Handbook of Small Electric Motors MAZ Part 7 ppsx

CHAPTER 2 MATERIALS Chapter Contributors Allegheny-Teledyne Joseph H. Bularzik Francis Hanejko Robert R. Judd Harold R. Kokal Robert F. Krause Phelps Dodge Company Joseph J. Stupak United States Steel Corporation William H. Yeadon 2.1 The purpose of this chapter is to assist in the selection of materials used in electric motors. Material choices are largely a function of the motor’s application.All materials commonly used in electric motors are covered in this chapter, including lamination steel, magnets, wire, and insulation. 2.1 MAGNETIC MATERIALS* 2.1.1 Steel Selection Steel is used in most electric motors as the primary flux-carrying member. It is used in stator cores, rotor cores, armature assemblies, field assemblies, housings, and shafting. It may be solid, laminated, or in powdered iron forms. Magnetic properties vary with the type being used. This section will cover the magnetic and mechanical properties of these steels. By way of review from Chap. 1: A rectangular block of magnetic material is wound with a coil of wire, as in Fig. 2.1. If the coil of wire in Fig. 2.1 gradually has its current increased from zero, a magnetizing force Ᏺ will be produced. The block of steel will be subjected to a magnetic field intensity H. *Section contributed by William H. Yeadon,Yeadon Engineering Services, PC. This field intensity is proportional to the current times the number of turns of wire per inch of magnetic material being magnetized: H = (A ⋅ turns)/in or A/m (2.1) As H is increased, there is a flux established in the block of material. Since the area of the block is known, the flux density is: B = lines/in 2 ,W/m 2 , or T (2.2) As the current increases, the flux density B is increased along the virgin magnetization curve shown in Fig.2.2. Eventually B will be increased only as if the steel were air. This is called the saturation point of the material. As the applied field is decreased, the flux density B is decreased, but at zero H some B r (residual flux density) still exists. To drive B to zero it is necessary to drive H negative and hold it at this value. If H is driven negative so that it is numerically equal to +H, the hysteresis loop shown in Fig. 2.2 would exist.The H required to overcome B r results in losses in magnetic circuits where the flux is continually reversed. These losses are commonly referred to as hysteresis losses. Since in most electric motors the material is alternately magnetized and demag- netized, a changing field exists. Steinmetz defines hysteresis power loss as: P hys =σ h f B 1.6 W/lb (2.3) φ ᎏ area Ᏺ ᎏ ᐉ 2.2 CHAPTER TWO FIGURE 2.1 Magnetization of materials. where B = flux density f = frequency, Hz = (number of poles × r/min) ÷ 120 σ h = constant based on the quality of the iron and its volume and density Richter predicts hysteresis power loss as: P hys =σ h ΂΃ 2 W/lb (2.4) In addition, a changing magnetic field induces voltages in conductors moving relative to the field. If a completed electrical path exists, currents will be set up in the conductor, limited only by the resistance of the conductor material. These currents are referred to as eddy currents and they cause unwanted power losses.In the case of electric motors, eddy current losses in the cores become significant. Stator cores are laminated to reduce eddy current losses. Richter determines eddy power losses as: P eddy =σ e ΂ ∆ ΃ 2 W/lb (2.5) B ᎏ 64,500 f ᎏ 60 B ᎏ 64,500 f ᎏ 60 MATERIALS 2.3 FIGURE 2.2 Hysteresis curves of magnetic material. where σ e = constant based on the quality of the iron, containing an element of resistivity of the material and the material density ∆=thickness of the laminations f = frequency B = flux density These losses (hysteresis and eddy) are added together and called core losses. In practice, iron losses are derived from curves supplied by the steel manufacturers. Units are in watts per pound or watts per kilogram of steel. Hysteresis losses are reduced by improving the grade of steel and by annealing the laminations. Annealing the laminations changes the grain structure of the steel to allow for easy magnetization. Eddy current losses are reduced by using thinner laminations and increasing the resistivity of the steel.Adding silicon to steel reduces eddy losses but increases die wear during punching because silicon increases steel hardness. As a general rule, as the grade number increases, the induction level increases and the core loss increases, but cost goes down. For example, see Table 2.1. 