Step-down transformer backward?

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

Why would this be so?

Reply to
Fester Bestertester
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** A tranny with no magnetising current is simply not possible.

For the ML4-2-10 ( 1 KVA e-core) expect to see around 0.5amps rms when the 115V winding is energised.

..... Phil

Reply to
pallison49

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
Reply to
RobertMacy

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?

Reply to
default

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...

Reply to
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?

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!

Reply to
RobertMacy

amdx wrote in news:ltcrgj$14d$ snipped-for-privacy@dont-email.me:

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

Reply to
petrus bitbyter

it's in rush current..

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

Jamie

Reply to
Maynard A. Philbrook Jr.

** 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

Reply to
Phil Allison

Measured: 0.92A primary steady-state current.

Not unreasonable for my needs.

Thanks to all who helped me. Much appreciated.

Cheers.

Reply to
Fester Bestertester

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 ?

Reply to
jurb6006

** 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

Reply to
Phil Allison

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.

Reply to
DaveC

** 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

Reply to
Phil Allison

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.

Reply to
jurb6006

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.

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No doubt now huh.

Reply to
jurb6006

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