Testing my Principal High Power 1 to 1 Mains 240vAC to Mains 240AC / 110vAC 50hz isolation transformer.

[inotarobot]

Hi all, just thought I share some pics of my testing of my Primary High Power 1 to 1 Mains 240vAC to Mains 240AC / 110vAC 50hz isolation transformer. .

I had opened the Transformer by removing its outer metal casing to do

1. a visual inspection of the lamination stack and the windings, as I had done a megger test at 500v and 1000v, and results were a little lower than they were 5 years back.

2. I needed to upgrade the input 240vac cable as it was at least 20+ years old. Also needed a longer length and a more flexible cable.

3. NEED to add a winding selection switch and wire 2 different output sockets, as this transformer has split secondary windings that give in series 240vac OR in parallel 110vac

So while I had core exposed :-

1. I did a low temp 'bake' of the unit, then a retest with my megger. Result was an acceptable improvement in the Insulation Resistance test.

2. I choose to set up unit and do a few measurement on i/p voltage to o/p voltage. You can see the setup in the pic. It consisted of :-

A 0 to 260vac Variac with its own meters, and 2 of my Tektronix Programmable Digital Multimeters to read the Isolation transformer's Input and output voltages.

The Tek meters were powered by a 'floating' UPS (no mains input) and you can see I did not connect their case ground in any way to this system under test.

So interestingly I found this ONE TO ONE isolation transformer was NOT quite 1 to 1. it had approx a 9 volt ac difference input to output. with 241.089vac input my output was 232.053vac

Still need to finish reassembling it and then I will do a load test on it; again monitoring voltage as in pic. However I will add 2 more of the Tektronix Programmable Digital Multimeters and use them to measure input and output AC Current.

Will also have look at I/P and O/P waveforms on my Tek 7834 Scope under 4 different load steps. 25% 50% 75% and 100% 15 mins at each should be adequate.

Not forgetting to check temperatures.

No real issue but just a potential 'got yar' if I am using this isolation transformer with this variac to test a Linear Power Supply.

Anyways was cleaning up my pics so decided to share and do this post as I felt this info could be helpful to someone on here in the future.

BTW you will notice I had not put any bolts through the Lamination Mounting holes while testing. The lamination stack was really tight and I was not doing any loading. Had lamination stack been a tad 'loose' ( lamination's ablke to be separated at corners by a finger nail) I certainly would have added bolts and tightened them.

 

[inotarobot]

You know sometimes being a 1 man band in your home lab / workshop, I on occasions, overlook things during bench work; for me especially when I am working on my test gear or tools.

I was sitting back eating dinner and looking at the pics I posted and I noticed in that first pics, a small white card with winding detail on it.

That's when it struck me ( no that was not the Nip of Whisky at now 9.44pm),

I had not added the info on that white card into my equipment maintenance book.

BUT ALSO even more important I don't recall doing an Insulation Resistance check between Input Windings and the Output Windings:

OR even an Insulation Resistance check of Metal Isolation barrier plate you can see in the left side of transformer in top pic in this post; that has a black with white striped lead connected to it with 2 yellow crimp terminals, to Input Windings and Output Windings

I know I likely would have, should have. BUT did I ?

Since this Transformer's primary purpose is ISOLATION of the Loads Input AC compared to my Building AC Supply these are some of the most important tests.

Thinking ...if Input windings to Lamination Core are Very High Resistance, as is Output Windings to Lamination core, then it should follow Input Windings to Out Windings Insulation Resistance should also be HIGH in the 10 to 100's of Meg.

BUT that does not rule out my doing it anyway.

 

[mark0x01]

That is quite a beast.
I see it has what looks like a grounded shield between the cores.
An improvement on a separate 1:1 isolation transformer and variac in series.
I tend to stay at very low voltage nowdays since the eyesight, brain and nerves are considerably more vintage than before