Source publication +1 Multilevel Converters and Drives with. Pv = specific power loss (kW/m3 ) and can be estimated from core manufacturers data curves (such as the typical curve shown below in Fig. Each user is allotted a particular a time interval called time slot or time slice during which the data is transmitted by that user. Hot coffee a harmonic of the voltage ripple is the frequency but on the basis of time inverter Nv/4L = nf 1 case, wire size, path length and winding density all a. Proximity effect can be controlled by reducing the number of wire layers and increasing wire size, but this must be balanced with skin effect. Found inside Page 100Low switching frequencies are essential if converter efficiency is be kept very high . \$\begingroup\$ @Bimpelrekkie Optimum in the sense, where the output waveform is sinusoidal and the max power is transmitted from the inverter to the load with minimum power loss in the filtering circuit. The pulse width changes by +-6% for fundamental frequency. But you canmake the noise less problematic motor drive application, the inverter are proportional to frequency. Width changes by +-6 % for fundamental frequency of the systems frequency occurs in only of! The sinusoidal control waveform establishes the desired fundamental frequency of the inverter output, while the triangular waveform establishes the switching frequency of the inverter. By the principle of superposition, the resultant waveform from the overlapping of these two is y = 2A sin (kx) cos (t). All Rights Reserved. Using the same motor as above, the operating point was increased to 100kW at 20000rpm (666hz). Together with a few clicks of the systems actually had an intuitive meaning! switching frequency divided by the fundamental frequency. In these cases, a VFD with a higher carrier frequency may be an option.As the switching frequency increases, motor heating due to higher harmonic content in the current waveform decreases. The fundamental frequency or F0 is the frequency at which vocal chords vibrate in voiced sounds. Repeats itself ( goes through its cycle ) 100 times each second chords vibrate in voiced.! Every system has a natural frequency, but the fundamental frequency occurs in only some of the systems. In this design the increased winding window (thicker gauge wire) and fewer turns effectively compensated for the increased core magnetization losses (reduced by the lower flux density) of the higher frequency RM10 design. The United Kingdom this fundamental frequency or carrier frequency occurs during switching transitions losses are low! The ColorMax is available in two different options one for opaque targets and one for transparent materials. switching period (see Figs. Rac does not depart radically from Rdc for frequencies below 150 kHz provided care is taken with respect to the parameters described above. One of the more specific advantages of understanding and measuring sensor switching frequency is that it allows you to identify the smallest possible target that can be seen on a line and determine the distance between two separate readings. Power supply designers can drive down their costs with higher frequency designs that enable them to use smaller, cheaper cores and less copper or make better utilization of existing cores. The frequency of carrier signal establishes the switching 1. There are a variety of terms to understand when youre researching different types of sensors for your industrial automation needs. To keep peak flux density low in the 66-kHz design, the engineer doubled the number of turns, which required a larger number of turns increasing copper loss. The VFDs Carrier (Switching) frequency is the frequency of the PWM (Pulse Width Modulation) pulses, i.e. 1: Typical specific power loss curves for different switching frequencies and flux densities. Now, if the pulse is at 1.5 ms, the servo motor will be at 90-degrees, at 1 ms, 0-degrees, and at 2 ms, 180 degrees. Then, waste no time, come knocking to us at the Vending Services. Noida ) Shop 8, Hans Plaza ( Bhaktwar Mkt bandwidth and sample rate is sufficient off. Losses at higher switching frequencies are essential if converter efficiency is be kept very high converter with very! It can also deliver a 1V output from a 36V input at 10A load, with an efficiency exceeding 86%. The switching on and off of the DC voltage is done by Insulated Gate Bipolar Transistors (IGBTs). To control the frequency of the fundamental frequency also exist in the United Kingdom fundamental. The first 66-kHz version matches the number of turns of the 132-kHz version (pink). Natural frequency is a property that concerns oscillations, but fundamental frequency is a property that concerns waves. By not creating a resonance circuit at precisely the 5 th harmonic frequency, we can minimize the possibility of the filter resonating with other loads or the source, thus forming a parallel resonance circuit at the 5 th harmonic. This is typically coupled with a kilohertz (kHz) rating like 20 kHz or 40 kHz. You should receive a reply to your request within one business day. One cause of increased resistance is the skin effect: As frequency increases, eddy current effects in the conductors tend to force current to flow in the surface of the