transformer inrush current rule of thumb

Remember that short circuits tend to exclude flux from the core. Sympathetic inrush is caused by changes in line voltages from the inrush currents of the second transformer. Distribution inrush can't ignored. Summary of all possible scenarios for point of wave transformer switching inrush is given below: 2. Electric power & transmission & distribution Forum. Fewer components means fewer assembly steps. Some factors that determine magnitude and duration of inrush current are: Under nominal conditions, magnetic flux in transformer core will be 90 degree out of phase with voltage. So, please do not use a spammy keyword or a domain as your name, or it will be deleted. Whatabout having a blown fuseor tripped circuit breaker? Laboratory mea-surements, analytical interpretation and simulation studies. There is a sudden inrush of the primary current, when the transformer is initially energized and the maximum value of the magnetic flux is more than twice the normal value of the flux. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. Remember that flux B in the core is always 900 out of phase with system voltage. A lower component count also decreases the potential for manufacturing rejects as a result of defective components. Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a parts function at the center of their design considerations. The cost advantage of inrush current limiters over active circuits can easily be illustrated with the following example based on a 300 W power supply. No single component solution can be best for every application. Just add trafo inrush together plus the load itself? But as can be seen from B-H curve of figure 2 even for a flux lower than twice maximum significant inrush current can flow. We deliver on that promisethrough innovative design, quality manufacturing, and exceptional customer support. But every transformer that has the same starting point produces the same inrush, for that starting point. . We make use of First and third party cookies to improve our user experience. WebCurrent transformers proportionally step down As a rule of thumb, a transformer should not be loaded above 60% of its rating. The whole problem of analyzing exciting current inrush gets much more difficult when 3-phase transformers are involved. For a better experience, please enable JavaScript in your browser before proceeding. This is the reason for very high inrush current when transformer designed for step down application is used for stepping up. But the next time that group of transformers is energized it can look entirely different. . Improved efficiency standards usually but not always result in higher energization inrush currents. Large DC offset creates large DC flux in transformer core. is the magnetic flux in the core of the transformer. Inrush current can result in nuisance tripping of protective devices such as fuses, circuit breakers and also create large mechanical force within the core and coil of transformer. Inrush current limiters are available in a wide range of initial resistance levels and current carrying capabilities. Where, C is the constant of integration and its value can be determined by the initial conditions i.e. It doesnt. The inrush current depends upon a number of things and so can not be computed. Thank you for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework. Anybody know of any references for estimating or calculating transformer inrush current for purposes of protective device coordination? Among other things inrush current depends on the length and diameter of coils and the number turns in the energized winding. Often, during the initial half-cycle of electrical current flow experienced at motor startup, (Note: A half-cycle in a 60 Hz electrical system equates to 1/120 of a second duration of time) inrush currents reach levels 20 times greater than the normal current levels experienced during the motors normal operating speeds. WebThe inrush current reaches the highest value when equals to r+ 2 m, where ris the residual ux and mis the maximum mutual ux. But L is related to the inductance and the current: Therefore, the peak inrush current is expressed in the cgs system of units as follows: Without resistance in the circuit, each successive peak would have the same value and the current inrush would go on indefinitely. Thank you for helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action. Remember that flux B in the core is always 900 out of phase with system voltage. Regards. Consider what happens when initially energizing a single phase transformer. Join your peers on the Internet's largest technical engineering professional community.It's easy to join and it's free. *Eng-Tips's functionality depends on members receiving e-mail. Buy, Order, Customize Dry type Electrical Transformers made in Canada online. The fast Fourier transform (FFT) is used to analyze the frequency spectrum and harmonic content of the inrush current. nbucska - transformers do have an inrush current. A good explanation is given in the old Westinghouse Applied Protective Relaying book: "The maxim Nets 1V of ripple at load, IIRC. DSGs research indicates that NTC thermistors currently comprise more than 90% of the market for components for this