120VAC input to variable 3 phase output PSU? - Page 1
Forum section: electronics / general

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 21, 2023, 02:40:52 am
content: TL;DR: I'm looking for a power supply that has standard US 120VAC input and gives variable voltage 3-phase output (say, from 200V to 480V).Hi all, I'm soon going to be picking up a large auction lot from a closed-down industrial facility and there's lots of mid-high voltage 3-phase equipment which I would like to test so I can sell it as 'used/tested powers on' instead of having to sell it for 'parts / not working' status.I don't have 3-phase service and there's no way I could get it - my power is all 120VAC US standard - I can access 240V from the plug my dryer uses if need be, though I would prefer not to.I'm not expecting to find anything that can supply much amperage, and I'm not going to worry about load-testing anything that has a high-amperage draw, but I would like to be able to power-on test equipment like 400V 3-phase to 24VDC power supplies and the like, to verify they are functional before resale.I'm having a hell of a time finding anything like this, because when I try searching for any keyword combination including 'power supply' and '3 phase' I get power supplies that use 3 phase as the input, not the output - which makes sense as that would be the far more common use-case.Any ideas?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 21, 2023, 03:14:15 am
content: You should be able to salvage a 480V to 120V "control transformer" which you can use backwards to get 480V single phase. Most 3 phase switching supplies will start up on single phase although they won't be able to run at full power.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: June 21, 2023, 03:18:39 am
content: Interesting. I did not know I could get power-on with a single phase. I'll keep an eye out for a suitable transformer, it this works it will be a very cost-effective solution, thank you.Edit: well then... it seems one of the lots I won includes 2 240/480 to 120 control transformers... Cool. The (potential) solution just happens to accompany the problem.

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 21, 2023, 04:06:41 am
content: Understand you’ll have to try all three different possible single phase connection possibilities.I routinely use a universal 150va transformer to power up variable frequency drives at 230/460/575 for testing. Checking inputs with a DMM in diode mode will find shorted input diode bridges thereby saving your expensive transformer fuses. You will fuse it, right?

author: edpalmer42
author_rank: Super Contributor
timestamp: June 21, 2023, 04:58:40 am
content: This situation frequently arises for people who buy old lathes, milling machines, shapers, etc. The solution is a 'phase converter'. Old ones are rotary converters that just look like a big motor. Newer ones are solid-state and are frequently called a VFD (Variable Frequency Drive). You might actually have one in your auction haul.Search on Youtube for 'phase converter' or VFD to learn more about them and how to improvise something.

author: EPAIII
author_rank: Super Contributor
timestamp: June 22, 2023, 08:09:01 am
content: The search term you want is "VFD". Well at least that is what you are going to wind up using. VFD stands for Variable Frequency Drive. And as the name says, it is the frequency that is varied to control the speed of three phase motors, not the Voltage. Yes, three phase motors do come with different Voltage ratings, but any given motor will always be operated at just one Voltage so you may need two or at most three different VFDs to run just about all three phase motors you may come across. And many motors will be dual Voltage with different connections for each. So they can be tested with either of the nameplate Voltages.Most three phase motors are going to be 230 Volts in a delta configuration so that is the first Voltage VFD that you should buy. The 230 Volt VFDs are very common and can be purchased at very reasonable prices - often less than the prices for three phase transformers which have a lot of steel and copper in them. Imported and used ones are available on Ebay all the time.Then there are 230/460 Volt type motors which can be run on 230 Volts. So a 230 Volt VFD will be OK for them too.Only the 460 Volt only motors will require a second VFD and those will be very rare as they would be used only on higher horsepower equipment that would require too much current if run on 230 Volts. The 460 Volt VFDs are built for higher power levels and not as numerous so the price is going to be high. I would not buy a 460 Volt VFD until absolutely necessary.The big problem with VFDs is they often require a 230 Volt input. This can save the cost of a transformer (steel and copper = $s). So you may need to use that dryer outlet or have one installed in your shop/garage.Machines that use three phase power will usually have their control circuitry use only one of those three phases. So it may be necessary to use a designated phase for that. You should read the instructions for the VFD before buying it and also be aware that a different machines may differ in this respect. Some also may have a separate, single phase power requirement.