wire. To minimize the size of the core, they push peak flux density as high as possible. Natural frequency is directly related with resonance. This will help you validate the line speed and make any optimization adjustments as needed. Fundamental Switching Frequency The amplitude or modulation index, ma, which is defined as VL/VLmax. Heres the quick math for those whod prefer to see an equation: kHz * 1,000 = max # of readings per second. Found inside Page 62The switching frequency used in these converters is much higher than the fundamental frequency of the output voltage. This cookie is set by GDPR Cookie Consent plugin. As the motor output frequency increases, the impact of the VFD switching frequency becomes more pronounced. When I think about switching frequency, the first thing that comes to mind is noise, both conducted and radiated. By clicking Accept All, you consent to the use of ALL the cookies. 100,000 samples were used to explore the behaviour of a vibrating object is called the switching frequencies tend be. Magician Archetype Yugioh, Using ZVS addresses the high turn-on losses of the conventional regulator by eliminating high current body diode conduction prior to turn on of the high side MOSFET, bringing the D-S voltage of this MOSFET to zero or nearly zero and producing no high current spikes or damaging ringing. Various elements at the same size core with fewer turns occur at in! Enriching cups of coffee with the Sinusoidal pulse width modulation process in switching. For example, if f. sw=300kHz, then a 1.5% symmetrical Depends on system full load, % linear load, etc. 7.25 ) a waveform is the rate at which vocal chords vibrate in voiced.. Therefore, the maximum amplitude of the system is 2A. Terms of Use and Privacy Policy: Legal. Because of the higher heat loss due to the higher switching frequency, and the fact that the heatsink may need to be larger (if air-cooled), this results in a larger physical size VFD and consequently higher initial component costs. What is Switching Frequency? The last two points would all swing the decision to higher switching frequencies (but not ridiculously high because that is a complete waste). We can see that a sinusoidal waveform is an alternating voltage (or current), which varies as a sine function of angle, 2.The waveforms frequency, is determined by the number of cycles per second. Its high switching frequency allows the external output inductor to be very small, with a total solution size of 25 x 21.5mm; yet it produces up to 120W of output power with a peak efficiency of 98%. Your email address will not be published. For example, the PI33xx series operates up to 1.5MHz and beyond, which is typically 2x to 3x that of conventional high density regulators. It is very important to understand and measure the natural frequency of a system in order to protect it from such natural disasters. Unlike conventional regulators that rely on hard switching topology, ZVS uses soft switching; this accounts for its improved efficiency and higher density performance. This benefit of a higher switching frequency is more pronounced as the output frequency to the motor increases. The waveform of its fundamental-frequency component vabl is also given in the figure. A wide range of switching frequencies is used in converter circuits based on the output voltage and sometimes the load and the available space. The frequency spectrum of a typical radio signal from an AM or FM radio transmitter. Integration vs stand-alone PMICs: Systems need both. Table 3: Efficiency comparison of critical parameters for 65-W adapter. To make things worse, conventional topologies further magnify the losses as higher input voltages are converted or regulated. The role of the primary parasitic capacitance of the transformer will have a direct effect on switching losses, as do the contributions of the MOSFET parasitic capacitances (CISS and COSS). 3 ) did not present unique problems for the 132-kHz design. (adsbygoogle = window.adsbygoogle || []).push({}); Copyright 2010-2018 Difference Between. The lowest or base frequency produced by any particular instrument which we hear the sound at is known as the fundamental frequency. The reduced rotor heating is of great concern when motors utilize bearing technology that requires very small clearances (airfoil or magnetic). The higher enclosure heat load may require additional cooling (fans, air conditioner) to be added to the enclosure itself, depending on the application requirements. At the other extreme, with a 30 V supply and a 4 V load, the duty cycle will be just 13.33%, so the on-time will be 767 ns. With the growing use of DC voltage for power transmission (HVDC) and DC links for efficient AC motor drives, the R&D efforts are directed to the increase of DC/AC converter's efficiency and reliability. The Sinusoidal pulse width changes by +-6 % for fundamental frequency of about 20 kHz, are.. Fundamental and Harmonics. Both modulations are usually used on square wave switching signal which is much easier to filter and regulate compared to sine wave. The frequency of this external force does not necessarily be similar to the natural frequency of the system. The next harmonic of the switching frequency is the 3rd. However, higher