Affichage des articles dont le libellé est Electrotechnical. Afficher tous les articles
Affichage des articles dont le libellé est Electrotechnical. Afficher tous les articles
In our first tutorial about Inductors, we looked briefly at the time constant of an inductor stating that the current flowing through an inductor could not change instantaneously, but would increase at a constant rate determined by the self-induced emf in the inductor.
In other words, an Inductor in an electrical circuit opposes the flow of current, ( i ) through it. While this is perfectly correct, we made the assumption in the tutorial that it was an ideal inductor which had no resistance or capacitance associated with its coil windings.
However, in the real world “ALL” coils whether they are chokes, solenoids, relays or any wound component will always have a certain amount of resistance no matter how small associated with the coils turns of wire being used to make it as the copper wire will have a resistive value.
Then for real world purposes we can consider our simple coil as being an “Inductance”, L in series with a “Resistance”, R. In other words forming an LR Series Circuit.
A LR Series Circuit consists basically of an inductor of inductance L connected in series with a resistor of resistance R. The resistance R is the DC resistive value of the wire turns or loops that goes into making up the inductors coil. Consider the LR series circuit below.
The LR Series Circuit
The above LR series circuit is connected across a constant voltage source, (the battery) and a switch. Assume that the switch, S is open until it is closed at a time t = 0, and then remains permanently closed producing a “step response” type voltage input. The current, i begins to flow through the circuit but does not rise rapidly to its maximum value of Imax as determined by the ratio of V / R (Ohms Law).
This limiting factor is due to the presence of the self induced emf within the inductor as a result of the growth of magnetic flux, (Lenz’s Law). After a time the voltage source neutralizes the effect of the self induced emf, the current flow becomes constant and the induced current and field are reduced to zero.
We can use Kirchoffs Voltage Law, (KVL) to define the individual voltage drops that exist around the circuit and then hopefully use it to give us an expression for the flow of current.
Kirchoff’s voltage law (KVL) gives us:
The voltage drop across the resistor, R is IR (Ohms Law).
The voltage drop across the inductor, L is by now our familiar expression L = di/dt
Then the final expression for the individual voltage drops around the LR series circuit can be given as:
We can see that the voltage drop across the resistor depends upon the current, i, while the voltage drop across the inductor depends upon the rate of change of the current, di/dt. When the current is equal to zero, ( i = 0 ) at time t = 0 the above expression, which is also a first order differential equation, can be rewritten to give the value of the current at any instant of time as:
Expression for the Current in an LR Series Circuit
- Where:
- V is in Volts
- R is in Ohms
- L is in Henries
- t is in Seconds
- e is the base of the Natural Logarithm = 2.71828
The R/L term in the above equation is known commonly as the Time Constant, ( τ ) of the LR series circuit and V/R also represents the final steady state current value in the circuit. Once the current reaches this maximum steady state value at 5τ, the inductance of the coil has reduced to zero acting more like a short circuit and effectively removing it from the circuit.
Therefore the current flowing through the coil is limited only by the resistive element in Ohms of the coils windings. A graphical representation of the current growth representing the voltage/time characteristics of the circuit can be presented as.
Transient Curves for an LR Series Circuit
Since the voltage drop across the resistor, VR is equal to IxR (Ohms Law), it will have the same exponential growth and shape as the current. However, the voltage drop across the inductor, VL will have a value equal to: Ve(-Rt/L). Then the voltage across the inductor, VL will have an initial value equal to the battery voltage at time t = 0 or when the switch is first closed and then decays exponentially to zero as represented in the above curves.
The time required for the current flowing in the LR series circuit to reach its maximum steady state value is equivalent to about 5 time constants or 5τ. This time constant τ, is measured by τ = L/R, in seconds, were R is the value of the resistor in ohms and L is the value of the inductor in Henries. This then forms the basis of an RL charging circuit were 5τ can also be thought of as “5 x L/R” or thetransient time of the circuit.
The transient time of any inductive circuit is determined by the relationship between the inductance and the resistance. For example, for a fixed value resistance the larger the inductance the slower will be the transient time and therefore a longer time constant for the LR series circuit. Likewise, for a fixed value inductance the smaller the resistance value the longer the transient time.
