air_gap
Differences
This shows you the differences between two versions of the page.
Previous revision | |||
— | air_gap [2023/10/18 14:15] (current) – stan_zurek | ||
---|---|---|---|
Line 1: | Line 1: | ||
+ | ====== Air gap ====== | ||
+ | |< 100% >| | ||
+ | | // | ||
+ | |||
+ | **Air gap**, also **airgap**[([[http:// | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Diagram of an air gap in a [[magnetic circuit]] | ||
+ | [[/file/Air gap diagram_png|{{/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Depending on application, | ||
+ | |||
+ | An air gap is a practically unavoidable part of any magnetic circuit in which there is a relative movement between different parts (e.g. in [[motor|motors]], | ||
+ | |||
+ | Due to increased [[reluctance]] of an air gap the [[magnetic flux|flux]] spreads into the surrounding medium causing the [[flux fringing]] effect. It is generally an unwanted phenomenon which usually increases [[proximity loss|proximity]] and [[eddy current loss]] in conductors located in the vicinity of the air gap. | ||
+ | |||
+ | <WRAP clear></ | ||
+ | {{page> | ||
+ | |||
+ | ===== Influence on B-H loop and permeability ===== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Influence of air gap ($l_g$ = 0.07 mm) on the shape of [[B-H loop]] for a [[cut core]] ($l_c$ = 250 mm), so $l_g / l_c$ = 0.00028 | ||
+ | [[/file/Air gap in BH loop_png|{{/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | The [[B-H loop]] of a magnetic circuit is affected by the presence of an air gap. Permeability of non-magnetic material is low and therefore it requires greater values of $H$ to obtain the same value of $B$ as compared with [[magnetically soft materials]]. With the introduction of an air gap the B-H loop of a [[magnetic circuit]] gets [[shearing of B-H loop|" | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Changes of [[effective permeability]] and linearisation of the B-H loops caused by increasing air gap | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Conversely, if no air gap is present then the slope becomes as steep as possible, and the B-H loop will represent the closest approximation of the characteristic of the magnetic material (for a given shape of the magnetic circuit). For this reason international standards defining [[magnetic measurement|magnetic measurements]] specify procedures to ensure that the influence of non-magnetic parts of a magnetic circuit remain within negligible levels. | ||
+ | |||
+ | This can be achieved for instance by careful [[polishing]] or [[lapping]] of the flat faces, in order to reduce the surface roughness and the amount of space between the magnetic surfaces. | ||
+ | [(SST> | ||
+ | |||
+ | <box 40% right #f0f0f0> | ||
+ | Even a very small air gap can have a significant impact on [[effective permeability]] | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | [[Split core|Split cores]] made out of ferrite are widely used in [[electronic transformer|electronic transformers]] because they greatly simplify assembly, so that low-cost devices are possible. However, by definition, a split core comprises some small air gap, even if the [[mating faces]] are very carefully polished or lapped. Typical air gaps for such ferrite cores are as follows: | ||
+ | * precision-ground or lapped ~ 1 μm (0.001 mm) | ||
+ | * normally ground ~ 10 μm (0.01 mm) | ||
+ | * purposely gapped > 10 μm (0.001 mm) | ||
+ | |||
+ | The effects depend on the [[magnetic path length]] of the core and the material permeability, | ||
+ | ===== Importance of air gap in practical applications ===== | ||
+ | |||
+ | The air gap can take different form, shape and size depending on the type of magnetic circuit and its shape. In some circuits it might be actually an integral part ensuring correct performance of the device, but in other cases it should be as small as possible. Such requirements will be determined by the operating principle, performance, | ||
+ | |||
+ | ==== Rotating machines ==== | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Air gap between [[lamination|laminations]] of a [[rotor]] and [[stator]] of an [[induction motor]] | ||
+ | [[/file/Air gap in induction motor laminations_jpg|{{/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | |||
+ | In [[rotating machine|rotating machines]] the air gap is usually unwanted yet unavoidable due to the necessity of physical movement required between the [[stator]] and the [[rotor]]. The smallest practical air gap for industrial machines is around 0.2 mm.[([[http:// | ||
