If compressor is without IGV, then there is no excitation force frequency due to IGV. However, there will be diffuser of compressor . If diffuser has vaneless type means plate type then there will not be excitation frequency due to Vaneless diffuser. But if there is vane diffuser plate, then excitation frequency is available which is expressed as 1 * RPM = 1 * Nos of Diffuser vanes * Speed (RPM) / 60 [Hz] and 2 * RPM frequency = 2 * Nos of Diffuser vanes * Speed (RPM) /60.. [Hz]
