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Some electrical sub-stations are large enough to have battery back-up for all the DC energised and controlled switchgear. Most times this will hover around the 110 or 220V mark. However, the front of the instrument states "85-265VAC @ 47-440Hz" - but this does not necessarily mean the recorder cannot operate on DC. The RPM, like 99% of equipment manufactured now-a-days, uses a Switch Mode Power Supply. What is forgotten by most power engineers is the first item found, apart from the EMC filters, is a bridge rectifier. Yes, this means the instrument actually operates on DC! In our case it will have an operating range of 120VDC (85VACrms * 1.414) to 375VDC (265VACrms * 1.414). The lower frequency limit is imposed owing to the smoothing capacitor not being able to hold enough charge between cycles thus increasing the ripple and becoming extremely hot. The upper limit is due to the above mentioned input filters starting to filter out the mains and they then become extremely sun-burned. One small point. Although the lower limit was stated as 120VDC it must be remembered that this was directly derived from the lower AC limit of 85VAC * 1.414. No thought was given to the fact that the smoothing capacitor would have some serious ripple on it, especially as the power supply draws more current at lower voltages. It is highly likely that it will comfortably operate on lower than 120V meaning the 110VDC battery bank will do the trick. As a sales point I've even been known to state that "the instrument is capable of 85-265VACrms @ 47-440Hz, or 110-375VDC with automatic reverse polarity detection".
© 30.11.01 |
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