Ferroresonant Transformer Design
"Why, linear power transformers are themselves not very popular these days, and audio amplifiers themselves are class D, mostly. So this discussion is entirely to satisfy our own itch."
Not necessarily so sir - linear power transformers are very much in use as are class-A/AB amps. The realm is different. Yes small hand-held devices, as also TVs use switching supplies (their chargers, for example) and use class-D amps internally. But not so with high-powered (10W or higher) audio systems. They all use linear transformers and class-AB amplifiers. Open up any AV receiver and you'll know what I mean. Yes class-D amps are getting more common (and naturally these benefit from switching supplies) but the good old linear amp and transformer is still going strong. The advantage of a linear transformer is less noise - there's no switching related noise like an SMPS has.
To remind you, I had already excluded SMPS based systems from the list of suspicion and only pointed out that systems made for US, that use linear transformers stand a greater chance of failure when used in a non-US environment. So the question is not so much of whether the linear ferroresonant transformer is commonly used or not - but one of using a transformer in a non-conducive environment. Why would a manufacturer tragetting US consumers use a transformer which is never going to be used with 50Hz (as far as that manufacturer is concerned) given that this transformer would be both bulkier and costlier than the 60Hz version?
S = k.I^2, where S is the amount of stupidity a species possesses, I is the intelligence the species has and k is the universal constant of stupidity. Ferroresonant Transformers are a special type of laminated saturating transformer which provides a regulated output.*
These are sometimes known as Constant Voltage Transformers (CVTs). Using a special magnetic structure and a capacitor, ferroresonant transformers can supply a well regulated output, voltage which remains constant despite changes in input voltage and load. The ferroresonant approach relies on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage.
Older designs of ferroresonant transformers had an output with high harmonic content, leading to a distorted output waveform. Modern devices are used to construct a perfect sine wave. The ferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage, even as the input voltage varies widely.
They offer high isolation (filtering), and short-circuit protection (current limitation).
Minimum maintenance is required, beyond frequent replacement of failed capacitors. Redundant capacitors, built into some units, allow several capacitors to fail between inspections, without any noticeable effect to the device's performance.
* “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 sine-wave 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” (Resonant LC 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). (see below)
