FFT Analysis During Simulation

 hi every body 
i have a problem understanding the model of power_fft.mdl in simulink.
im using this model as a block to calculate the harmunics of a ups system but i cant understand why is it necessary to  have 3 signals as input to this block (power_fft.mdl)?
im trying to calculate the voltage harmunics in the output of the ups.and my input signal is only one voltage and when i want to change the gain parameters from [1 2 3 ] to [1] or any thing else , the model(power_fft.mdl)returns an error.

Error in port widths or dimensions. Output port 1 of 'power_fft/FFT/Discrete Shift Register' is a one dimensional vector with 40 elements.

and

Error in port widths or dimensions. Input port 1 of 'power_fft/FFT/Reshape' has 120 elements. This port does not accept the dimensions (or orientation) specified by the input signal.

please do help if possible.and tell me how can i change the model or its parameters so that there is only one input and one out put. and my 2nd question is about the output of FFT block, imean f(n) and RMS. how are this numbers explained. thank you every body.