DC Transformer

[Corky]

why is it we cant use DC transformers?

i know we need a change in magnetic flux to create a current but when i say DC i mean a DC offset sinewave so the voltages ranging from 0-10v say but it is always changing,

Would this create a magnetic flux which we could in turn use in a transformer and lower the voltage for the output?

in my understanding this is possible but it isnt in use as we dont have a need for it, imlooking at reasons we use AC in the uk over dc for the power grid,

would saying we can easily use transformers with ac and not dc be wrong? again i understand with DC (---------------) this wouldnt work but dc /\/\/\/\?

thanks guys

 

[alec_t]

A varying DC (e.g. 0-10V) input to a transformer would certainly produce an output, but the DC current could cause saturation of the core and result in overheating.

 

[ronsimpson]

The primary (and secondary) of a transformer is a inductor. A transformer is drawn as two inductors connected together.

A inductor apposes high frequency and passes low frequency current.
Example: The AC resistance is an inductor at 100khz might be 5000 ohms. At 1khz=5 ohms, 1hz= 0.005 ohms and at DC=0 ohms.
I am trying to show that the inductor shorts out at DC.

Yes I know "AC resistance"="ohms" is not the rights word but......

 

[Reloadron]

imlooking at reasons we use AC in the uk over dc for the power grid,

The original and earliest power grids in the US were Thomas Edison designs and distributed DC. DC simply is not practical for a large power grid distribution for a number of reasons starting with line loss. Then came Tesla and the War of the Currents. The link is a good read. My grandparents house in New York City was originally served by Edison's DC Power Grid. Today's high voltage transmission lines carry power over thousands of miles of cable. This would never work with DC current due to line losses. The guy at the end of the line would get no power.

AC distribution makes it possible to have 750,000 volt lines and using transformers step the voltage down as required. The higher voltage has a lower current as when stepped down we can easily trade off voltage for current. It's all power right? Read the link as you may find it interesting.

Ron

 

[Nigel Goodwin]

The original and earliest power grids in the US were Thomas Edison designs and distributed DC. DC simply is not practical for a large power grid distribution for a number of reasons starting with line loss. Then came Tesla and the War of the Currents. The link is a good read. My grandparents house in New York City was originally served by Edison's DC Power Grid. Today's high voltage transmission lines carry power over thousands of miles of cable. This would never work with DC current due to line losses. The guy at the end of the line would get no power.

In the UK (in the past) there have been various voltages, frequencies, and even DC as well - but obviously not since the National Grid became universal.

In fact only a few miles from me in the town of Staveley they used to use power generated locally (with a strange specification) at Staveley Works, the major employer in the area.

 

[Corky]

The primary (and secondary) of a transformer is a inductor. A transformer is drawn as two inductors connected together.

A inductor apposes high frequency and passes low frequency current.
Example: The AC resistance is an inductor at 100khz might be 5000 ohms. At 1khz=5 ohms, 1hz= 0.005 ohms and at DC=0 ohms.
I am trying to show that the inductor shorts out at DC.

Yes I know "AC resistance"="ohms" is not the rights word but......

but could we not use a DC frequency, as in just on off on off on off,

im happy with the info i have now and you were right reload it was a good read

cheers again

 

[audioguru]

A transformer is an inductor. When you quickly turn off a DC current in an inductor it produces a VERY high voltage spike.

You talked about feeding a sinewave to a transformer and luckily a sinewave is smooth and does not quickly turn on and off. But you have a DC offset voltage. Then the peak current in the transformer primary winding is doubled compared to a sinewave at the same level but without a DC offset voltage. The DC wastes a lot of power producing heat.
If the signal source is push-pull then simply add a coupling capacitor to block the DC but pass the AC.

 

[Reloadron]

but could we not use a DC frequency, as in just on off on off on off,

im happy with the info i have now and you were right reload it was a good read

cheers again

The frequency of DC is zero since DC current does not change direction. However, we can pulse DC resulting in a frequency. Just remember that when we pulse DC the current is not bi directional, it's as you mention On/Off On/Off ... Inverters are a good example of using pulsed DC with transformers. I can start with 12 Volts DC for example, no frequency and turn that DC On/Off rapidly creating pulsed DC and apply that to the primary of a transformer either stepping it up or down.

Matter of fact, and not to date myself, early car radios used valves (vacuum tubes on my side of the pond). Valves required high voltage to operate far exceeding the 6 volt and later 12 volt automotive systems. The 12 volt power would drive a mechanical multivibrator making pulsed DC that in turn drove a step up transformer to a high voltage that was then rectified and filtered to supply the valve plates.

