Minimum, Maximum,
and their mate Average
 

The saying "Tom, Dick, & Harry" would be incomplete if any of the names were removed. Trend readings are the same. You cannot seperate Minimum, Maximum, and Average from one another and still have a worthwhile result. Each tells only a part of the story, but having all three does not just give you a story with three parts. There are in fact six parts as the relationship between the values adds to the picture, and these tell more than just the values alone.

 

UNDERSTANDING THE VALUES:

Before explaining how to interpret the values, a brief description of each. We'll use the analogy of the older pen-chart recorder where possible.

Measured Value explained (as used in descriptions):
On simple recorders there is normally an 'RMS to DC convertor' and the output of this is sampled at a specific interval. The measured value is therefore, by derision, the sampled RMS.

On semi-complex and complex recorders the measured value is the calculated RMS of any half or full cycle, this depending on the recorder. On most recorders this value is available for use under the heading of 'current reading' (the value usually updated at the sample interval). For more explanation on intervals, see Sample And Recording Periods Explained.

Average:
This is probably the easiest to explain, and we'll get it out of the way first. This is the absolute mean of the measured value over the recording interval. At the end of the interval this mean is stored to memory as the Average. This mean may be derived from the average of the means from all the samples - as defined by the sample period - taken during the recording interval.

On older pen-charts this would be the area where the most ink would be deposited on the paper, especially if slow moving.

Minimum and Maximum:
This is the absolute minimum or maximum measured value read during the recording interval. This will mean that should a sag or swell occur at any time during an interval, the lowest and/or highest measured values are then taken as the minimum and maximum readings respectively.

What can confuse some is that these readings are not in any way an average, but are absolute values of the measured value. Furthermore, these are values (especially on semi-complex and complex recorders) that hand-held meters cannot interpret through purely being too short for the circuitry to react (specifically because the human would not see such incidents).

On older pen-chart recorders this would be the absolute edges of the pen trace. The only difference is on pen charts there are no 'time slots' whereas on more modern recorders the minimum and maximum are 'drawn' only at the end of every recording interval.

 

INTERPRETATIONS:

Below is a set of interpretations that can be made from the Min, Avg, and Max graphs (especially with semi-complex recorders).

What Is Considered Normal (a baseline):
A standard graph with Min, Max & Avg separated by no more than 1% is typical of commercial and domestic supplies. Obviously the less the separation, the better the quality (especially impedance) of the supply. However, it must never be expected that the three will line up, especially during long recording intervals. This is owing to the analogue to digital conversion accuracy which will always be at least ±1 count (Telog Linecorder = 0.3 or 0.6V, depending on model).

Determining Swells and Sags:
We will, for the sake of simplicity, deal with determining sags as they are more common. Swells are seen by the same phenomena with the Max and Min traces swapped.

The recording interval is extremely important when trying to determine the causes of sags. Having a recording interval too long will mask the cause as there is little means of determining how often the sags are occurring - this being the main source of recognition. It is strongly suggested when trying to determine sags that the recording interval be set at the fastest possible, at least 1 minute, preferably 30 seconds (exercising caution as too small may make the recording period too short).

During any sag the measured value is kept a watch on and the lowest seen is kept till the end of the recording interval, and then written away as the Min.

Determining Sag/Swell direction.
By this we mean is the sag/swell caused by external or internal influences (before or after the test point respectively).

Internal influences will show a distinct rise in current with a corresponding drop in voltage i.e. the voltage trace will have a low minimum while the current trace a high maximum.

External will have the current recording a sag as the load will have less supply voltage. There is a word of warning here as there are certain loads (e.g. PCs) that draw excess current on the 'rising edge' of the sag. The difference is the current surge is nowhere near as high as what it would be to cause the voltage to dip should it have been an internal influence.

 

SIGNATURES:

RECOGNISING TAP CHANGES:

These usually change the voltage by between 2 and 5%, and especially when the voltage is approaching the lower or higher accepted limits. Examples of a change up and change down are show alongside. Note the relationship of the three values and the time relationship between them.

RECOGNISING FLICKER:

'Odd Flicker' (the odd sag) is seen by a recognisable dip in the minimum trend but no unusual change in the Avg-Max relationship (the sag did not impact the average value).

Permanent flicker (periodic sags) is seen by a constantly larger separation between the Avg and Min trends than found between the Avg and Max trends. Such sags are usually the result of welding machines etc. with high but intermittent draw of current.

Phasing flicker is seen by a large and equal separation between the Avg and the Min & Max trends. What separates this from normal changes is the is the large variation in voltage from cycle to cycle. Flicker of this nature is usually associated with motor speed controllers that are running only a few Hz off the incoming fundamental.

RECOGNISING 'SPIKY' POWER:

Recognised by a large Max-Avg difference while the Min-Avg difference is small. One must be careful of such readings as the spikes may be too short to significantly alter the rms value of the half-cycle in which they occured. This will result in a Max-Avg ratio that looks normal yet sensitive equipment is being damaged.

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© 05.09.02