of magnetic flux.

A given component of a magnetic circuit's reluctance is related to its length and inversely proportional to its cross-sectional area and permeability, a magnetic property of the supplied material. Iron, for example, has a very high permeability in comparison to air, therefore it has a very low reluctance, or gives very little resistance to the presence of magnetic flux. In a series magnetic circuit, the total reluctance is the sum of the individual reluctances encountered around the closed flux channel.

Magnetomotive Force Definition

The presence of electromotive force causes current to flow in an electric circuit, while the presence of magnetomotive force (MMF) causes magnetic flux to flow in a magnetic circuit. Magnetomotive Force is the magnetic pressure that creates the magnetic flux in a magnetic circuit. The SI unit of magnetomotive force is ampere - turn (AT) and in CGS system is G (gilbert). The MMF for the inductive coil seen in the diagram below is,

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The product of the current around the turns and the number of turns in the coil equals the MMF's strength. Under work law, MMF is defined as the force required to move the unit's magnetic pole once around the magnetic circuit. Therefore, the magnetomotive force formula is expressed as,


Here, Fis the magnetomotive force, N is the number of turns and I is the current flowing through a coil.

The magnetic potential is another name for the MMF. It is the property of a substance that causes a magnetic field to form. The product of magnetic flux and magnetic reluctance is used to compute the magnetomotive force. The resistance is the magnetic field's opposition to establishing the magnetic flux on it. The reluctance and magnetic flux MMF is given as,


Here, Ф is the magnetic flux and R is the reluctance of a circuit.

The magnetomotive force may be measured in terms of magnetic field intensity and material length. The magnetic field strength is the force acting on the magnetic field's unit pole. The MMF for field intensity is stated as,


Here, H is the magnetic field intensity and l is the material length.

FAQs on Magnetomotive Force (MMF)

Q1. What is the purpose of an air gap in a magnetic circuit?

Ans: A magnetic circuit is a confined space or passage in which a magnetic flux is circulated or followed. An air gap is a non-magnetic component of a magnetic circuit that is normally linked to the remainder of the circuit magnetically in series. This allows a large quantity of magnetic flux to travel through the gap. The air gap can be filled with a non-magnetic substance other than air, such as gas, water, vacuum, plastic, wood, and so on, depending on the application. One of the primary reasons for an Air Gap is to enhance the magnetic circuit's reluctance. The quantity of air or another non-magnetic substance in the circuit, such as a fibre plate or fibre board, increases the reluctance of the circuit, allowing us to put more current in a coil before reaching saturation.

Q2. What is the relationship between reluctance flux and machine mmf ?

Ans: Magnetic reluctance, often known as magnetic resistance, is a term used in the study of magnetic circuits. It is defined as the magnetomotive force (mmf) to magnetic flux ratio. It indicates the magnetic flux's opposition and is affected by an object's shape and composition. In a magnetic circuit, magnetic reluctance is equivalent to electrical resistance in an electrical circuit, where resistance is a measure of the resistance to the electric current. In this way, the notion of magnetic reluctance is similar to Ohm's law. However, unlike current flowing through a resistance, magnetic flux travelling through a reluctance does not result in heat dissipation.

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