Any ideas about this oddity?
Any ideas about this oddity?
(OP)
I have this little control transformer I've used for years to provide me with 230, or 460V for testing or powering controls I've built.

For the first time I need it for 208V. So I fed it 115.0V and checked the 200V tap and found it only had 186V. I brought it home and plugged it into 120.0V +5.0 more and.... all I get is:

Is this reasonable?

For the first time I need it for 208V. So I fed it 115.0V and checked the 200V tap and found it only had 186V. I brought it home and plugged it into 120.0V +5.0 more and.... all I get is:

Is this reasonable?
Keith Cress
kcress - http://www.flaminsystems.com





RE: Any ideas about this oddity?
You should have a Variac variable transformer to adjust AC voltages to meet a precise spec for testing. I've got a half dozen because they keep showing up at flea markets for $5.
RE: Any ideas about this oddity?
Just for fun, try putting 240 Volts into the 230 Volt input and check the voltages at the 115 Volt and the 200 Volt connections.
I'm guessing about 208 Volts at the 200 Volt tap.
115 Volts x 240/230 Volts = 120 Volts plus 2% turns ratio error = 122 Volts at the 115 Volt output..
Bill
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"Why not the best?"
Jimmy Carter
RE: Any ideas about this oddity?
Brainstorming what might be different about your situation:
1 - small transformers (0.35kva) act different than large (>5MVA)? Maybe higher p.u. leakage reactance and lower p.u. magnetizing reactance.
2 - lv leakage reactance higher than hv leakage reactance? (you're supplying magnetization thru low side vs TTR set typically on high side).
3 - measurement accuracy? Did you confirm 115V and 120V at the LV terminals during the test.
I've never played with small transformers but my guess would be #1 for what little it's worth.
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(2B)+(2B)' ?
RE: Any ideas about this oddity?
What do you have on the H1-H3 terminals while you're making your measurement? If that's loading the transformer it could be compounding the problem.
RE: Any ideas about this oddity?
Gunnar Englund
www.gke.org
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Half full - Half empty? I don't mind. It's what in it that counts.
RE: Any ideas about this oddity?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Any ideas about this oddity?
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Any ideas about this oddity?
https://www.geindustrial.com/sites/geis/files/gall...
So if the transformer is set up to provide a higher-than-nameplate no-load "output" voltage (for nameplate input) when we define in/out = hv/lv, then it will provide a lower-then-nameplate no-load "output" voltage when we reverse it to operate as in/out = lv/hv.
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(2B)+(2B)' ?
RE: Any ideas about this oddity?
I'll run it "forward" and tell you what happens. Bill et al.
Pete; Thanks. Yeah, I checked the input 115V and 120V maybe a second before reading the outputs I reported.
David; Hi, I had nothing on those outputs when I did the above. I meant to point that out because it would certainly have made a notable difference. I'm using it to run a panel while I make the big clacky things rattle and freak my dog out with a PLC, but during the test the disconnect was open.
Gunnar; A Squariac?! Now I've heard everything. For a while there was a bench-top power supply company that made a standard lab supply that worked with only a linear regulator. As you turned up the output voltage you could hear it changing transformer taps. I thought that was really clever.
I have two Variacs but one is enormous and completely unenclosed making it scary to use and relegated to last resorts. The other is nice but weighs about 30 lbs and is a huge box with a cord that's an 1" in diameter so I tend not to drag it out either. I really should find a petite desk model for issues like this.
Thanks Jeff for the technical. I've always kind of thought about transformers as the simple thing sold to us in school. I need to keep these non linear details in mind.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Any ideas about this oddity?
(Bill's comments didn't register with me the first time I skimmed them)
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(2B)+(2B)' ?
RE: Any ideas about this oddity?
All the small control transformers we use produce a higher no-load voltage then their nameplate would suggest. For a 600:120 transformer, instead of actually getting a 5:1 voltage ratio it's more like a 4.8:1 or 4.7:1 voltage ratio.
This is why you don't use a control power transformer for relays or voltage meters. You use a potential transformer.