+ | But with larger machines there are greater dimensional tolerances and the gap is sized accordingly to the diameter of the rotor to be at the order of 0.1% of the machine diameter. | ||
+ | |||
+ | An air gap increases the value of magnetising current and lowers the achievable [[flux density]].[([[http:// | ||
+ | |||
+ | In rotating machines it may lead to increased [[magnetic loss]] on the surface of the rotor due to increased [[space harmonic|space harmonics]]. Thin laminations are thus required to reduce the effect of eddy currents in the parts of the motor which are exposed to AC flux.[([[http:// | ||
+ | |||
+ | However, there are also special motors with oversized air gap required for non-magnetic reasons (electrical isolation, access to other parts, etc.)[([[https:// | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | The air gap in this prototype of a [[linear motor]] is filled partly with [[Tufnol]] \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | ==== Linear motors ==== | ||
+ | The cylindrical shapes of rotating machines can be manufactured with tighter tolerances than it is the case for instance for [[linear motor|linear motors]]. As a consequence the linear machines must operate with larger air gaps, which might impact on their [[efficiency]].[([[http:// | ||
+ | |||
+ | Trains using magnetic levitation technology are propelled by using the principle of linear motors. The air gap between the train suspension and the tracks can be as large as 13 mm for [[electromagnetic suspension]] (Germany) or 102 mm for [[electrodynamic suspension]] (Japan).[([[http:// | ||
+ | |||
+ | For structural reasons the gap can be filled partly with air and partly with a solid non-magnetic material (see photo). | ||
+ | |||
+ | ==== Gapped and air-cored inductors ==== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | [[Short-circuit]] current limiting [[air-cored choke]] rated at 1 [[kA]] | ||
+ | [[/ | ||
+ | < | ||
+ | </ | ||
+ | |||
+ | === Energy storing inductors === | ||
+ | |||
+ | Air gaps are an integral part of [[gapped inductor|gapped inductors]]. The gap reduces [[effective permeability]] of a given [[magnetic circuit]] and allows storing much greater energy before [[magnetic saturation|saturation]] is reached. Increasing the gap reduces the [[inductance]], | ||
+ | |||
+ | For a given size of inductor the amount of stored energy versus applied air gap can be represented by a [[Hanna curve]].[(Goldman)] | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Electronic [[choke]] with large air gap (comparable to the length of the ferrite rod) \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | If operation with high currents is required then the air gap might be very large, so that the [[magnetic circuit]] is quite " | ||
+ | |||
+ | In some cases the currents are so high that it is very difficult or cost prohibitive to design the inductor with a magnetic core. In such case a so-called "[[air core]]" | ||
+ | |||
+ | In order to reduce the [[flux fringing]] effect and the losses associated with it, in some inductors the gap is distributed into many smaller gaps.[([[http:// | ||
+ | |||
+ | The [[distributed air gap|distribution of air gap]] can be also extended even further. There are magnetic materials, which are made from small particles (mostly based on [[powder iron]], [[sendust]] or [[moly permalloy powder]]) bound together in such a way as to contain certain percentage of non-magnetic volume in them. The resultant [[effective permeability]] is much lower, but the air gap is uniformly distributed throughout the whole material.[([[http:// | ||
+ | The fringing effect and leakage flux is greatly reduced, which is especially important for high-frequency applications. Such cores are usually more expensive (either in initial production or further [[coil winding]]) than alternative technologies with a single gap. | ||
+ | |||
+ | === Variable and signal inductors === | ||
+ | Air gap is frequently used as means of [[tuning]], or adjusting inductance to the required value. The variation can be done just once, for instance with a fixed shim introduced between parts of the core to create an air gap. But in some cases the variation must be carried out more frequently, or even cyclically. An important phenomena which necessitates tuning of an LC circuit, is to either achieve the [[resonance]] point (e.g. signal transmission through [[electromagnetic wave|electromagnetic waves]]) or to be as far as possible from it (e.g. for filtering reasons). | ||
