Transformer rating with secondaries in series or parallel question - Page 1
Forum section: electronics / projects

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 18, 2020, 09:02:46 pm
content: 6660V single phase transformer rated 5.75 kVA. I wire the secondary windings in parallel instead of series to give 3330V How many amps will it deliver within its ratings please? Thanks. Photo of the beast attached below.

author: Gyro
author_rank: Super Contributor
timestamp: December 18, 2020, 09:17:03 pm
content: Firstly, are you really sure you want to power it up at all? It's instant and messy death!The answer is worryingly simple, it's still 5.75 kVA. Divide Power by volts.Some observations though...- The transformer indicates that it is designed for forced air cooling. It might even suffer heat build-up without load, I don't know. There doesn't seem to be any indication of required airflow. Without cooling, output current will depend on thermal mass and time.- The secondary windings may not be exactly balanced. You may get additional heating if you parallel them.- 5.75 kVA is well above what you could pull through a 13A plug - that's steady state, not considering the, probably very high, inrush current. It would require a dedicated breaker and directly wired radial circuit from the consumer unit.

author: Zero999
author_rank: Super Contributor
timestamp: December 18, 2020, 09:40:32 pm
content: Just one question: why? This is one of those questions which should never be asked, because if you need to ask, you lack the knowledge to do it safely!

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 18, 2020, 10:27:50 pm
content: Quote from: Zero999 on December 18, 2020, 09:40:32 pmSpare transformer for a Henry 3CX3000 valved RF generator. 6.6kV through a full wave bridge is too much, 3.3kV would be OK. It runs on a dedicated industrial mains supply, same as runs a *big* single phase TIG set, so no 13 Amp plugsI would be getting a pro to install if required. The existing transformer is force air cooled with a centrifugal blower. Thanks. I don't see many single phase transformers like this in the UK and shipping from the States is horribly expensive. It works fine, I dried it out by feeding the secondary for a week as it came from unknown storage conditions. I fully appreciate 6 or 3kV is dangerous

author: IanB
author_rank: Super Contributor
timestamp: December 18, 2020, 10:37:26 pm
content: I see two bushings where it says 6660 V, which makes me think this is a utility transformer and that side is normally the primary. I guess you are going to run it backwards and there are two independent secondary windings you can parallel up?Often running such transformers backwards creates a huge surge current on start up and either some ballasting or a soft start mechanism is required.

author: Chris Wilson
author_rank: Super Contributor
timestamp: December 18, 2020, 10:49:35 pm
content: Quote from: IanB on December 18, 2020, 10:37:26 pmIt came from a different make of RF generator so was purpose made for a similar application, definitely not a utility transformer Ian. It seems happy with a modest load, even with no blower, so I am interested in how many amps it should be happy with, with the forced air cooling. In parallel into a full wave bridge it gives around 4500V DC from memory. Thank you.