However, for a fixed value inductance, by increasing the resistance value the transient time and therefore the time constant of the circuit becomes shorter. This is because as the resistance increases the circuit becomes more and more resistive as the value of the inductance becomes negligible compared to the resistance. If the value of the resistance is increased sufficiently large compared to the inductance the transient time would effectively be reduced to almost zero.
LR Series Circuit Example No1
A coil which has an inductance of 40mH and a resistance of 2Ωs is connected together to form a LR series circuit. If they are connected to a 20V DC supply.
a). What will be the final steady state value of the current.
b) What will be the time constant of the RL series circuit.
c) What will be the transient time of the RL series circuit.
c) What will be the value of the induced emf after 10mS.
d) What will be the value of the circuit current one time constant after the switch is closed.
The Time Constant, τ of the circuit was calculated in question b) as being 20mS. Then the circuit current at this time is given as:
You may have noticed that the answer for question (d) which gives a value of 6.32 Amps at one time constant, is equal to 63.2% of the final steady state current value of 10 Amps we calculated in question (a). This value of 63.2% or 0.632 x IMAX also corresponds with the transient curves shown above.
Power in a LR Series Circuit
Then from above, the power in a LR series circuit is given as:
The instantaneous rate at which the voltage source delivers power to the circuit is given as:
The instantaneous rate at which power is dissipated by the resistor in the form of heat is given as:
The rate at which energy is stored in the inductor in the form of magnetic potential energy is given as:
Then we can find the total power in a RL series circuit by multiplying by i and is therefore:
Where the first I2R term represents the power dissipated by the resistor in heat, and the second term represents the power absorbed by the inductor, its magnetic energy.
Thus far we have seen a selection of Input devices that can be used to detect or “sense” a variety of physical variables and signals and are therefore called Sensors. But there are also a variety of electrical and electronic devices which are classed as Output devices used to control or operate some external physical process. These output devices are commonly called Actuators.
Actuators convert an electrical signal into a corresponding physical quantity such as movement, force, sound etc. An actuator is also classed as a Transducer because it changes one type of physical quantity into another and is usually activated or operated by a low voltage command signal. Actuators can be classed as either binary or continuous devices based upon the number of stable states their output has.
For example, a relay is a binary actuator as it has two stable states, either energised and latched or de-energised and unlatched, while a motor is a continuous actuator because it can rotate through a full 360o motion. The most common types of actuators or output devices are Electrical Relays,Lights, Motors and Loudspeakers.
We saw previously that solenoids can be used to electrically open latches, doors, open or close valves, and in a variety of robotic and mechatronic applications, etc. However, if the solenoid plunger is used to operate one or more sets of electrical contacts, we have a device called a relaythat is so useful it can be used in an infinite number of different ways and in this tutorial we will look at Electrical Relays.
Electrical Relays can also be divided into mechanical action relays called “Electromechanical Relays” and those which use semiconductor transistors, thyristors, triacs, etc, as their switching device called “Solid State Relays” or SSR’s.
The Electromechanical Relay
The term Relay generally refers to a device that provides an electrical connection between two or more points in response to the application of a control signal. The most common and widely used type of electrical relay is the electromechanical relay or EMR.
An Electrical Relay
The most fundamental control of any equipment is the ability to turn it “ON” and “OFF”. The easiest way to do this is using switches to interrupt the electrical supply. Although switches can be used to control something, they have their disadvantages. The biggest one is that they have to be manually (physically) turned “ON” or “OFF”. Also, they are relatively large, slow and only switch small electrical currents.
Electrical Relays however, are basically electrically operated switches that come in many shapes, sizes and power ratings suitable for all types of applications. Relays can also have single or multiple contacts within a single package with the larger power relays used for mains voltage or high current switching applications being called “Contactors”.
In this tutorial about electrical relays we are just concerned with the fundamental operating principles of “light duty” electromechanical relays we can use in motor control or robotic circuits. Such relays are used in general electrical and electronic control or switching circuits either mounted directly onto PCB boards or connected free standing and in which the load currents are normally fractions of an ampere up to 20+ amperes. The relay circuit are common in Electronics applications.