My thinking on this is that DC has no frequency, the frequency of DC is zero Hz. Pulsed DC does have a frequency but again, it is pulsed. DC will not drive a transformer where pulsed DC will.

Ron

 

[Reloadron]

In the UK (in the past) there have been various voltages, frequencies, and even DC as well - but obviously not since the National Grid became universal.

In fact only a few miles from me in the town of Staveley they used to use power generated locally (with a strange specification) at Staveley Works, the major employer in the area.

Nigel, nice to see you. Hope all on your side of the pond is well.

Ron

 

[audioguru]

Inverters are a good example of using pulsed DC with transformers. I can start with 12 Volts DC for example, no frequency and turn that DC On/Off rapidly creating pulsed DC and apply that to the primary of a transformer either stepping it up or down.

Nope.
A simple cheap square wave inverter uses a center-tapped primary winding that has each end driven to ground with transistors. The center-tap is connected to +12V.
When one end is driven to ground then the other end swings up to double the supply voltage and the resulting signal in the transformer is AC, not dc pulses.

 

[KeepItSimpleStupid]

There is some useful stuff for renewing the possibility of a DC power grid or at least a DC powered home, primarily because most of our devices are DC now.
But then what would you do with all of the vintage 120 VAC stuff.

Interesting, most AC-DC converters start by rectifying the mains to get like 400 VDC for both 120 VAC and 240 VAC.

Your washers and heaters may have 3 phase BLDC motors in them already.

https://www.scientificamerican.com/...ge-will-direct-current-make-a-comeback-in-us/

 

[crutschow]

A flyback circuit does use switched DC into a single winding with the output being a series of pulses. But it's not as efficient as other types of inverters for general DC-DC conversion.

 

[Diver300]

DC is used for some high voltage power transmission, as the peak voltage, for which the insulation has to be designed, is the same as the average voltage. On AC, the peak voltage is 1.4 times as big for the same RMS voltage.

Against that has to be factored the costs of converting the low voltage up to the high voltage for power transmission. Before semiconductors, that would have meant big rotary converters, so using a transformer and keeping the whole lot at AC was easier. Nowadays, it is a lot easier to convert DC voltages, by chopping it up into AC and transforming that. As DC-DC conversion becomes easier, it is done a lot more.

https://en.wikipedia.org/wiki/High-voltage_direct_current

 

[Nigel Goodwin]

DC is used for some high voltage power transmission, as the peak voltage, for which the insulation has to be designed, is the same as the average voltage. On AC, the peak voltage is 1.4 times as big for the same RMS voltage.

Against that has to be factored the costs of converting the low voltage up to the high voltage for power transmission. Before semiconductors, that would have meant big rotary converters, so using a transformer and keeping the whole lot at AC was easier. Nowadays, it is a lot easier to convert DC voltages, by chopping it up into AC and transforming that. As DC-DC conversion becomes easier, it is done a lot more.

https://en.wikipedia.org/wiki/High-voltage_direct_current

It is used for VERY occasional special purposes (I seem to recall for undersea power transfer?), in fact I was talking to a guy the other year and his son does just exactly that - and is VERY highly paid for it, as he's one of very few people qualified to do so (and travels extensively plying his trade).

I wish I could remember who it was, and where I was talking to him?.

 

[Reloadron]

Nope.
A simple cheap square wave inverter uses a center-tapped primary winding that has each end driven to ground with transistors. The center-tap is connected to +12V.
When one end is driven to ground then the other end swings up to double the supply voltage and the resulting signal in the transformer is AC, not dc pulses.

My bad and that is correct. We are driving the transformer with a square or rectangular waveform that is in fact AC and not pulsed DC.

Thank You AG
Ron

 

[tvtech]

I wish I could remember who it was, and where I was talking to him?.

Hi Nigel

I sometimes battle myself....so much stuff to remember. Ah well, not getting younger either, am I.

Last time I posted about Choppers...not failing and all....I forgot about the ones we did have fail way back where the Technology was relatively new.

And you corrected me and that was totally cool

I.must.write.things.down.

I should have said" In the last 8 Years ....blank page with Choppers...etc". Anyway I forgot. And you remembered.

Oh well, at least one of us must try to remember stuff, so accurate info is shared with all and sundry that visit here

Regards,
tvtech