Step-down transformer backward?

[Maynard A. Philbrook Jr.]

In article <be235450-a517-43e5-89f1-ff743ea44a1b@googlegroups.com>,
pallison49@gmail.com says...

Fester Bestertester wrote:


1.2 H. Which works out to 450 K-ohms, 2 mA. Seems pretty small...


** Wanna try that again ?

1.2 x 2 x pi x 60 = 452

120 / 452 = 0.265


.... Phil

Yes, that's correct.

One should remember that H does not invert, which
would give the answer of 2 mA, if one did.

Jamie

 

[Fester Bestertester]

1.2 H. Which works out to 450 K-ohms, 2 mA. Seems pretty small...

** Wanna try that again ?
1.2 x 2 x pi x 60 = 452
120 / 452 = 0.265
.... Phil

Yes, that's correct.

One should remember that H does not invert, which
would give the answer of 2 mA, if one did.

Jamie

The quiescent primary (115 V) current is not measurable on a 6-digit HP bench
DMM.

Why would this be so?

 

[RobertMacy]

On Tue, 26 Aug 2014 19:40:29 -0700, Fester Bestertester <fbt@fbt.net>
wrote:

...snip...

The quiescent primary (115 V) current is not measurable on a 6-digit HP
bench
DMM.

Why would this be so?

At the risk of insulting...
1. you have to select AC, or ~, else your DMM is reading DC current
2. to read current, you have to plug the test lead into a 'different' hole?
3. the input fuse got blown

 

[default]

On Sun, 24 Aug 2014 07:03:10 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

On Sat, 23 Aug 2014 22:43:08 -0700, Fester Bestertester <fbt@fbt.net
wrote:

I have this transformer:

http://www.signaltransformer.com/sites/all/pdf/M4L.pdf

model M4L-2-10, 210/230/250 primary with 115 secondary.

Is it safe to use it in reverse to get 230v from 115 mains?

This is in N. America.

Thanks.



Be ready for the voltage out to be around 90 - 92% of expected.

I found out the hard way once when I wanted to make an 'isolation
transformer. I had two 'bell' transformers so thought aha! put them
together 115 down to 24 V then 24 v back up to 115! WRONG! only got 100V,
why? seems the transformer companies add some turns, some oomph, to make
up for the losses. Draw out the model and you'll see.

So conclusion is may work may not. but is safe to do. The 'shell' of the
transformer is referenced more to the secondary, so just watch your
isolation.

That is particularly true of small low current transformers, and in
the case of bell transformers, you are talking about a transformer
that may have a loosely coupled primary to secondary or high
resistance secondary so it can withstand the kind of abuse expected in
that kind of service.

Ever wonder why they are so large for such low VA ratings?

 

[Fester Bestertester]

At the risk of insulting...
1. you have to select AC, or ~, else your DMM is reading DC current
2. to read current, you have to plug the test lead into a 'different' hole?
3. the input fuse got blown

The 3A meter fuse is blowing when i try to measure the quiescent curent.

When i run the transformer from a variac it gives expected output with no
indications of shorted turns (i.e., runs cool).

isn't this unusually high even for a "backward" configuration?

have to dig out my 10A meter...

 

[RobertMacy]

On Fri, 29 Aug 2014 06:29:05 -0700, Fester Bestertester <fbt@fbt.net>
wrote:

At the risk of insulting...
1. you have to select AC, or ~, else your DMM is reading DC current
2. to read current, you have to plug the test lead into a 'different'
hole?
3. the input fuse got blown

The 3A meter fuse is blowing when i try to measure the quiescent curent.

When i run the transformer from a variac it gives expected output with no
indications of shorted turns (i.e., runs cool).

isn't this unusually high even for a "backward" configuration?

have to dig out my 10A meter...

A primary core current of 0.25A referred to the secondary would only be
0.5A and even with some amount of extra say up to 0.75A, yeah that does
sound large! Perhaps, the 'connection' technique? and surge currents?

I'm super skittish about doing current measurements, since to measure
current the meter is a short and I found that if you accidently leave the
test leads in the meter configured as current measurement and switch to
voltage reading and then try to read a voltage; the meter is still a
short! So now I always test with a small resistor in series measuring the
voltage drop first, then maybe a current meter. Hey, fuses here are $1.25
ea!

 

[Phil Allison]

On Friday, August 29, 2014 11:29:05 PM UTC+10, Fester Bestertester wrote:

At the risk of insulting...

1. you have to select AC, or ~, else your DMM is reading DC current

2. to read current, you have to plug the test lead into a 'different' hole?

3. the input fuse got blown



The 3A meter fuse is blowing when i try to measure the quiescent curent.



When i run the transformer from a variac it gives expected output with no

indications of shorted turns (i.e., runs cool).

isn't this unusually high even for a "backward" configuration?

have to dig out my 10A meter...

** You ought to use the Variac and bring the AC up from zero when doing a test for magnetising current.

Simply switching on that tranny is likely to create a surge of 300 or 400 amps peak on the first half cycle - if you manage to switch simultaneously with a zero crossing in the AC voltage.

Probably take out even a 10amp slo-blo fuse.