As their name implies, electromechanical relays are electro-magnetic devices that convert a magnetic flux generated by the application of a low voltage electrical control signal either AC or DC across the relay terminals, into a pulling mechanical force which operates the electrical contacts within the relay. The most common form of electromechanical relay consist of an energizing coil called the “primary circuit” wound around a permeable iron core.
This iron core has both a fixed portion called the yoke, and a moveable spring loaded part called the armature, that completes the magnetic field circuit by closing the air gap between the fixed electrical coil and the moveable armature. The armature is hinged or pivoted allowing it to freely move within the generated magnetic field closing the electrical contacts that are attached to it. Connected between the yoke and armature is normally a spring (or springs) for the return stroke to “reset” the contacts back to their initial rest position when the relay coil is in the “de-energized” condition, ie. turned “OFF”.
Electromechanical Relay Construction
In our simple relay above, we have two sets of electrically conductive contacts. Relays may be “Normally Open”, or “Normally Closed”. One pair of contacts are classed as Normally Open, (NO)or make contacts and another set which are classed as Normally Closed, (NC) or break contacts. In the normally open position, the contacts are closed only when the field current is “ON” and the switch contacts are pulled towards the inductive coil.
In the normally closed position, the contacts are permanently closed when the field current is “OFF” as the switch contacts return to their normal position. These terms Normally Open, Normally Closed or Make and Break Contacts refer to the state of the electrical contacts when the relay coil is “de-energized”, i.e, no supply voltage connected to the inductive coil. An example of this arrangement is given below.
The relays contacts are electrically conductive pieces of metal which touch together completing a circuit and allow the circuit current to flow, just like a switch. When the contacts are open the resistance between the contacts is very high in the Mega-Ohms, producing an open circuit condition and no circuit current flows.
When the contacts are closed the contact resistance should be zero, a short circuit, but this is not always the case. All relay contacts have a certain amount of “contact resistance” when they are closed and this is called the “On-Resistance”, similar to FET’s.
With a new relay and contacts this ON-resistance will be very small, generally less than 0.2Ω’s because the tips are new and clean, but over time the tip resistance will increase.
For example. If the contacts are passing a load current of say 10A, then the voltage drop across the contacts using Ohms Law is 0.2 x 10 = 2 volts, which if the supply voltage is say 12 volts then the load voltage will be only 10 volts (12 – 2). As the contact tips begin to wear, and if they are not properly protected from high inductive or capacitive loads, they will start to show signs of arcing damage as the circuit current still wants to flow as the contacts begin to open when the relay coil is de-energized.
This arcing or sparking across the contacts will cause the contact resistance of the tips to increase further as the contact tips become damaged. If allowed to continue the contact tips may become so burnt and damaged to the point were they are physically closed but do not pass any or very little current.
If this arcing damage becomes to severe the contacts will eventually “weld” together producing a short circuit condition and possible damage to the circuit they are controlling. If now the contact resistance has increased due to arcing to say 1Ω’s the volt drop across the contacts for the same load current increases to 1 x 10 = 10 volts dc. This high voltage drop across the contacts may be unacceptable for the load circuit especially if operating at 12 or even 24 volts, then the faulty relay will have to be replaced.
To reduce the effects of contact arcing and high “On-resistances”, modern contact tips are made of, or coated with, a variety of silver based alloys to extend their life span as given in the following table.
Electrical Relay Contact Tip Materials
- Ag (fine silver)
- 1. Electrical and thermal conductivity are the highest of all the metals.
- 2. Exhibits low contact resistance, is inexpensive and widely used.
- 3. Contacts tarnish easily through sulphurisation influence.
- AgCu (silver copper)
- 1. Known as “Hard silver” contacts.
- Better wear resistance and less tendency to arc and weld, but slightly higher contact resistance.
- AgCdO (silver cadmium oxide)
- 1. Very little tendency to arc and weld, good wear resistance and arc extinguishing properties.
- AgW (silver tungsten)
- 1. Hardness and melting point are high, arc resistance is excellent.
- 2. Not a precious metal.
- 3. High contact pressure is required to reduce resistance.
- 4. Contact resistance is relatively high, and resistance to corrosion is poor.
- AgNi (silver nickel)
- 1. Equals the electrical conductivity of silver, excellent arc resistance.
