Handbook of Small Electric Motors MAZ Part 7 ppsx

The inclusion of the maximum core loss in theproduct description eliminates the need to cross-index the M grade with a core losschart.To illustrate the system,26N174 is the description o

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 electricmotors Material choices are largely a function of the motor’s application All mate-rials commonly used in electric motors are covered in this chapter, including lami-nation 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, andshafting It may be solid, laminated, or in powdered iron forms Magnetic propertiesvary with the type being used This section will cover the magnetic and mechanicalproperties of these steels

By way of review from Chap 1: A rectangular block of magnetic material iswound with a coil of wire, as in Fig 2.1 If the coil of wire in Fig 2.1 gradually has itscurrent increased from zero, a magnetizing force  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 ofwire per inch of magnetic material being magnetized:

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/in2, W/m2, or T (2.2)

As the current increases, the flux density B is increased along the virgin zation 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 den-

magneti-sity) 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 rresults in losses inmagnetic 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 netized, a changing field exists

demag-Steinmetz defines hysteresis power loss as:

where B=flux density

f=frequency, Hz =(number of poles ×r/min) ÷120

σh=constant based on the quality of the iron and its volume and densityRichter predicts hysteresis power loss as:

Phys= σh  2

In addition, a changing magnetic field induces voltages in conductors moving tive to the field If a completed electrical path exists, currents will be set up in theconductor, limited only by the resistance of the conductor material These currents

rela-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 determineseddy power losses as:

f

60

FIGURE 2.2 Hysteresis curves of magnetic material.

whereσe=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 ers Units are in watts per pound or watts per kilogram of steel

manufactur-Hysteresis losses are reduced by improving the grade of steel and by annealingthe laminations Annealing the laminations changes the grain structure of the steel

to allow for easy magnetization Eddy current losses are reduced by using thinnerlaminations and increasing the resistivity of the steel Adding silicon to steel reduceseddy losses but increases die wear during punching because silicon increases steelhardness

As a general rule, as the grade number increases, the induction level increasesand 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 finalgoal Most motor producers want a minimum of two steel suppliers for a given lam-ination type This means that someone has to find more than one steel sheet supplierthat 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 steelgrades that is little help in inferring magnetic quality The old American Iron andSteel 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 andWCI product lines and in older Temple steel material specifications, but all threespecifications having the same M number may not have the same magnetic charac-teristics

The American Society for Testing and Materials (ASTM) has attempted to unifysteel specifications by means of a universal naming system that is published inASTM specification A664 The result is a mixed-unit alphanumeric string, such as47S200, 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 designa-tion, 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” isappended to the string to indicate a metric core loss measurement Because of the

TABLE 2.1 Comparison of Steel Grades

Material type Flux density B @ 100 Oe W/lb @ 18 kG Relative cost/lb

mixed units, this effort is not intellectually pleasing, but no one can deny the overallneed for it.

Many foreign manufacturers and standardization bodies have recognized theneed for meaningful electrical sheet specifications and have adopted a specificationname similar to that of the ASTM effort All specifications have much more mag-netic property detail than can be conveyed in an identifying name For instance, nopermeability or magnetization curve shape is indicated in the steel name, but someindications of minimum permeability or minimum induction at a designated magne-tizing field will be given in the specification detail

The punchability of steel sheet with identical magnetic quality from two differentsuppliers is rarely the same This forces the press shop to have a set of dies for eachsteel supplier of a given part This can raise the costs of keeping several steel suppli-ers for one part.Also, the subtleties of producing flat, round laminations from a largesheet usually involve a trial-and-error procedure for the die shop This means thatparts for which there are multiple steel suppliers are a multiple headache for the dieshop

The information in Table 2.1 illustrates the M-grade (motor grade) steels rization system

catego-Magnetic properties are given in a variety of units The conversion chart in Table2.2 is provided for convenience

