Magnetic Circuits

Till now we have talked about standard circuits like linear and non-linear circuits but in the electronics industry, there are another kind of circuits called ‘magnetic circuits’. These circuits like any other circuits have a closed path but the path is followed by magnetic lines of forces creating a field of magnetic flux instead of a flowing current. In this article, we will study what are magnetic circuits, and what components make up the circuit. We will also represent the equation used to represent these circuits mathematically. Later we will discuss the need of having magnetic circuits and some applications of magnetic circuits in everyday life. We will conclude the article by summing up what we have learned. The electronics industry especially the communication system uses these signals in various circuits and readers can read about these other signals as well.

What are Magnetic Circuits?

Magnetic circuits are similar to normal electrical circuits that have a closed path followed by magnetic lines of force. It is important to know that in a magnetic circuit, the magnetic lines of force originate from a point and end at the same point after completing the full path. Despite being a circuit, it is important to note that nothing flows in a magnetic circuit like current that flows in a standard electrical circuit.

As the name suggests, a magnetic circuit consists of magnetic materials which have high permeability, these materials are usually steel or iron. Magnetic circuits also include electric motors, transformers, generators galvanometers, etc.

Applications of Magnetic Circuits

Magnetic circuits remain indispensable in electrical engineering thereby having various applications in real-day life like.

• Magnetic circuits are majorly used in transformers. This is mainly because of their ability to transfer energy efficiently. These circuits result in efficient power transfer by giving direct control over power supply in circuit.
• The circuits are used in inductors for storage of energy efficiently. The magnetic circuits generate a magnetic field of electrons and the energy of these electrons is stored in inductors. Using magnetic circuits also reduces electrical noise.
• Other appliances using magnetic circuits includes electric motors and generators. They help to operate motors by converting electrical energy to mechanical energy. The generators use these circuits for performing the exact opposite task i.e. converting mechanical energy to electrical energy.
• Some general devices that work on the principles of magnetism involve usage of magnetic circuits. Some of the examples of these devices are magnetic sensors, magnetic amplifiers, magnetic recording devices and magnetic couplers. They are general use items that involve magnetic circuits.
• Hospitals make use of magnetic circuits in operating devices and performing surgeries. Medical imaging in machines like MRI(Magnetic Resonance Imaging) involve strong magnets and magnetic circuits which generate detailed images for medical diagnosis.

Circuit Diagram of Magnetic Circuits

The magnetic circuit diagram shown below consist of a magnetic material, generally an iron core which is used for making the solenoid part of the circuit. The solenoid with N turns creates a magnetic flux which generates a current I in the solenoid.

Circuit Diagram

Mathematical Representation

For a material with l as mean length of the magnetic circuit, A is cross-sectional area of the core and

µ_r is relative permeability of the core, The magnetic flux of ø Weber sets up in the core when the current of I ampere is passed through a solenoid

Flux Density in core material can be written as

Flux Density = Magnetic flux per unit area i.e B = ø /A

We can also define the magnetizing force as

Magnetising Force = Flux Density/ (µ_r×µ₀) Magnetising Force = B/ (µ_r×µ₀) H = ø /A (µ_r×µ₀)

Ohm’s Law For Magnetic Circuits

Consider the magnetic circuit given below.

Magnetic Circuit

We have already seen that magnetic circuits are analogous to electric circuits i.e. we can compare magnetic circuits to electric circuits in certain respects. That being said ,we can say that there must be a law analogous to ohm’s law for magnetic circuits. This indeed is true that there is a law called Rowland’s law which is very similar to ohm’s law. The statement of this law is:

Rowland’s law states that the number of magnetic lines of force (Φ) is proportional to the magnetomotive force (F_m) and inversely proportional to the circuit’s reluctance (R_m).

Let us see the analogy, According to ohm’s law

V = IR

Here, V is the voltage I is the current in circuit R is circuit resistance.

Similarly for magnetic circuits

F = ΦR

Here, F is the magnetomotive force Φ is the magnetic flux R is the reluctance of the material.

The magnetomotive force can be compared to the voltage of the source while the magnetic flux can be compared to the current flow in electric circuit. The magnetic reluctance is synonymous to the resistance in electric circuit. This makes Rowland’s law an equivalent version of ohm’s law in magnetic circuit.

Properties of Magnetic Circuits

There are two major properties of magnetic circuits namely magnetic flux and magnetomotive force . We will discuss them here.

1. Magnetic Flux

Magnetic Flux is the term used for defining magnetic induction in magnetic circuits. It is often referred to when discussion is about strength of magnetic field in a particular region. Formally magnetic flux can be defined as a quantitative measure of magnetic field lines passing through a given region. It is generally represented by symbol Φ and can be written as

Φ = B⋅A⋅cos(θ)

Φ is the magnetic flux measured in Weber (Wb), B is the magnetic field strength measured in Tesla (T), A is the area of magnetic field lines measured in square meters (m²), θ is the angle between the magnetic field lines and the normal to the surface.