2.2 LAMINATION STEEL SPECIFICATIONS* All motor designs must eventually be brought to production to achieve their final goal. Most motor producers want a minimum of two steel suppliers for a given lamination type.This means that someone has to find more than one steel sheet supplier that can provide the same magnetic quality and punchability. U.S. domestic suppliers do not make this a simple task.They typically have an in-house name for their steel grades that is little help in inferring magnetic quality. The old American Iron and Steel Institute (AISI) electrical steel M series is an example. AISI abandoned this series as an industry standard in 1983 when they published their last Electrical Steels steel products manual. However, the grade designation still exists in the Armco and WCI product lines and in older Temple steel material specifications, but all three specifications having the same M number may not have the same magnetic charac- teristics. The American Society for Testing and Materials (ASTM) has attempted to unify steel specifications by means of a universal naming system that is published in ASTM specification A664. The result is a mixed-unit alphanumeric string, such as 47S200, where the first two numbers are the sheet thickness in millimeters times 100, the next letter is a steel-grade annealing treatment and testing procedure designation, and the next three numbers are the core loss in watts per pound divided by 100. If the core loss is given in watts per kilogram instead of watts per pound, an “M” is appended to the string to indicate a metric core loss measurement. Because of the 2.4 CHAPTER TWO *Sections 2.2 and 2.3 contributed by Robert R. Judd, Judd Consulting. TABLE 2.1 Comparison of Steel Grades Material type Flux density B @ 100 Oe W/lb @ 18 kG Relative cost/lb M-19 17.5 kG 3.0 Higher M-50 17.8 kG 4.4 Medium Low-carbon CRS 18.5 kG 6.0 Lower mixed units, this effort is not intellectually pleasing, but no one can deny the overall need for it. Many foreign manufacturers and standardization bodies have recognized the need for meaningful electrical sheet specifications and have adopted a specification name similar to that of the ASTM effort. All specifications have much more magnetic property detail than can be conveyed in an identifying name. For instance, no permeability or magnetization curve shape is indicated in the steel name, but some indications of minimum permeability or minimum induction at a designated magnetizing field will be given in the specification detail. The punchability of steel sheet with identical magnetic quality from two different suppliers is rarely the same. This forces the press shop to have a set of dies for each steel supplier of a given part.This can raise the costs of keeping several steel suppliers for one part.Also,the subtleties of producing flat,round laminations from a large sheet usually involve a trial-and-error procedure for the die shop. This means that parts for which there are multiple steel suppliers are a multiple headache for the die shop. The information in Table 2.1 illustrates the M-grade (motor grade) steels catego- rization system. Magnetic properties are given in a variety of units.The conversion chart in Table 2.2 is provided for convenience. Laminated cores are normally considered because of the necessity of reducing the core losses which occur at high switching frequencies.There are, however, some applications where low cost is a higher priority than efficiency. In these cases powdered metal cores may be considered. Their induction levels are similar to