[ARC5] UPS Transformers

[Brian]

Hello Wayne,

Most quasi-sine output UPS devices have two transformers.
There is one for charging the batteries and running the management circuitry 
off the mains. And there is one that takes the output of the oscillator / 
low impedance power amplifier and shifts it to your desired line Voltage.
It would be nice to get the design data on the latter transformer to 
discover what duty cycle it was designed to meet. Some UPS devices, whose 
specs I have read, have a hold up time at 70% of rated load of 5 minutes. 
When I did the maths, the overall energy conversion efficiency was about 
40%. I suspect some loss of efficiency would be down to a cheap transformer 
designed by an accountant. When I was designing transformers, if the 
efficiency test results didn't get near 99 to 99.5%, I would dismantle and 
reassemble.
When a UPS device's specs claim an efficiency in excess of 80%, I look very 
carefully at the output waveform. It is very easy to get high efficiency the 
closer the output resembles a square wave. With a square-wave output, less 
cooling is required than if the output resembles a sine-wave. The maximum 
efficiency you can get out of a Class AB amplifier is pi/4, or 78.5%. No-one 
uses Class A with its maximum efficiency of 25%. However, Classes C, D E, F, 
G ... can easily exceed 80% efficiency at the expense of waveform purity.

With respect to adding outboard batteries to increase the hold-up time, look 
at the back panel. If there is an external battery socket, then you can 
safely add batteries.

73 de Brian, VK2GCE.

On Friday, November 20, 2015 2:45 AM , you wondered:

I would assume that you would want to use transformers of a similar 
size/capacity if you want to approach the full capability of the device.

I have seen people afflicted with an extended power outage ask if they can 
charge the battery for their UPS and then use it to run their computer.  I 
assume the answer to this is "No," that the UPS is designed not only to use 
a battery of a given capacity but also to run on that battery power for that 
length of time, not several times longer.  I have seen very little evidence 
of active cooling or extensive heatsinking in UPS units either for the 
transformers or the solid state components.

Wayne
WB5WSV