2. Magnetomotive Force (MMF)

This concept comes into picture when we talk about maintaining magnetic flux in a magnetic circuit. Similar to electromotive force in electrical circuits we talk about MMF as an analogous force. This is an important force that helps in sustaining the magnetic circuit. Formally, It is the total magnetic potential difference in a magnetic circuit and which performs the task of maintaining magnetic flux in a magnetic circuit. It can be written as

F = N⋅I

F is the magnetomotive force in ampere-turns, N is the number of turns of the coil or winding, I is the current flowing through the coil in amperes.

Difference Between Electric Circuit and Magnetic Circuit

Lets see the difference between electric and magnetic circuits.

Electric Circuits	Magnetic Circuits
It is the path for electric current to flow.	It is the path traced by magnetic flux.
It follows ohms law V=IR	It follows law analogous to ohms law F=ΦR
The force involved in maintaining circuits is electromotive force.	The force involved in maintaining circuits is magnetomotive force.
Resistance opposes the flow of current.	Reluctance is opposed by magnetic path to flux.
The flow of electrons decides current in conductor.	The magnetic lines of force decide the flux in core.

Advantages of Magnetic Circuits

These are some advantage associated with magnetic circuits

• Magnetic circuits are efficient in terms of energy transfer between components of the circuit. Transformers are some devices which use magnetic circuits for transfer of electrical energy between coils. They help to increase efficiency by using magnetic cores with high permeability.
• Designing of magnetic circuits can be versatile depending on our needs. Based on the requirements of certain devices we can select different core materials, wire thickness, shapes, and sizes. This makes magnetic circuits a common choice of use.
• Magnetic circuits provide a way to perform electrical isolation and voltage management in electrical devices like transformers. Separation or electric isolation is necessary to eliminate failures in circuit and management of voltage ensures transfer of energy over long distances with minimal losses.
• Although magnetic circuits don’t completely remove eddy currents but they can be reduced by magnetic circuits. The presence of laminated cores in magnetic circuits improves the overall efficiency of circuit by reducing eddy currents.
• Magnetic circuit helps regulating the performance of circuit by giving direct access to the magnetic properties of system like magnetic flux. This is a crucial factor and must be taken into consideration.

Disadvantages of Magnetic Circuits

There are few disadvantages of some magnetic circuit

• The major disadvantage associated with magnetic circuits is the hysteresis loss associated with losses due to the magnetic core in the circuit. This loss occurs due to the energy which is used to magnetize and demagnetize the core material.
• Another loss associated with magnetic circuits is Eddy Current Loss. In appliances that work with high-frequency, eddy current losses can still be a significant factor leading to additional problems like power loss, unnecessary heating.
• The designing of magnetic circuits involves a complex procedure this is mainly due to the specification of particular equipment. While choosing a circuit one needs to consider many factors like type of core material, geometry, and winding configurations.
• The cost of manufacturing these circuits can be high due to various components involved like magnetic core, wiring and other components which are expensive.
• The properties of magnetic circuit depends largely on factors like temperature .This is because the core inside circuit is made up of permeable material whose magnetic properties are temperature dependent. Thereby affecting performance and efficiency of magnetic circuits.

Conclusion

As we see that magnetic circuit is analogous to standard electrical circuit in many terms some of which have been mentioned above. The circuit has analogous values for resistance and electric current which have been mentioned in the analogy of this circuit. We have also seen the difference between electric and magnetic circuit and have compared then on the basis of various factors. It is important to note that although magnetic circuit serves various advantages yet there are certain limitations of the circuit which can be solved by other circuits. Some real-life applications of circuit have also been discussed. It is important to study the characteristics of this circuit and understand their working to implement them in real-life.

Frequently Asked Questions on Magnetic Circuits – FAQs

What is the core of magnetic circuit made up of?

The core of circuit is made up of magnetic materials. Some of the examples are iron and soft steel.

How does the flux change with increased number of turns in solenoid?

From the formula of flux (ø =NI) it is directly proportional to number of turns hence magnetic flux increases with increased number of turns.

Can we use wood as the core of solenoid?

No, we specifically choose a magnetic material as the core of solenoid due to its properties like permeability. Other materials like wood can’t show magnetic behaviour hence we can’t use it.

Can ohm’s law be applied to magnetic circuits?

It is not possible to apply ohm’s law to magnetic circuits rather we can apply and equivalent version of ohm’s law known according to which the number of magnetic force lines(Φ) is proportional to the magnetomotive force (F_m) and inversely proportional to the circuit’s reluctance (R_m).