Can Circular Rotational Losses of Non-Oriented Soft Magnetic Materials Be Estimated From Alternating Losses?

Non-oriented (NO) electrical steels are widely used as core materials in rotating electrical machines, whereas for measurements under alternating magnetization (AM), standardized methods exist; measurements under rotational magnetization (RM) prove to represent a significant problem. In this paper, we performed measurements of both circular rotational and alternating losses in rolling and transverse directions for four types of NO materials of different Si contents and thicknesses by means of a hexagonal rotational single-sheet tester at different frequencies and flux density values. The losses were measured by the electrodynamical method and for confirmation also by the thermal (rise-of-temperature) method. The estimated rotational losses by means of established well-known methods are correlated with the measured ones. Literature reports attempt to estimate losses $P^{\mathrm {(RM)}}$ for circular RM on the basis of losses $P^{\mathrm {(AM)}}$ for AM. A loss ratio factor $\Gamma = P^{\mathrm {(RM)}}/P^{\mathrm {(AM)}}$ is defined which is reported to be close to $\Gamma = 2$ . In this paper, $\Gamma $ factors were determined for three different grades of material. The results indicate a rather weak dependence of $\Gamma $ on the grade, however, a strong one on the intensity $B$ of induction. In approximation, $\Gamma = 2$ is valid for rather weak induction of the order $B = 0.6$ T. Induction increases in steps of $\Delta B = 0.1$ T yield decreases of loss ratio by about $\Delta \Gamma = 3$ %. This enables a rough estimation of rotational losses. However, the possibility of estimation is restricted to the circular case of constant modulus and angular velocity of the $B$ vector. Other cases are characterized by different values of both eddy current losses and hysteresis losses, RM involving completely different domain reconstructions compared to AM. For increased frequency, the decrease in $\Gamma $ becomes weaker due to more dominant eddy current losses.