+ | |||
+ | LC resonance and tuning is very important for many electromagnetic circuits, ranging from millimetre-sized low-power high-frequency signal transmission[([[http:// | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Air gap in [[relay]] between the movable [[armature]] and the [[core]] (top right corner) \\ | ||
+ | [[/ | ||
+ | < | ||
+ | </ | ||
+ | |||
+ | Completely air-cored inductors can also be employed in radio frequencies, | ||
+ | |||
+ | ==== Relays ==== | ||
+ | |||
+ | In [[relay|relays]] the air gap is usually an integral part, as it facilitates the movement between the fixed parts (e.g. [[winding]] and [[magnetic core]]) and the active [[armature]], | ||
+ | |||
+ | In relays, the length of the gap is a compromise between the required mechanical force, mechanical movement, and the available excitation. There are numerous constructions which allow optimisation of these different factors, according to requirement of specific application[([[http:// | ||
+ | |||
+ | ==== Solenoids and linear actuators ==== | ||
+ | |||
+ | [[Solenoid|Solenoids]] and [[linear actuator|linear actuators]] work similarly to [[relay|relays]]. The magnetic circuit comprises [[yoke]] or [[core]] and a movable armature, usually called [[plunger]]. | ||
+ | |||
+ | <box 25% right #f0f0f0> | ||
+ | [[/ | ||
+ | [[/ | ||
+ | < | ||
+ | </ | ||
+ | |||
+ | In the simplest form a plunger is being pulled into the space inside the coil. However, as with relays, there are multiple different designs and approaches, including those with a rotating armature, whose construction becomes therefore close to electric motors.[([[https:// | ||
+ | |||
+ | ==== Transformers ==== | ||
+ | |||
+ | In classical [[transformer|transformers]] air gap is usually avoided. The role of transformer is to deliver the energy from the primary winding to the secondary winding instantaneously, | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Laminations in larger transformers are overlapped to distribute the air gap[([[http:// | ||
+ | [[/ | ||
+ | < | ||
+ | </ | ||
+ | |||
+ | Any air gap in the [[magnetic core]] increases [[leakage inductance]] and stores additional energy, which needs to be cyclically transferred or dissipated. All these factors impact [[efficiency]] of energy transformation. | ||
+ | |||
+ | The air gap lowers the total inductance of the primary winding and causes an increase in [[apparent power]] | ||
+ | [(Marketos> | ||
+ | through the increase of magnetising current.[([[http:// | ||
+ | |||
+ | If a transformer core is made of laminations, | ||
+ | |||
+ | Similar practice is employed with cores wound from [[amorphous ribbon]] [([[http:// | ||
+ | |||
+ | === Current and voltage transformers === | ||
+ | Current and voltage transformers of the common design follow the same principles as regular [[transformer|transformers]] - the air gap is to be minimised as it limits the measurement accuracy of both amplitude and phase. | ||
+ | |||
+ | However, special designs (for instance used in [[protection]]) might include air gap to lower the effective permeability and widen the operating range of primary current ([[linear current transformer|linear CTs]], [[transactors]]), | ||
+ | |||
+ | === Flyback transformers === | ||
+ | |||
+ | <box 25% left #f0f0f0> | ||
+ | A [[gapped core]] is used in [[flyback transformer|flyback transformers]] | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | <box 25% right #f0f0f0> | ||
+ | "Air gap" can be made by inserting non-magnetic spacers (circle shows " | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | The operating principle of [[flyback transformer|flyback transformers]] used in [[switched-mode power supply|switched-mode power supplies]] is more akin to energy-storing [[inductor|inductors]] than to classical transformers. For this reason should be more correctly referred to as [[coupled inductor]]. | ||
+ | [([[http:// | ||
+ | All energy must be first stored in the magnetic field in the first part of the cycle, and only passed to the secondary winding in the second part of the cycle. The energy storing capability is usually achieved by means of an air gap (or more precisely: a gap filled with [[non-magnetic material]]). | ||
