Abstract:
A novel method of and apparatus for accurately measuring surge currents such as motor-starting inrush currents is provided. An input signal from a current sensor is monitored, and when the input signal changes and exceeds a predetermined threshold, a surge current is detected. The input signal is acquired over a predetermined time period by a fast sampling ADC, which converts the input signal into a series of digitized samples representative of instantaneous current values. These values are processed to compute average current or RMS current, which is then displayed.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates generally to measuring electrical current, and in particular to measuring surge current.  
           [0002]    A special form of digital multimeter known as a clampmeter includes a current sensing jaw that may be opened and placed around a wire to measure current. Conventional current clampmeters typically have a range switch for selecting a desired current range, and internal processing circuitry including a dual-slope analog-to-digital converter (ADC) for accurately measuring the value of current being measured. The final current reading is displayed on a liquid-crystal display (LCD) screen.  
           [0003]    Accurate measurement of inrush surge current of an AC electric motor at startup would be useful because it reflects on the health of the motor, since a weak motor may draw excessive current. Conventional current clampmeters do not offer this feature.  
           [0004]    Short duration surge currents have heretofore been extremely difficult to measure with any degree of accuracy for a number of reasons. While dual-slope ADCs provide extremely accurate measurements, particularly for DC measurements, the ADC operation is asynchronous to the surge current. Consequently, it is very difficult to capture the surge current as it occurs. Also, the measurement time of the dual-slope ADC is typically longer than the time duration of the surge current, or has dead time during which nothing is measured, resulting in faulty measurements.  
           [0005]    For root-mean-square (RMS)- or average-responding instruments using analog detection methods, the time for the converter to respond makes such methods impractical for measuring motor inrush current. A very recently-produced current clampmeter instrument using an RMS-converter takes several seconds at lower current levels to reach final value.  
           [0006]    It would be desirable to provide a clampmeter with an inrush current feature to measure current surges without resorting to expensive oscilloscope triggering and processing circuitry.  
         SUMMARY OF THE INVENTION  
         [0007]    In accordance with the present invention, a novel method of and apparatus for accurately measuring surge currents such as motor-starting inrush currents is provided for a current-measuring instrument such as a clampmeter.  
           [0008]    An input signal from a current sensor is monitored, and so long as the signal level is below a predetermined threshold, the display indicates that the current-measuring instrument is armed and ready to measure a surge current. When the input signal changes and exceeds the predetermined threshold, a surge current is detected, triggering the instrument, and the measurement process begins.  
           [0009]    The input signal is acquired over a predetermined time period after the trigger point by a fast sampling ADC, which converts the input signal into a series of digitized samples representative of instantaneous current values. The absolute values of the digitized samples are accumulated (added together), and the total divided by the number of samples acquired to produce the average. The average is then formatted for display in terms of average current or RMS current, and finally displayed.  
           [0010]    In a proposed commercial embodiment, a microcomputer with a fast ADC is utilized to carry out the measurement process. The predetermined time period is one hundred milliseconds, which is sufficient to capture and measure motor inrush current of a AC electric motor. Processing for surge current detection and sample accumulation is carried out “one the fly” on a sample-by-sample basis, so extra storage and post-acquisition processing are not needed. Once a surge current is detected, a readout of the average or RMS value is available in a fraction of a second.  
