Auto transformer with sketchy specifications - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: December 10, 2023, 01:08:02 am
content: I need a 60Hz, 240V to 220V transformer to power some ac servos. I found this transformer, supposedly rated for 3kVa. Fortunately I need considerably less than 3kVa and I suppose this thing might actually deliver 3kVa for short spurts, but it looks grossly overrated by CSA standards.Salient points:1) Input power cord (120V in) uses 1.5mm^2 (AWG16ish), not sure what UL/CSA lets manufacturers get away with for chassis wiring, but I'm sure this would not fly.2) The winding on the transformer measured to be 0.0625" so appears to be close to AWG143) the circuit breaker (looks like a circuit breaker, might just be a switch) is rated for 20A4) The stickers on the back claim 13.6A out at 220V. that's close enough to 3kVA, if it actually does that, input current would be close to 25A5) no ground.I guess I'm going to load test it,what do you think an acceptable transformer temperature is as measured on outer windings?Comments welcome.

author: jonpaul
author_rank: Super Contributor
timestamp: December 10, 2023, 10:11:02 am
content: Bonjour: Vague post:" 60Hz, 240V to 220V transformer "What do you mean: Input to a variable transformer is between 220 and 240V OR you need to convert 220 V to 240V?"to power some ac servos" No idea what you mean.Motor size? Voltage? VA? locked rotor possible?What is the core dimension and weight of the transformer?From China? Korea?j

author: soldar
author_rank: Super Contributor
timestamp: December 10, 2023, 11:04:03 am
content: That looks way too small for 3kv. you could give dimensions, resistance of windings, etc.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: December 10, 2023, 11:34:28 am
content: looks like you have been skammed, sorry the thinn wires you mantion do give it away unless they are made from special super conductor

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2023, 11:43:36 am
content: To buck 240V down to 220V, with a 3KVA load, you only need a 20V 250VA (220V in) transformer!However it must be wired as an autotransformer with its original primary and secondary in series and in phase, with 240V applied to the top end of the former secondary, so the currents in the two windings sum at the output. DO THE MATH and confirm the winding currents are in spec for a 250VA transformer . . .

author: langwadt
author_rank: Super Contributor
timestamp: December 10, 2023, 01:41:16 pm
content: Quote from: Ian.M on December 10, 2023, 11:43:36 amit only has to supply the (3000VA/220V) = 13.6A times (240V-220V) = 20V, so ~= 273VAhttps://youtu.be/mb75-kVVtW8?si=9EdSMera4T85s0hN&t=880

author: CaptDon
author_rank: Super Contributor
timestamp: December 10, 2023, 01:42:31 pm
content: The core size of that transformer looks similar to a microwave oven transformer and as such would support only around 1KVA. Is that a multi-tapped transformer? It isn't built like a variac unless I am missing something in the picture. If it works like a buck / boost transformer providing a few volts above and below the input voltage than perhaps it could do 3KVA although it still lacks durability in other areas.

author: CaptDon
author_rank: Super Contributor
timestamp: December 10, 2023, 01:45:22 pm
content: I see on closer inspection it is a buck / boost autoformer tapped arrangement, so the core could support 3KVA in that fashion of connection but overall the build quality is barely acceptable.

author: soldar
author_rank: Super Contributor
timestamp: December 10, 2023, 01:53:31 pm
content: Quote from: CaptDon on December 10, 2023, 01:42:31 pmIt looks smaller than that to me.Also I think input is 120V and output range 220 - 240.The OP is not clear.

author: CaptDon
author_rank: Super Contributor
timestamp: December 10, 2023, 02:14:02 pm
content: Wow, if indeed it is 120 in and slightly variable 240 out it is way underbuilt and over spec'd!!!

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2023, 02:16:18 pm
content: Quote from: langwadt on December 10, 2023, 01:41:16 pmThat's true if you wire it as a simple buck transformer (i.e. 240V primary, secondary phased to subtract from line voltage, but wired as an autotransformer, the current direction in the lower winding is opposite that in the upper winding, so in a step-down configuration (where they add at the tap), you need less 'secondary' current, so a lower VA rating is sufficient.Of course, in the real world, pushing it to the bleeding edge for anything running 24/7 is a *BAD* idea so speccing a 300VA transformer would be sensible unless its intermittent duty.Quote from: CaptDon on December 10, 2023, 02:14:02 pm... and ungrounded. At best, its a transformer kit good for maybe 1.5 - 2 KVA, needing a three core line cord fitted, probably 14 AWG, but at worst its another far east death trap, like the infamous'Red Peril' Variacthat Tkamiya bought, tested, tore down, then had no choice but to scrap it as it could not sensibly be brought up to code!

author: Jester
author_rank: Frequent Contributor
timestamp: December 10, 2023, 02:23:22 pm
content: Quote from: langwadt on December 10, 2023, 01:41:16 pmThis device as designed routes the 120V input to one of the various taps based on the selector switch. With 120V in, the possible output voltages available (unloaded) with the selector switch is 218, 225, 232, 244, 252V.The core is 5" x 4" x 2.25"I was planning on powering this from 120V and use the 218V tap, I don't see any way to power from 240V and get 220V, to lower input current required.The ac servos (750W) are actually 3-phase however they have a driver that requires 220Vac in with an absolute maximum upper limit of 242V. Mains here are 240V but can and do wander above 240, I just measured and found 244VThere will be four servos, however at a given time at most two will be functional and in actual practice will typically only require about 200W or less.I'm guessing the transformer is configured as shown below.

author: langwadt
author_rank: Super Contributor
timestamp: December 10, 2023, 02:50:49 pm
content: Quote from: Jester on December 10, 2023, 02:23:22 pmgoing from 120V to 218V the transformer has to handle ~100V at the load currentto go from 240V to 220V you'd have to wire it in reverse, i.e. power in on the Vout and output from one of the taps

author: Ian.M
author_rank: Super Contributor
timestamp: December 10, 2023, 02:58:27 pm
content: I suspect the taps are mid-winding, so (short of rewinding) you cant buck 240V to 220V with it. OTOH unless the machine has other 220-240 V loads, its not worth wiring it for 240V in based on the O.P's description of the peak and average loads.

author: langwadt
author_rank: Super Contributor
timestamp: December 10, 2023, 02:59:38 pm
content: Quote from: Ian.M on December 10, 2023, 02:16:18 pmyeh, easier to see when redrawn with a regular transformer