UPS Output Voltage

[nhcf]

APC SmartUPS UPS System has an output voltage of 208 VAC. Its modular battery system provides battery packs at 192 VDC. How does the inverter produce a 208 VAC sine wave from 192 VDC voltage?

 

[ScottyUK]

Inverter and a transformer.

 

[Comcokid]

UPS systems are much like DC-to-AC inverters. Except instead of starting with a 12V or 24V battery, they store the energy at a higher voltage. Higher voltage means less current, less copper, less conversion losses, etc.

A standard inverter takes the battery voltage and using high-frequency PWM (say 20kHz to 100 kHz range) through a transformer (designed for this higher frequency), to a high-voltage DC. For 110 to 120 VAC this is generally 165 to 200 VDC. The voltage comes from the need to produce the peak of the sine wave, at Vrms * SQRT(2). Then in a second stage the high voltage is PWM through a H-bridge to produce a sine wave output. This second stage electrically is not much unlike an AC motor VFD drive running at a fixed frequency of 60 Hz.

There are several popular techniques that vary slightly in the details. Solar inverters are not much different. Of course, the intermediate high voltage and output frequency are different if you're going 12V to 230V 50 Hz (nod to ScottyUK). I'm sure there other differences if your end goal is 208VAC, and for some design or cost reason 192 VDC batteries make the best solution for APC.

 

[itsmoked]

The differences can be subtle. Inverters that are transformer output rather than semiconductor output are much more robust. They're also heavier and can cost more. Your unit can be exactly as Com described or it could be switching the 190V into the primary of a step-up transformer. Either way the voltage was likely decided by the kwhr capacity of the battery bank and its size and/or the available semiconductors (voltage or current ratings) that provided the best economy.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[nhcf]

thanks for the replies. I had not considered the obvious answer of a transformer in the UPS, mainly because it did not look big enough to house a 5 KVA xfmr, and the output of the UPS is fed directly into a 208 - 120 VAC transformer.

I assume the main UPS transformer is configured like a buck-boost, only boosting as needed to the 208 VAC?

 

[itsmoked]

If you feel the unit is too small to house a 5kVA transformer then Com's more common implementation is what you've got. They're using a much smaller high frequency transformer to step the voltage up to 293V then switching that out. Not buck-boost, step-up.

Keith Cress
kcress -

http://www.flaminsystems.com