Robust and low-maintenance - these properties characterize the squirrel-cage induction motor and make it the ideal electrical machine for drive tasks where reliability is paramount. The rotor of this machine consists of a stacked steel sheet package with embedded conductive bars, which are electrically connected at each end by a so-called short-circuit ring. The stator is also of laminated design and carries a polyphase field winding. When supplied with a three-phase current, this generates a rotating magnetic field inside the stator, which passes through the rotor and induces voltages in the conductive bars. The short-circuit rings lead to a current flow on the rotor and thus to a rotor magnetic field. The interaction of the rotor and stator magnetic fields generates torque, which is available at the shaft.

The conductive bars and short-circuit rings are manufactured from aluminum or copper using a die-casting process. Without permanent magnets from limited available resources, the squirrel-cage induction machine is ecologically advantageous compared to the permanent-magnet excited synchronous machine. At the same time, the price fluctuations for aluminum and copper are much lower than for rare earth for permanent magnets.

For smoothing the torque, the conductor rods are not parallel with the shaft but at an angle by stacking the individual layers of the rotor laminations twisted relative to each other. This method leads to inclined slots on the rotor cast with the conductor bars in the subsequent casting process. This sloping of the casting channels leads to problems during the casting process due to the increased flow resistance. As a result, air inclusions, so-called blowholes, occur in the conductor bars and short-circuit rings.

Air inclusions lead to a local increase in electrical resistance, which has a negative effect on the efficiency of the machine. Due to the random distribution of the air inclusions in the conductor bars, there is an uneven current distribution leading to an asymmetry of the rotor magnetic field and thus to an increase in torque ripple. Depending on the size of the blowholes and the particular application, the enhanced torque ripple and the reduced efficiency lead to significant problems.

After the casting process, the production of the active rotor part is completed. During further processing, the surface of the active part gets first smoothed by machining, followed by shrinking onto the rotor shaft and mounting the bearings. Depending on the application, additional components, such as a fan wheel, are mounted. In the final step, the complete rotor is balanced. The finishing process of the castings represents a large part of the manufacturing process. However, the quality of the cast components can only be evaluated after their complete assembly, installation in the stator, and during operation. If an unacceptable torque ripple is detected, the entire rotor is out of use, and the complete reworking process has resulted in unnecessary costs.

A reliable assessment of the casting even before post-processing could therefore save a lot of time and costs. This is where X-ray technology and especially three-dimensional computed tomography can provide support. In a joint project based on a defective asynchronous motor, the Institute for Electrical Energy Conversion (iew) and YXLON International gained interesting insights.

Due to the material density and the complex structures, the inspection was performed with the YXLON FF85 CT system by use of the line detector CTScan 3 and 450 kV. In so-called fan-beam CT, the object is scanned layer by layer in full horizontal rotation of 360°. Although this CT method is considerably more time-consuming than cone-beam CT with a flat-panel detector, it enables very dense material to be penetrated without disturbing artifacts.

Fig. 3: Illustration of fan-beam technology.

Fig. 4: CT volume - overview of the part

Fig. 5 + 6: Cut through the part

The two-dimensional slice images can be viewed individually or reconstructed into a high-resolution, three-dimensional volume, which in turn can be virtually sliced and analyzed at any point.

Porosity is clearly visible in the short-circuit rings as well as in some conductive bars.

Fig. 7: Porosity in the short-circuit ring

Fig. 5: Porosity in the conductive bars

Fig. 6 + 7: CT volume with pore analysis

To what extent such pore occurrences are functionally impairing for which application and where tolerances can be defined must be determined in further investigations.

Even if computed tomography appears costly at first glance and ultrasonic technology is already widely used for series testing of induction machine, imaging CT methods offer more comprehensive and precise test results. Modern software programs enable automated defect detection and can perform automatic evaluations of anomalies within predefined tolerances. It would not only provide the manufacturer with objective assessments of his products, but much more, the inspections could happen in parallel with the manufacturing process without additional effort by experienced specialists. The early determination of the quality of a motor even before further processing would save time, resources, material, and costs, an economic factor that should not be underestimated within the steadily increasing demand for electromobility.

The Institute for Electrical Energy Conversion (iew) was founded in June 2011 at the Department of Electrical Engineering and Information Technology at the University of Stuttgart. The research work of the institute focuses on two main areas: electrical machines and contactless energy transmission. Both fields are among the main topics of electromobility. The scientists at iew are researching the design of electric motors with very high torque density and position-tolerant inductive charging systems. The aim is to develop highly efficient components for electric vehicles of the future.