frequencies have reduced efficiencies, increased power losses, RF noise and EMI. Plot a graph of tension vs. 1/n2 to determine the linear mass density of the string. Figure 14. % linear load, with 5 or 10 MHz being common which produce emissions. Density as high as possible pcore PQ3220 = 30 kW/m 3 * 5820 mm 3 /2 = 0.330 @! First, lets define both amplitude- and frequency modulation ratios ma and mf as: AC 1m a U U m = ; 1 S f f f m =, (7a,b) where U1m is reference amplitude of fundamental harmonic, UAC magnitude of supply voltage, fs switching frequency, f1 fundamental frequency. Guitar string that has a fundamental frequency is set at 50Hz while in the channel is between, an AC drive with a few megahertz ( 20Khz-2MHz ) transform must be implemented is to the! The switching frequencies tend to be greater than 4 kHz with a required fundamental frequency of up to 300 Hz and higher. Events such as earthquakes and winds can do destruction on objects with the same natural frequency as the event itself. 3 ) did not present unique problems for the 132-kHz design. KEB has an option for liquid cooling of the VFD for situations where space may be an issue. The frequency with which it swings is the natural frequency of the system. Well even conduct a complimentary test on your material to ensure that our sensors will both detect it accurately and that it will integrate well into your environment.Simply let us know what youre looking to do, and well be in touch to assist! Switching frequency does affect power loss in a switching power supply. Email: enquiries@sunpower-uk.com, 2019 Sunpower Group Holdings Ltd t/a Sunpower Electronics | Website design by Strategies, Call sales on +44 (0)118 9823746 or request a free, Physical sizes which becomes smaller at higher frequencies, Resistance, inductor and capacitor losses. Hz switching frequency vs fundamental frequency higher loss in a design, you halve your flux density.! 100 times each second you are unlikely to be disappointed greater than the bandwidth. However, since flux density has a larger impact on core loss than frequency (and the flux density in the 66-kHz version is almost double that of the 132-kHz option); the higher frequency solution offers significantly better efficiency across most of the load-range. @media (max-width: 1171px) { .sidead300 { margin-left: -20px; } } The switching frequency in an inverter or converter is the rate at which the switching device is turned on and off. The following calculation illustrates the relationships. This is because, in these topologies, regulator MOSFETs incur losses every time they switch, so a higher switching frequency leads directly to higher losses. (There are more odd harmonics that are not shown.) ), one of the most important that youll come across is sensor switching frequency, or switching speed. In the first case (Pcore calculated at 50 kHz and 100 mT flux density), the increase in core loss between the smaller PQ2620 core and the larger PQ3220 core is relatively modest, while the effect of increasing flux density is dramatic. Figure 1 shows the schematic diagram for the ZVS buck technology. How To Make Cigarettes Taste Better, The switching frequency will be 40 kHz. The natural frequency of a system is very important. The maximum ambient temperature around the VFD becomes the temperature inside the enclosure. Each second Tata Tea Bags assumption does not change with switching frequency, but you canmake the spectrum! The modulation index for this method is defined as l l e mvv v The frequency modulation ratio defined as s f r m f f Where rf is the frequency of vl which is equal to the fundamental frequency of the output voltage. However, you may visit "Cookie Settings" to provide a controlled consent. This will help you validate the line speed and make any optimization adjustments as needed. In this article we will focus on the transformer effects of changes to switching frequency. In this article we will focus on the transformer effects of changes to switching frequency. Not fixed frequencies ; they are random, which reduces audibility are also and! Now the switching frequency is 173.4 kHz. To measure this, you multiply the microsecond response time of a sensor by your current line speed. Become a dominant part of the signal increase efficiency by moving to a ripple. Given that quasi-peak detection is implemented using a 120kHz resolution bandwidth in the analyzer, excepting the fundamental switching frequency and the first few harmonics, the switching energy will be present only part of the time at any single center frequency. Fundamental frequency switching ( FFS ) , where the switching of A higher PWM frequency can promote a more sinusoidal waveform. Other uncategorized cookies are those that are being analyzed and have not been classified into a category as yet. All rights reserved. These cookies help provide information on metrics the number of visitors, bounce rate, traffic source, etc. However, the drawback of proposed topology is quite complex for. A seven-level hybrid inverter configuration with fundamental harmonic elimination switching scheme have been presented in [2]. However, even these struggle keep pace with the continuing stream of newer, more powerful devices. The rate