- AgPd (silver palladium)
- 1. Low contact wear, greater hardness.
- 2. Expensive.
- Platinum, Gold and Silver Alloys
- 1. Excellent corrosion resistance, used mainly for low-current circuits.
Relay manufacturers data sheets give maximum contact ratings for resistive DC loads only and this rating is greatly reduced for either AC loads or highly inductive or capacitive loads. In order to achieve long life and high reliability when switching AC currents with inductive or capacitive loads some form of arc suppression or filtering is required across the relay contacts.
Extending the life of relay tips by reducing the amount of arcing generated as they open is achieved by connecting a Resistor-Capacitor network called an RC Snubber Network electrically in parallel with an electrical relay contact tips. The voltage peak, which occurs at the instant the contacts open, will be safely short circuited by the RC network, thus suppressing any arc generated at the contact tips. For example.
Electrical Relay Snubber Circuit
Electrical Relay Contact Types.
As well as the standard descriptions of Normally Open, (NO) and Normally Closed, (NC) used to describe how the relays contacts are connected, relay contact arrangements can also be classed by their actions. Electrical relays can be made up of one or more individual switch contacts with each “contact” being referred to as a “pole”. Each one of these contacts or poles can be connected or “thrown” together by energizing the relays coil and this gives rise to the description of the contact types as being:
- SPST – Single Pole Single Throw
- SPDT – Single Pole Double Throw
- DPST – Double Pole Single Throw
- DPDT – Double Pole Double Throw
with the action of the contacts being described as “Make” (M) or “Break” (B). Then a simple relay with one set of contacts as shown above can have a contact description of:
"Single Pole Double Throw – (Break before Make)", or SPDT – (B-M)
Examples of just some of the more common diagrams used for electrical relay contact types to identify relays in circuit or schematic diagrams is given below but there are many more possible configurations.
Electrical Relay Contact Configurations
- Where:
- C is the Common terminal
- NO is the Normally Open contact
- NC is the Normally Closed contact
One final point to remember about using electrical relays. It is not advisable at all to connect relay contacts in parallel to handle higher load currents. For example, never attempt to supply a 10A load with two relays in parallel that have 5A contact ratings each, as the mechanically operated relay contacts never close or open at exactly the same instant of time. The result is that one relay contact will always be overloaded even for a brief instant in time resulting in premature failure of the relay over time.
Also, while electrical relays can be used to allow low power electronic or computer type circuits to switch relatively high currents or voltages both “ON” or “OFF”. Never mix different load voltages through adjacent contacts within the same relay such as for example, high voltage AC (240v) and low voltage DC (12v), always use separate relays for safety.
One of the more important parts of any electrical relay is its coil. This converts electrical current into an electromagnetic flux which is used to mechanically operate the relays contacts. The main problem with relay coils is that they are “highly inductive loads” as they are made from coils of wire. Any coil of wire has an impedance value made up of resistance ( R ) and inductance ( L ) in series (LR Series Circuit).
As the current flows through the coil a self induced magnetic field is generated around it. When the current in the coil is turned “OFF”, a large back emf (electromotive force) voltage is produced as the magnetic flux collapses within the coil (transformer theory). This induced reverse voltage value may be very high in comparison to the switching voltage, and may damage any semiconductor device such as a transistor, FET or micro-controller used to operate the relay coil.
One way of preventing damage to the transistor or any switching semiconductor device, is to connect a reverse biased diode across the relay coil.
When the current flowing through the coil is switched “OFF”, an induced back emf is generated as the magnetic flux collapses in the coil.
This reverse voltage forward biases the diode which conducts and dissipates the stored energy preventing any damage to the semiconductor transistor.
When used in this type of application the diode is generally known as a Flywheel Diode, Free-wheeling Diode and even Fly-back Diode, but they all mean the same thing. Other types of inductive loads which require a flywheel diode for protection are solenoids, motors and inductive coils.
As well as using flywheel Diodes for protection of semiconductor components, other devices used for protection include RC Snubber Networks, Metal Oxide Varistors or MOV and Zener Diodes.
The Solid State Relay.
