* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Inductor Basics notes
Survey
Document related concepts
Magnetotellurics wikipedia , lookup
Electromagnetic field wikipedia , lookup
Magnetoreception wikipedia , lookup
Lorentz force wikipedia , lookup
History of geomagnetism wikipedia , lookup
Ferromagnetism wikipedia , lookup
Friction-plate electromagnetic couplings wikipedia , lookup
Superconducting magnet wikipedia , lookup
Skin effect wikipedia , lookup
Electromagnet wikipedia , lookup
Electrical resistance and conductance wikipedia , lookup
Induction heater wikipedia , lookup
Electromotive force wikipedia , lookup
Transcript
Introductory Analog Electronics Ctec 101. Inductor Basics Supplement Prepared by Mike Crompton. (Rev. 31 March 2005) Inductance An inductor is a series of turns of wire around a core material. The core can be air, soft iron in solid of powdered form, graphite or alloys of different metals chosen for their magnetic properties. The inductance of a coil (inductor) is dependant on: 1. Number of turns 2. Area (How close the turns are to each other) 3. Core material The theoretical D.C. resistance of a pure inductor is 0, therefore they pass (offer no resistance) to DC. In practice there is some resistance due to the resistance of the wire. The principal of induction relies on two theories. 1. When current flows through a wire, a magnetic field is established around the wire. The size of the field is determined by the amount of current. If the current varies, the field varies. 2. When a moving magnetic field “cuts” a conductor, a voltage is induced. (Generator Principle). In 2 above, if the moving magnetic field is the result of varying current flow through a conductor, then the varying current is created by a varying voltage source (VSUPPLY) somewhere in the circuit. The induced voltage in 2 above is always the opposite polarity to VSUPPLY. Inductance is measured in Henry’s (H, mH or H) and the letter ‘L’ is used to denote an inductor. The simple circuit of Fig 1 will be used to explain induction. The coil ‘L’ has very few turns, the first turn (T1) and the last turn (T5) are deliberately spaced far apart. Fig. 1 L1 T1 T2 T3 T4 T5 S1 12V At the instant S1 is closed the l2VDC will appear across the coil and current will start to increase from 0A and flow into the first turn (T1). In doing so it creates an expanding (moving) magnetic field around T1. This expanding field cuts the other turns (T2-T5) and induces a voltage in them. The induced voltage is the opposite polarity to the DC source and tries to force current the opposite way. This of course stops the source current from increasing. (Doesn’t stop it, just stops it from increasing further). Once the source current stops increasing the magnetic field stops moving (still there, just isn’t moving). With no movement of the 2 I Vind in each turn (T1-T5) field the induced voltage disappears, the stops current from current starts to increase again and flows rising at each of into T2. The increasing current forms an T5 these points expanding magnetic field that induces a T4 voltage into the other turns (T1 & T3 – T5). This induced voltage is in opposition T3 to the DC source and tries to force current T2 the opposite way ..etc…etc. Eventually the current (which actually increases in an T1 exponential curve, not in steps as shown in Fig 2) will reach maximum value but will Imax Switch have been delayed considerably by reached Closed comparison to the circuit voltage, which reached max. the instant SW1 closed. Notice that the delay or “opposition to current flow” is created by the induced voltage forcing current to flow in the opposite direction, and that the greater, or quicker the current changes the greater the induced voltage and the greater the opposition to current flow. When the switch is opened again, the current will drop to zero almost instantly and the magnetic field will collapse very quickly. This very fast collapse can induce an extremely large voltage in the coil that can be in the many thousands of volts, creating spectacular effects or doing severe damage. The amount of voltage induced in an inductor, some times called “Back EMF” or “Kick-Back” is calculated by the formula: VIND = L(dI / dt) Where L is inductance in Henrys dI is the CHANGE in current. dt is the CHANGE in time. Notice that the amount of induced voltage has little or nothing to do with the supply voltage and everything to do with the rate of change of current. The foregoing is an over-simplified description of “self inductance” but should help to create a “mental picture” of what happens every time the current tries to change in any way (increase or decrease) when there is an inductor in the circuit. When an AC is applied to an inductor the circuit current will change with the voltage of the applied AC. This means there will always be a changing magnetic field and always an induced voltage that will always oppose the current flow. This is one of the major attributes of inductors, they tend to block or oppose AC and pass DC. This will be studied at length in a later course. 3 Fig.2 time Switch Opened