Stefan Schmitt

BLOCK Transformatoren-Elektronik GmbH & Co. KG

Inductors for PWM operation are usually constructed using ferrite, powder or nanocrystalline material. This is due to their low loss at high frequencies. However, the cost of the inductor is significantly higher in comparison to an inductor made of electrical steel. In regenerative energy these additional costs for fewer losses can be covered easily by the feed-in revenues. In regenerative drives, the cost of the device is more important. Electrical steel also has much more potential to emit acoustical noise. Causes for the noise emissions are described (magnetostriction, forces) and a modification is proposed that does not affect the main inductor design parameters (inductance, temperature rise) and can thus be applied to existing designs. This modification aims on changing the geometry of the inductor by changing the shape of the air gap filler. Usually the filler completely covers all three limbs of a three-phase inductor. Here it is replaced by three smaller pieces only covering a square region in the center. There, only base-frequency flux occurs; the pulse frequency of the PWM cannot penetrate the electrical steel sheets that deep due to eddy currents. The effectiveness of this modification is shown by excerpts of extensive measurements, where an overall reduction of the sound pressure level of 8 dB was achieved, especially at high frequencies.