What Is a Constant-Voltage Transformer?

A well-known solution for electrical “noise” in industrial plants has been the constant-voltage transformer, or CVT.

The typical components of a CVT are shown in Figure 2.8.2. The magnetic shunt on the central core has the following effects on the core’s reluctance. It reduces the reluctance of the core.

 This can be thought of as introducing more resistance in parallel to an existing resistance. The magnetic shunt in the CVT design allows the portion of the core below the magnetic shunt to become saturated while the upper portion of the core remains unsaturated.

This condition occurs because of the presence of the air-gap between the magnetic shunt and the core limbs. Air has a much higher reluctance than the iron core.

Therefore, most of the flux passes through the lower portion of the core, as shown by the thick lines in Figure 2.8.2.

In terms of an electrical analogy, this configuration can be thought of as two resistances of unequal values in parallel.

The smaller resistance carries the larger current, and the larger resistance carries the smaller current.

The CVT is designed such that:

• The lower portion of the central limb is saturated under normal operating conditions, and the secondary and the resonating windings operate in the nonlinear portion of the flux-current curve.

• Because of saturation in the central limb, the voltage in the secondary winding is not linearly related to the voltage in the primary winding.

There is consonance between the resonating winding on the saturated core and the capacitor. This arrangement acts as a tank circuit, drawing power from the primary. This results in sustained, regulated
oscillations at the secondary with the applied line frequency.

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