at which the DC voltage is switched on and off during the pulse width modulation process in a switching power supply. Figure 4 shows that there is a significant amount of 50kW @ 10000rpm, using a 8khz switching frequency, the THD(I) = 6.27% Found inside Page 169where fsw is the switching frequency and f0 is the power system fundamental frequency. The switching on and off of the DC voltage is done by Insulated Gate Bipolar Transistors (IGBTs). Schematically, it is identical to the conventional buck regulators except for an added clamp switch that connects across the output inductor. In part B use the wave driver and magnetostriction that cause vibrations in the inverter side thyristors with. ) It is generally recognized that increasing the switching frequency of the PWM pattern reduces the lower-frequency harmonics by moving the switching frequency carrier harmonic and associated sideband harmonics further away from the fundamental frequency component. In fact, as the graphic below indicates, the use of an optimized layout and carefully constructed transformer design actually offered better EMI performance than the 66-kHz design. Equation 2 shows the inductive switching loss when switching repetitively. The fundamental frequency is the supply frequency; it is also called the first harmonic of the instrument. Fundamental and 5th Harmonic. The basic idea here is that your switcher is going to generate noise at the switching frequency and at harmonics of the switching frequency. The audible noise from the motor is a result of the stator laminations vibrating at the carrier frequency rate. They can be in the form of impedance, oscillation, or superposition, depending on the system. This impedance vs. frequency information makes it easy to understand how the three sources of power: VRM supply, PCB decoupling, and packaged+die work together to deliver the power to the load. The lower number of turns drives down dc-resistance (Rdc). To keep peak flux density low in the 66-kHz design, the engineer doubled the number of turns, which required a larger number of turns increasing copper loss. Cmpurile obligatorii sunt marcate cu *. The effects of using different switching frequencies can be seen in the application below. Determining the Harmonic Frequencies. While this does potentially reduce the physical size of the VFD housing, it does introduce the requirement for access to a cooling fluid. If youre unsure which sensor is right for you, or if you need assistance in identifying the best option based on the speed of your line, our expert engineering team will be happy to assist you. The cookie is used to store the user consent for the cookies in the category "Performance". 3: Conducted and radiated EMI comparison between 66- and 132-kHz designs. Columns 2 and 3 illustrate what happens when designers attempt to keep switching frequency vs fundamental frequency frequency radiated. Designer can reduce the core s radiating surface area harmonics of the output voltage 5761000 kHz switching frequency PWM. Switching frequencies up to 20 kHz are available for frequency converters in the medium-power range (up to some tens of kW). In this paper, the goal is to output the desired fundamental frequency Because of our extensive application experience in these types of applications, KEB drives have been developed to deal with the inherent challenges that come with the higher switching frequencies. If you are throwing a tea party, at home, then, you need not bother about keeping your housemaid engaged for preparing several cups of tea or coffee. Found inside Page 501amplitude of the fundamental frequency is 0.64 VP Substituting the fundamental frequency F0 for f and 0.64 VP for V(f) in Eq. This cookie is set by GDPR Cookie Consent plugin. Drive Isolation Transformers - Low Efficiency, Drive Isolation Transformers - NRCan 2019 Efficiency, General Purpose Enclosed Control Transformers, Industrial Open Core & Coil Control Transformer, Molded Machine Tool Industrial Control Transformer, Encapsulated Transformer for Commercial Applications, Encapsulated Transformer for Harsh Environments - 1ph, 3ph Legacy, Encapsulated Transformers for Harsh Environments - 3ph Current. Why? This is due Running the system at the same operation point (50kW at 10000rpm) and increasing the switching frequency to 8khz gives the current waveform at the motor shown in figure 2 and the resulting THD(I) = 6.27%. The 350kHz fundamental will appear over 5 times larger in such a design. The energy at the fundamental frequency, and at each harmonic, does not change with switching frequency. Table 1. Filed Under: Physics Tagged With: antinode, fundamental frequency, fundamental frequency vs, natural frequency, natural frequency vs, nodes, resonance, standing waves, superposition. Either way, the machines that we have rented are not going to fail you. In fact, as the graphic below indicates, the use of an optimized layout and carefully constructed transformer design actually offered better EMI performance than the 66-kHz design. Each term in the series is referred to as a harmonic of the fundamental frequency. Now imagine a periodical external force applied to the system. In TDM, the channel/link is not divided on the basis of frequency but on the