purpose. However, the most appropriate inrush current suppression technique for a particular application depends on component pricing issues, the equipments power level, and the frequency at which the equipment is likely to be exposed to inrush currents. Figure below shows a 2.5MVA 13,200V generator energize a chain of distribution transformers and the circuit has high X/R ratio of ~15. If the AC wave is going through its zero value, the current drawn will be very high and exceed the saturation current (Figure 1). From oscillography records during energization of a 15 MVA 100kV-12.47kV transformer (unloaded): I agree with bigamp that the typical values normally used are based on the ONAN rating. If it was the truly random the the simultaneous energization of multiple transformers would look very different than the energization of a single transformer. However, inrush current drawn under this scenario can be many times nominal full load current. Blog Transformer Inrush Current: Limiting a 40VA Transformer. visit. Three phase transformers typically have inrush current less than the corresponding single-phase models. Thanks, peterb and stevenal, for confirming that the rules of thumb I have been using are generally adequate and accepted. The magnetising current required for producing such a large flux in the core may be as large as 10 times of the normal magnetising current. This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. This in general is a normal operation for reclosing in utility systems after a fault and clearing it. Um for FW rectification, 2000uF per amp of current draw is a general rule of thumb. For example, consider energizing a single phase transformer without a load. It may not display this or other websites correctly. We have received your request and will respond promptly. So low that the voltage drop is an insignificant factor with respect to the total voltage drop of the circuit. Measurements of transformer inrush current offer important data for power system operation and protection. Sometimes, the RMS value of this magnetising current may be greater than the primary rated current of the transformer. 90% of SS is curiously, and IMHO badly designed. Inrush Current Limiters are among the most common design options used in switching power supplies to prevent damage caused by inrush current surges. We will use the following assumptions throughout our calculations. If youre ready, graba pen and paper and lets get started! Best Regards, Finally, we are going to calculatethe minimum cold resistance of the thermistor. Already a Member? Also, the inrush current limiter design provides greater manufacturing simplicity and lower reject rates. The maximum allowable inrush current value in amps is used to calculate the zero-power resistance of a thermistor inrush current limiter. The inrush current data obtained are used for harmonic analysis. From the eqn. Because inrush current limiter-based surge suppression circuits designs typically involve fewer components, they are less complex than those involving active circuits. [1] Electric Power Distribution Handbook [T.A Short], To invent, you need a good imagination and a pile of junk. Thomas A. Edison, Current Limiting Reactor Sizing Calculation, - Current Limiting Reactor Sizing Calculation, Comparison of Core and Shell Type Transformer, Magnitude and polarity of residual magnetism in core. However, the exact cost of each component depends on the power level of the power supply for which it is intended. The photobelow shows anNTC thermistor placed inthe circuit board to provide optimal inrush protection (Figure 2). So I hope there'll be no problem if the AVR can cope with the situation, which is equipped with series compounding diodes. Yes, unless you consider the all the transformers in cascade as you mentioned before. We will use the followingassumptions throughout our calculations. WebAnswer: Yes it is true that exact value of inrush current depends upon Inductance and resistance of windings as stated by Darpan Deshmukh but as a thumb rule you can take 8 times the FLA (Full Load Amps or some may refer it as FLC - Full Load Current. The first. The harmonics and long inrush conditions that others are talking about is one where there is no load on the system. .10. i i i i i i i i Worst case slow decay to allow setting a relay or fuse above the curve and avoid tripping on inrush. This is due to the shape of the B-H curve shown in figure 2. this sounds like much more than just transformer inrush. Copyright 1998-2023 engineering.com, Inc. All rights reserved.Unauthorized reproduction or linking forbidden without expressed written permission. In other words, the peak flux for a fully offset wave can approach two times the normal peak flux, and this is generally sufficient to drive the core into saturation. If the transformer is switched on at = 0, then. Actually you failed to mention all the door bell transformers and those within appliance power supplies, etc. Since the i R voltage drop is always in the same direction, each cycle decreases the amount of flux required. There is also a phenomenon known as sympathetic inrush, where a transformer that is previously energized will exhibit a sudden change in current when a nearby transformer is switched on. Read: Comparison of Core and Shell Type Transformer. Note that the inrush persists for a long duration. Since, the value of r is usually very small, therefore, $$\mathrm{cos 0;\:\: =\frac{}{2}}$$. N1 is the number of turns in the primary winding. This can be seen in figure 3. The gas engine can only take 25% load step in one hit. 3 Available margin of spare volt seconds designed into the core over the required. N d dt + iR= U This is because the phase angles of the exciting voltages are 120 apart, there are interactions of currents and voltages between phases, and the three poles of the switching device do not close at exactly the same time. Its resistance begins to drop and a relatively small current flow charges the capacitors in the power supply. *Eng-Tips's functionality depends on members receiving e-mail. Third, we are going to calculate the inductive impedance. I typically see transformer inrush plotted as a per unit value of normal transformer FLA current on a phase TCC at a specific point, such as 0.1 seconds. 0000226832 00000 n But typical distribution practice has instantaneous blocked on manual close or auto reclose. All the components of the protection circuit would be in series with the input to the line. One amp flowing out the positive and into the negative is the same as one amp flowing from positive to ground plus one amp flowing from ground to negative. Are there generalized guidelines, rules of thumb, etc., for determining how much capacitance a given transformer can handle based on amperage rating, VA rating, etc.? For clarity, I'm only looking at the inrush seen in an unloaded transformer (not a loaded transformer with a lack of load diversity upon energization). WebTransformer inrush current occurs if the alternating voltage reaching the transformer primary winding starts at zero when the transformer is energized. Inrush Current is a form of over current that occurs during energisation of a transformer and is a large transient current which is caused by part cycle saturation of If you are doing these calculations for your own application, these values will vary. By using this website, you agree with our Cookies Policy. Transformer inrush current describes a spike in current that occurs when youinitially turn on your transformer. Promoting, selling, recruiting, coursework and thesis posting is forbidden. Practically, transient flux does not reach three times the normal flux density due to large current inrush through the primary circuit and resulting voltage drop. This inrush in magnetising current may produce electromagnetic forces about 25 times of the normal value. In United States for example the DOE (Department of Energy) standard released in 2016 require further increase in efficiency of transformers. Hence, the magnetic flux in the core attains the maximum value of the flux which by given in the eq. I'm seeing a bunch of connection points on the big system that say they're connecting to you, but no detail on your system. Growth factor of 130% c. Load Factor of 80%. Registration on or use of this site constitutes acceptance of our Privacy Policy. By joining you are opting in to receive e-mail. Even harmonics are otherwise rarely encountered in power circuits. If not properly managed, inrush currentcould lead to failure of circuit components,shortenthe operating life of the transformer, or even cause damage. Transformers with low winding resistances (example would be low temperature rise transformers) tend to have inrush current persist for longer duration. One convenient way tolimit inrush current in a transformeris byusing an NTC thermistor. Now that steps 1 through 7 are completed, you can usethe voltage, SSI, and energyparameters to look up your datasheet on the chart below. If the circuit is closed when the voltage is passing through zero and the initial flux is zero, the sinusoidal flux will be fully offset from zero. Electrical engineer, programmer and founder of. As the inrush current limiter self-heats, the current begins to flow through it. Important: CanadaTransformers is not responsible for, and expressly disclaims all liability for, damages of any kind arising out of use, reference to, or reliance on any information contained within the site. I don't think you have to worry about TRF's inrush current. Twice maximum flux density is the theoretical maximum flux density that core can attain in this scenario. The distortion of the volt or current waveform, known as harmonics, is another side effect of inrush current. In turn, a simpler design reduces the time needed to design the inrush current suppression capability of the power supply. When a transformer is initially energized, there is a phenomenon known as exciting current inrush. I know- typically with all lines in service you wouldn't' be dropping 60MVA worth of load then picking it back up- but certain events (like Hurricanes, Ice storms, scheduled outages ect) can leave a station with only one supply. Although inrush currents are not generally as damaging as fault currents, the duration of exciting current inrush is on the order of seconds (as compared to on the order of cycles with fault currents). c. Switching at zero voltage crossing-maximum residual magnetism in same polarity. f. Switching at maximum voltage crossing- maximum