           [0011]    Other objects, features, and advantages of the present invention will become obvious to those having ordinary skill in the art upon a reading of the following description when taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is typical AC current waveform associated with startup of an electric motor;  
         [0013]    [0013]FIG. 2 is a block diagram of an exemplary circuit for measuring surge current in accordance with the present invention;  
         [0014]    [0014]FIGS. 3A and 3B are exemplary displays showing respectively instrument status before inrush current is detected and a value of inrush current; and  
         [0015]    [0015]FIG. 4 is an exemplary program to explain the operation of the circuit of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    Referring to FIG. 1 of the drawings, there is shown for purposes of understanding the invention a typical armature winding AC current waveform for a single phase three horsepower electric motor during its startup. Initially, the current in the armature winding is high as the motor overcomes its inertia and begins to rotate. In the example shown, this current varies between approximately +/−120 amperes for approximately 100 milliseconds. Then, as the motor increases its rotational speed, the current drops until the motor is running at its normal speed. Once the motor reaches its normal speed, the current in the armature winding stabilizes at a normal run rate of approximately +/−20 amperes. The high current during the first 100 milliseconds is what is referred to as inrush current, and is what will be measured by the present invention. Inrush current is typically specified for AC electric motors as about 6 to 10 times the normal operating current. Excessively high inrush current, or inrush current that increases over the life of the motor are indicators of a defective motor.  
         [0017]    [0017]FIG. 2 is an exemplary circuit employed in a clampmeter for measuring surge current, including motor inrush current. A current-carrying conductor  10  induces current into a current sensor  12 , which may suitably be any of a number of current transducers which produce voltage output proportional to a current that is sensed. Examples of such current sensors include Hall-effect devices and coils. In a proposed commercial embodiment, the current sensing device of a clampmeter may suitably comprise either a current transformer or a Hall-effect device. The clampmeter has what is known in the industry as a “jaw” that can be opened to accept a wire and then closed to encompass the wire. This type of jawed device is also known as a current clamp, and hence the instrument is known as clampmeter.  
         [0018]    The input signal from current sensor  12  is applied to a signal conditioning circuit  14 , where it may be buffered, amplified or attenuated as necessary to meet the input requirements of ADC  16 . ADC  16  samples the input signal and produces digital representations of instantaneous signal values at a rate determined by sample clock  18 . In this exemplary embodiment, the sample clock free runs at a four-kilohertz rate, producing a digitized sample every 250 microseconds. Also in this embodiment, ADC  16  and clock  18  are part of an integrated circuit package  20  which also includes a CPU  22  along with its associated bus structure  24 , a memory  26  (both RAM and ROM), an input/output (I/O) port  28  and an LCD driver  30 . Integrated circuit package  20  is an MPS4300C32 14-bit microcomputer manufactured by and commercially available from Texas Instruments.  
         [0019]    Because the clampmeter has functions other than measuring surge or inrush currents, operating mode, function, and measurement routines are provided through the I/O port  28  to CPU  22  from a mode/function block  32  that includes suitable selector switches and firmware from which measurement routines may be selected. The mode/function block  32  in turn is controlled by user control  34 , which may suitably include selection knobs and buttons on the front panel of the clampmeter.  
         [0020]    A liquid crystal display  36  provides alpha-numeric or digital readout of display information. Refer to FIGS. 3A and 3B for exemplary displays. Let us suppose that the user has selected the motor inrush current mode. Mode/function block  32  provides this information to CPU  22 , which through LCD driver  30  places the word INRUSH on the display  36  to let the user know that the inrush mode has been selected. Before a measurement is made, CPU  22  places four segments  40  on the display as shown in FIG. 3A to let the user know that the clampmeter instrument is prepared to measure motor inrush current, but no inrush current has been detected. Once the inrush current has been detected, an alpha-numeric display is generated as shown in FIG. 3B. The value of “119A” displayed is simply selected as an example of a typical display.  
         [0021]    Operation of the circuit of FIG. 2 will be discussed in connection with the exemplary program shown in FIG. 4. Prior to initiation of the inrush current measurement, ADC  16  continuously samples the input signal under the control of free-running sample clock  18 ; however, nothing is done with the digitized samples until a measurement is triggered. In step  50 , the inrush current measurement mode is selected to start the measurement algorithm. The program is downloaded from the firmware in mode/function block  32  to CPU  22 .  
