Handbook of Small Electric Motors Episode 3 potx

dc applications, the designer has the choice of a number of material systems, each with unique properties that can be customized to meet specific part requirements. Care must be taken in the manufacture of the sintered P/M parts to minimize the carbon and nitrogen pickup. By varying the alloying, sintered density, and process- ing, the resulting magnetic properties cover a broad range of applications. For ac applications, the P/M industry has developed two families of powders that can be utilized in alternating magnetic fields. The choice between the iron powder– polymer composites and a powder that can be given a low-temperature annealing is based on the part function and ultimate part requirements. 2.7 MAGNETIC TEST METHODS* The characteristics of the magnetic core materials discussed in this chapter must be determined by certain specified procedures, which are usually in accordance with the standard test methods of the ASTM.The data obtained from such measurements can be used in a comparison of the electrical and magnetic properties and as a guide in the selection of the best material for a particular design.The test methods utilized can be classified in two general groups: those using direct current and those using alternating current as a source of power. 2.7.1 Direct-Current Tests All direct-current test data are obtained by ballistic test methods. Ring samples, or an appropriate test sample in one of several different permeameters, were used with the basic circuit illustrated in Fig. 2.80 or in a modification of this circuit. MATERIALS 2.71 FIGURE 2.79 Core loss at 200 Hz. *Section courtesy of Allegheny-Teledyne. For a more detailed discussion of this circuit and its adaptation to various test methods, see ASTM A-34 on standard methods of test for normal induction and hys- teresis of magnetic materials. The test methods described by ASTM cover most of the generally accepted test methods likely to be used for obtaining direct-current magnetic properties. Test Samples. In general, the form of the test sample will be determined by the type of material to be tested,the type of test desired,and the availability of the mate- rial from which the test sample will be made. The following is a partial list of sample forms which might be used in obtaining direct-current test data on soft magnetic materials. ● Stamped or machined rings or links ● Wound tape toroid or other form ● Epstein strips ● Split strips ● Small strips or bars ● Bars and rods ● Cut cores and laminations Tape-wound cores and Epstein strips are not practical for heavy-gauge materials. If the material being tested has no pronounced directional magnetic properties, the ring or toroidal winding is the preferred sample form. For nonoriented materials having some directional properties, an Epstein sample cut one-half in the direction of best properties or in the direction of rolling and one-half across this direction is the preferred form. With oriented materials, the wound toroidal core or Epstein strips cut parallel to the rolling direction are preferred over other types of speci- mens.Where the size and shape of material to be tested are limited, it may be neces- sary, with some sacrifice in accuracy, to use other forms of test specimens such as bars, rods, small strips, cut cores, laminations,or other special shapes.For reliable test results, permeameters require properly selected samples of correct size and shape and are limited in ranges of magnetizing force, permeability, and induction. Perme- ameter tests generally are not as reliable as those made on ring or toroidal speci- mens of proper dimensions. In most cases, manufactured parts are of such size and shape that they cannot be adapted to fit into standard test equipment and therefore cannot be tested accurately for material characteristic properties. 