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Sophisticated ICT systems sometimes use local engine driven generators for backup in case of mains failure
Some modern small UPS boxes are designed to operate primarily from traditional mains supplies and some care should be taken when operating them from a small generator
These notes provide some guidance on the potential problems which can be met together with suggested practice
The average modern UPS need a much larger generator than one might expect. Allowance must be made for re-charging the batteries as well as providing full load and the effects of UPS efficiency.
Typically Advance recommends a generator capable of delivering at least TWICE the output power of the UPS however some generator manufacturers quote unrealistic performance specifications and it is best to check with our engineering team.
In addition some UPS input circuits have high crest factors and generator control circuits cannot handle them.
The Master Elite range of UPS has a specially developed input filter for use with generators and other sensitive sources.
Some generators do not refer the low side of the output to earth. This MUST be `tied down' to avoid possible damage to the UPS
Care must also be taken that the generator neutral is connected to the UPS input `low'
Conventional mains phasing is expected by the UPS and the generator should be wired through to the UPS input in the normal way
The generator must run close to 50 Hz for the unit to operate correctly if the frequency goes outside the UPS operating window it will operate from the UPS battery which will not be re-charged
Electrical noise
Some medium sized generators may be connected to other loads. In this case care must be taken to ensure that the other loads do not create excessive spikes or other electrical noise
The generator exhibits a much higher impedance than a conventional mains supply. Noise from triac controlled motors etc. may affect the UPS rather than just disappear in the distribution system
Clearly any generator which is not brushless may also cause a similar problem
A generator providing a clean waveform with a reasonably stable frequency and voltage correctly connected only to the UPS is the most attractive arrangement
Some thought needs to be given to the Regulations regarding out of phase supplies in the same work area
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