Motors, particularly electrical motors, play a key role in industry. Such motors are used to drive fans, pumps, compressors, valves, and many other machines. It is potentially very costly to allow a significant problem to go on unnoticed in either the motor or the motor driven machine. It is also costly and very time consuming to attempt to repair a nonexistent problem. With present less definitive methods, these costly situations often occur. Thus there is a need for an improved diagnostic method and apparatus as described in the invention for use with motors and motor driven machines.
Present conventional techniques have been developed to monitor motor systems. One commonly used conventional technique involves analyzing the signature developed from a single motor current probe in order to ascertain motor fault conditions. However, there are shortcomings associated with this and other standard motor monitoring techniques which prevent optimal motor analysis.
A first problem with conventional current probe technology for motor system analysis is that current and voltage envelopes are generally estimated using inexact demodulation techniques such as RMS to D.C. conversion or rectification, followed by low-pass filtering. These signal processing methods exhibit a slow response to rapidly changing current or voltage signals since they rely heavily on low-pass filtering. This reliance results in an inaccurate response to highly dynamic current features such as inrush current.
A second problem with such conventional technology for motor analysis is non-linearity in the current probes, especially at the lower part of their detection range. This problem is often dealt with by specifying a lesser accuracy at low currents changing the specification from percent reading to percent full scale, or simply not specifying the accuracy over a portion of the detection range.
A third problem with such conventional technology is the distorting influence of filters, non-simultaneous analog-to-digital converter sampling, and other hardware-induced signal processing errors in which the true phase relation between measurement channels is impaired.
A fourth shortcoming of some conventional methods is the need to take the motor off-line in order to ascertain resistive and inductive imbalance. This is a great inconvenience when dealing with continuous-duty motors. In addition, the off-line measurements may not be truly representative of the on-line resistive or inductive balance, since operational stresses may significantly change the state of balance.
A fifth shortcoming with present conventional technology is the exclusive reliance on electrical measurements alone to monitor and diagnose all problems in such motor systems. A motor is not just electrical; it is an electromechanical device, encompassing as well the mechanical aspects of the driven machine, and analyzing either the electrical or the mechanical aspects in isolation ignores important monitoring and diagnostic information.
The mechanical performance of an electrical motor is traditionally evaluated using a dynamometer coupled to the output shaft of the motor to determine the torque and horsepower at various steady state operating conditions. Normally, electrical voltage and current input and shaft speed are measured at a series of torque loadings specified by the dynamometer.
A Heyland diagram is often constructed to estimate a motor's performance capabilities when full dynamometer testing is not possible. The Heyland diagram provides an approximation of the mechanical performance which may be expected from the motor. The Heyland diagram is constructed by conducting two steady-state tests on the motor, one with the shaft uncoupled from any load (no-load) and the other with the shaft locked to prevent rotation (locked rotor). Electrical measurement of the voltages applied to the motor and the currents entering the motor are made during the no-load and locked rotor tests. The voltages and currents are analyzed to determine the real power dissipated by the motor and the reactive power that oscillates through the motor without dissipation. A no-load test is performed by operating the motor with the motor shaft uncoupled from any external load. In a locked rotor test, the motor shaft is mechanically restrained so that it cannot rotate. Voltage and current measurements are made during a brief application of power to the motor. It is normally necessary to conduct locked rotor tests using deliberately reduced line voltages, since currents five to eight times the rated value would otherwise be incurred. Measured currents are then scaled by the ratio of normal to reduced main voltages prior to plotting the points on a diagram. The two tests are used to plot two points on a plot of real power against reactive power. A circular arc is curve fitted to these two data points. Then, from this arc (the Heyland diagram), the behavior of the motor between these extremes can be predicted, providing reasonable estimates of the maximum horsepower, horsepower at most efficient operation, maximum torque, starting torque and efficiency.
The present invention overcomes many of the problems of present conventional monitoring techniques by providing a method and apparatus which improves the quality of voltage/current envelope estimation by utilizing an analytic signal approach in which the signal is uniquely defined in terms of quadrature components, and to do so for polyphase motors so that the estimation of polyphase variables such as motor power is improved as well as that of parameters primarily associated with individual phases, such as voltage and current. The present method and apparatus overcomes the non-linear effects of a current probe by modeling the true probe response in software, estimating the model parameters using actual calibration data, and subsequently correcting all measured current values utilizing the model. The present method and apparatus also overcomes the effect of sampling skew and other measurement channel time delay effects by accurately estimating channel-to-channel phase delays using an analytic signal approach in which quadrature components are used to calculate the instantaneous phases of the multichannel signals, and the instantaneous values are statistically combined to yield accurate cross-channel phase corrections. In addition, the present method and apparatus provides a measure of inphase and reactive imbalance during operation by comparing the voltage and current phasers. Finally, the present method and apparatus enhances the monitoring and diagnostic capability of motor monitoring techniques by simultaneously measuring electrical and mechanical variables, using both types of variables together to more definitively diagnose the existence of certain faults, some of which both types of measurements are sensitive to, and some of which only electrical or only mechanical measurements are sensitive to.
In a further embodiment, the present invention provides a method and apparatus for evaluating the output torque and shaft horsepower without employing a dynamometer or other torque measuring equipment. The present invention eliminates the need for locked rotor testing and measures the entire Heyland locus from a no-load to locked rotor condition whenever the motor is turned on.