Ferroresonant Transformer

A Ferroresonant Transformer, also know as constant voltage transformer (CVT) is designed to achieve regulation with non-linear operation. It provides line regulation, reduce harmonics, and is current limiting.

In addition to providing above benefits, it also provides line isolation and some models come built in with additional output transient voltage suppression (TVS) mechanism.

There is no method in a conventional transformer for the regulation of the output against changing input voltages because it is designed to operate on the linear portion of the magnetization curve (below the knee).

A basic ferroresonant transformer consists of a core, a primary winding, two secondary windings - one for the load and one for the capacitor - and a magnetic shunt that separates the primary and secondary windings. The load regulation will be higher because of the inherent internal regulation of the transformer.

In other words, ferroresonant transformer uses a transformer core that is operated in saturation. So, a change of magnitude of flux density is relatively independent of the magnetic flux inside the core. This means, since the transformer core is operated under saturation, small change in input voltage do not cause any significant change in the output voltage.

Note that the secondary circuit of the ferroresonant transformer comprises of a resonant tank circuit. If the resonant tank circuit is not there, the output voltage will be a square wave with high harmonic voltage distortion. By adding the resonant circuit, a quasi-sine wave voltage can be obtained.

To sum-up, ferro-resonant transformer is a special lamination transformer. Sometimes it may be preferred as constant-voltage-transformer (CVT) or voltage stabilizer because it provides a relatively constant output voltage with less distortion using a wide range of input voltages. 

Benefiting from the features of regulated output, quasi-square waveform and wide input voltage range, ferroresonant transformer is widely used in power supply, voltage regulator, lighting, inverter, battery charger and other industrial tools.

In industry, ferroresonant transformers are used in application that could be negatively affected by voltage sags or voltage dips in the power system.

Keep note that a Constant Voltage Normal (CVN) transformer produces a square wave output, while a Constant Voltage Sinusoidal (CVS) transformer produces a sinewave output!

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