Laminated cores are normally considered because of the necessity of reducingthe core losses which occur at high switching frequencies There are, however, someapplications where low cost is a higher priority than efficiency In these cases pow-dered metal cores may be considered Their induction levels are similar to those ofannealed sheet steel, but the core losses may be four to five times greater There aresome recent advances in powdered iron that make them suitable for these applica-tions They are discussed in a later section

The following figures show magnetic property curves of several materials Notethat 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 lossinto a concise, six-character label The first two characters in the new descriptionindicate the thickness of the material, for example, 29 for 0.014 in thick The thirdcharacter is a letter which indicates the material family, such as “G” for grain ori-

TABLE 2.2 Electromagnetic Unit Systems

density 1 T =6.452×104lines/in2 1 G =6.452 lines/in2 1 kline/in2=0.155 kG

1 T =104G 1 kline/in2=1.55×10−2TMagnetic 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 Oeintensity

flux 1 Wb =108maxwells 1 maxwell =1 line 1 kline =10−5Wb

Permeability µ0 4π*10−7Wb/(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 definethe material’s maximum core loss The inclusion of the maximum core loss in theproduct description eliminates the need to cross-index the M grade with a core losschart.To illustrate the system,26N174 is the description of 26-gauge,nonoriented sil-icon steel with a maximum core loss of 1.74 W/lb.

Figures 2.3 through 2.33 show typical properties of magnetic motor steels, tesy of Temple Steel Company

cour-Allegheny-Teledyne company also produces alloy steels with varying propertiesfor 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 laminationspunched from semiprocessed electrical sheets Other types of annealing thatenhance the quality of laminations are the stress relief annealing of laminationspunched from fully processed electrical sheet and the annealing of hot band coilsbefore cold rolling Stress relief annealing is done to flatten laminations and torecrystallize the crystals damaged during punching This damage extends from thepunched edge to a distance from the edge equal to the sheet thickness, and itseverely degrades the magnetic quality of the affected volume In a small motor, this

can be an appreciable percentage of the lamination teeth cross section.Because theteeth carry a very high flux density, punching damage can severely reduce smallmotor 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 ples; 75 percent parallel grain and 25 percent cross grain at 60 Hz after annealing.

sam-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.

MATERIALS 2.19

FIGURE 2.28 Core loss versus frequency for 29G066 (29 06).

2.20 CHAPTER TWO

FIGURE 2.29 Exciting power versus frequency for 29G066 (29 06); 100 percent parallel grain.

MATERIALS 2.21

FIGURE 2.30 Core loss versus frequency for 29G066 (29 06); 75 percent parallel grain and 25 cent cross grain.

per-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 2.23

FIGURE 2.32 Core loss versus frequency for 26N174 (26 19); half parallel grain and half cross grain.

2.24 CHAPTER TWO

FIGURE 2.33 Exciting power versus frequency for 29G066 (29 06); half parallel grain and half cross grain.

MATERIALS 2.25

FIGURE 2.34 Induction and permeability of vanadium permendur.

2.26 CHAPTER TWO

FIGURE 2.35 Core loss and apparent core loss of 0.006-in vanadium permendur.

MATERIALS 2.27

FIGURE 2.36 Core loss and apparent core loss of 0.008-in vanadium permendur.

2.28 CHAPTER TWO

FIGURE 2.37 Core loss and apparent core loss of 0.010-in vanadium permendur.

MATERIALS 2.29

FIGURE 2.38 Core loss and apparent core loss of 0.012-in vanadium permendur.

2.30 CHAPTER TWO

FIGURE 2.39 DC hysteresis loop for vanadium permendur.

MATERIALS 2.31

FIGURE 2.40 Magnetization curves for armature grade (AISI M-43), metric units.

FIGURE 2.41 Magnetization curves for armature grade (AISI M-43), English units.

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 2.33

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.

2.34 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 2.35

FIGURE 2.48 Magnetization curves for super dynamo grade, metric units.

FIGURE 2.49 Magnetization curves for super dynamo grade, English units.

