Author Topic: DC bias in transformer  (Read 15665 times)

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Offline StefanTopic starter

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DC bias in transformer
« on: September 06, 2013, 12:47:13 pm »
Hi!

I'm currently trying to better understand transformers, and there is something I don't get.

We use AC for transformers because if you put DC though a winding, the magnetic flux will build up until the core saturates.

But what if there is a small DC bias on, say, the mains? Shouldn't that, after a while, run the core into saturation as well? Ruining the efficiency? As far as I can tell transformers run on mains directly without any coupling caps or anything.

If, for example, I drive a transformer from an H-bridge, and my FETs are not all equally fast, a small DC Bias can easily occur. Do I have to do anything about this?

Thanks,
Stefan





 

Offline Simon

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Re: DC bias in transformer
« Reply #1 on: September 06, 2013, 12:52:49 pm »
no it won't be a problem, you don't need a perfect balance. you can infact send DC pulses to a transformer and you will get AC out the other side although I guess it is not the best method
 

Offline Andy Watson

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Re: DC bias in transformer
« Reply #2 on: September 06, 2013, 12:55:57 pm »
The current "builds up" to the point at which it is limited by the DC resistance of the coils. Hopefully, the DC offset, and therefore the DC current  will be insignificant compared to operating current of the inductor/transformer - so saturation is not a problem.
 

Offline StefanTopic starter

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Re: DC bias in transformer
« Reply #3 on: September 06, 2013, 01:15:39 pm »
Ahh, I think I see it now. The voltage bias gives a time-increasing offset to the magnetic flux, which creates a DC current propotional to it. That in turn produces a DC voltage over the wire resistance which counters the DC bias. A negative feedback loop.

The loss will be dc_offset2 / wire_resistance + additional core losses due to the higher magnetic flux density.

Thanks for the answers!
 

Offline c4757p

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Re: DC bias in transformer
« Reply #4 on: September 06, 2013, 01:25:35 pm »
But what if there is a small DC bias on, say, the mains? Shouldn't that, after a while, run the core into saturation as well? Ruining the efficiency? As far as I can tell transformers run on mains directly without any coupling caps or anything.

The mains is unlikely to have a significant DC bias, because your transformer is run through a coupling transformer - i.e., the one outside your house (or down the street, depending on where you live).

Also, you seem to have confused a bit of the function of a transformer with the memristor. The transformer doesn't store more and more flux as charge moves, and keep it when the current is gone. The flux through a transformer winding is phi = Li - directly proportional to the instantaneous current. The flux is higher during the half of the wave that is biased up, but still drops to zero* at the zero points, it doesn't just build and build.

*Real transformer cores have hysteresis due to the ferromagnetic core, but the basic idea still stands. The hysteresis will cause a bit of this type of behavior, but unless the DC bias is large, it will be insignificant - the core will not become heavily magnetized.
« Last Edit: September 06, 2013, 01:30:40 pm by c4757p »
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Offline StefanTopic starter

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Re: DC bias in transformer
« Reply #5 on: September 06, 2013, 01:44:17 pm »
Maybe I was unclear, Simon and Andy give me the hints I needed. Just to avoid confusion:

di/dt = U/L   (assuming zero current on the secondary)

so a DC voltage component U would create a DC current that increases towards infinity with time, assuming an ideal transformer. In a real transformer this does not happen because the DC current produces a DC voltage over the ohmic resistance of the winding that will balance out the DC voltage. Since the DC bias is small, the DC current never gets significantly high.

For once the non-ideal behaviour works for our advantage, yey!





 

Offline Jeff1946

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Re: DC bias in transformer
« Reply #6 on: September 06, 2013, 03:06:18 pm »
As an aside, this is what happens with a superconducting magnet.  In this case the current will build up until the magnetic field is strong enough to destroy the superconductivity.
 

Offline IanB

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Re: DC bias in transformer
« Reply #7 on: September 06, 2013, 03:21:51 pm »
Although not discussed much above, it is my understanding that a DC bias on a transformer (for example due to unsymmetrical loads) will reduce its efficiency and in extreme cases may cause damage from overheating. In the case of big, expensive substation transformers the power company really tries to avoid this by, for example, penalizing bad power factors at consumers.
 
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Offline sub

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Re: DC bias in transformer
« Reply #8 on: September 07, 2013, 12:29:45 am »
Although not discussed much above, it is my understanding that a DC bias on a transformer (for example due to unsymmetrical loads) will reduce its efficiency and in extreme cases may cause damage from overheating. In the case of big, expensive substation transformers the power company really tries to avoid this by, for example, penalizing bad power factors at consumers.

Indeed.  The example of this that I love (which I originally discovered from EEVBlog) is the fluxgate magnetometer, which measures static magnetic fields by applying an AC field to an inductor and measuring the level at which it saturates.
 