2.72 CHAPTER TWO FIGURE 2.80 Basic circuit diagram for demagnetic testing. When testing ring or toroidal specimens, first the B pickup coil, then the mag- netizing winding are wound almost directly on the specimen. Care must be taken to avoid strain due to pressure of insulation or winding. Frequently, a close-fitting, rigid insulating case is used to avoid these strains. Magnetizing forces are calcu- lated from the number of turns in the magnetizing coil N, the magnetizing current I, and the mean length of magnetic path in the test specimen ᐉ m , using the follow- ing relation: H = Oe (2.35) The corresponding induction is measured with the aid of the B coil and the bal- listic circuit, the ballistic galvanometer having previously been calibrated to become direct reading in terms of gauss per millimeter of deflection. This calibration is obtained by calculating a current which must be reversed in the primary of the stan- dard mutual inductor to give the desired number of millimeters of deflection for any chosen induction in gauss. The number of B coil pickup turns N, the cross-sectional area in square centimeters of test sample A, the value of the mutual inductance in henries L m , and the induction in gauss B, at the chosen value of deflection, are used in the following relation to determine the value of the calibrating current: I = A (2.36) The galvanometer series resistance is adjusted to give, on current reversal, a deflec- tion equal to that chosen to represent the value of B used in the formula. If accurate machined test samples are available, the cross-sectional area is calcu- lated from physical measurements. For all other samples of uniform cross section, it must be calculated from the weight in grams, the density δ in grams per cubic cen- timeter, and the mean length ᐉ m in centimeters, as follows: A = cm 2 (2.37) Normal induction curves and hysteresis loops are run in accordance with ASTM designation A-34. In all cases, care must be taken to properly demagnetize the test specimen prior to the measurement of magnetic properties.A drift in low-induction characteristics may be observed subsequent to demagnetization. For best results, a 24-hour storage period in a magnetically shielded container should precede the test run. In practice, however, a reasonable time is allowed to elapse, and the tests are then performed. To realize fully the benefits of the previous magnetization, the normal induction curve must be obtained by taking a regular series of test points beginning with the lowest value of induction and proceeding upward toward saturation. In addition, the sample must be in a uniform cyclic condition for each of these test values. Direct-current demagnetization is achieved by first magnetizing to a high induc- tion, then by a long series of slow reversals the current is reduced in small incre- ments to zero. When alternating currents must be used, the lowest available power frequency is chosen, and demagnetization is obtained by first magnetizing to high induction, then slowly reducing the applied field to zero. It is most complete after heat treatment above the Curie point. This condition may not be stable, however, and the first curve obtained cannot always be repeated, even after careful demagne- tization using one of the other methods. weight ᎏ δᐉ m BNA ᎏ L m × 10 8 0.4πNI ᎏ ᐉ m MATERIALS 2.73 Permanent magnets are generally tested in the same manner as described here, with the exception that suitable test specimens are normally of solid bar or rod form and are usually run in a saturation or high H permeameters such as described in Bureau of Standards RP548 and RP1242. Direct-current tests on all classes of magnetic materials may be run over a wide range of temperatures in the same manner as that used at room temperature. Proper care must be taken to maintain the test specimen at the desired constant tempera- ture for each test run. In many cases, this requires auxiliary equipment of a special nature not generally available for normal test work. 2.7.2 Alternating-Current Tests Many magnetic materials have widespread use at commercial power frequencies. For this reason, the 60-cycle properties of most magnetic materials have been gen- erated. Epstein Frame. Core loss and permeability testing at 60 cycles per second are fairly well standardized over a wide range of magnetic inductions for both the 50-cm butt joint and the 25-cm standard double lap joint Epstein