2.36 CHAPTER TWO

FIGURE 2.50 Magnetization curves for transformer C grade (AISI M-19), metric units.

FIGURE 2.51 Magnetization curves for transformer C grade (AISI M-19), English units.

MATERIALS 2.37

FIGURE 2.52 Magnetization curves for transformer A grade (AISI M-15), metric units.

FIGURE 2.53 Magnetization curves for transformer A grade (AISI M-15), English units.

2.38 CHAPTER TWO

FIGURE 2.54 Core loss of 29-gauge silicon-iron electrical steels at 60 cps.

MATERIALS 2.39

FIGURE 2.55 Core loss of 26-gauge silicon-iron electrical steels at 60 cps.

2.40 CHAPTER TWO

FIGURE 2.56 Core loss of 24-gauge silicon-iron electrical steels at 60 cps.

MATERIALS 2.41

FIGURE 2.57 Core loss of 29-gauge silicon-iron electrical steels at 50 cps.

2.42 CHAPTER TWO

FIGURE 2.58 Core loss of 26-gauge silicon-iron electrical steels at 50 cps.

MATERIALS 2.43

FIGURE 2.59 Core loss of 24-gauge silicon-iron electrical steels at 50 cps.

2.3.1 Stator Lamination Annealing

Semiprocessed lamination sheet is received from the producing mill in the heavilytemper-rolled condition This condition enhances the punchability of the sheet andprovides energy for the metallurgical process of grain growth that takes place duringthe annealing treatment Annealing of the laminations is done for several reasons.Among them are the following

Cleaning. Punched laminations carry some of the punching lubricant on their faces This can be a water-based or a petroleum-based lubricant It must be removedbefore the laminations enter the high-temperature zone of the annealing furnace toavoid sticking and carburization problems This is done by preheating the lamina-tions in an air or open-flame atmosphere to 260 to 427°C (500 to 800°F)

sur-Carbon Control. Carbon in solution in steel can form iron carbides during mill cessing, annealing, and electromagnetic device service These carbides have severaleffects on properties—all detrimental They affect metallurgical processing in theproducing mill, degrading permeability and, to some extent, core loss They pin grainboundaries during annealing, slowing grain growth They pin magnetic domain walls

pro-in devices, pro-inhibitpro-ing magnetization and thus pro-increaspro-ing core losses and magnetizpro-ing

current If the carbides precipitate during device use, the process is called aging.

Because of these problems, the amount of carbon is kept as low as is practicalduring mill processing The best lamination steels are produced to carbon contents

of less than 50 ppm Steels of lesser quality can be produced with up to 600-ppm bon, but in the United States, 400 ppm is presently a practical upper limit Laminatedcores cannot run efficiently with these high carbon contents, so the carbon isremoved by decarburization during annealing The annealing atmosphere containswater vapor and carbon dioxide, which react with carbon in the steel to form carbonmonoxide The carbon monoxide is removed as a gas from the furnace This processworks well for low-alloy steels, but for steels with appreciable amounts of silicon andaluminum, the same water vapor and carbon dioxide provide oxygen that diffusesinto the steel, forming subsurface silicates and aluminates These subsurface oxidesimpede magnetic domain wall motion, lowering permeability and raising core loss

car-Grain Growth. The grain diameter that minimizes losses in laminations driven atcommon power frequencies is 80 to 180 µm As the driving frequency increases, thisdiameter will decrease Presently, the temper-rolling percentage and the annealingtime and temperature are designed to achieve grain diameters of 80 to 180 µm

Coating. Laminations punched from semiprocessed steel are uncoated, whilethose punched from fully processed sheet are typically coated at the steel mill with a

core plate coating This coating insulates laminations from each other to reduce

interlamination eddy currents, protects the steel from rust, reduces contact betweenlaminations from burrs, and reduces die wear by acting as a lubricant during stamp-ing The semiprocessed steel laminations are also improved by a coating, but eco-nomics precludes coating them at the steel mill Instead, they are coated at the end