Patent ID: 6671639
Filing Date: 2003-12-30
Classification: G01P

Abstract:
A method of determining a speed of rotation of a squirrel-cage motor, the method comprising:determining a nominal frequency of the voltage to be supplied to the squirrel-cage motor; determining a pole pair number of the squirrel-cage motor; arranging an electric current measuring device to measure a current taken by the squirrel-cage motor; setting a measurement period length; setting a sampling frequency; collecting samples with the electric current measuring device from the current taken by the squirrel-cage motor at the sampling frequency for the duration of the measurement period length; storing the samples in the memory of a data carrier as measurement data (S1) of a fixed format, determining the rotation speed of the squirrel-cage motor from the measurement data (S1) by determining a base frequency of the current taken by the squirrel-cage motor, detecting substantially measurable current values at side frequencies, a first side frequency appearing at a frequency formed of the base frequency from which is subtracted a rotation speed frequency, and a second side frequency appearing at a frequency formed of the base frequency to which is added the rotation speed frequency, whereby by estimating the side frequencies in which the measurable current values appear, the rotation speed frequency of the squirrel-cage motor can be determined by subtracting the first side frequency from the second side frequency and by dividing the difference thus obtained by two, and determining the rotation speed frequency of the squirrel-cage motor by estimating the side frequencies from measurement data (S2) in the time domain by using a maximum likelihood estimate calculated by maximizing a maximum likelihood function adjusted to the measurement data (S2), the side frequencies being found at frequency points where the maximum likelihood function obtains a maximum value.