frames.Air gap and strain effects have been reduced in the 25-cm double lap joint test frame (employing a 28- cm test sample, thereby permitting more dependable permeability measurements). The basic circuit used for 60-cycle core loss and permeability measurements in this type of testing is illustrated by Fig. 2.81.A detailed description of the test method is given in ASTM designation A-343. Because of the large sample size required by the Epstein frame and the desir- ability of testing certain highly oriented materials and materials of extremely high permeability,a test sample in the form of a wound toroid or ring carrying its own test windings is frequently substituted for the Epstein test frame in the circuit illustrated in Fig. 2.81.The test is made in the same manner as with the standard Epstein frame. When making ac core loss and permeability measurements, it is customary to maintain sinusoidal voltages. ASTM standard test methods assume this condition and, for core loss measurements, apply corrections when the form factor departs from 1.11 by more than 1 percent. In this test method, when true root-mean-square- reading instruments are used, it becomes important to know the waveform of the voltage or current being measured. At inductions above the point of maximum permeability of the normal magneti- zation curve, ac permeabilities may be determined by the method previously 2.74 CHAPTER TWO FIGURE 2.81 Basic circuit diagram for 60-cycle ac core loss and permeability measurements. described. Note that this method assumes that the ratio of the magnetizing compo- nent of the current to the total current is nearly unity. Due to the presence of loss components in the current, which lower this ratio, permeabilities obtained by this method may not be the same as those obtained from a direct-current ballistic test. The magnetizing force for individual test points, in oersteds, is calculated from the following: H = (2.38) where N = number of magnetizing winding turns I m = peak amperes ᐉ m = mean length of magnetic path, cm H = magnetizing force for the given test point, Oe For the standard 25-cm double lap joint test frame, this formula reduces to H = 10 I m (2.39) For all samples made from sheet or strip materials, the specimen cross-sectional area is calculated from the weight, density, and length. For Epstein samples, the length used to calculate the area is four times the sample length. For other sample forms of uniform cross section, the length is the mean length of the magnetic path. Induction is calculated from the measured voltage using an average, or root- mean-square volts on a sinusoidal waveform.The formula becomes B m = (2.40) This test method is usable for core loss and volt-ampere measurements from mod- erately low inductions up to those approaching saturation. In the upper region, exciting currents become large, and instrumentation and other problems are magni- fied. Incremental core loss and ac permeability tests can be made using the preceding test method. Where the operating inductions permit, these tests are usually made with a bridge or electronic instruments. Owen Bridge. The standard Owen bridge test frame for Epstein samples has 100- and 1000-turn windings, but other sample forms, with appropriate windings, may also be used. This bridge circuit is illustrated in Fig. 2.82 and described in ASTM desig- nation A-343. E × 10 8 ᎏ 4.44fNA 0.4πNI m ᎏ ᐉ m MATERIALS 2.75 FIGURE 2.82 Circuit diagrams for Owen bridge test method. Hay and Maxwell Bridges. Hay and Maxwell bridges are also adaptable to these measurements. The bridge methods are the most widely used means for obtaining low-induction properties, but methods using direct reading meters or electronic instruments are also popular. Alternating-current potentiometers may be used, but they are not readily available. The modified Hay bridge is rapidly gaining popularity for bridge-type measure- ments. It has been adopted by ASTM and appears in the A-343 standards. This method may be used with Epstein-type test frames as well as with other sample forms. Its circuit diagram is illustrated in Fig. 2.83. Permeability Measurements. Permeability