+ | |||
+ | However, the air gap also increases [[leakage inductance]], | ||
+ | |||
+ | ==== Current sensors ==== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Current sensor might employ a [[/gapped core]] with a [[Hall-effect]] sensing element \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Multiple approaches are used in [[current sensor|current sensors]]. The importance and influence of the gap depends on the given technology.[([[http:// | ||
+ | |||
+ | A common approach used by sensor manufacturers is the " | ||
+ | |||
+ | Another common approach is the " | ||
+ | |||
+ | However, there is a compensating winding on the core and the sensor is used as a zero detector - for a condition where the magnetic field from the primary current is compensated by the current in the winding located on the core. The value of the compensating current is proportional and hence a measure of the primary current. Closed-loop technique offers better linearity and accuracy than the open loop, but it is more costly to implement. | ||
+ | |||
+ | === Rogowski coils === | ||
+ | |||
+ | [[Rogowski coil]] is wound on a non-magnetic former - it is therefore an [[air-cored transformer]], | ||
+ | |||
+ | <box 25% left #f0f0f0> | ||
+ | Typical [[Rogowski coil]] is wound on a flexible [[non-magnetic]] rod \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | For accurate measurements the Rogowski coil must fully enclose the current to be measured. But the coil must also open to enable the coil to be closed around a conductor. Therefore, there is a gap between the meeting ends, and this introduces a measurement error. There are various ways of dealing with this gap, for instance overwinding of the coil ends or positioning them in a specific way. [([[http:// | ||
+ | |||
+ | Due to low permeability of the non-magnetic core the sensitivity does not allow to measure very low currents, and coils with "high sensitivity" | ||
+ | [([[http:// | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | [[Induction heating]] of a metal bar \\ | ||
+ | [[file/ | ||
+ | //< | ||
+ | </ | ||
+ | |||
+ | ==== Induction heating ==== | ||
+ | |||
+ | Induction heating employs a [[coil]] (or set of coils) which generates alternating magnetic field, usually at kHz frequency. The coil does not need to directly touch the heated object, because the electromagnetic field induces enough [[power loss]] (e.g. through [[eddy currents]]) in the object that it is possible even to achieve melting temperatures of metals. | ||
+ | |||
+ | Because of the level of current the coils are commonly water cooled from inside. Apart from the heated object, the coils often do not have any magnetic core. Popularity of the so-called [[flux concentrator|flux concentrators]] [(IH_handbook> | ||
+ | keeps growing, but applying them to already existing designs can change inductance of the coil, which might require re-tuning of the system for resonating power supplies. [(Ruffini> | ||
+ | |||
+ | The air gap also provides electrical insulation, so that the coil is not short-circuited by the heated object or flux concentrator. [(IH_handbook)] | ||
+ | |||
+ | ==== Electromagnets ==== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Large [[/ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | A common performance expected from an [[electromagnet]] is to generate magnetic field within a given volume of an air gap. This could be done for a number of tasks, for instance: | ||
+ | * to exert mechanical force on a designed part - this operation is similar to [[electromagnetic actuator|electromagnetic actuators]] | ||
+ | * to exert mechanical force on inclusions or other elements suspended in non-magnetic matter - a principle used in [[magnetic separator|magnetic separators]][([[http:// | ||
+ | * to provide magnetic field required for material processing[([[http:// | ||
+ | |||
+ | ===== Energy stored in air gap ===== | ||
+ | A magnetic circuit behaves like a " | ||
+ | |||
+ | This energy storing property is utilised for instance in energy storing [[inductor|inductors]] and [[flyback transformer|flyback transformers]], | ||
+ | |||
+ | For a simple magnetic circuit with a single air gap (see the first image at the top), for which the core is made out of high-permeability material such that $μ_{material} >> μ_0$, with the air gap itself and the flux density in the air gap being uniform, and if the [[flux fringing]] can be neglected, it can be derived that the stored energy is: | ||
