| ADVANCED LESSON 19 | |
| LEARNING OBJECTIVES and NOTE | |
| Inductance | |
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Understand the term ‘self inductance’ and recall that a ‘back EMF.’ is
produced as current flow changes in an inductor. Recall that the inductance of a coil increases with increasing number of turns, increasing coil diameter and decreasing spacing between turns |
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In this lesson we
are going to learn about what happens when an electric current is
passed through wire. If an electric current passes through a straight wire a magnetic field is created around the wire. This is called self inductance and can be detected by placing a compass next to a wire. When the current flows through the wire, a magnetic field is created and the needle of the compass placed near the wire will move. If the wire is wound into a coil connected to a battery and the switch closed, the magnetic field increases and the compass needle can be moved further away and still be seen to deflect. Further increases in self inductance can be achieved by further increasing the number of turns on the coil; increasing the diameter of the coil; decreasing the spacing between the turns on the coil; placing a high permeability core inside the coil. The henry is the basic unit used to measure inductance and has the symbol H. Smaller units are often more convenient for radio work: a millihenry (mH) is one thousandth of a henry. |
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Understand the rise and fall of current in an LR circuit. When a DC power source is connected to an inductor via a resistor, the current will start to increase as shown in the graph. The increasing current in the inductor will cause an instantaneous negative voltage . This is known as the back EMF. The two voltages will be equal, but opposite in polarity and so no current will flow. The back EMF will start to rise eventually reaching zero. As the voltage rises, so will the current. Eventually the voltage will reach a maximum and the current will level off. When the source voltage is removed from the coil by switching to the opposite position, the current will flow through the resistor and be reduced to zero over a period of time. The induced voltage will also be reduced to zero. In a DC circuit, the time taken to reach a steady current will be very short and only occur once as the power is connected and once when the power is disconnected. In an AC circuit this will be happening continuously as the voltage moves from positive, to zero, to negative and then back to positive. Changes will also depend on the frequency of the applied voltage. |
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| Understand
the use of high permeability cores and slug tuning. In modern miniaturised circuits it is important to reduce the size of components. Inductors in the past have been quite large. Nowadays. they are miniaturised by winding them on cores with a high permeability. This means that only a few turns of very thin wire are required to produce a small inductor but with a high inductance. To allow the value of inductors to be adjusted they are wound on formers with a small cylinder of a material inside the former such as ferrite which has a permeability higher than 1, for example ferrite.. The ferrite can be screwed in and out of the coil thus adjusting the inductance to tune a circuit. You have to be very careful using metal screwdrivers as the slugs are easily broken . Us a special tool designed for the job. The inductance can be lowered using a slug with a permeability less than 1 such as brass. |
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| Understand and apply the formulae for calculating the combined values
of inductors in series and
in parallel In the same way that capacitors and resistors can be connected in series or parallel to create a new value, inductors can also be connected in series and parallel. The formula to calculate the total value of three inductors in series is:  |
Inductors in series  Example 1  |
The formula to calculate the total value of three inductors in parallel is: |
Inductors in parallel Example 2 
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