measurements over very wide ranges of inductions and frequencies may be made using electronic instruments and the cir- cuit diagram of Fig. 2.84. These meters will withstand large overloads and may be calibrated to read directly in terms of magnetizing force and induction. Frequently, test methods which are designed especially for quality control pur- poses have sufficient accuracy for other tests and at the same time are fast and con- venient to use. The direct impedance substitution method for determining low-induction ac permeability is one type. It uses a more versatile arrangement of the simplified circuit and is shown in Fig. 2.85. When using this test method, it is desirable to keep the resistance of the test coil and the inductance of the decade resistors as low as possible. The core materials under test have relatively high permeability, but the coil resistance and core losses still produce in-phase exciting current components.The type of core, gauge, size,and other pertinent facts are always known; therefore, for comparative test purposes, it 2.76 CHAPTER TWO FIGURE 2.83 Circuit diagram of modified Hay bridge. FIGURE 2.84 Circuit diagram for ac permeability. is reasonably accurate to assume that the voltage drop across the test winding is entirely reactive. Under these conditions, the formulas L = HX L = 2πfL (2.41) are combined and developed to create the following working equation: µ= (2.42) where L = inductance, H N = test coil turns µ=effective permeability A = cross section, cm 2 ᐉ m = mean length of magnetic path, cm f = frequency X L = inductive reactance E R = voltage drop across the series decade resistor AC Hysteresis Loop Tracer. The dynamic hysteresis loops are also of value in design applications employing many of the newer magnetic materials. These loops are most conveniently obtained with the aid of a suitable oscilloscope with wide- band dc amplifiers, using the test circuit illustrated in Fig. 2.86. In obtaining this type of data, care must be taken to ensure that none of the har- monics present in either the voltage or current waveforms are attenuated by the ampli- fiers and that the phase shift in the amplifiers and the integrator circuit is held within certain limits. In this circuit, R 1 should be as low as possible and R 2 should have a value at least 10 times the capacitative reactance of condenser C at the test frequency. constant ᎏ E R 4πN 2 µA ᎏ 10 9 ᐉ m MATERIALS 2.77 FIGURE 2.85 Circuit diagram for ac permeability measurement by direct impedance substitution. FIGURE 2.86 Schematic diagram for ac hysteresis loop tracer. Dynamic hysteresis loops are of interest under two conditions of excitation. The condition of most general interest is one in which the sinusoidal flux in the core is maintained at all times and the exciting current is allowed to distort to the nonsinu- soidal form required to maintain this flux. The other condition of general interest is that in which the sinusoidal exciting current is maintained and the core flux is per- mitted to distort as required to maintain sinusoidal excitation. Under conditions of sinusoidal core flux, no harmonics are present in the voltage wave being integrated, and the R-C integrator illustrated in the circuit in Fig. 2.86 is adequate, provided its phase shift is within required limits. In the case of the core with sinusoidal exciting current, however, substantial percentages of harmonics may be present in the integrated voltage, and a simple R-C integrator may no longer suf- fice to give a reliable presentation of the dynamic hysteresis loop. Constant-Current Flux Resetting. Another type of test, which is becoming very popular as a means of evaluating core materials for magnetic amplifiers and sat- urable reactor applications, is the constant-current flux resetting test. This method uses the basic circuit of Fig. 2.87, which employs a half wave of excitation to drive the core into saturation. A constant value of direct current is used as a means of reset- ting the core flux during the interval between the half waves of exciting current.An integrating voltmeter is normally used to measure the change in peak induction as a function of the dc resetting current, with a given constant value of half-wave excita- tion. Under these conditions, this function is a type of magnetization curve similar to the control characteristic curve of a magnetic amplifier. 