         [0022]    In step  52 , the CPU is initialized by setting a threshold level provided by the firmware. The threshold level is something above zero amperes, and may be, for example, five amperes, to ensure signal detection. The threshold level may be adjustable by a user through the user control  34  if desired to prevent false readings initiated by noise. CPU  22  also sets the initial display as shown in FIG. 3A.  
         [0023]    In step  54 , an acquired digitized sample of the input signal is compared with the threshold level. If the input has not changed from zero and does not match the threshold level, another sample is acquired and process repeated until the input signal increases from zero and matches the threshold level.  
         [0024]    Once the input has changed, CPU  22  waits a short time period, for example, approximately 1 millisecond, and then in step  56  a newly-acquired digitized sample is compared with the previously acquired sample. If the input signal has not increased after the short time delay, it is an indication that the first digitized sample was part of a transient or spike which went away by the time the second sample was acquired. If this is the case, then CPU  22  returns to its initialized state and steps  54  and  56  are repeated until an increase in signal level is detected.  
         [0025]    In step  58 , assuming the signal level detected in step  54  has increased, it is assumed that inrush current has been detected, triggering a measurement cycle. The measurement process begins as the first of 400 digitized samples is acquired. It should be noted for this example that since the sample clock  18  causes ADC  16  to acquire a sample every 250 microseconds, 400 samples represents a stream of points over a time period of 100 milliseconds. For example, assuming that the motor inrush current is nearly sinusoidal at 60 hertz, six complete cycles of the input signal will be processed and used to compute the average value, and at 50 hertz, five complete cycles of the input signal likewise will be processed. For other time periods and sampling rates, a different number of samples may be acquired rather than the 400 used in this example; however, it is a relatively straightforward computation to determine the actual number of samples needed to complete a measurement.  
         [0026]    In step  60 , the absolute value (ABS) of the digitized sample is determined, which is analogous to full-wave rectification, and then stored in an accumulator register (M+). It should be noted that the M+ function is similar to that commonly found on calculators.  
         [0027]    In step  62 , a count of the number of samples is kept. If the number of samples is less than 400 samples for this example, the program returns to step  58  and another digitized sample is acquired and added to previously acquired samples in step  60 . When the count reaches 400, a measurement period of 100 milliseconds is complete and 400 samples have been acquired and added together in the aforementioned accumulator register (M+).  
         [0028]    In step  64 , the average current value is calculated by dividing the accumulated value of 400 digitized samples by 400.  
         [0029]    In step  66 , the results are formatted for display. The AC average values or RMS values, or even an average of the DC value (the signal envelope) may be calculated as determined by the program instructions. For example, if the RMS value is desired, the average value is multiplied by a factor of 1.111.  
         [0030]    In step  68 , the inrush current is displayed.  
         [0031]    Thereafter, the program ends as indicated by step  70 .  
         [0032]    It should be noted that ADC  16  has only 14 bits of resolution, and so apparent resolution is increased by oversampling and averaging. The increase in counts of resolution is the square root of the number of samples taken. This is achieved by using an acquisition interval of 100 milliseconds, since six full cycles of a 60-hertz AC waveform can be sampled as discussed above.  
         [0033]    It can be readily seen that other operating modes are easily facilitated, such as peak detection, min/max, measuring true RMS and average responding RMS. True RMS can be calculated directly from the samples acquired over 100 milliseconds. By taking an average of the absolute value of an AC waveform over 100 milliseconds and multiplying by a factor of 1.11, the average responding RMS value can be calculated.  
         [0034]    While I have shown and described the preferred embodiment of my invention, it will be apparent to those skilled in the art that many changes and modifications may be made without departing from my invention in its broader aspects. For example, it would be a relatively simple matter to acquire all of the digitized samples, store them in a separate memory, and employ post-acquisition processing to determine the value of surge currents. The trade-off, of course, is increased complexity and longer processing times. It is therefore contemplated that the appended claims will cover all such changes and modifications as fall within the true scope of the invention.