transformer question

[qsiguy]

I know that you can wire transformers in reverse. Say you have a transformer that goes from 220V on the primary to 110V on the secondary. If you feed it 110V on the secondary you'll get 220V on the primary. (correct me if I'm wrong). My question is does the voltage you feed it need to be what it's rated at or are you mainly concerned with the ratio of the transformer? For instance, if you have a transformer with a primary voltage of 110V/220V and a secondary of 24V can you only feed the secondary with 24V? What would happen if you wire 110V to the secondary? Would you just toast the transformer or if not, what will you get on the primary?

Thanks for the information.

 

[audioguru]

Transformers have pretty good insulatuion that can probably stand up to a couple of thousand volts. Some transformers don't have enough turns because they are very cheap and thier efficiency doesn't matter like in microwave ovens that are used only for a few minutes at a time at nearly full power. If one of those cheap transformers is used in reverse then it would draw a high current without a load and would get too hot with a load.

Some cheap wall-wart transformers also don't have enough turns and get hot without a load. They would melt if used in reverse.

 

[qsiguy]

So does it just matter what the ratio is to determine what voltage you get out based on the voltage you put in on either the primary or secondary?

 

[audioguru]

The voltage ratio of a transformer that operates at its rated frequency and power is the turns ratio. Without a load then the voltage output will be a little higher.

 

[qsiguy]

Ok, so if I have a transformer that has a primary of 120V and a secondary of 24V that transformer has a turns ratio of 5:1, correct? If I feed 120V into the secondary then I'll get 600V at the primary. Am I calculating this correctly?

Sorry to keep asking basically the same question. This question came up at work where one of my field techs was told to wire up a transformer that showed 220/110 for primary and 24 for the secondary and another one of my techs told him he could feed 110V and get 220 out but I told him I don't think he as the right transformer. He didn't have many details on the transformer he was trying so his info was a little vague.

 

[audioguru]

A transformer with a 220/110 primary has two primary windings. Connect them in parallel for a 110V mains and connect them in series for a 220V mains.

It won't be isolated if you feed 110V to one winding and get 220V out of both windings in series and therefore would be dangerous.

 

[eblc1388]

.... a transformer that showed 220/110 for primary and 24 for the secondary and another one of my techs told him he could feed 110V and get 220 out but I told him I don't think he as the right transformer.

One can actually connects up the transformer in an auto-transformer fashion to obtain a 220V output while feeding in 110V. The turns or voltage on the secondary does not matter and no connection to the secondary is required.

The winding ratio here is 1:2 and not 5:1. You can lookup on autotransformer for more details.

 

[qsiguy]

Ah, that makes more sense now. So if the primary of a given transformer has inputs on the primary side for 220 and 110 you could put 110 to the center and one end terminal and you'd get 220 on the two ends and you wouldn't even need to wire anything to the secondary. Is that what you are talking about?

 

[Nigel Goodwin]

Ah, that makes more sense now. So if the primary of a given transformer has inputs on the primary side for 220 and 110 you could put 110 to the center and one end terminal and you'd get 220 on the two ends and you wouldn't even need to wire anything to the secondary. Is that what you are talking about?

Yes, it makes a crude auto-transformer - I actually use a transformer like that to power a 110V UV light off 230V UK mains.

As long as you don't push the transformer near it's maximum ratings it works fine.

 

[grim]

Ok, so if I have a transformer that has a primary of 120V and a secondary of 24V that transformer has a turns ratio of 5:1, correct? If I feed 120V into the secondary then I'll get 600V at the primary. Am I calculating this correctly?

Sorry to keep asking basically the same question. This question came up at work where one of my field techs was told to wire up a transformer that showed 220/110 for primary and 24 for the secondary and another one of my techs told him he could feed 110V and get 220 out but I told him I don't think he as the right transformer. He didn't have many details on the transformer he was trying so his info was a little vague.

Whilst your calculations are correct in a theoretical sense, in practice the transformer would saturate, blowing your input fuse. If you want to run a transformer at a higher than rated voltage, then you need to increase the frequency too - by the same percentage. So if this is a 50Hz transformer then you would need 250Hz

Also when you run a transformer backwards, you do not get the correct volts out of the primary, because of regulation.

The classic educational models of transformers say things like "a 240V tx with 2400 turns on the primary has 120 turns on the secondary, what is the output?" and the answer is of course 12V.
But that isn't a 12V transformer, unless you specifiy your open circuit volts.

In reality a 12V transformer could have 150turns, making it appear to be a 15V transformer, but when loaded up, the voltage drops (mainly due to resistance) to 12V. It's called regulation, and most transformer specs wil list it.

so take that same transformer and apply 12v to the secondary
12/150x2400 = 192V and that is open circuit. load it up, and it will probably drop to 150 or so.

i am not saying you cannot run a transformer backwards, you can. But you need to know the specs of the transformer, in particular the regulation.

for a 12v to 230 step up, one with a 9v secondary MAY be a better bet.

 

[audioguru]

Hi Grim,
Good points!

 

[qsiguy]

That was very informative. I knew the basics of transformers and use them often in the normal configuration. Nice to know a little more about other uses for them in unconventional configurations. The issue that prompted me to ask was one of my techs was trying to get 220 from a 110 line.

Thank you all for your input.

 

[audioguru]

I had a 220VAC fan that wouldn't run on 110VAC. So I tuned it in a series resonant circuit at 60Hz with a series capacitor of about 4.7uF and it ran fine. I got about 180VAC across the inductance of the fan.

 

[Claude Abraham]

Ok, so if I have a transformer that has a primary of 120V and a secondary of 24V that transformer has a turns ratio of 5:1, correct? If I feed 120V into the secondary then I'll get 600V at the primary. Am I calculating this correctly?

Sorry to keep asking basically the same question. This question came up at work where one of my field techs was told to wire up a transformer that showed 220/110 for primary and 24 for the secondary and another one of my techs told him he could feed 110V and get 220 out but I told him I don't think he as the right transformer. He didn't have many details on the transformer he was trying so his info was a little vague.

No! Please don't do that. If the primary is rated at 120V with the secondary rated at 24V, you must stay within those limits. It is perfectly fine to drive the secondary with 24V and get 120V out at the primary. However, do not exceed 24V on the secondary. It's not just the turns ratio that matters, but the magnitude of the voltages as well.

Although the insulation is good for a few thousand volts, if the rated voltages are exceeded the core will saturate and the current draw will be destructively large. Driving the 24V secondary with 120V will severely saturate the core and the exciting current will go through the roof. Large power will be dissipated resulting in a large temperature rise and ultimately failure and possibly injury.

If you observe the voltage and frequency ratings and work accordingly, you should have no problems.

 

[Diver300]

That was very informative. I knew the basics of transformers and use them often in the normal configuration. Nice to know a little more about other uses for them in unconventional configurations. The issue that prompted me to ask was one of my techs was trying to get 220 from a 110 line.

Thank you all for your input.

The easiest way is to find a dual voltage transformer. They usually have two input windings that are put in parallel for 110 / 120 V and in series for 220 / 230 V

Put the windings in series, and feed one winding from 110 V. The voltage across the pair should be about 220 V. Leave the secondary not connected.

You shouldn't take more power than about the rating of the transformer.