A method for estimating amplitudes of a periodic component in a measured signal, the measured signal corresponding to measurements of current flowing through an electric motor, wherein the method is performed by a control device configured to control a variable speed drive in charge of the electric motor, wherein the control device comprises at least two filters, including a first filter and a second filter, the method comprising the following consecutive operations: - controlling the variable speed drive to inject a motor voltage comprising a control law voltage and an additional voltage component into the electric motor; - receiving said measured signal, the measured signal being analog; - performing a delta-sigma modulation on the received analog signal to obtain a digital signal; - applying (204.1-204.n) the first filter to the product of the digital signal and a first periodic signal to estimate a first amplitude signal among said amplitudes, and applying the second filter to the product of the digital signal and a second periodic signal to estimate a second amplitude signal among said amplitudes, wherein the first and second periodic signals are independent orthogonal functions, and wherein the at least two filters are Finite Impulse Response Filters, being a linear combination of basic blocks M, basic blocks M being iteratively defined by: M 0 f t = f t M k + 1 f t = 1 T ∫ t − T t M k f τ dτ where T is an integer multiple of the period of the periodic signals, and where f is a dummy function variable; - adapting a control law of the electric motor based on at least the first and second amplitude signals, or calculating a monitoring value based on at least the first and second amplitude signals. - controlling the variable speed drive to inject a motor voltage comprising a control law voltage and an additional voltage component into the electric motor; - receiving said measured signal, the measured signal being analog; - performing a delta-sigma modulation on the received analog signal to obtain a digital signal; - applying (204.1-204.n) the first filter to the product of the digital signal and a first periodic signal to estimate a first amplitude signal among said amplitudes, and applying the second filter to the product of the digital signal and a second periodic signal to estimate a second amplitude signal among said amplitudes, wherein the first and second periodic signals are independent orthogonal functions, and wherein the at least two filters are Finite Impulse Response Filters, being a linear combination of basic blocks M, basic blocks M being iteratively defined by: M 0 f t = f t M k + 1 f t = 1 T ∫ t − T t M k f τ dτ where T is an integer multiple of the period of the periodic signals, and where f is a dummy function variable; applying the second filter to the product of the digital signal and a second periodic signal to estimate a second amplitude signal among said amplitudes, wherein the first and second periodic signals are independent orthogonal functions, and wherein the at least two filters are Finite Impulse Response Filters, being a linear combination of basic blocks M, basic blocks M being iteratively defined by: M 0 f t = f t M k + 1 f t = 1 T ∫ t − T t M k f τ dτ where T is an integer multiple of the period of the periodic signals, and where f is a dummy function variable; - adapting a control law of the electric motor based on at least the first and second amplitude signals, or calculating a monitoring value based on at least the first and second amplitude signals.