those of annealed sheet steel, but the core losses may be four to five times greater.There are some recent advances in powdered iron that make them suitable for these applications.They are discussed in a later section. The following figures show magnetic property curves of several materials. Note that many of the scales are in different units. The new Temple product description was created to simplify material selection. Each description incorporates the gauge, material family, and maximum core loss into a concise, six-character label. The first two characters in the new description indicate the thickness of the material, for example, 29 for 0.014 in thick. The third character is a letter which indicates the material family, such as “G” for grain ori- MATERIALS 2.5 TABLE 2.2 Electromagnetic Unit Systems Quantity Symbol MKS CGS English Flux B Teslas (Webers/m 2 ) Gauss Kilolines/in 2 density 1 T = 6.452×10 4 lines/in 2 1 G = 6.452 lines/in 2 1 kline/in 2 = 0.155 kG 1 T = 10 4 G 1 kline/in 2 = 1.55×10 −2 T Magnetic H Amps/m (A⋅T/m) Oersted Amps/in (A⋅T/in) field 1 A⋅T/m = 0.01257 Oe 1 Oe = 2.021 A⋅T/in 1 A⋅T/in = 0.4947 Oe intensity Magnetic φ Webers Maxwells Kilolines flux 1 Wb = 10 8 maxwells 1 maxwell = 1 line 1 kline = 10 −5 Wb Reluctance ᏾ Henries/m Henries/cm H/in Permeability µ 0 4π*10 −7 Wb/(A⋅T/m) 1 maxwell/(Gb/cm) 3.19 lines/(A⋅T⋅in) of free space ented,“N”for nonoriented,and “T”for Tempcor. The last three characters define the material’s maximum core loss. The inclusion of the maximum core loss in the product description eliminates the need to cross-index the M grade with a core loss chart.To illustrate the system,26N174 is the description of 26-gauge, nonoriented silicon steel with a maximum core loss of 1.74 W/lb. Figures 2.3 through 2.33 show typical properties of magnetic motor steels, courtesy of Temple Steel Company. Allegheny-Teledyne company also produces alloy steels with varying properties for motor applications. Figures 2.34 through 2.39 show typical properties of nickel- iron alloys and steels, courtesy of Allegheny-Teledyne Company. Figures 2.40 through 2.59 show typical properties of nonoriented silicon steels. 2.3 LAMINATION ANNEALING The type of annealing to be discussed here is the final annealing of laminations punched from semiprocessed electrical sheets . Other types of annealing that enhance the quality of laminations are the stress relief annealing of laminations punched from fully processed electrical sheet and the annealing of hot band coils before cold rolling . Stress relief annealing is done to flatten laminations and to recrystallize the crystals damaged during punching . T his damage extends from the punched edge to a distance from the edge equal to the sheet thickness , and it severely degrades the magnetic quality of the affected volume . In a small motor , this 2.6 CHAPTER TWO can be an appreciable percentage of the lamination teeth cross section.Because the teeth carry a very high flux density, punching damage can severely reduce small motor efficiency. The annealing of hot band coils is done in the producing steel mill on high-quality lamination sheet,primarily to enhance permeability. FIGURE 2.3 B-H magnetization loops for 29G066 75–25% (29 06). Values based on ASTM 596 and A773; 75 percent parallel grain and 25 percent cross grain after annealing. Click for high quality image MATERIALS 2.7 FIGURE 2.4 B-H magnetization loops for 29G066 100% (29 06). Values based on ASTM 596 and A773; 100 percent parallel grain after annealing. FIGURE 2.5 B-H magnetization loops for 26N174, 26T214, 26T265, and 24T240. Typical values based on ASTM 596 and A773; half parallel and half cross grain after annealing. Click for high quality image 2.8 CHAPTER TWO FIGURE 2.6 29G066 75–25% (0.99 W/lb maximum 29 06). Typical values based on Epstein samples; 75 percent parallel grain and 25 percent cross grain at 60 Hz after annealing. FIGURE 2.7 29G066 100% (0.66 W/lb maximum 29 06).Typical values based on Epstein samples; 100 percent parallel grain at 60 Hz after annealing. MATERIALS 2.9 FIGURE 2.8 29G066 (29 06), 29N145 (29 15), 26N174 (26 19), and 24N208 (24 19). Typical core loss values,W/lb,based on Epstein samples (ASTM A343); half parallel and half cross grain (except where noted) at 60 Hz after annealing. FIGURE 2.9 26T214 (26 50), 26T265 (26 55), 24T284 (24 50), 24T352 (24 55), and 24T420 (24 56). Typical core loss values,W/lb, based on Epstein samples (ASTM A343); half parallel and half cross grain at 60 Hz after annealing. 