HOW DOES A CONSTANT VOLTAGE TRANSFORMER WORK?

Constant voltage transformers (CVTs), also known as ferroresonant transformers, is a type of transformer that provides a stable output voltage despite variations in input voltage, primarily using the ferroresonant principle. Spread of PCs have made CVTs quite popular. The CVT has special construction with a capacitor connected across the secondary winding. CVTs are basically 1:1 ratio transformers, excited high on their saturation curves, thereby providing an output voltage not significantly affected by input voltage variations.

Ferroresonance

Ferroresonance is a nonlinear resonance phenomenon in electrical circuits involving a saturable inductor (like a transformer core), capacitance, and often resistance.​ As core flux density B approaches the saturation knee on the B-H curve, inductive reactance drops sharply, varying cyclically with voltage peaks. This makes the effective permeability oscillate between high (unsaturated) and low (saturated) values.​ Ferroresonant method has no active components and relies on the tank circuit’s square loop saturation properties to absorb fluctuations in average input voltage. This is why CVTs are known as ferroresonant transformers. Ferroresonance rejects noise/spikes without electronics. Constant voltage transformers (CVTs), or ferroresonant transformers, regulate output voltage through core saturation and resonance, independent of input fluctuations.

CVT Construction

A CVT is wound on a special magnetic core (a three-limb shell) and consists of a primary winding, a secondary winding, and a resonant capacitor forming a tank circuit tuned to line frequency (50/60 Hz). An air gap or magnetic shunt separates primary and secondary paths, enabling flux transfer while minimizing losses.

A CVT carries an LC tank circuit on the secondary side, using a large capacitor, along with a magnetic shunt that ‘short circuits’ part of the magnetic flux between the primary and secondary side. This keeps the primary side well below saturation point, while the secondary side is within saturation region. AC input in the primary induces flux in the core and drives the secondary and resonant circuit into shallow saturation. LC tank oscillates at line frequency, clamping flux density constant beyond saturation. Output voltage stays stable, as flux and frequency remain fixed across a wide range of input voltages.

When input voltage varies (e.g., 170-270 V), the core saturates, restricting secondary flux to a constant value and producing a flattened sine wave output (say, 220 V ±1%). This ferroresonance effect provides inherent regulation without electronics or moving parts. Part of the magnetic core (over which the secondary winding is wound) is saturated, while that covered by the primary is not saturated.  Ratings of CVT may range from 50 VA to as high as 2000 VA or even greater.

Output voltage will not be a pure, but approximate sinewave with peaks flattened approaching a square wave. A compensating winding is connected in series with secondary to improve performance.

Advantages of CVT

Constant voltage transformers offer several advantages over other types of voltage stabilizers:

• High Reliability: CVTs are robust and durable, with no semiconductor devices to wear out.
• Wide Input Range: They can accept a wide range of input voltages, keeping output stable.
• Isolation and Noise Reduction: Input- output isolation reduces electrical noise and disturbances.
• Stable Output Voltage: Output voltage is very stable to close tolerance.
• Harmonic suppression: Significant elimination of 3rd to 7th harmonics, (suppresses third harmonics by over 90%).
• Simple and Reliable Structure: Working components are only transformers and capacitors.
• Protection Function: In the event of overload or short circuit, output voltage drops to zero, automatically limiting the current. It can automatically recover after the fault is eliminated.

Limitations of CVT

• Unsuitable in certain specific situations like extremely large transient loads, or input voltage fluctuation range beyond their design range.
• Cost may be higher than other stabilizing devices. This is permissible in critical applications,
• Waste heat due to secondary side being in saturation.
• Distinct whine, and sensitive to grid frequency changes than a regular transformer.

Applications of CVT

CVT is preferred over a voltage stabilizer for computer applications since voltage stabilizer has relays. Their switching may cause the output voltage to interrupt for a short time. Such transients are not desirable for computers, which may cause the computer to reboot. Further, CVT provides a clean spike-free output voltage. CVTs provide better voltage regulation.

• Medical Equipment: Hospitals and healthcare facilities rely on CVTs to safeguard sensitive medical equipment from voltage fluctuations.
• Telecommunications: CVTs are used to protect telecommunication systems, ensuring continuous operation and preventing damage to network infrastructure.
• Data Centers: CVTs maintain stable voltage levels for servers and networking equipment.
• Laboratory Instruments: Essential for precision instruments used in research and development.
• Industrial Machinery: To maintain consistent voltage levels to prevent production disruptions and protect equipment.
• Auto Industry: Stabilize battery and EV charging circuits against grid volatility.
• Process Control: Programmable logic controllers, motor starter coils, and electrotechnology circuits, sustains critical functions amid grid instability.​

Common ratings for constant voltage transformers (CVTs) in factories range from 100 VA to 10 kVA, tailored for protecting sensitive control and automation equipment. Ratings match load demands (e.g., PLCs at 500-1000 VA, motor controls at 2-5 kVA), with input ranges of 160-280 V common in India. Multiple CVTs may be used in parallel circuits for larger needs.