This article is about the design of a measurement system for measuring iron losses in soft magnetic composites under controlled circular flux density patterns. With conventional measurement setups for iron loss determination, material samples are usually magnetized along a single spatial direction. However, in some parts of electrical machines magnetization loci over time substantially divert from the patterns used during conventional alternating loss measurements. Based on previous findings, iron losses strongly depend on the actual two dimensional magnetization locus over time. Therefore, the measurement with alternating magnetization cannot fully reflect the magnetizing conditions and iron losses present during machine operation. Available data especially for soft magnetic composites (SMC) regarding rotational losses is very limited. Hence, in this article a new measurement setup for the investigation of iron losses under controlled circular flux density loci was designed. It turns out, that iron losses show a significant deviation when comparing alternating and rotational losses with regard to the flux density magnitude and frequency.