While the electromechanical relay (EMR) are inexpensive, easy to use and allow the switching of a load circuit controlled by a low power, electrically isolated input signal, one of the main disadvantages of an electromechanical relay is that it is a “mechanical device”, that is it has moving parts so their switching speed (response time) due to physically movement of the metal contacts using a magnetic field is slow.
Over a period of time these moving parts will wear out and fail, or that the contact resistance through the constant arcing and erosion may make the relay unusable and shortens its life. Also, they are electrically noisy with the contacts suffering from contact bounce which may affect any electronic circuits to which they are connected.
To overcome these disadvantages of the electrical relay, another type of relay called a Solid State Relay or (SSR) for short was developed which is a solid state contactless, pure electronic relay.
The solid state relay being a purely electronic device has no moving parts within its design as the mechanical contacts have been replaced by power transistors, thyristors or triac’s. The electrical separation between the input control signal and the output load voltage is accomplished with the aid of an opto-coupler type Light Sensor.
The Solid State Relay provides a high degree of reliability, long life and reduced electromagnetic interference (EMI), (no arcing contacts or magnetic fields), together with a much faster almost instant response time, as compared to the conventional electromechanical relay.
Also the input control power requirements of the solid state relay are generally low enough to make them compatible with most IC logic families without the need for additional buffers, drivers or amplifiers. However, being a semiconductor device they must be mounted onto suitable heatsinks to prevent the output switching semiconductor device from over heating.
Solid State Relay
The AC type Solid State Relay turns “ON” at the zero crossing point of the AC sinusoidal waveform, prevents high inrush currents when switching inductive or capacitive loads while the inherent turn “OFF” feature of Thyristors and Triacs provides an improvement over the arcing contacts of the electromechanical relays.
Like the electromechanical relays, a Resistor-Capacitor (RC) snubber network is generally required across the output terminals of the SSR to protect the semiconductor output switching device from noise and voltage transient spikes when used to switch highly inductive or capacitive loads. In most modern SSR’s this RC snubber network is built as standard into the relay itself reducing the need for additional external components.
Non-zero crossing detection switching (instant “ON”) type SSR’s are also available for phase controlled applications such as the dimming or fading of lights at concerts, shows, disco lighting etc, or for motor speed control type applications.
As the output switching device of a solid state relay is a semiconductor device (Transistor for DC switching applications, or a Triac/Thyristor combination for AC switching), the voltage drop across the output terminals of an SSR when “ON” is much higher than that of the electromechanical relay, typically 1.5 – 2.0 volts. If switching large currents for long periods of time an additional heat sink will be required.
Input/Output Interface Modules.
Input/Output Interface Modules, (I/O Modules) are another type of solid state relay designed specifically to interface computers, micro-controller or PIC’s to “real world” loads and switches. There are four basic types of I/O modules available, AC or DC Input voltage to TTL or CMOS logic level output, and TTL or CMOS logic input to an AC or DC Output voltage with each module containing all the necessary circuitry to provide a complete interface and isolation within one small device. They are available as individual solid state modules or integrated into 4, 8 or 16 channel devices.
Modular Input/Output Interface System.
The main disadvantages of solid state relays (SSR’s) compared to that of an equivalent wattage electromechanical relay is their higher costs, the fact that only single pole single throw (SPST) types are available, “OFF”-state leakage currents flow through the switching device, and a high “ON”-state voltage drop and power dissipation resulting in additional heat sinking requirements. Also they can not switch very small load currents or high frequency signals such as audio or video signals although special Solid State Switches are available for this type of application.
In this tutorial about Electrical Relays, we have looked at both the electromechanical relay and the solid state relay which can be used as an output device (actuator) to control a physical process. In the next tutorial we will continue our look at output devices called Actuators and especially one that converts a small electrical signal into a corresponding physical movement using electromagnetism. The output device is called a Solenoid.
Electromagnetism is produced when an electrical current flows through a simple conductor such as a piece of wire or cable. A small magnetic field is created around the conductor with the direction of this magnetic field with regards to its “North” and “South” poles being determined by the direction of the current flowing through the conductor.
Magnetism plays an important role in Electrical and Electronic Engineering because without it components such as relays, solenoids, inductors, chokes, coils, loudspeakers, motors, generators, transformers, and electricity meters etc, would not work if magnetism did not exist.
