MOTOR MAGNETIC CORE LOSSES BASICS AND TUTORIALS

Magnetic core losses consist of the eddy current and hysteresis losses, including the surface losses, in the magnetic structure of the motor. A number of factors influence these losses:

1. The flux density in the magnetic structure is a major factor in determining these magnetic losses. The core loss can be decreased by increasing the length of the magnetic structure and, as a consequence, decreasing the flux density in the core.

This will decrease the magnetic loss per unit of weight but, since the total weight will increase, the improvement in losses will not be proportional to the unit loss reduction.

The decrease in magnetic loading in the motor also decreases the magnetizing current and thus influences the power factor.

2. The magnetic core loss can also be reduced by using thinner laminations in the magnetic structure. Typically, many standard motors use 24-gauge (0.025-in. thick) laminations. By using thinner laminations, such as 26- gauge (0.0185-in. thick) or 29-gauge (0.014-in. thick), the magnetic core loss can be reduced.

The reduction in the magnetic core loss by the use of thinner laminations ranges from 10 to 25%, depending on the method of processing the lamination steel and the method of assembling the magnetic core.

3. There has been considerable progress made by the steel companies to obtain lower magnetic losses in both silicon and cold-rolled (low-silicon) grades of electrical steel. The magnetic core loss (Epstein loss) can be reduced by using silicon grades of electrical steel or the improved grades of cold-rolled electrical steel.

The type of steel used by the motor manufacturer depends on his process capability. The cold-rolled electrical steel requires a proper anneal after punching to develop its electrical properties, whereas the silicon grades of electrical steel are available as fully processed material.

However, because of variables in the processing of the lamination steel into finished motor cores, the reduction in core loss in watts per pound equivalent to the Epstein data on flat strips of the lamination steel is seldom achieved.

Magnetic core loss reductions on the order of 15–40% can be achieved by the use of thinner-gauge
silicon-grade electrical steels. A disadvantage of the higher-silicon lamination steel is that, at high inductions, the permeability may be lower, thus increasing the magnetizing current required. This will tend to decrease the motor power factor.

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