Method for the diagnostics of electromechanical system based on impedance analysis
The present invention is concerned with a method for diagnosing the state of electromechanical systems in which electrical rotating machinery is used on the basis of analysis of impedance estimated from at least two currents and two voltages, measured during an operation of the electromechanical system. The method may be especially useful in the condition monitoring of electric motors and generators. The invention combines the information from both the voltage and current signals measurable at the motor terminals. Specifically, the measurements of voltage and current from two or more phases of a polyphase electrical machine are combined to estimate the impedance of the machine, impedance being the resistance to the flow of current that a circuit exhibits when a voltage is applied to it.
1. A method for monitoring a condition of an electromechanical system in which an electrical rotating machine is used to measure currents and voltages of more than one phase to be more sensitive to and prevent insensitivity to developing faults, comprising the steps of:
providing an electrical rotating machine supplied with electrical power by alternating current supply cables;
connecting a measuring device with an analog to digital convertor to said alternating current supply cables;
measuring at least two phase currents and two phase voltages for the same respective phases supplying the electrical rotating machine with said analog to digital convertor;
processing the measured phase currents and phase voltage values using a computer processing device connected to said analog to digital convertor, wherein said computer processing device contains a processor and a processing module which are adapted to transform the measured phase current and phase voltage values from the time domain to the frequency domain, creating voltage spectrum signals V 1 F , V 2 F , V 3 F , . . . Vn F and current spectrum signals I 1 F , I 2 F , I 3 F , . . . In F ;
processing in said computer processing device the current spectrum signals I 1 F , I 2 F , I 3 F , . . . In F to obtain an inverted current spectra I 1 F,inv , I 2 F,inv , I 3 F,inv , . . . In F,inv ;
processing in said computer processing device the inverted current spectra I 1 F,inv , I 2 F,inv , I 3 F,inv , . . . In F,inv to obtain impedance spectrum signals Y 1 F , Y 2 F , Y 3 F , . . . Yn F of the rotating machine;
extracting a vector of amplitudes at frequencies of interest from the impedance spectrum signals with an analyzer contained in said computer processing device which are needed to diagnose the electromechanical system;
calculating a maximum value of a difference between the amplitudes at frequencies of interest extracted from the impedance spectrum signals in said computer processing device;
comparing a maximum value of the difference between the amplitudes at frequencies of interest extracted from the impedance spectrum signals with a threshold which is given as a limit in said computer processing device so as to reduce the possibility of missed alarms; and
indicating an alarm to the user when the limit is exceeded.
2. A method according to claim 1 , characterized in that the vector of amplitudes frequency of interest is a third harmonic of a supply frequency on said alternating current supply cables.
3. A method according to claim 2 , characterized in that the vector of amplitudes at frequency of interest indicate the presence of an eccentricity in the electromechanical system.
4. A method according to claim 1 characterized in that the electrical rotating machine in the electromechanical system is a motor or a generator.
5. A system for monitoring a condition of an electromechanical system in which electrical rotating machinery is used to detect development of electrical faults in the electromechanical system by measuring currents and voltages of more than one phase to be more sensitive to and prevent insensitivity to developing faults, the system comprising:
an electrical rotating machine supplied with electrical power by alternating current supply cables;
means for measuring at least two phase currents and two phase voltages for the same respective phases supplying a rotating machine, wherein said means for measuring includes an analog to digital convertor connected to said alternating current supply cables;
a computer processing device that receives the measured phase current and phase voltage values and calculates impedance spectrum signals Y 1 F , Y 2 F , Y 3 F , . . . Yn F of the rotating machine;
an analyzer module for extracting data of interest from the impedance spectrum signals;
an alarm to indicate a fault to the user, wherein said computer processing device:
calculates the maximum value of the difference between the amplitudes at frequencies of interest extracted from the impedance spectrum signals; and
compares a maximum value of the difference between the amplitudes at frequencies of interest extracted from the impedance spectrum signals with a threshold which is given as a limit so as to reduce the possibility of missed alarms.