Transformer +/- 15v

[Brad93]

If using a 400v transformer, with -15v 0v and +15v terminals
on the primary.

I should use 400v and +15v right since we are on 415v three phase here?

 

[Seadog]

Yes

 

[minimutly]

Yes too, although in theory we are now on 400V...

 

[m_c]

Theory is all good, but input voltage can vary between areas, and fluctuate throughout the day.

I'd probably check what the output voltage tolerance is, then adjust accordingly, but at 15V output, a 15V variation on the input is only a 0.6V change on the output.
The output probably varies more due to load variation.

 

[Woody.v8]

Yes too, although in theory we are now on 400V...

Tell my test meter that

 

[minimutly]

If it’s an isolating transformer I would expect the output to be 415v? Or did I read it wrong?

 

[m_c]

If it’s an isolating transformer I would expect the output to be 415v? Or did I read it wrong?

You read it right, but isolating doesn't mean it simply outputs what you put in.
Isolating means the input and output are electrically separate circuits.

 

[Seadog]

415 in plus 2x 115v out. Or reverse it for 230v in and 415 out (plus all of the interim steps from the tappings).

 

[m_c]

Just realised something, you'll want to use the -15V terminal.
Using the +15V will result in an expected input voltage of 385V.

 

[Seadog]

Minimutly

415 in plus 2x 115v out. Or reverse it for 230v in and 415 out (plus all of the interim steps from the tappings).

 

[Brad93]

I think i left it on 0V.

My fluke tells me the voltage between L1 and L2 is 415v today

 

[daleyd]

Connect it up

Measure output volts

Move input if not within spec

Re measure output

Move other way if still wrong

Job jobbed.

 

[Brad93]

Connect it up

Measure output volts

Move input if not within spec

Re measure output

Move other way if still wrong

Job jobbed.

I did measure the output volts with input on both 0v and +15v and the output was the same

 

[daleyd]

I did measure the output volts with input on both 0v and +15v and the output was the same

Is it just an isolating transformer, no step up/down? Did you try it on -15?

I’ve only ever used step down for controls voltage so 230/415 in and 110 out - the different inputs always made a difference to the output when setting those up, maybe it doesn’t make a difference on a plain isolating transformer?

I would have thought it would do though

 

[m_c]

I did measure the output volts with input on both 0v and +15v and the output was the same

What was the output voltage?

400/415 is less than 4% difference, so isn't going to have much noticeable effect on lower voltage outputs.
Bigger issue is possibly the transformer running hotter due to a slightly higher voltage than the actual rating, but I'd guess there is a fairly large margin before it became a problem.

 

[Brad93]

Is it just an isolating transformer, no step up/down? Did you try it on -15?

I’ve only ever used step down for controls voltage so 230/415 in and 110 out - the different inputs always made a difference to the output when setting those up, maybe it doesn’t make a difference on a plain isolating transformer?

I would have thought it would do though

Yes it's for control voltage

 

[Brad93]

What was the output voltage?

400/415 is less than 4% difference, so isn't going to have much noticeable effect on lower voltage outputs.
Bigger issue is possibly the transformer running hotter due to a slightly higher voltage than the actual rating, but I'd guess there is a fairly large margin before it became a problem.

Didn't make a note but 115.-- something volts on either setting.

So i should have it on the -15v setting you're saying?

I wish RS' documentation was a bit clearer

 

[daleyd]

Didn't make a note but 115.-- something volts on either setting.

So i should have it on the -15v setting you're saying?

I wish RS' documentation was a bit clearer

I’d try it on all 3 connections - if negligible difference then don’t worry too much about it.

Whenever I’ve done it there’s always been a small, but noticeable difference. Our machines came from France so they were set up to be tested at the factory with 400 - when commissioned on site over here then they needed changing.

Can’t remember which way as it’s been about 10 years since I worked there but they were pretty much always needed adjustment - and it did make a difference to the output.

Again, I wouldn’t have thought so - but maybe some load might show up a difference.

 

[Brad93]

I've just found this via Schneider so when i go back to replace the main supply cable i will alter the transformer to be on +15v.

Output voltage was negligible it's only a control circuit with no lamps etc anyway.

The equipment did have a 400v Primary to 115v & 20v secondary transformer.

Price from OEM, $1015.

The 20v powers a motor protection board which monitors two thermistors in the motor.

Since a aftermarket motor overload has been fitted, i've disconnected the motor protection board.

What i'm hoping is the motor protection board will run from a small separate transformer possibly 400v to 24v.
Although the 24v may upset the board or the thermistor temperature resistance.

 

[m_c]

Didn't make a note but 115.-- something volts on either setting.

So i should have it on the -15v setting you're saying?

I wish RS' documentation was a bit clearer

I'd have expected a bit more variation on a 115volt output, but it could be a saturation issue limiting the voltage.

From my interpretation, I take the windings to be -15, 0, 15, 240, 400, so if you think of them as being connections along the windings in that order, 0 & 400 gives 400. 15 & 400 gives 385. -15 & 400 gives 415. And if you wanted to try something daft, 240&400 would give 160.
You're right in that the datasheet isn't that clear, but my interpretation of the wording -

Standard primary winding 0-230-400VAC with a +/- 15V adjustment possibility around 0.

is that it means the +/-15 is relative to the 0V point.
A schematic of the coil would certainly help clarify things.

You could check by measuring voltages between the 0 and +/-15V terminals, as the input side should act like an auto-wound transformer with the terminals having the approximate voltage present on them.