Resonant Transformer

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No exposition on transformer regulation could be called complete without mention of an unusual device called a ferroresonant transformer. "Ferroresonance" is a phenomenon associated with the behavior of iron cores while operating near a point of magnetic saturation (where the core is so strongly magnetized that further increases in winding current results in little or no increase in magnetic flux).

While being somewhat difficult to describe without going deep into electromagnetic theory, the ferroresonant transformer is a power transformer engineered to operate in a condition of persistent core saturation. That is, its iron core is "stuffed full" of magnetic lines of flux for a large portion of the AC cycle so that variations in supply voltage (primary winding current) have little effect on the core's magnetic flux density, which means the secondary winding outputs a nearly constant voltage despite significant variations in supply (primary winding) voltage. Normally, core saturation in a transformer results in distortion of the sinewave shape, and the ferroresonant transformer is no exception. To combat this side effect, ferroresonant transformers have an auxiliary secondary winding paralleled with one or more capacitors, forming a resonant circuit tuned to the power supply frequency. This "tank circuit" serves as a filter to reject harmonics created by the core saturation, and provides the added benefit of storing energy in the form of AC oscillations, which is available for sustaining output winding voltage for brief periods of input voltage loss (milliseconds' worth of time, but certainly better than nothing).

In addition to blocking harmonics created by the saturated core, this resonant circuit also "filters out" harmonic frequencies generated by nonlinear (switching) loads in the secondary winding circuit and any harmonics present in the source voltage, providing "clean" power to the load.

Ferroresonant transformers offer several features useful in AC power conditioning: constant output voltage given substantial variations in input voltage, harmonic filtering between the power source and the load, and the ability to "ride through" brief losses in power by keeping a reserve of energy in its resonant tank circuit. These transformers are also highly tolerant of excessive loading and transient (momentary) voltage surges. They are so tolerant, in fact, that some may be briefly paralleled with unsynchronized AC power sources, allowing a load to be switched from one source of power to another in a "make-before-break" fashion with no interruption of power on the secondary side!

Unfortunately, these devices have equally noteworthy disadvantages: they waste a lot of energy (due to hysteresis losses in the saturated core), generating significant heat in the process, and are intolerant of frequency variations, which means they don't work very well when powered by small engine-driven generators having poor speed regulation. Voltages produced in the resonant winding/capacitor circuit tend to be very high, necessitating expensive capacitors and presenting the service technician with very dangerous working voltages. Some applications, though, may prioritize the ferroresonant transformer's advantages over its disadvantages. Semiconductor circuits exist to "condition" AC power as an alternative to ferroresonant devices, but none can compete with this transformer in terms of sheer simplicity.

Very simple. A resonant transform trashes the waveform by dinking with the third and other harmonics.

These have been illegal in most civilized countries for decades. And even in parts of New Mexico.

IEE 455 or some such. I've got the exact reference somewhere in Tech Musings

Thou shalt not trash thy power line with harmonics if thine power is above 50 watts. The law now in Europe for a decade and the US for several years.

Which is what modern power factor correction is all about.

Resonant transformers are ridiculously above the harmonic limits.

There are some here, up to ~5000va; no mention of them being contraband:

-- john

Why would anyone want to make a particular type of transformer (of all things) *illegal* for God's sake? Why the proscription??

Hi Paul,

Probably because it could mess up the power grid if you had too many of those online. Depending on how well they are equipped with caps, input inductors and the like their input current can deviate so much from sinusoidal that it could become a problem. I had seen one where the lamps connected to the same power phase actually began to 'sing'.

That is also a reason why power factor correction (PFC) is mandatory for heavy duty gear in some countries. Meaning you can't just slap in a rectifier plus some large electrolytics and then happily run your big switcher from that like in the good old days. In the same way that you can't replace the muffler on an MG-B with a couple of straight pipes for that nice loud roar...

Regards, Joerg

Hi John,

Unless I misunderstand something, I read under "Operating Characteristics" that the input current without a load can be up to 50% or more of the full load current. Whoa!

Regards, Joerg

I read in sci.electronics.design that Don Lancaster wrote (in ) about 'Resonant Transformer', on Thu, 6 Jan 2005:

Agreed that's very simple, but is it an explanation?

I don't know of any law against them in Europe that's been in place for decades. Higher-power examples (e.g. 250 VA) probably won't meet EN

I read in sci.electronics.design that Don Lancaster wrote (in ) about 'Resonant Transformer', on Thu, 6 Jan 2005:

Did you mean 'IEEE'?

No. Not in Europe.

Since 1998 there has been control of mains harmonics emissions in Europe. But nothing there about 50 W.

Only IEEE 519 and that applies to sites, not individual pieces of equipment. The US adamantly refuses to produce a 120 V 60 Hz version of IEC 61000-3-2.

Low power ones may not violate the Class A limits of IEC/EN 61000-3-2, which are absolute current limit, so very lax for low-power stuff.

In really simple terms, you have a transformer which has the core in saturation all the time. This provides a constant AC voltage output, since the voltage depends on the max flux swing of the core. Then you must have some large inductance in series with the input, to limit the max current, and also an LC filter on the output, resonant at power frequency to restore some semblance of a sinewave. After all that, you can put some diodes and a cap to make raw DC.

Sounds terrible, but it works, and needs no active components to get reasonable regulation.

Concerning the use of a ferroresonant transformeer with small engine-driven generators, as long as the generator is set near the correct frequency (do not remember higher or lower), the transformer tends to force the generator to be "on frequency"!

I bet those "singing" lamps were switchers (ie: had solid state controllers).

Hi Robert,

Nope, plain old incandescent bulbs. No switchers, just switches ;-)

And the singing was nearly gone when the res xfmr was loaded with something heavy. So theoretically you could do morse code via load on-off.

Regards, Joerg

I had a couple of special CVTs made once, with custom secondary voltages. At low loads they oscillated, with about a 10Hz amplitude modulation of the secondary voltage. The oscillation disappeared as the load was increased.

The mfrs knew immediately what the problem was, and took them back for mending. The mend seemed to involve shims and wooden mallets, "to correct the magnetic shunt".

Something like no more than 3 Amps of 3rd harmonic in the mains too... (Probably not exactly the right value)

Now they want to screw everything up by forcing everyone to use lead free solder and components. It's bound to be a royal pain.

boB