Then every coil of wire uses the effect of electromagnetism when an electrical current flows through it. But before we can look at Magnetism and especially Electromagnetism in more detail we need to remember back to our physics classes of how magnets and magnetism works.
The Nature of Magnetism
Magnets can be found in a natural state in the form of a magnetic ore, with the two main types being Magnetite also called “iron oxide”, ( FE3O4 ) and Lodestone, also called “leading stone”. If these two natural magnets are suspended from a piece of string, they will take up a position in-line with the Earth’s magnetic field always pointing north.
A good example of this effect is the needle of a compass. For most practical applications these natural occurring magnets can be disregarded as their magnetism is very low and because nowadays, man-made artificial magnets can be produced in many different shapes, sizes and magnetic strengths.
There are basically two forms of magnetism, “Permanent Magnets” and “Temporary Magnets”, with the type being used dependant upon its application. There are many different types of materials available to make magnets such as iron, nickel, nickel alloys, chromium and cobalt and in their natural state some of these elements such as nickel and cobalt show very poor magnetic quantities on their own.
However, when mixed or “alloyed” together with other materials such as iron or aluminium peroxide they become very strong magnets producing unusual names such as “alcomax”, “hycomax”, “alni” and “alnico”.
Magnetic material in the non-magnetic state has its molecular structure in the form of loose magnetic chains or individual tiny magnets loosely arranged in a random pattern. The overall effect of this type of arrangement results in zero or very weak magnetism as this haphazard arrangement of each molecular magnet tends to neutralise its neighbour.
When the material is Magnetised this random arrangement of the molecules changes and the tiny unaligned and random molecular magnets become “lined-up” in such a way that they produce a series magnetic arrangement. This idea of the molecular alignment of ferromagnetic materials is known as Weber’s Theory and is illustrated below.
Magnetic Molecule Alignment of a Piece of Iron and a Magnet
Weber’s theory is based on the fact that all atoms have magnetic properties due to the spinning action of the atoms electrons. Groups of atoms join together so that their magnetic fields are all rotating in the same direction. Magnetic materials are composed of groups of tiny magnets at a molecular level around the atoms, and a magnetised material will have most of its tiny magnets lined up in one direction only to produce a north pole in one direction and a south pole in the other direction.
Likewise, a material that has its tiny molecular magnets pointing in all directions will have its molecular magnets neutralised by its neighbouring magnet, thereby neutralising any magnetic effect. These areas of molecular magnets are called “domains”.
Any magnetic material will produce a magnetic field itself which depends on the degree of alignment of magnetic domains in the material set up by orbital and spinning electrons. This degree of alignment can be specified by a quantity known as magnetisation, M.
In an unmagnetised material, M = 0, but some of the domains remain aligned over small regions in the material once the magnetic field is removed. The effect of applying a magnetising force to the material is to align some of the domains to produce a non-zero magnetisation value.
Once the magnetising force has been removed, the magnetism within the material will either remain or decay away quiet quickly depending on the magnetic material being used. This ability of a material to retain its magnetism is called Retentivity.
Materials which are required to retain their magnetism will have a fairly high retentivity and as such are used to make permanent magnets, while those materials required to loose their magnetism quickly such as soft iron cores for relays and solenoids will have a very low retentivity.
Magnetic Flux
All magnets, no matter what their shape, have two regions called magnetic poles with the magnetism both in and around a magnetic circuit producing a definite chain of organised and balanced pattern of invisible lines of flux around it. These lines of flux are collectively referred to as the “magnetic field” of the magnet. The shape of this magnetic field is more intense in some parts than others with the area of the magnet that has the greatest magnetism being called “poles”. At each end of a magnet is a pole.
These lines of flux (called a vector field) can not be seen by the naked eye, but they can be seen visually by using iron fillings sprinkled onto a sheet of paper or by using a small compass to trace them out. Magnetic poles are always present in pairs, there is always a region of the magnet called the North-pole and there is always an opposite region called the South-pole.