2.78 CHAPTER TWO FIGURE 2.87 Constant-current flux resetting test circuit. Low-Induction Tests. Low-induction tests are usually made with bridge equip- ment or with ac potentiometers, mentioned previously (see ASTM A-343).Very use- ful information over a broad range of inductions, extending to extremely low inductions, may be obtained with the circuit of Fig. 2.84. Tests below 20 G may be made at 60 Hz, provided adequate isolation with elec- trostatic and magnetic shielding is incorporated into the test equipment. Filters may also be used if necessary to eliminate interference. These measurements normally require amplifiers and electronic equipment, which are supplied from 60-Hz power sources.Without the isolation and shielding, 60-Hz pickup and hum are likely to lead to erroneous test values at this frequency. For this reason, it may be desirable to select test frequencies which are not a multiple of the power frequency; 100 Hz is a commonly used low-level test frequency for such measurements.An audio oscillator may be used directly as a power source at low levels of induction. Core Loss and AC Permeability Tests at Audio and Ultrasonic Frequencies. The methods of testing for 60-Hz core loss and ac permeability as previously described can be expanded for measurements at higher frequencies.As test frequencies go up, many additional problems are introduced, notably instrumentation and power sup- ply. Because of capacity and stray field effects, improved techniques must be employed to obtain dependable test results. The circuit diagram of the 60-Hz test in Fig. 2.81 is usually modified to the form shown in Fig. 2.88. Test samples may be either Epstein strips, tape-wound cores, stamped rings, lam- inations, or special shapes having uniform closed magnetic paths. Restrictions on geometrical shape must be observed for accurate testing.For ring or toroidal shapes, a ratio of mean diameter to radial width of magnetic path of 10 to 1 or greater is desirable. Special test frames are prepared for Epstein strips and usually have primary and secondary winding in the range of 24 to 240 turns. For the lower-frequency range, a standard 60-Hz, 700-turn test frame may be used for convenience. Watch for reso- nant effects, which are likely to appear in this frame. In all test frames where double lap joints are used, the vertical dimension of the frame must be kept as small as pos- sible to avoid unnecessary calculations required to correct for air flux in the pickup coil. If care is used to minimize resistance, interturn or interlayer capacitance, stray pickup, and so on, compensating mutual inductors may be used with these test frames. For the lower frequencies, direct indicating meters of good quality are now avail- able. Because of frequency limitations on these meters, electronic instruments must be used over most of the frequency range. High-quality power sources are essential. They must be capable of maintaining sinusoidal flux for all inductions at which tests will be made. Calculations for this method are essentially the same as for the 60-Hz core loss test method described previously. When incremental permeability measurements are to be made, it will be neces- sary to provide a third winding to supply the dc field. The ac blocking impedance used in the dc circuit must be designed to function effectively for any harmonics pres- ent as well as at the fundamental test frequency. Oscilloscope measurements are frequently made at audio and ultrasonic fre- quencies, particularly on hysteresis loops, peak exciting current,and peak inductions or voltage. When hysteresis loops are being examined under sinusoidal flux condi- tions, the integrator and amplifiers used should have negligible phase shift, and the amplifiers must be capable of passing all harmonics produced in the exciting current MATERIALS 2.79 FIGURE 2.88 Circuit diagram for ac core loss and permeability measurements at