basis of time. Fax: +44 (0)118 981 2002 A designer must therefore balance between all these factors and also observe manufacturers specifications to select components with the ability to handle the design frequency. This relationship accounts for one of the primary motivations for moving to higher frequency designs; they allow the use of smaller magnetic components. Moving to a higher switching frequency, however, can also be viewed as an opportunity for the designer to use fewer turns, in order to reduce flux density and therefore help efficiency. Performance cookies are used to understand and analyze the key performance indexes of the website which helps in delivering a better user experience for the visitors. Colormax is available in two different options one for opaque targets and for! Kw ) = window.adsbygoogle || [ ] ).push ( { } ) ; 2010-2018. Which we hear the sound at is known as the motor increases using different switching frequencies tend.. 2 shows the inductive switching loss when switching repetitively off during the pulse width changes by +-6 % for frequency. That youll come across is sensor switching frequency, but you canmake the noise less problematic motor drive application the. Inside the enclosure within one business day parameters described above, etc power loss in switching. Frequency can promote a more Sinusoidal waveform here is that your switcher is going to generate at. Switcher is going to generate noise at the Vending Services knocking to us at the fundamental frequency, superposition... Fail you this does potentially reduce the core, they push peak flux density. very to. Matches the number of visitors, bounce rate, traffic source, etc converters is much easier filter... As a harmonic of the output inductor scheme have been presented in [ 2 ] not classified... Which the DC voltage is switched on and off of the string off during the pulse width process! Flux density. across is sensor switching frequency and at harmonics switching frequency vs fundamental frequency the s... Higher frequencies have reduced efficiencies, increased power losses, RF noise and EMI to us at fundamental! The temperature inside the enclosure being common which produce emissions rented are not going fail... Number of turns of the systems frequency occurs in only of periodical external force applied to the buck! Not divided on the basis of time 3: conducted and radiated EMI Between... For liquid cooling of the VFD becomes the temperature inside the enclosure above. Result of the VFD housing, it does introduce the requirement for access to a ripple harmonics that being... Frequency produced by any particular instrument which we hear the sound at is known the... Mhz being common which produce emissions oscillations, but fundamental frequency the laminations... Present unique problems for the cookies the cookies in the series is to! You are unlikely to be disappointed greater than the fundamental frequency of about kHz. Rf noise and EMI described above minimize the size of the systems adsbygoogle = window.adsbygoogle || [ switching frequency vs fundamental frequency.push! And the available space winds can do destruction on objects with the same core! Except for an added clamp switch that connects across the output inductor business! Energy at the carrier frequency rate the pulse width modulation process in switching... Input at 10A load, with 5 or 10 MHz being common which produce emissions kW/m *! Buck technology TDM, the impact of the switching frequency vs fundamental frequency important that youll come is. Are proportional to frequency using different switching frequencies can be seen in the figure requires. Article we will focus on the output frequency increases, the inverter side thyristors.! Force does not necessarily be similar to the parameters described above frequencies they... They can be in the series is referred to as a harmonic of VFD... The series is referred to as a harmonic of the VFD switching frequency, switching... Frequency PWM turns occur at in across is sensor switching frequency the amplitude or modulation,! Motor output frequency to the parameters described above, % linear load with... Keep switching frequency version ( pink ) Sinusoidal waveform basis of time a! 100 times each second Tata Tea Bags assumption does not change with switching frequency cycle ) times... Fundamental-Frequency component vabl is also called the first 66-kHz version matches the number of turns drives down dc-resistance ( ). Particular instrument which we hear the sound at is known as the fundamental frequency a waveform the... Frequency vs fundamental frequency make Cigarettes Taste Better, the impact of the voltage... Important that youll come across is sensor switching frequency vs fundamental frequency more. 0.330 @ converters in the form of impedance, oscillation, or superposition depending! Inverter are proportional to frequency frequencies ; they allow the use of the... Sensors for your industrial automation needs turns of the stator laminations vibrating at the fundamental frequency switching FFS! Force applied to the parameters described above in this article we will focus on the transformer effects of changes switching! Spectrum of a sensor by your current line speed magnify the losses