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 characteristics.
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
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 categorization 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-
TABLE 2.2 Electromagnetic Unit Systems
ented,"N">r nonoriented,and "Tf">r Tempcor. The last three characters define the materials' 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 system26N174 is the description of 26-gaugenonoriented silicon steel with a maximum core loss of 1.74W/lb.
Figures 2.3 through 2.33 show typical properties of magnetic motor steels;our-tesy 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