audio and ultra- sonic frequencies. without attenuation. When hysteresis loops under sinusoidal exciting currents are being examined,the integrators must also be capable of integration over a wide range of frequencies, particularly when the fundamental frequency involved is rather high. 2.8 CHARACTERISTICS OF PERMANENT MAGNETS* 2.8.1 The Meaning of Magnetic North and South In order to avoid confusion,it is important to have a single, consistent convention for the meaning of magnetic north and south. Of course, it has been known for thou- sands of years that like poles repel each other and opposite poles attract. It was also known for a long time before the nature of magnets was understood that a magnet suspended from a thread or allowed to float on a block of wood or in a ceramic cup would tend to rotate until one of its two poles would point to the earth’s geographic north pole, and the other would turn toward the south. It was not understood, how- ever, that the earth itself was a giant magnet. The pole of the magnet which turned toward the earth’s north pole was called the north-seeking pole, or simply the north magnetic pole of the magnet.This is the ancient and present meaning of a magnetic north pole. Since opposite poles attract, however, it can be seen that the earth’s geo- graphic north pole must be a magnetic south pole! Many have had difficulty with this convention, but it has been so well established over hundreds or thousands of years that it is impossible to change now. If uncertainty exists about which polarity a magnet has, it is not difficult to repeat the old experiment. Hang the magnet on a thread (but not one which has a great deal of twist, as a torque will be exerted on the magnet due to the twist and its weight) or float it in a nonmetallic cup, away from any steel object (such as a steel basin or belt buckle), and if the direction of geo- graphic north is known, the polarity will follow. Flux Density B and Coercivity H. Two important properties of permanent magnet materials are the flux density (also called the magnetic induction) B and the coerciv- ity (also somewhat ambiguously referred to as the magnetic field strength) H. These two quantities are related, exist at every point in the magnet and its surroundings, and in general vary from one position to another. They are vectors—that is to say, each has a scalar (i.e., a number) value attached to it and also a direction. In free space the two have the same direction at a given point and are related by a simple constant called the permeability of free space µ 0 , but within a magnet the relationship is more complicated. In some materials the two do not even have the same direction. These two quantities are fundamentally different, the flux density playing a similar role in magnetic circuits as current (per-unit area) in electrical circuits, and the coer- civity of magnetic circuits resembling the electrical voltage (per-unit length). B =µ 0 H in free space or, practically, in air, plastic, etc. In most materials which are not more magnetic than space, including air, organic substances such as plastic and wood, and most metals, however, the magnetic per- meability is almost indistinguishable from that of free space. It is frequently convenient to specify permeability not in absolute units but in relationship to that of free space. This is defined as the relative permeability µ r : 2.80 CHAPTER TWO *Section contributed by Joseph J. Stupak, Oersted Technologies. [...]... Alnico 5 930 ° 32 00 5400 Alnico 5-7 930 ° 5700 5000 1 630 ° Alnico 5DG 930 ° 35 00 5200 1 630 ° Alnico 6 930 ° 38 00 230 00 50 1610° Alnico 8 932 ° 8000 10000 50 1500° Alnico 9 932 ° 7500 7000 50 1 630 ° 10000 10000 10000 10000 10000 4000 4000 4000 Samarium cobalt SmCo5 480° 30 000 5800 55 134 0° Sm2Co17 572° 35 000 2800 86 1472° Sm2Co17 bonded 30 2° 20000 2800 0. 43 × 106 Neodymium-iron Neo 35 30 2° Neo 40 32 9° Neo 45 32 9°... Ceramic 1 1.0 /3. 3 Ceramic 5 3. 4/2.5 Ceramic 7 2.7/4.0 Ceramic 8 3. 5 /3. 1 Ceramic 0.4 /3. 5 bonded Br, G Hc, Oe 4800 4400 1.4 1.65 1 .35 1.45 5.5 7.5 6.5 3. 65 6.75 10.5 7100 7750 6400 6000 12700 134 00 133 00 10500 9000 10500 450 580 560 660 640 740 685 760 1600 1500 1 3. 