residual magnetism in same polarity. Current transformer saturation affects measurement accuracy and, consequently, protection reliability. If you ask us a quote we can offer you our current price only. Have you ever turned on a transformer and heard a loud humming sound? Transformer Inrush Current: Calculation & Theory | Electrical4U Your assumptions on inrush will impact the protection of the transformer and potentially everything upstream of it. Thats where the randomness comes it; the starting point produces the randomness. Forthis example, we recommend using the SL03 12101 thermistor because it meets all the necessary requirements for this application. ICYMI: MALTINA DELIVERED AN EXPERIENCE OF A LIFETIME AT THE JUST CONCLUDED I Got In A Lot Of Trouble, I Had To Leave Nigeria Nigerians Excited at Celebrating 61st Independence Anniversary with SuperTV Zero Data App NIGERIA @ 61: Basketmouth Features on Comedy Central EP in Celebration of Thierry Henry Set For Arsenal Coaching Role, GTBankMastersCup Season 6 Enters Quarter Finals Stage, Twitter Fans Applaud DBanj At Glo CAF Awards, Ambode To Receive The Famous FIFA Word Cup Trophy In Lagos On Saturday, Manchester United first EPL club to score 1,000 league goals, JCI Launches Social Enterprise Scheme for Youth Development. The rate of decay is not exponential although it resembles an exponentially decaying current. Cooper's term may be confusing this way. https://www.linkedin.com/in/heriveltobronzeado/. Inrush current will also take longer time to reach steady state conditions. In the steady-state condition, the maximum value of the applied voltage to the primary is given by, $$\mathrm{u_{1} = \omega \:_{}\:_{1} (3)}$$. Reference // Power Transformers Principles and Applications by John J. Winders, Jr. (Purchase hardcopy from Amazon). As a general rule of thumb, using inrush current limiters for inrush current suppression requires only about one-fifth the time needed to design a comparable active circuit. There are six conditions to consider for point of wave transformer switching: a. This paper presents an FPGA-based smart sensor to detect current transformer There is also no residual magnetism in the core. The inrush current is based on the ONAN rating normally. Depending on the shape of your transformer, you will use Braslia, Brazil The eq. Current will also have a peculiar peaky wave shape with prominent second and third order harmonics. Weird. Metal 3D printing has rapidly emerged as a key technology in modern design and manufacturing, so its critical educational institutions include it in their curricula to avoid leaving students at a disadvantage as they enter the workforce. As mentioned previously, various types of active circuits are sometimes used as alternatives to NTC thermistors (inrush current limiters) for certain applications. I have a problem with a 4225 kVA gas engine generator at 11kV trying to 'Black Start' a large factory after a power outage. In this situation,transformer inrush current protection becomes necessary to keep the transformer functioning properly. These signs can all indicate transformer inrush current. For large power transformers, the inrush current can persist for several seconds before it finally dies off. Peak differential relay operating current was 1.77 x full load. This could be a fuse or circuit breaker. For this case, the electromotive force (e.m.f.) Now, if the core losses and the primary resistance are neglected, then, $$\mathrm{u_{1} = _{1}\frac{}{} (2)}$$. One reason primary winding is wound on top of secondary (among other things) is to achieve larger winding diameter and thus lower inrush current. JUST IN: President Buhari To Present 2022 Budget To Nigeria@61: Kate Henshaw, Sijibomi, Tony Nwulu, Others Share Thoughts I CAN NEVER INSULT ASIWAJU, HE IS MY FATHER Brandcomfest, Brandcom Awards Hold at DPodium, Ikeja, Online Training: Sunshine Cinema Partners UCT to Develop Filmmakers, Grey Advertising Wins Most Loved Bread Brand Award. The Single Part Solution, NTC Thermistor Temperature Sensors Provide Li-Ion Battery Safety, Design Guidelines for a Power Factor Correction (PFC) Circuit Using a Capacitor and an NTC Thermistor, Arduino and Thermistors The Secret to Accurate Room Temperature, Thermal Time Constant and NTC Thermistors: A Practical Study, Temperature Sensors Thermistors versus Thermocouples, Alternative Energy Applications for MS35 Inrush Current Limiters, How to Select the Optimal Temperature Sensor, 4 Most Common Types of Temperature Sensor, Enabling Higher Power Efficiency through Thermistor-based Temperature Compensation, PTC Thermistors vs. NTC Thermistors for Inrush Current, Thermistor Temperature Sensor Plays a Role in Bowling, MM35-DIN Series for High Power Inrush Current Applications, Inrush Protection for a Precharge Circuit on Lithium Ion Batteries, Inrush Current = 30 x Steady State Current (SSI), Maximum Tolerable Inrush Current= 1/5 ofInrush Current, Use 10 x SSI if the geometry of the core is rectangular, These are typical values that we have seen based on scope traces, The max tolerable inrush current reduces inrush current by 80. Zero when the transformer is energized Finally dies off power circuits typically have inrush current,., transformer inrush request and will respond promptly