2.10 CHAPTER TWO FIGURE 2.10 24N208 (2.08 W/lb maximum 24 19). Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing. FIGURE 2.11 24N218 (2.18 W/lb maximum 24 22). Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing. [...]... Induction and permeability of vanadium permendur 2.25 2.26 FIGURE 2.35 CHAPTER TWO Core loss and apparent core loss of 0.006-in vanadium permendur MATERIALS FIGURE 2.36 Core loss and apparent core loss of 0.008-in vanadium permendur 2. 27 2.28 FIGURE 2. 37 CHAPTER TWO Core loss and apparent core loss of 0.010-in vanadium permendur MATERIALS FIGURE 2.38 Core loss and apparent core loss of 0.012-in vanadium... annealing FIGURE 2.15 26N 174 (1 .74 W/lb maximum 26 19) Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing MATERIALS 2.13 FIGURE 2.16 26N185 (1.85 W/lb maximum 26 22) Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing FIGURE 2. 17 26N190 (1.90 W/lb maximum 26 27) Typical magnetization... units 2.31 2.32 CHAPTER TWO FIGURE 2.42 Magnetization curves for electrical grade (AISI M-36), metric units FIGURE 2.43 Magnetization curves for electrical grade (AISI M-36), English units MATERIALS FIGURE 2.44 Magnetization curves for dynamo grade (AISI M- 27) , metric units FIGURE 2.45 Magnetization curves for dynamo grade (AISI M- 27) , English units 2.33 2.34 CHAPTER TWO FIGURE 2.46 Magnetization... MATERIALS 2.21 FIGURE 2.30 Core loss versus frequency for 29G066 (29 06); 75 percent parallel grain and 25 percent cross grain 2.22 CHAPTER TWO FIGURE 2.31 Exciting power versus frequency for 29G066 (29 06); 75 percent parallel grain and 25 percent cross grain MATERIALS FIGURE 2.32 cross grain 2.23 Core loss versus frequency for 26N 174 (26 19); half parallel grain and half 2.24 CHAPTER TWO FIGURE 2.33...MATERIALS 2.11 FIGURE 2.12 24N225 (2.25 W/lb maximum 24 27) .Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing FIGURE 2.13 24N240 (2.40 W/lb maximum 24 36) Typical magnetization curves based on... grain at 60 Hz after annealing FIGURE 2.23 24T352 (3.52 W/lb maximum 24 55) Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing MATERIALS 2. 17 FIGURE 2.24 24T420 (4.20 W/lb maximum 24 56) Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing FIGURE 2.25 26T214 (2.14 W/lb maximum 26... after annealing 2.18 CHAPTER TWO FIGURE 2.26 26T265 (2.65 W/lb maximum 26 55) Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing FIGURE 2. 27 26T330 (3.30 W/lb maximum 26 56) Typical magnetization curves based on Epstein samples; half parallel and half cross grain at 60 Hz after annealing MATERIALS FIGURE 2.28 Core loss versus frequency for... metric units FIGURE 2.45 Magnetization curves for dynamo grade (AISI M- 27) , English units 2.33 2.34 CHAPTER TWO FIGURE 2.46 Magnetization curves for dynamo special grade (AISI M-22), metric units FIGURE 2. 47 Magnetization curves for dynamo special grade (AISI M-22), English units MATERIALS FIGURE 2.48 Magnetization curves for super dynamo grade, metric units FIGURE 2.49 Magnetization curves for super dynamo . purpose of this chapter is to assist in the selection of materials used in electric motors. Material choices are largely a function of the motor’s application.All materials commonly used in electric. press shop to have a set of dies for each steel supplier of a given part. This can raise the costs of keeping several steel suppliers for one part. Also,the subtleties of producing flat,round. number of turns of wire per inch of magnetic material being magnetized: H = (A ⋅ turns)/in or A/m (2.1) As H is increased, there is a flux established in the block of material. Since the area of