# Relationship between flux and mmf

### Magnetism: quantities, units and relationships

Hence the term magnetic circuit is used in relation to producing flux in the core by applying mmf (= Ni). We shall see more similarities between an electrical. H gives rise to magnetic flux density via the magnetic permeability of the You cannot calculate flux density with MMF alone - you need a. The Relation between B(flux density),H(field strength). MMF(magnetmotive force) and φ(flux). We need to define and distinguish magnetic Field intensity H.

Just as the external electrical source is called an electro motive force, the external magnetic source is called a magneto motive force mmfand is measured in ampere turns.

The resistance to the flow of magnetic flux in the core is called Reluctance Rm Fig A coil the primary supplied with an AC current is wound around one side of the core to provide a source of mmf. On the other side of the core a separate coil the secondary is wound, which supplies a measuring instrument to measure the amount of current in the coil. The current in this coil will be proportional to the amount of flux flowing in the core. This arrangement therefore provides a means of measuring magnetic flux.

A is the cross sectional area of the core and L is the mean length of the flux path around the core.

### Transformer Cores, Reluctance and Permeability

Electrical resistance also depends not only on the dimensions of the conductor but also on the material of the conductor and its resistivity. Relative and Absolute Permeability Permeability is often expressed as: Quoting the absolute permeability of materials used in cores would involve similarly awkward numbers.

The permeability of iron can be many hundreds, so having a magnetic circuit path of iron rather than air greatly increases the flux, which is why iron is a common choice of material for inductor and transformer cores. Common Types of Transformer Cores. However, in a gapped core a small section of the flux path is replaced by a non-magnetic medium - such as air. The term 'air gapped core' is still used even if the gap is filled not by air but by nylon or some other material immune to saturation.

## Magnetomotive force

Some core types may be purchased with a gap prefabricated, such as the RM7 shown above; but the photo at right shows a mH EC70 core in which the gap has been created by inserting sheets of cardboard between the core halves.

Adhesive tape can hold the core together adequately because clamping force requirements are less than for an ungapped core. Your first thought may be that if the core is overloaded then what you need is a larger one, and that the last thing you would want to do is remove part of it!

Well, magnetics is a curious business. However, you don't want six amps to flow round this circuit - that's too much; you only want four amps.

### magnetic flux - Difference between Fm and H - Electrical Engineering Stack Exchange

What's the simplest way to achieve that? The answer is a no-brainer: You now have a total of three ohms in the circuit and so all that a 12 volt battery can deliver into it is four amps. The problem which an air gap solves in a core is the excessive flux produced by a high level of current in the windings.

Transferring the previous analogy, suppose that your windings produce an MMF of 12 ampere turns and that your magnetic core your 'load' has a reluctance of two ampere-turns per weber. However, a flux of six webers is too much; you can only tolerate four webers.

To solve the problem you just add in a 'resistor' call it a 'reluctor' of one ampere-turn per weber. Now you have a total reluctance of three ampere-turns per weber 'in circuit' so that now four webers is all that the MMF can push round the core.

You may have guessed that the role of this extra reluctor is taken on by the air gap.