A DC motor consists of two main components: a rotor and a stator. The rotor has a toroidal core with slots for holding the coils or windings. According to Faraday’s law, when the core rotates in a magnetic field, a voltage or electric potential is induced in the coil, and this induced electric potential will cause a current flow, called eddy current.

Eddy currents are the result of the rotation of the core in the magnetic field

Eddy current is a form of magnetic loss, and the power loss due to the flow of eddy current is called eddy current loss. Hysteresis loss is another component of magnetic loss, and these losses generate heat and reduce the efficiency of the motor.

The development of eddy current is influenced by the resistance of its flowing material

For any magnetic material, there is an inverse relationship between the cross-sectional area of the material and its resistance, which means that the decreased area leads to an increase in resistance, which in turn leads to a decrease in eddy currents. One way to reduce the cross-sectional area is to make the material thinner.

This explains why the motor core is made of many thin iron sheets (called electric motor laminations) rather than one large and solid piece of iron sheets. These individual sheets have a higher resistance than one solid sheet, and therefore produce less eddy current and lower eddy current losses.

The sum of eddy currents in the laminated cores is less than that in the solid cores

These lamination stacks are insulated from each other, and a layer of lacquer is usually used to prevent eddy currents “jumping” from stack to stack. The inverse square relationship between material thickness and eddy current loss means that any reduction in thickness will have a significant effect on the amount of loss. Therefore, Gator, a China satisfactory rotor factory, strives to make motor core laminations as thin as possible from the perspective of manufacturing and cost, with modern DC motors typically using laminations of 0.1 to 0.5 mm thick.

Conclusion

The eddy current loss mechanism requires the motor to be stacked with insulating layers of stacks to prevent eddy currents “jumping” from laminations to laminations.