6 3. 3 4 .3 0.4 230 0 39 50 39 50 430 0 2450 Hci Relative recoil perme- Density, ability lb/in3 Mechanical state 2.200 0 .31 1 Ductile 640 740 670 800... max 10 3 10 3 (T⋅m)/ (T⋅m)/ kA/m Oe kA/m G/Oe kA G/Oe kA Induction at max energy product G mT Oe 5.50 6.50 3. 90 8.10 43. 8 51.7 30 .0 64.5 12,500 13, 200 10,800 13, 700 1250 132 0 1080 137 0 30 00 30 00 30 00 30 00 240 240 240 240 640 675 750 740 51 54 60 59 3. 7 2.4 5.6 2.0 4.6 3. 0 7.0 2.5 19.0 18.5 14.0 18.0 24.0 10,000 1000 23. 0 11,000 1100 17.5 7,400 740 22.5 12,000 1200 G mT 8.10 64.5 13, 200 132 0 30 00 240... 12000 1.050 1.050 1.050 0 .30 0 0 .30 0 0.252 Brittle Brittle Somewhat brittle Neodymium-iron Neo 35 35 /14 Neo 40 40/12 Neo 45 45/17 Neodymium 10/ bonded 35 40 45 10 1 230 0 12900 133 75 7400 1 130 0 12100 130 00 6000 14000 12000 17000 1.090 1.090 1.090 1.200 0.270 0.271 0.271 0. 230 Brittle Brittle Brittle Somewhat brittle Material names Max suggested use temp, °F Coercive field to Electrical magnetize Tensile... 770° 30 00 1 230 00 Alnico Alnico 1 Alnico 2 840° 840° 2250 2900 4000 30 00 65 1 430 ° 1500° Alnico 3 840° 2800 12000 60 1900° Alnico 4 840° 33 00 9000 65 1 430 ° Composition Copper, nickel, iron Aluminum, nickel, cobalt Aluminum, nickel, cobalt Aluminum, nickel, cobalt Notes 2.89 MATERIALS TABLE 2.11 Properties of Magnetic Materials (Continued) Material names Max suggested use temp, °F Coercive field to Electrical... (Courtesy of Arnold Engineering Company.) TABLE 2.14 Magnetic and Physical Properties (Typical Values) Max energy, product Bd × Hd MGOe kJ/m3 Anico 8B Alnico 8HE Alnico 8H Alnico 9 5.50 6.00 5.50 10.50 43. 8 47.7 43. 8 83. 6 Residual induction Br G mT 8 ,30 0 830 9 ,30 0 930 7,400 740 11,200 1120 Peak magnetic force Oe 6000 6000 6000 6000 Coercive force Hc kA/m Oe 480 480 480 480 1650 1550 1900 137 5 Permeance... material of the same particle density may be as much as 2.88 CHAPTER TWO TABLE 2.11 Properties of Magnetic Materials Material name MMPA brief designation Cunife Alnico Alnico 1 Alnico 2 Alnico 3 Alnico 4 Alnico 5 Alnico 5-7 Alnico 5DG Alnico 6 Alnico 8 Alnico 9 Max energy product 1 1.4/0.48 1.7/0.58 1 .35 /0.50 5.5/0.64 7.5/0.74 6.5/0.67 3. 9/0.80 5 .3/ 1.9 9.0/1.5 Ceramic (ferrite) 32 50 Ceramic 1 1.0 /3. 3 Ceramic... following are representative of typical commercial magnetic materials (see Tables 2.12–2.71) These curves do not cover all available materials Figures 2.99 through 2. 130 are supplied courtesy of Arnold Engineering Company Curves in Figs 2. 131 through 2.145 are supplied courtesy of Magnequench 2.1 03 MATERIALS FIGURE 2.99 Typical demagnetization curves for alnico 2, 3, and 4 (Courtesy of Arnold Engineering... 1860 1500 7.000 6.400 7.000 4.500 2.200 1.900 2.000 4.200 2.000 1.500 0.249 0.256 0.249 0.2 53 0.265 0 .36 5 0.2 63 0.268 0.2 63 0.264 Brittle Brittle Brittle Brittle Brittle Brittle Brittle Brittle Brittle Brittle 1850 2400 2400 2500 2200 32 50 2500 4000 30 50 35 00 1.100 1.060 1.060 1.060 1.040 0.180 0.178 0.178 0.179 0. 134 Brittle Brittle Brittle Brittle Flexible 480 580 500 Samarium cobalt SmCo5 19/20 Sm2Co17... within the shell of plastic, which is invisibly small in diameter 2.102 CHAPTER TWO Many layers of these spheres (perhaps 30 ) are deposited onto a plastic sheet, which forms a support The support sheet may be on the order of 0.005 in thick, and the layers of spheres may add on the order of 0.002 in to the total thickness When no magnetic field is present, the flakes lie flat in the bottom of their spheres, . 1.0 /3. 3 1 230 0 1850 32 50 1.100 0.180 Brittle Ceramic 5 3. 4/2.5 3. 6 39 50 2400 2500 1.060 0.178 Brittle Ceramic 7 2.7/4.0 3. 3 39 50 2400 4000 1.060 0.178 Brittle Ceramic 8 3. 5 /3. 1 4 .3 430 0 2500 30 50. bonded surface Neodymium-iron Neo 35 30 2° 32 000 12000 150 Neo 40 32 9° 32 000 19200 150 Neo 45 32 9° 32 000 19200 150 Neodymium 30 2° 20000 0. 43 × 10 6 bonded instead of ferrite). It is as difficult. cobalt Alnico 5-7 930 ° 5700 5000 1 630 ° Aluminum, nickel, cobalt Alnico 5DG 930 ° 35 00 5200 1 630 ° Aluminum, nickel, cobalt Alnico 6 930 ° 38 00 230 00 50 1610° Aluminum, nickel, cobalt Alnico 8 932 ° 8000 10000