How does a double conversion online UPS know how much power to draw? - Page 1
Forum section: electronics / general

author: nbritton
author_rank: Frequent Contributor
timestamp: November 15, 2020, 12:52:50 am
content: So if I understand correctly, a double conversion UPS first rectifies AC into DC, which is then fed into the batteries and inverter. This implies the input is decoupled from the output, so how then does the UPS know how much power to draw from the wall?I ask because I just got an APC SURT7500XLT and four battery packs and 160x 12v @ 5Ah batteries as part of another bigger deal.I have two ideas of how to use this, the first one is to get a 96v solar charge controller and solar panels and run it in off grid mode, i.g. as a power wall.The second idea is I have a detached garage that only has a single 120v @ 15a circuit and I want to use that to feed the UPS, as I have computer equipment that will only run on 240v and the equipment has very variable power draw depending on load.

author: james_s
author_rank: Super Contributor
timestamp: November 15, 2020, 02:15:40 am
content: When load on the inverter increases, the amount of current it draws from the low voltage bus increases, and the power supply/charger on the input will draw more current to maintain the voltage on the bus. A power supply doesn't "know" how much to draw, even with just a simple power transformer loading the output causes the current in the primary to increase proportional to current through the secondary, the energy has to come from somewhere.A double conversion UPS is essentially an inverter running from a battery, and then there is a charger that is also connected to the battery. If you load up the output it will pull down the battery voltage and this will cause the charger to deliver more current trying to hold it up. When it delivers more current to the battery it will draw that current from the wall.

author: nbritton
author_rank: Frequent Contributor
timestamp: November 15, 2020, 09:10:59 am
content: Thanks for the reply. So if I have something like an APC 208v to 120v step down transformer and then backfeed it through the 5-15R socket then it will output 208v on the primary side and it will also be current limited due to saturation?

author: james_s
author_rank: Super Contributor
timestamp: November 15, 2020, 07:38:13 pm
content: I'm not sure I follow. I'm not familiar with that particular transformer but if it is in fact just a transformer then you can wire it backward and feed the 120V winding with 120V and get 208V out the 208V winding. Current draw on the primary will follow current drawn from the secondary, the current will be limited by the circuit breaker tripping if you try to draw too much. The power drawn by the transformer will be equal to the power you draw from the transformer, plus maybe 5% losses.

author: ejeffrey
author_rank: Super Contributor
timestamp: November 15, 2020, 10:03:37 pm
content: Quote from: nbritton on November 15, 2020, 09:10:59 amA couple things:1) Don't do that. Backfeeding through a socket is incredibly dangerous. If you want to reverse a transformer that is wired up with plugs and sockets, you need to rewire it to use an appropriate plugs and sockets for the desired configuration.2) A simple transformer is bi-directional. When built into an instrument that has additional functionality such as switching and monitoring it may or may not work as expected. It depends on what that "APC 208 to 120 V transformer" actually is.3) Most transformers don't operate with significant saturation under any normal circumstances and it isn't a part of transformer regulation. If the secondary is open circuit, the primary acts like a simple inductor and the current is limited by the inductive impedance to a small value. As the secondary is loaded the secondary current generates a flux opposite and partially cancelling the flux created by the primary. This decreases the flux change and allows more primary current to flow.

author: langwadt
author_rank: Super Contributor
timestamp: November 16, 2020, 12:02:10 am
content: Quote from: james_s on November 15, 2020, 02:15:40 amisn't it more like an inverter running from either a powersupply or a battery (of roughly the same voltage) and then a charger for the battery

author: james_s
author_rank: Super Contributor
timestamp: November 16, 2020, 02:46:17 am
content: Oh, I misunderstood exactly what was being asked. Yeah don't backfeed through the female socket, rewire the transformer so that you never have a situation where you can have live prongs on a plug.

author: james_s
author_rank: Super Contributor
timestamp: November 16, 2020, 02:49:10 am
content: Quote from: langwadt on November 16, 2020, 12:02:10 amThere may be variations on the design, I haven't looked at the actual schematic from one but my understanding is that they essentially float charge the battery which is connected to the inverter at all times. Due to this there is no switchover time the way there is with an off-line UPS, the inverter is already connected to the battery and running all the time, so if the the input is lost the battery picks up the load instantaneously.