At one time, the Standby-Ferro UPS was the dominant form of UPS in the 3-15kVA range. This design depends on a special saturating transformer that has three power connections. The main power supply comes from AC input, through a transfer switch, then through the transformer and to the output. In the case of a power failure, the transfer switch is opened, and the inverter picks up the output load.
The inverter, in the Standby-Ferro design, is in the standby mode, and is energized when the input power fails, which opens the transfer switch. The transformer has a special “Ferro-resonant” capability, which provides limited voltage regulation and output waveform “shaping” The isolation from AC power transients provided by the Ferro transformer is as good as, or better than any filter available.
However, the Ferro transformer itself creates severe output voltage distortion and transients, which can be worse than a poor AC connection. Even though it is a standby UPS by design, the Standby-Ferro generates a great deal of heat because the Ferro-resonant transformer is inherently inefficient. These transformers are also large relative to regular isolation transformers; so the unit is generally large and heavy.
The Standby-Ferro UPS unit’s strengths are excellent line filtering and high reliability. However, the design’s efficiency is very low combined with instability when used with some generators and newer power-factor corrected computers, causing the popularity of this design to decrease significantly.
The principal reason why Standby-Ferro UPS systems are no longer commonly used is that they can be fundamentally unstable when operating a modern computer power supply load. All large servers and routers use “Power Factor Corrected” power supplies which draw only sinusoidal current from the utility, much like an incandescent bulb.
This smooth current draw is achieved using capacitors, devices which ‘lead’ the applied voltage, Ferro resonant UPS system utilize heavy core transformers which have an inductive characteristic, meaning that the current ‘lags’ the voltage. The combination of these two items form what is referred to as a ‘tank’ circuit. Resonance or ‘ringing’ in a tank circuit can cause high currents, which jeopardize the connected load.
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