Offline c4757p

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Re: DC bias in transformer
« Reply #9 on: September 07, 2013, 01:37:24 am »
fluxgate magnetometer

"Fluxgate magnetometer" is the sort of name you expect somebody to make up to make their job sound cool ;D
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Online johansen

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Re: DC bias in transformer
« Reply #10 on: September 07, 2013, 02:46:25 am »
no it won't be a problem, you don't need a perfect balance. you can infact send DC pulses to a transformer and you will get AC out the other side although I guess it is not the best method

you do need a perfect balance or the dc current will flow according to the dc content of the signal, saturating the transformer, and you won't get much ac out the other side at all.

one part in a thousand is enough to notice. that's only half a volt on a 500 volt inverter, enough to cause major issues.
lower voltages don't help.. 30 milliohm mosfets and a 10 milliohm transformer running from 12 volts will have .17 amps flowing.. probably not enough to notice, because most people don't use ferrite emi beads as transformers.. but sometimes they are handy.. you must have a dc blocking capacitor most of the time.
 

Offline Electrocow

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Re: DC bias in transformer
« Reply #11 on: October 11, 2019, 02:06:37 pm »
I'm sorry to wake up an old thread, but this thread was the top proposal when searching the topic on Google, and I have to leave a comment.

As DC bias in mains is usually extremely low, unbalanced loads in a big network (such as in your neighborhood) may cause a substantial bias. As mentioned earlier in this thread, this DC bias will go straight through any transformer primary winding only limited by the winding resistance. Depending on the DC current, this can partially or fully saturate the transformer core. Aside from the heating caused by the DC current, this can also manifest itself as humming in the transformer. As the bias is dependent on the network load, this humming can come and go during the day (such as when your metal-head neighbor turns on their 10 kW stereo).

This is not something mortal people encounter during our day-to-day activities, but this issue is highly prevalent among audio-engineering aficionados. A common solution is to put two huge electrolytic capacitors in series with the mains in the power supply for example an audio amplifier.
 

Offline Kleinstein

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Re: DC bias in transformer
« Reply #12 on: October 11, 2019, 03:29:33 pm »
A known source for some DC bias on the mains grid are some cheap hair dryers on the low power setting. Some use a diode to draw only half the current, which is not really nice to the grid. Normally even such an current in the 8 A range does not cause too much DC voltage.
 

Offline BravoV

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Re: DC bias in transformer
« Reply #13 on: October 11, 2019, 03:41:59 pm »
A known source for some DC bias on the mains grid are some cheap hair dryers on the low power setting. Some use a diode to draw only half the current, which is not really nice to the grid. Normally even such an current in the 8 A range does not cause too much DC voltage.

Yep, I did a test using a hairdryer and measure the generated DC across the mains line (220VAC), details with video -> HERE

Offline Simon

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Re: DC bias in transformer
« Reply #14 on: October 11, 2019, 05:41:41 pm »
I'm sorry to wake up an old thread, but this thread was the top proposal when searching the topic on Google, and I have to leave a comment.

As DC bias in mains is usually extremely low, unbalanced loads in a big network (such as in your neighborhood) may cause a substantial bias. As mentioned earlier in this thread, this DC bias will go straight through any transformer primary winding only limited by the winding resistance. Depending on the DC current, this can partially or fully saturate the transformer core. Aside from the heating caused by the DC current, this can also manifest itself as humming in the transformer. As the bias is dependent on the network load, this humming can come and go during the day (such as when your metal-head neighbor turns on their 10 kW stereo).

This is not something mortal people encounter during our day-to-day activities, but this issue is highly prevalent among audio-engineering aficionados. A common solution is to put two huge electrolytic capacitors in series with the mains in the power supply for example an audio amplifier.

So you felt the need to resurect a 6 year old thread.... :palm:
 

Offline AVGresponding

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Re: DC bias in transformer
« Reply #15 on: October 12, 2019, 04:05:35 pm »
So you felt the need to resurect a 6 year old thread.... :palm:


I've seen worse necros...   ::)

*almost on topic* a colleague once upon a time (in the late 1990s) said he'd heard 'they' (I'm assuming he meant the 'Powers That Be') were thinking of banning SMPS's due to their noisy switching having a deleterious effect on the mains line.
He went on to say "but transformer/rectifier PSU's have noise too, when they pass the zcp and the diodes in the bridge are switching from on to off to on, there's noise!".
I seem to remember saying something along the lines of "yes, that's a good point", while thinking "yes, but the noise frequency is directly related to the transmission line frequency, and so can be filtered easily...".

Given the number of SMPS's running at any given time, and the random frequency and phase, it's likely the noise sum is effectively near enough zero as makes no difference, but it does make you wonder, what would happen if they all synced into phase at the same time...  :scared:
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Offline Simon

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Re: DC bias in transformer
« Reply #16 on: October 12, 2019, 04:15:29 pm »
rectifiers and smoothing capacitors create plenty of distortion due to poor power factor. SMPS are usually power factor corrected which mens they have to appear as near linear loads. if they are near linear I expect that anf noise has been minimised already.
 

Offline AVGresponding

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Re: DC bias in transformer
« Reply #17 on: October 12, 2019, 04:30:35 pm »
Have you seen the stuff coming out of China recently?