author: EPAIII
author_rank: Super Contributor
timestamp: June 22, 2023, 08:27:36 am
content: One more thought:While I have seen power supplies that did use all three phases of the three phase input power, they did so to produce over 100 kW of DC power that was used as a primary power in that piece of equipment. They used all three phases to produce that very high power level in a practical manner.A power supply that produces 24 Volts from three phase power is, in all likelihood providing that 24 Volts for the control circuitry in the machine which will, in all likelihood provide a power level more like 10 or 20 or 50 W - that's Watts, not kiloWatts. It will be powered by only ONE phase of the three phase power to the machine and only single phase power, of the correct Voltage, will be required to test it. You can use your 115V outlets in your shop/garage or your dryer outlet for that.

author: Wolfram
author_rank: Frequent Contributor
timestamp: June 25, 2023, 03:00:55 pm
content: Absolutely do not use a generic VFD to power random 3 phase equipment, they should generally only be used to power motors. VFDs are made to power motors, and a vast majority of them have hundreds of volts of high frequency carrier (2 - 30 kHz typically) on the output terminals, along with the programmed output voltage and frequency. This can easily kill electronics in connected equipment.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: July 02, 2023, 09:54:32 pm
content: So I got around to trying the 240/480 to 120VAC transformer in reverse (it's designed to take either 240 or 480 and give 120VAC) - running 120VAC into the output side yeilded 1/10th the expected voltage on the input side - 240 gave approx 20VAC and 480 gave ~46VAC. I'm not sure specifically why I am getting this result, perhaps it is something to do with the dual-input design to accommodate either 240 or 480 input.I may just need to search for and find a 120 to 240 and/or 480VAC transformer.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 02, 2023, 10:22:15 pm
content: Confirm that the transformer is wired correctly. Can you post a picture of it?

author: neverendingstudent
author_rank: Regular Contributor
timestamp: July 02, 2023, 11:23:37 pm
content: Picture attached. Transformer is a Daykin model DKN200A, part number TF501, wired as shown. I have standard US wall voltage (120VAC) wired to the secondary (labeled as 120V) and was checking... well, all output combinations really. I have also tried with and without the H3/H2 bridge bar seen screwed in (transformer came with that bridge wired in) - I saw no change in output H1/H3 or H2/H4. I also checked H1/H4 with and without but, although I don't remember the voltage offhand, it wasn't 240 or 480. I specifically remember 46VAC being the highest measured voltage. Checked with 2 different meters to make sure I wasn't reading scale wrong or something simple like that.Hmm. I suppose one thing I didn't try was swapping the connector legs on the 120VAC side.

author: WattsThat
author_rank: Frequent Contributor
timestamp: July 02, 2023, 11:38:46 pm
content: With no power: confirm low ohmic values (2-5 ohms likely) between:X1-X2H1-H2H3-H4If any of the three windings are open, it’s shot and will not work for the intended use.Always verify meter function first. Don’t use auto-ranging (if your meter has it), select lowest ohm range manually, usually 200 ohms and shorting probes together to verify zero ohms. If all three windings are good, you’ve probably got a measurement error.

author: neverendingstudent
author_rank: Regular Contributor
timestamp: July 02, 2023, 11:43:22 pm
content: Yep, that was it - H1-H2 is shot, 97 Meg-ohms.Well, I do have a second one and that one is showing good - ~5 ohms on each winding. I'll try it next.Edit: sweet, the other one works, I have my 240/480VAC test source!

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 03, 2023, 12:09:40 am
content: There's usually a thermal fuse on the primary side, if that's the problem it would be an easy fix.

author: dmills
author_rank: Super Contributor
timestamp: July 03, 2023, 08:27:46 am
content: Just a note that this works for powering up most control circuits, but will NOT (Without extra help) run a three phase motor load.I run a three phase motor on my lathe by combining a 1.5KVA control transformer wired backwards (Gets me 400V single phase give or take) and a smallish three phase input VFD (Only two phases connected) to get three phase delta for the lathe. Turns out it is a bit on the small side, and that switching to high speed makes the VFD frequency sag until the thing gets to about half speed, looks like I really need about a 5kVA inverter to handle the starting surge.The earth connections take a little thinking about, particularly if you don't have a centre tap on the 400V winding.

author: mairo
author_rank: Unknown
timestamp: July 03, 2023, 12:31:47 pm
content: Maybe something like PacificPower 308-AMX, if you can find it for cheap.https://pacificpower.com/products/amx/308AMX_Datasheet.pdf