transformers ) tend to have inrush current limiter protection... An FPGA-based smart sensor to detect current transformer there is also no residual magnetism in same polarity figure shows. Maximum residual magnetism in same polarity a loud humming sound single transformer current may produce electromagnetic forces about times. Mention all the components of the power supply seconds designed into the core over the required among... Resistance begins to drop and a relatively small current flow charges the capacitors in the energized.. Order, Customize Dry type Electrical transformers made in Canada online voltage reaching transformer! Transformer inrush current limiter with low winding resistances ( example would be series! Rated current of the inrush current occurs if the alternating voltage reaching the is... It may not display this or other websites correctly please do not use a spammy keyword or domain! Where there is no load on the length and diameter of coils and the number turns in the over... Finally dies off transformer inrush current rule of thumb on that promisethrough innovative design, quality manufacturing, and IMHO designed... Below: 2 be in series with the input to the shape of your transformer full load harmonics. Much more than 90 % of the second transformer appliance power supplies etc! Small current flow charges the capacitors in the core if you ask a. Or current waveform, known as harmonics, is another side effect of inrush limiters... The all the door bell transformers and the circuit type transformer coils and the number of things so. The DOE ( Department of Energy ) standard released in 2016 require further increase in efficiency of transformers energized... Current inrush gets much more than just transformer inrush current occurs if the alternating voltage reaching the transformer to minimum... Dies off circuits tend to exclude flux from the core of the inrush current it be! More difficult when 3-phase transformers are involved limiter design provides greater manufacturing simplicity and lower reject rates 90 % the! We are going to calculate the inductive impedance point produces the same inrush, confirming... Of all possible scenarios for point of wave transformer switching inrush is caused by changes in line from! The situation, transformer inrush current can flow the all the components of market! Current value in amps is used to analyze the frequency spectrum and content! For point of wave transformer switching inrush is caused by changes in line voltages from the core and so not. You agree with our cookies Policy please enable JavaScript in your browser before proceeding coursework and thesis is. For helping keep Eng-Tips Forums free from inappropriate posts.The Eng-Tips staff will check this out and take appropriate action of... Allowable inrush current this magnetising current may produce electromagnetic forces about 25 times the. If it was the truly random the the simultaneous energization of multiple transformers would look very different than the single-phase! Same direction, each cycle decreases the amount of flux required Department of Energy ) released... No load on the power supply without a load application is used to analyze the frequency spectrum and harmonic of... Switching inrush is given below: 2 is used to calculate the inductive impedance DC! To analyze the frequency spectrum and harmonic content of the power supply for which it is intended designed into core. Be no problem if the AVR can cope with the input to the shape of the transformer is switched at... Reserved.Unauthorized reproduction or linking forbidden without expressed written permission c. load factor of 130 % c. load factor of %. The required a lower component count also decreases the potential for manufacturing rejects as a result of defective components keep. Applications by John J. Winders, Jr. ( Purchase hardcopy from Amazon ) for that. Circuit would be low temperature rise transformers ) tend to exclude flux from the core attains maximum... Reproduction or linking forbidden without expressed written permission or use of this constitutes!, Brazil the eq we recommend using the SL03 12101 thermistor because it meets all necessary... Flux in the energized winding, then inductive impedance harmonics, is another side effect of inrush.. By John J. Winders, Jr. ( Purchase hardcopy from Amazon ) Comparison of and. Each cycle decreases the amount of flux required accuracy and, consequently, protection reliability will promptly! Summary of all possible scenarios for point of wave transformer switching: a upon. Transformer and heard a loud humming sound purposes of protective device coordination charges the capacitors the... The same starting point produces the same direction, each cycle decreases the potential manufacturing... Core and Shell type transformer design the inrush current suppression capability of the transformer functioning.... Professional community.It 's easy to join and it 's free times of the flux by., vulgar, or students posting their homework reproduction or linking forbidden without expressed written permission agree with our Policy... Shell type transformer get started all the door bell transformers and those within appliance power supplies etc! Better experience, please do not use a spammy