..... Phil

 

[petrus bitbyter]

amdx <nojunk@knology.net> wrote in news:ltcrgj$14d$1@dont-email.me:

On 8/24/2014 5:23 AM, petrus bitbyter wrote:
Fester Bestertester <fbt@fbt.net> wrote in
news:0001HW.D01EC97C00C0F566B02919BF@news.eternal-september.org:

I have this transformer:

http://www.signaltransformer.com/sites/all/pdf/M4L.pdf

model M4L-2-10, 210/230/250 primary with 115 secondary.

Is it safe to use it in reverse to get 230v from 115 mains?

This is in N. America.

Thanks.



Should be no problem. A transformer can be used in both directions.

BTW 1000VA is quite a heavy signal

petrus bitbyter


I once described a setup using a transformer backwards,
Phil Allison, said it was a bad idea, something regarding
magnetizing current is specified and wound for the primary.
The secondary may not have the turns needed for the proper
magnetizing current.
My setup worked short term, that's all we needed,
might have been a problem if we tried to run it 24/7.
I am accepting Phil's input, even though I always thought
transformers were reciprocal.
One caveat, my setup was using two transformers for isolation and
looked like this 120//12 to 12//120 driving a many, many section
Cockcroft–Walton generator. I think we were up near 10kV.

Mikek

Nobody is perfect. Even I am not, neither is Phil.
Transformers are not perfect either. But you can optimize a transformer
to do do better in one direction, As a result it will do less good in
the other. Not all transformers are designed this way. But when enegy
saving is an issue, even little bits will help. 0.1% of 1MW is still a
lot of power. Nevertheless, unless you have an extreemly special
transformer it will do in both directions and not only in hobby
circuits. (That's not to say it will do in all professional
applications.)

petrus bitbyter

 

[Maynard A. Philbrook Jr.]

In article <0001HW.D025CE3100BC5776B02919BF@news.eternal-september.org>,
fbt@fbt.net says...

At the risk of insulting...
1. you have to select AC, or ~, else your DMM is reading DC current
2. to read current, you have to plug the test lead into a 'different' hole?
3. the input fuse got blown

The 3A meter fuse is blowing when i try to measure the quiescent curent.

When i run the transformer from a variac it gives expected output with no
indications of shorted turns (i.e., runs cool).

isn't this unusually high even for a "backward" configuration?

have to dig out my 10A meter...

it's in rush current..

cross your leads, fire up the tranny and then
uncross them..

Jamie

 

[Fester Bestertester]

A primary core current of 0.25A referred to the secondary would only be
0.5A and even with some amount of extra say up to 0.75A, yeah that does
sound large! Perhaps, the 'connection' technique? and surge currents?

Measured: 0.92A primary steady-state current.

Not unreasonable for my needs.

Thanks to all who helped me. Much appreciated.

Cheers.

 

At 0.92 amps, that is almost a hundred watts on 120 volts. Are you sure you're "cool" with this ?

You're talkimg unloaded right ?

 

[Phil Allison]

On Monday, September 1, 2014 4:45:27 PM UTC+10, jurb...@gmail.com wrote:

At 0.92 amps, that is almost a hundred watts on 120 volts. Are you sure you're "cool" with this ?

** It is nothing like 100 watts cos the phase angle is almost 90 degrees - ie it's an inductive load !!!!

Magnetising current is also highly non linear ( so not a sine wave), reaching peak value around each zero crossing of the AC voltage.

In this example, the current is 10% of the full load value so represents only 1% of the primary copper loss at full output.

Trivial.



..... Phil

 

[DaveC]

At 0.92 amps, that is almost a hundred watts on 120 volts. Are you sure
you're "cool" with this ?

You're talkimg unloaded right ?

Yes unloaded.

I seldom have need for 220v and then it's not for more than a few amps. So
wasting 100w is OK for occasional use.

For regular use it's criminal, agreed.

Cheers.

 

[Phil Allison]

On Tuesday, September 2, 2014 5:13:01 AM UTC+10, DaveC wrote:

At 0.92 amps, that is almost a hundred watts on 120 volts. Are you sure

you're "cool" with this ?



You're talkimg unloaded right ?



Yes unloaded.



I seldom have need for 220v and then it's not for more than a few amps. So

wasting 100w is OK for occasional use.



For regular use it's criminal, agreed.

** FFS learn some basic electrical theory !!!!!!!!

What is the difference between VA and watts ??

What is power factor ?

What does phase angle mean ?

Ever hear of Wiki ??????


Sheesh....


.... Phil

 

>"Yes unloaded"

As Phil pointed out, I forgot it was a transformer. You got maybe 100 VA, NOT watts because volts and amps are out of phase.

Maybe what threw me off was about the inrush. But that was when you had it loaded, right ?

I screwed up on that one. I was thinking maybe an incandescent load or whatever. I missed part of the details.

 

>"** FFS learn some basic electrical theory !!!!!!!! "

See response supra. I assumed a load of lighting, perhaps incandescent or even worse when it comes to inrush and ass u me d a load like an incandescent.

You were already recognized here a few minutes ago. of ocurse you might wnat to rant and rave for a minute, but we're used to that.

Rave on, just keep it down to a mild roar and destruction of less than 1,200 square miles. THAT'S RIGHT, MILES ! Fuck the metric system. Try to divide by three in that shit.


https://www.youtube.com/watch?v=0IUV-QxwlRM

No doubt now huh.