Step down transformer voltage too high ! 4

[IntegrityTesting]

Well not really... I need to drop 240 to 120. The transformer is doing precisely what it is supposed to do. My input voltage of 250V is giving me an output of 136V, far too high for my comfort! Definitely my fault for casually picking out a transformer and not thinking about my higher source voltage…

I have a fairly common Acme transformer, 2kva 240/120 with multiple taps… Is there an elegant solution utilizing different tap combinations to get me to an acceptable range for 120V equipment/appliances???

In attached the wiring diagram, and a trans spec sheet…

Thanks for your help!!!

 

[IntegrityTesting]

 

[ScottyUK]

Do you need the full 2kVA? If you can live with a significant de-rate then I think there are options but you will under-utilise the core and will essentially have a transformer which can only deliver about half of what it should be capable of.

 

[IntegrityTesting]

Hi ScottyUK,

I can definitely live with 1/2 the power no problem. Powering several small items that don't need to be running at the same time...

Thanks!

 

[ScottyUK]

Ok, so you have two identical windings with taps at:

H1(H6) 0V
H2(H7) 190V
H3(H8) 200V
H4(H9) 208V
H5(H10) 220V

For 250V you need to connect one full 220V winding in series with the 30V tap of the second winding. To do this you should connect the supply line and neutral across H1 and H10 with a link between H5 and H7. The secondaries stay in the standard configuration for 120V.

 

[waross]

I agree Scotty.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Before you go off too far though, are you measuring the output voltage with the full expected load? Because if your meter is the only thing connected, it is always going to read high. Control Power Transformers are like that on purpose.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[IntegrityTesting]

Spectacular!

Thanks Scotty and the confirmation waross!

I figured just there was a relatively simple solution with the available taps...

jraef, I'm reading the input 250 on a panel meter verified by a DVM with an average 10A load on a 50A circuit... The secondary voltage was indeed just measured with a dvm on the secondary's unloaded, but I was also concerned that the regular 5A/120 load (refrig compressor) was going to really not do well very long at 135V

It sure seems like 15% IS A BIT TOO MUCH OVER???

Thanks guys!

 

[ScottyUK]

If the secondary voltage drops too low under load then use the H8 or H9 taps instead of H7 for nominal primary voltages of 240V or 228V nominal respectively instead of 250V.

 

[IntegrityTesting]

Issue resolved!

Thanks a ton...

122V without load, absently forgot to measure under load. 2 desired units - A battery charger and small refrig compressor pulling 5.3A

 

[ScottyUK]

Thank you for the feedback - glad you got the problem sorted out.