Magnetic fields are always shown visually as lines of force that give a definite pole at each end of the material where the flux lines are more dense and concentrated. The lines which go to make up a magnetic field showing the direction and intensity are called Lines of Force or more commonly “Magnetic Flux” and are given the Greek symbol, Phi ( Φ ) as shown below.
Lines of Force from a Bar Magnets Magnetic Field
As shown above, the magnetic field is strongest near to the poles of the magnet were the lines of flux are more closely spaced. The general direction for the magnetic flux flow is from the North ( N ) to the South ( S ) pole. In addition, these magnetic lines form closed loops that leave at the north pole of the magnet and enter at the south pole. Magnetic poles are always in pairs.
However, magnetic flux does not actually flow from the north to the south pole or flow anywhere for that matter as magnetic flux is a static region around a magnet in which the magnetic force exists. In other words magnetic flux does not flow or move it is just there and is not influenced by gravity. Some important facts emerge when plotting lines of force:
- Lines of force NEVER cross.
- Lines of force are CONTINUOUS.
- Lines of force always form individual CLOSED LOOPS around the magnet.
- Lines of force have a definite DIRECTION from North to South.
- Lines of force that are close together indicate a STRONG magnetic field.
- Lines of force that are farther apart indicate a WEAK magnetic field.
Magnetic forces attract and repel like electric forces and when two lines of force are brought close together the interaction between the two magnetic fields causes one of two things to occur:
- 1. – When adjacent poles are the same, (north-north or south-south) they REPEL each other.
- 2. – When adjacent poles are not the same, (north-south or south-north) they ATTRACT each other.
This effect is easily remembered by the famous expression that “opposites attract” and this interaction of magnetic fields can be easily demonstrated using iron fillings to show the lines of force around a magnet. The effect upon the magnetic fields of the various combinations of poles as like poles repel and unlike poles attract can be seen below.
Magnetic Field of Like and Unlike Poles
When plotting magnetic field lines with a compass it will be seen that the lines of force are produced in such a way as to give a definite pole at each end of the magnet where the lines of force leave the North pole and re-enter at the South pole. Magnetism can be destroyed by heating or hammering the magnetic material, but cannot be destroyed or isolated by simply breaking the magnet into two pieces.
So if you take a normal bar magnet and break it into two pieces, you do not have two halves of a magnet but instead each broken piece will somehow have its own North pole and a South pole. If you take one of those pieces and break it into two again, each of the smaller pieces will have a North pole and a South pole and so on. No matter how small the pieces of the magnet become, each piece will still have a North pole and a South pole, crazy!.
Then in order for us to make use of magnetism in electrical or electronic calculations, it is necessary to define what are the various aspects of magnetism.
The Magnitude of Magnetism
We now know that the lines of force or more commonly the magnetic flux around a magnetic material is given the Greek symbol, Phi, ( Φ ) with the unit of flux being the Weber, ( Wb ) after Wilhelm Eduard Weber. But the number of lines of force within a given unit area is called the “Flux Density” and since flux ( Φ ) is measured in ( Wb ) and area ( A ) in metres squared, ( m2 ), flux density is therefore measured in Webers/Metre2 or ( Wb/m2 ) and is given the symbol B.
However, when referring to flux density in magnetism, flux density is given the unit of the Teslaafter Nikola Tesla so therefore one Wb/m2 is equal to one Tesla, 1Wb/m2 = 1T. Flux density is proportional to the lines of force and inversely proportional to area so we can define Flux Densityas:
Magnetic Flux Density
The symbol for magnetic flux density is B and the unit of magnetic flux density is the Tesla, T.
It is important to remember that all calculations for flux density are done in the same units, e.g., flux in webers, area in m2 and flux density in Teslas.
Magnetism Example No1
The amount of flux present in a round magnetic bar was measured at 0.013 webers. If the material has a diameter of 12cm, calculate the flux density.
The cross sectional area of the magnetic material in m2 is given as:
The magnetic flux is given as 0.013 webers, therefore the flux density can be calculated as:
So the flux density is calculated as 1.15 Teslas.
When dealing with magnetism in electrical circuits it must be remembered that one Tesla is the density of a magnetic field such that a conductor carrying 1 ampere at right angles to the magnetic field experiences a force of one newton-metre length on it and this will be demonstrated in the next tutorial about Electromagnetism.
