Running Voltage

The question once asked of a group of power engineers "what voltage is 230VAC?" was met with a host of seriously blank, puzzled looks! Let's investigate what prompted this confusing question.

In all power supplies there is a rectifier. This may be directly coupled to the mains (usually after an EMC filter), or after a power transformer. The basic purpose is to convert the AC supplied into DC. The output from a rectifier would be a series of pulses from zero to full peak voltage, but this would play havoc with the electronics it is supplying so a smoothing capacitor is placed after the rectifier to capture the voltage at or near the maximum.

When it was defined that a voltage whose RMS is 230V actually swings from zero to 325V at the crests, and that it is these peaks that are captured by the likes of rectifiers and smoothing capacitors, the penny suddenly seems to fall. To the electronics engineer of modern power supplies, 230VAC actually represents a running voltage of 325VDC.

To make matters even worse, many mains supplies do not rest at 230VAC but are 'pushed' to above 240V, sometimes even higher as the statutory limit is 253V (although there is no stopping here!). To the electronics individual this upper limit represents a running voltage of almost 360VDC.

When it comes to the running voltage of hi-tech equipment the RMS value of the incoming supply is of absolutely no use to man or beast. It is extremely important when calculating the amount of power drawn by a resistive load, but this is as far as it goes.

Alongside is shown two waveforms. The first has an RMS of 230V, the second an RMS of 240V. According to the user of a "true RMS" multimeter it is the first, as opposed to the second, that is well within limits and has quite some way to go before power supply damage becomes a concern. In actual fact it is the first that has a hidden danger while the second is in fact being kind to hi-tech gear.

The reason for this phenomena is the first has a triangular waveshape with a peak voltage of 353V, the second is more rounded and only a peak of 339V. This difference is indicted in what is called "crest factor" - the ratio of Peak/RMS. A pure sinewave has a crest factor of 1.414.

In locations where a high amount of hi-tech is used this has been seen lower than 1.35, while some UPS systems have been known to generate waveforms as high as what was seen above at 1.53 especially when subjected to a load with a bad power factor.

At 230VACrms a crest factor of 1.35 represents a whole 15V cropped off the top of the waveform! (see alongside) It may not sound like a lot, but this means the incoming voltage may be 11Vrms higher before the same running voltage for hi-tech equipment is reached.

If we take this a little further; 253VACrms is the accepted maximum according to EN50160. Using 1.414 as the pure crest factor, the peak would reach 358V. Reversing the calculation using the crest factor of 1.35 means the input could rise to 265V before hi-tech equipment is subjected to the same running voltage, although resistive loads will be breaking into a hot sweat!

There is one further complication when it comes to running voltage for hi-tech devices and that is any mismatch in the positive and negative peaks. Should one be higher than the other then what should be full-wave ('2-pulse') rectification becomes half-wave if the imbalance is sufficiently large enough. This makes one set of rectifiers work overtime while the others take a holiday. If forced to continue this could lead to premature failure of the diodes that are overworked.

This clearly indicates that when it comes to power quality investigations on hi-tech suites RMS is required for all the resistive loads, but the peak voltages, both +pk and -pk, must be recorded too (or at least the crest factor, although any DC imbalance is then lost).

The true running voltage is therefore the incoming RMS voltage multiplied by the 'crest factor', and added to 'DC component'. If unknown then it is fairly safe to use 1.414 (the peak:RMS ratio of a pure sinewave) when dealing with mains input systems as these tend to have lower than perfect crest factors. If dealing with UPSs then the peak voltages must be measured to be absolutely sure.

It does make one wonder why facilities managers of hi-tech data centres insist on "True RMS" multimeters! Er, yes, well,.....

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© 27.08.02