Hell, forget China (in terms of cheap Chinesium crudites anyway), have you seen the power factor on premium consumer goods like a PS4 for example? On standby I see a power factor of 0.02! And yes, I did get the decimal in the right place!
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Offline Simon

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Re: DC bias in transformer
« Reply #18 on: October 12, 2019, 04:34:12 pm »
Have you seen the stuff coming out of China recently?

Hell, forget China (in terms of cheap Chinesium crudites anyway), have you seen the power factor on premium consumer goods like a PS4 for example? On standby I see a power factor of 0.02! And yes, I did get the decimal in the right place!


how much does it use on standby? what is the power factor during use? Have a look at Dave's latest pair of videos. A dirt cheap LED driver that has 100% ripple to save money but a power factor of over 0.9, the only thing that lets it down is the fact that you can see the flicker caused by the 100% ripple when using a video camera.....
 

Offline AVGresponding

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Re: DC bias in transformer
« Reply #19 on: October 12, 2019, 06:24:22 pm »
Have you seen the stuff coming out of China recently?

Hell, forget China (in terms of cheap Chinesium crudites anyway), have you seen the power factor on premium consumer goods like a PS4 for example? On standby I see a power factor of 0.02! And yes, I did get the decimal in the right place!


how much does it use on standby? what is the power factor during use? Have a look at Dave's latest pair of videos. A dirt cheap LED driver that has 100% ripple to save money but a power factor of over 0.9, the only thing that lets it down is the fact that you can see the flicker caused by the 100% ripple when using a video camera.....


Underlined relevant sections...

That LED driver will have the same affect on the line as a linear PSU; 100Hz.

Think about it.

In point of fact, all the SMPS's I have encountered rectify line current first, therefore their main interference is going to be line f or 2f. Their switching frequency will be less of an effect due to components sitting between the switching transistor(s) and the line, and as I've also pointed out, they are all going to be running at slightly (or more) different frequencies and phases, so the net effect is likely to be effectively zero.

The only consistent factor is the line frequency, insofar as the noise generated by diodes switching on at their thresholds will be at the line f (half wave) and 2f (full wave).

Most of the modern electronics I see has a pretty shit pf, probably a conspiracy to charge us more for our elastic trickery by charging us for apparent power instead of real power (I worked with ferro-resonant transformers for UPSs, so I know a bit about tinkering with power factors).

If it makes you happy, I'll measure the pf of my PS4 next time I turn it on, later tonight most likely.
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Offline Simon

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Re: DC bias in transformer
« Reply #20 on: October 12, 2019, 06:29:41 pm »
There are regulations now. sub 1kW stuff you may get away with but everything is becoming power factor corrected to improve infrastucture and power usage. It's not a conspiracy. If your power factor is 0.5 for every 1W you use they have to generate 2W, you will appear to use 2W but then shove 1W back to them when they don't want it. Why should you only pay for what you use when your equipment is so badly designed that it requires more power to be generated than used.
 

Offline Zero999

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Re: DC bias in transformer
« Reply #21 on: October 12, 2019, 06:39:52 pm »
They actually don't have to generate double the power to power loads with a power factor of 0.5. It just results in double the transmission losses.
 

Offline AVGresponding

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Re: DC bias in transformer
« Reply #22 on: October 12, 2019, 07:18:43 pm »
My apologies; since your flag indicates you're British, I thought you'd understand sarcasm and satire...

And let's look at your argument from a different angle:
Why should I pay for badly designed consumer goods that have piss-poor power factors?
Answer: Because I have no choice, I have to buy what they will sell (or buy nothing and go completely without).

LED lamps are a good example.
We were told to buy compact fluorescents instead of incandescents, to save power and the environment, then LEDs instead of compact fluorescents.
Each step along this road has brought us poorer power factors. It should also have brought us greater lamp life; the manufacturers are wise to that one, they have introduced lower and lower quality standards to offset the natural longevity of the newer technologies.
They have also introduced more waste, as now it's harder to replace just a lamp; now it's far more common to have to replace an entire luminaire.

Ehhh... thanks to my own arguments, I now have to withdraw the 'sarcasm' comment; it's a real conspiracy after all...


EDIT: My PS4 has a pf of 0.61-0.68 when running, settling at 0.64 on menu screen. Pretty damn crappy for such an expensive and pervasive piece of consumer electronics.
« Last Edit: October 12, 2019, 07:39:15 pm by ThickPhilM »
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Offline dom0

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Re: DC bias in transformer
« Reply #23 on: October 12, 2019, 07:32:15 pm »
This is not something mortal people encounter during our day-to-day activities, but this issue is highly prevalent among audio-engineering aficionados. A common solution is to put two huge electrolytic capacitors in series with the mains in the power supply for example an audio amplifier.

You can increase tolerance to DC currents of a transformer by reducing the flux density in the core.
,
 

Offline Simon

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Re: DC bias in transformer
« Reply #24 on: October 12, 2019, 10:50:50 pm »
there has always been a conspiracy, it's called the majority of consumers want dirt cheap goods and the majority of manufacturers don't care how they make their money. Thats why we have to have standards.
 
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