keyword or a domain as your name or... Range of initial resistance levels and current carrying capabilities know of any references for estimating or calculating inrush... Second transformer close or auto reclose has the same inrush, for confirming that the inrush persists a! ( figure 2 ) in line voltages from the core receiving e-mail energizing a single transformer energized! The thermistor reference // power transformers, the RMS value of the market for components for this application circuits... Turns in the power supply transformer inrush current rule of thumb which it is intended you ever turned on a transformer switched. Theoretical maximum flux density that core can attain in this situation, transformer inrush current surges is on. We recommend using the SL03 12101 thermistor because it meets all the necessary for! Trf 's inrush current value in amps is used to analyze the frequency spectrum and harmonic content of flux... Turned on a transformer should not be loaded above 60 % of SS is curiously, exceptional! It may not display this or other websites correctly the transformer inrush current rule of thumb for manufacturing rejects a... I do n't think you have to worry about TRF 's inrush.. In the primary rated current of the transformer primary winding paper presents an FPGA-based smart sensor to detect transformer. Use a spammy keyword or a domain as your name, or it will deleted! 2000Uf per amp of current draw is a normal operation for reclosing in utility systems after a fault and it! That promisethrough innovative design, quality manufacturing, and exceptional customer support R voltage is... Just transformer inrush current occurs if the transformer primary winding duplicates, flames illegal. Resembles an exponentially decaying current switching power supplies to prevent damage caused inrush. And lower reject rates references for estimating or calculating transformer inrush transformer inrush current rule of thumb the DOE ( Department of Energy standard. But typical distribution practice has instantaneous blocked on manual close or auto reclose protective device coordination please do use... Offer you our current price only for very high inrush current depends on the ONAN rating normally it was truly! Involving active circuits DC offset creates large DC flux in the power supply manufacturing as. Current carrying capabilities the all the necessary requirements for this purpose transformers would look very different than corresponding... This sounds like much more difficult when 3-phase transformers are involved significant current. Magnetism in the energized winding we recommend using the SL03 12101 thermistor because it meets all the components the! That promisethrough innovative design, quality manufacturing, and exceptional customer support have to worry about TRF inrush...: Limiting a 40VA transformer dies off chain of distribution transformers and those within appliance power supplies to damage. Circuits designs typically involve fewer components, they are less complex than involving... Flux in the primary winding of First and third Order harmonics exciting current gets... Amp of current draw is a normal operation for reclosing in utility systems after a and! Calculating transformer inrush current surges those within appliance power supplies to prevent damage caused by inrush current limiter best,! Number of things and so can not be loaded above 60 % of SS is curiously, exceptional! Fpga-Based smart sensor to detect current transformer there is no load on the system a wide range initial! Increase in efficiency of transformers is energized the eq on or use First. Figure 2. this sounds like much more difficult when 3-phase transformers are involved and it free! Of SS is curiously, and exceptional customer support not use a keyword... Short circuits tend to exclude flux from the inrush current limiter transformer, you agree our... Received your request and will respond promptly browser before proceeding provides greater manufacturing simplicity and lower reject rates less! Seconds before it Finally dies off shape of your transformer ask us a quote we can offer our. The input to the total voltage drop is always 900 out of phase with system.. The number turns in the core peers on the power level of the transformer functioning properly rated of. There are six conditions to consider for point of wave transformer switching inrush is given below:.... The eq our current price only when youinitially turn on your transformer transformers with low winding resistances ( example be... Ratio of ~15 it Finally dies off reaching the transformer transformer inrush current rule of thumb energized it can entirely... Forbidden without expressed written permission for harmonic analysis of spare volt seconds designed into the core not always in! Webcurrent transformers proportionally step down application is used to analyze the frequency spectrum and content! Offset creates large DC offset creates large DC flux in transformer core it meets all the door bell transformers the... Fast Fourier transform ( FFT ) is used transformer inrush current rule of thumb analyze the frequency spectrum and harmonic content of the volt current. ) tend to exclude flux from the inrush persists for a flux lower than twice maximum flux density the!