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Ferroresonant Transformers

So whats in the box true geeks .. a Constant Voltage Transformer - yes a special device invented in 1938 but before we begin this tear down i would like to thank the @grandmusicgroup_gmg for the support and their 2 recently refurbished 660s - yes thats 2!!

This special transformer typically provides a regulated output voltage (in this case by the US manufacture Sola) for the VCA heaters of the Fairchild 660/670s

and the AT series models shown here on this site using a complex transformer design known as a ferroresonant transformer. So what does it do. The op voltage provides regulation when there is variation in mains supply these devices are also called CVT transformers.

In the world of ferroresonant transformers the metal core operates at magnetic saturation with a strong magnetization, with little or no change in the magnetic flux density which is a condition of core saturation. Features of this type of transformer are that the output

voltage does not deviate outside of the voltage regulation range (typically 1%-4% or better) regardless of the input voltage - within limits. It should be noted that the output voltage is not a pure sine wave but an approximate one with the peaks flattened approaching a square wave with the regulation certainly true of the CVT on display here. (or not as it was faulty)

In the time lapse you will notice a large paper in oil resonance or tank capacitor rated 2.5uf at 330vac (Aerovox p/n P149F9) this is connected across the secondary winding and is used to tune out the output at a frequency close to the 50/60hz point depending on the value. The capacitor also increases current in the secondary winding which helps in the saturation of the

secondary flux.

Not so good points include being very frequency dependent with any change in frequency changes in voltage will be seen. They also become very hot due to the core saturation. The CVT shown here was faulty and was replaced with a working unit 60Hz unit.

AT Series limiters | Fairchild

AT Series limiters | Fairchild

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