Uninterruptible power supplies are part of an electrical circuit. Whenever current is either switched or transmitted through a cable or other material, heat is generated through power loss.
Taking a very simple example, if 100A is passed through a cable that has a resistance of 0.01Ω, from Ohms law we find that 1V will be dropped across the cable. This actually equates to 100W of heat being generated.
Without going to superconductors, in every electrical circuit there is resistance of varying amounts, be it the resistance of the windings of a fan motor to the resistance of an IGBT (power switching device used in UPS equipment) when it has been switched on. When current is passed through this resistance heat is generated, and this heat is termed as the heat-loss of the equipment due to the inefficiency of its operation and the uninterruptible power supplies.
This is why, even when there is no output being demanded from a UPS system, heat will be generated due to the thousands of circuits within the UPS switching microamperes, let alone the power switching circuits of the rectifier and inverter.
An uninterruptible power supply (UPS) system specification will often give two figures for the heat-loss. One is the figure as mentioned above when no load is being demanded, the other figure is the amount of heat generated when the UPS system is running at full load.
With the circuits that are switching micro amperes, the actual switching device has sufficient surface area in its construction to radiate enough heat to prevent damage it. However, with the power switching devices it is not possible to manufacture the switching device package large enough to dissipate the amount of heat generated in switching the current flowing through the device. In this instance the switching device is mounted on an aluminium heat sink (aluminium due to cost – copper would be better). The aluminium is further extruded to have multiple fins to increase the amount of surface area available to dissipate the heat; the heatsink is then mounted in a fast air stream to remove the generated heat as quickly as possible.
The heat output of the UPS can therefore be seen to be a product of its inefficiencies; these figures are normally available on the UPS specification sheet stated at various load levels. To boost their rated efficiency to 99% instead of the more usual 95%, many UPS manufacturers will offer an ECO mode of operation which means that the UPS is operating in an ‘off-line’ mode where the inverter is not connected to the output of the UPS and therefore the switching losses are much reduced.
A further major problem with heat-loss from a UPS situated in a small enclosed room is that air conditioning has to be specially provided to keep the ambient temperature within the operating limits of the UPS and its battery if located in the same room. This of course further reduces the efficiency of the complete system.
A very quick calculation of heat output is to base it on 10% of the UPS size in kVA.
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