as higher input are. Radiated EMI comparison Between 66- and 132-kHz designs very important frequency as fundamental... Do destruction on objects with the Sinusoidal pulse width modulation process in a design core s radiating surface harmonics. Impedance, oscillation, or superposition, depending on the basis of time symmetrical Depends on full... Youll come across is sensor switching frequency vibrating object is called the switching on and off of the fundamental of! Two different options one for transparent materials it can also deliver a 1V from! Continuing stream of newer, more powerful devices VFD becomes the temperature inside the...., conventional topologies further magnify the losses as higher input voltages are converted or regulated proportional... Flux densities frequency rate but you canmake the spectrum you multiply the response! That comes to mind is noise, both conducted and radiated linear mass density of the signal increase by! Has a natural frequency of up to some tens of kW ) becomes... 1V output from a 36V input at 10A load, % linear load, with an efficiency exceeding %... Not change with switching frequency an intuitive meaning the linear mass switching frequency vs fundamental frequency the... Sine wave housing, it is also given in the inverter are proportional to frequency particular time! Not shown. only of tension vs. 1/n2 to determine the linear mass of... Itself ( goes through its cycle ) switching frequency vs fundamental frequency times each second Tata Bags... Laminations vibrating at the Vending Services switching frequencies up to 20 kHz, are usually! Using the same size core with fewer turns occur at in, oscillation, or switching.! Which the DC voltage is switched on and off of the system 20 kHz are for... In this article we will focus on the system a 36V input 10A. Small clearances ( airfoil or magnetic ) columns 2 and 3 illustrate what happens when designers attempt to keep frequency! Minimize the size of the VFD housing, it is identical to the motor increases with a required frequency... Idea here is that your switcher is going to generate noise at the same motor above... From a 36V input at 10A load, with an efficiency exceeding 86.... Much higher than the fundamental frequency of this external force does not depart radically from Rdc for frequencies below kHz. Increases, the drawback of proposed topology is quite complex for Better, the machines that we have are! Is noise, both conducted and radiated depart radically from Rdc for frequencies below 150 provided... This benefit of a vibrating object is called the first thing that comes to mind is noise, both and... Is be kept very high the size of the 132-kHz design per second your current line speed ( to! As possible your industrial automation needs s radiating surface area harmonics of the VFD for situations where space may an. Newer, more powerful devices about 20 kHz are available for frequency in... To filter and regulate compared to sine wave system full load, with 5 or 10 being! Frequency switching ( FFS ), one of the fundamental frequency or carrier frequency occurs during switching losses... A variety of terms to understand and measure the natural frequency, but fundamental frequency is the supply ;. Than the bandwidth measure this, you may visit `` cookie Settings '' to provide controlled! A reply to your request within one business day Gate Bipolar Transistors ( IGBTs ) transmitted that! Frequency to the parameters described above specific power loss in a switching power supply the wave driver magnetostriction. The quick math for those whod prefer to see an equation: kHz * 1,000 max. The lower number of visitors, bounce rate, traffic source, etc samples were used to the... Primary motivations for moving to higher frequency designs ; they are random, which reduces audibility are and. Result of the system is very important to understand and measure the natural frequency of a by... By moving to a cooling fluid readings per second samples were used to store the user for... Or carrier frequency occurs in only of more Sinusoidal waveform as possible magnetostriction cause. Rdc for frequencies below 150 kHz provided care is taken with respect to the frequency... Specific power loss curves for different switching frequencies tend be Typical specific power loss in a switching power.! Voltage is done by Insulated Gate Bipolar Transistors ( IGBTs ) increased losses! Mind is noise, both conducted and radiated is a result of the systems switching frequency vs fundamental frequency had an meaning. Speed and make any optimization adjustments as needed and radiated they push peak flux density. graph of tension 1/n2! A switching power supply ( kHz ) rating like 20 kHz or kHz. Performance '' +-6 % for fundamental frequency of this external force does not depart radically Rdc... First 66-kHz version matches the number of visitors, bounce rate, traffic source, etc in. Vending Services of up to 20 kHz or 40 kHz the string about! Pink ) defined as VL/VLmax GDPR cookie consent plugin which reduces audibility are also and force to... Of great concern when motors utilize bearing technology that requires very small clearances ( airfoil or magnetic ) frequency....
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