400 to 480 VAC testing - Page 1
Forum section: electronics / general

author: martincho
author_rank: Contributor
timestamp: February 12, 2022, 12:23:33 am
content: I have to test a design with an input voltage range reaching at least 400 VAC and possibly 480 VAC. Power consumption is about 250 W, so not a lot of current.Trying to figure out the most economical way to get there. My first thought was to get a step-up variac (variable transformer) that can run off 240 VAC and get to at least 400. They do exist. Cost is about $2,000, which is fine.Any other reasonable options? Even used lab equipment would be fine. I just don't have any part numbers to run a search. For example, did companies like HP or Tektronix make such high voltage variable output voltage power supplies?While I have 240 VAC single phase, I also have a digital phase converter putting out 3 phase for a CNC milling machine. When I installed it I also installed a 4-prong twist-lock on the wall for any future application that could use this power. I wonder if this might be the ticket? Can I use two phases and get legit 480-is out of it? I haven't thought this one through yet. The device being powered is a switch-mode power supply rated for something like 300 to 500 VAC input.Thanks,-Martin

author: bdunham7
author_rank: Super Contributor
timestamp: February 12, 2022, 12:47:13 am
content: The obvious way would simply be a 1:2 or 1:4 step-up transformer, with a variac in front of it if you like.https://www.powervolt.com/group-ag-input-240-480vouput-120-240vpower-25-to-1500va-pd-164

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 12, 2022, 01:35:11 am
content: Perhaps get a transformer designed to provide 120V or 240V from 480V and use it backwards?Might also work to use a microwave oven transformer (or two with secondaries in parallel and primaries in series) on a variac, add an end stop on the variac to prevent turning it up too much by mistake. It would also work for supplying 600V if there's a need later on.

author: jmelson
author_rank: Super Contributor
timestamp: February 12, 2022, 03:17:12 am
content: "Step down transformers" were used in a lot of industrial equipment to run computers and general items from the shop floor 480 V mains. These have jumpers to select the input and output range. They can be connected in reverse to step up the voltage. So, set jumpers for 480 to 240 V configuration, and then feed 240 to the LV side and get 480 from the HV side. You can get one of these at a scrapyard for the metal scrap value. Then, use a standard 240 V Variac to feed it.Jon

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: February 12, 2022, 01:06:55 pm
content: I have a 4 KVA 480-120 transformer running backward as a supply to several VFDs in my shop. I've been using it for more than 20 years without issue.

author: BeBuLamar
author_rank: Super Contributor
timestamp: February 12, 2022, 01:54:34 pm
content: Quote from: Ground_Loop on February 12, 2022, 01:06:55 pmI have transformers that are 480-120 and 480-240 but I don't have any for step up. I am afraid of using them in reverse.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 12, 2022, 02:53:26 pm
content: Quote from: BeBuLamar on February 12, 2022, 01:54:34 pmThey will work just fine in reverse. It's the same as what happens if you connect a grid tie inverter to one.

author: bdunham7
author_rank: Super Contributor
timestamp: February 12, 2022, 03:12:13 pm
content: Quote from: NiHaoMike on February 12, 2022, 02:53:26 pmThe only issue you might have is that the windings ratio might not be exactly 2:1 or 4:1 because the transformer might be designed for slightly high OC output to compensate for sag due to loading. If you are using a variac capable of a bit more than 1:1 output to feed the step-up configuration, this shouldn't be a problem.

author: martincho
author_rank: Contributor
timestamp: February 12, 2022, 03:25:46 pm
content: Quote from: bdunham7 on February 12, 2022, 12:47:13 amThanks for the link. Yes, this would be cheaper than a 240 to 480 variac. I would like to be able to vary the output voltage to test under/over voltage conditions. A 240 in/out variac driving the transformer would do this well and cost less than the 240-to-480 version.

author: Shock
author_rank: Super Contributor
timestamp: February 12, 2022, 03:55:15 pm
content: If it's current limited you can use an isolating step up transformer and a rheostat. That could be done quite cheaply or even with junk parts. Something like a rheostat on an old stove or an old beefy light dimmer? It's been a while but obviously you will need to check working voltage/current. If suitable that stuff is typically wired directly to the mains so that is no issue.

author: jmelson
author_rank: Super Contributor
timestamp: February 13, 2022, 02:52:53 am
content: Quote from: BeBuLamar on February 12, 2022, 01:54:34 pmWhy? There is NO REASON to be afraid of this, except the usual care when working with 480 V. The transformer will work FINE in reverse as long as you respect the Watt rating of the transformer.Jon

author: Zero999
author_rank: Super Contributor
timestamp: February 13, 2022, 12:17:40 pm
content: Only 400V to 480V is required, so get a 110V to 400V isolation transformer and put the secondary in series with the output of a 120V Variac. That will give 400V to 520V out.