+ | [(Hurley> | ||
+ | |||
+ | | $$E \approx \frac{B^2 ⋅ V}{2 ⋅ \mu_0}$$ | ||
+ | |||
+ | where: $E$ - stored energy (J), $B$ - [[flux density]] in the air gap (T), $V$ - volume of the air gap (m< | ||
+ | |||
+ | ===== Flux fringing ===== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | [[Flux fringing]] (red arc) around an air gap in magnetic core \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | [[Flux fringing]] is caused by the fact that the [[reluctance]] of the concentrated air gap is much greater than that of the core. The flux tries to spread as wide as possible in order to minimise the drop of [[magnetomotive force]] across the air gap. As a result of flux fringing the total reluctance of the circuit is somewhat lower. This has several major effects. | ||
+ | |||
+ | In energy-storing inductors the inductance is related to the reluctance of the air gap. The fringing lowers the overall reluctance, so that the resulting inductance is somewhat higher. This needs to be taken into account so that the inductance value is appropriate for a given design. There are various [[empirical equation|empirical equations]] suggested in literature for calculating the correction of this effect. | ||
+ | |||
+ | For instance McLyman suggest the following " | ||
+ | |||
+ | | $$F = 1 + \frac{l_{gap}}{\sqrt{A}} ⋅ ln \left( \frac{2 ⋅ l_{window}}{l_{gap}} \right)$$ | ||
+ | |||
+ | |||
+ | where: $F$ - factor by which the inductance is increased (unitless), $l_{gap}$ - length of the air gap (m), $A$ - cross-section area of the core (m< | ||
+ | |||
+ | Another example is when the area of the air gap is scaled according to its length. For instance if the magnetic core cross-section is a rectangle then Kazimierczuk proposes to use the following calculation: | ||
+ | |||
+ | | $$F = 1 + \frac{l_{gap} ⋅ (a + b + 2 ⋅ l_{gap})}{a ⋅ b}$$ | (unitless) | ||
+ | |||
+ | where: $a$ and $b$ are the lengths of each side of the rectangular cross-section of the magnetic core (m). | ||
+ | |||
+ | Yet another approximating equation is given by Hurley and Wölfle[([[http:// | ||
+ | |||
+ | | $$F = \frac{(a + l_{gap} ) ⋅ (b + l_{gap}) }{a ⋅ b}$$ | (unitless) | ||
+ | |||
+ | However, all such equations are only approximate, | ||
+ | |||
+ | The second effect is additional [[copper loss]] due to the fact that fringing flux " | ||
+ | |||
+ | The third effect is that the fringing flux enters the core perpendicularly to the normal flow of magnetic field. In [[soft ferrite|soft ferrites]] this is not a problem. But in [[laminated core|laminated cores]] this flux does not travel along the laminations, | ||
+ | |||
+ | ===== Distributed air gap ===== | ||
+ | |||
+ | <box 30% right #f0f0f0> | ||
+ | Changing one large air gap into several smaller reduces [[flux fringing]] \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | In high-power energy storing inductors the air gaps can be quite large. This would cause for the inductance to differ by unacceptable amount and also the losses would be too large. | ||
+ | |||
+ | The air gap is therefore " | ||
+ | |||
+ | <box 30% left #f0f0f0> | ||
+ | Air gaps between particles in a [[powder core]] \\ | ||
+ | [[/ | ||
+ | {{page> | ||
+ | </ | ||
+ | |||
+ | Magnetic cores of high-frequency [[choke|chokes]] can be made from [[powder core|powder cores]]. High-permeability magnetic particles are compressed into a core, but a certain amount of non-magnetic volume is left on purpose. This causes a very good distribution of the resulting air gap over the whole volume of such core. As a result the fringing flux greatly reduced (as compared to a core with a concentrated gap), yet a large amount of energy can be stored. | ||
+ | |||
+ | In the powder cores the transition into magnetic saturation (when overexcited) is less sharp, which is also an important feature reducing severity of faults in electronic circuits using such solutions. However, such cores are usually more expensive and exhibit different [[magnetic loss]] (e.g. higher than soft ferrites), so magnetic design must take these factors into account. | ||
+ | |||
+ | |||
+ | ===== See also ===== | ||
+ | * [[Effective magnetic permeability]] | ||
+ | * [[Flux fringing]] | ||
+ | * [[Leakage flux]] | ||
+ | |||
+ | ===== References ===== | ||
+ | ~~REFNOTES~~ | ||
+ | |||
+ | {{tag> |