2.1
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 3> will be produced. The block of steel will be subjected to a magnetic field intensity H.
FIGURE 2.1 Magnetization of materials.
This field intensity is proportional to the current times the number of turns of wire 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:
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 Br (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 Br 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 demagnetized, a changing field exists.
Steinmetz defines hysteresis power loss as:
FIGURE 2.2 Hysteresis curves of magnetic material.
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:
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.
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 |