Manufacturer Of Servo Voltage Stabilizer With
Inbuilt Isolation Transformer.

Purevolt’s Constant Voltage Transformer (CVT) is designed to provide complete protection against power-related problems. It effectively regulates voltage variations, suppresses transients through isolation, and bridges short interruptions and voltage dips, ensuring a stable and reliable power supply.

We also export these Constant Voltage Transformers to various countries, meeting international quality and performance standards.

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Factors Affecting
Performance / Queries / Notes :

1. Selection of CVT

The CVT power rating should match the maximum load power, including the initial surge or starting current. While selecting the rating, always consider future expansion, such as adding extra computer drives, additional EPABX lines, or other equipment.

Since machines like computers, fax machines, and EPABX systems come with different power ratings and features, it is important to select the CVT carefully. Avoid assuming that one CVT rating is suitable for all types of equipment.

If the CVT is under-rated, the output voltage may collapse during overload or starting inrush conditions. This can cause computers or microprocessor-based systems to hang or malfunction.
When in doubt, it is always safer to select the next higher CVT rating.

2. CVT Voltage Measurement

All CVT specifications are based on True RMS voltage. Therefore, any voltage testing or measurement must be done using a True RMS meter.

Purevolt CVTs are equipped with a specially designed, shielded, and calibrated voltmeter to ensure accurate voltage readings.

3. No-Load Operation and Heating

Ferro-resonant transformers operate at a high magnetic flux density, and the resonant tank circuit remains active even when no load is connected. Because of this, the no-load current can be as high as 40%, which causes heating even under no-load conditions.

To avoid unnecessary heating:

  • Ensure that at least 25% of the rated load is connected at all times, or

  • Switch off the CVT when it is not in use.

Purevolt CVTs are designed to operate under worst-case conditions, including:

  • No load to full load

  • Minimum to maximum input voltage

  • Continuous operation at ambient temperatures up to 50°C

For proper cooling, the CVT should be installed in a well-ventilated area.
Do not place the unit inside a closed or enclosed cabinet, as this can restrict heat dissipation.

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4. Monitor Screen Shaking

A Constant Voltage Transformer (CVT) is a frequency-dependent system. When the input frequency moves outside the specified range of 49.5 Hz to 50.5 Hz, the CVT’s resonant circuit attempts to lock onto the basic frequency.

In monitors where the scan frequency is synchronized with the line frequency, this may cause continuous screen or raster shaking. However, this issue does not occur in monitors that use an internal crystal-controlled scan frequency.

Screen shaking can also occur if the CVT is placed too close to the monitor. This happens due to leakage magnetic flux from the CVT, which interferes with the electromagnetic deflection system of CRT monitors.

Recommended practice:

  • Keep the CVT at least 1 meter away from the monitor

  • Position the CVT in a way that minimizes magnetic flux interference

⚠️ Caution

Due to high leakage flux, magnetic storage media, sensitive electronic components, and display devices should be kept away from the CVT.

5. Computer Keyboard Hanging

In some computer systems, after connecting to a CVT, the computer may hang when heavy loads such as printers are switched ON. This issue is generally related to the Switch Mode Power Supply (SMPS) of the computer.

SMPS units have a preset “Power Good” threshold on the 5V DC bus. If the SMPS is marginally rated, switching ON a heavy load can momentarily disable the Power Good signal, causing the computer to hang or freeze.

Solutions:

  • Adjust the Power Good threshold of the SMPS

  • If the problem persists, upgrade or replace the SMPS with a higher power rating

6. Audible Noise

All CVTs generate some level of audible noise during operation. This occurs because the secondary winding operates in resonance, causing the core laminations to vibrate at the resonant frequency.

The intensity and frequency of the noise depend on:

  • Core geometry and mass

  • Power rating of the CVT

  • Voltage regulation tolerance

Higher-rated CVTs generally produce more audible noise, and CVTs with tighter voltage regulation may also have slightly higher noise levels.

Purevolt CVTs are designed to provide excellent voltage regulation under extreme load and voltage conditions, while ensuring that audible noise remains within acceptable limits, allowing smooth operation without disturbing the working environment.

7. Wider Input Line Voltage Range

To control very wide input voltage fluctuations (beyond the standard specified range), it is recommended to use a CVT with a higher power rating.
A higher-rated CVT provides better stability and prevents voltage collapse under extreme input conditions.

8. Operation with Motor Loads

When using a CVT with motor-based loads, special care must be taken due to the current-limiting characteristics of the CVT.

In general:

  • The CVT rating should be almost equal to the maximum power drawn during motor starting

  • Motors draw high inrush current during startup

For highly inductive motors, the starting current can be 2 to 8 times higher than the normal running current.
Hence, selecting a properly rated CVT is critical to avoid voltage drop or malfunction during motor startup.

9. Effect of Temperature

As the CVT warms up and reaches its stable operating temperature, a small variation in output voltage may occur.

  • The voltage change is approximately 1% for every 40°C rise in temperature

  • This is a normal characteristic of ferroresonant transformers

Proper ventilation should be ensured to maintain stable performance.

10. Output Voltage Setting

Since the commonly available incoming frequency is around 49.5 Hz, and considering the minor voltage variation during warm-up, the CVT is factory-set under controlled conditions.

Factory adjustment details:

  • Output voltage set to +2% / –0% of rated value

  • Input voltage applied at rated nominal value

  • Frequency maintained at 50 Hz

  • Full rated load applied at unity power factor

  • Calibration done at room temperature

This ensures that the CVT delivers a stable and accurate output voltage under real operating conditions.

11. Neutral to Ground Bonding

Neutral-to-ground bonding is factory provided as an optional feature.
The user can:

  • Remove the existing bond

  • Create their own neutral connection

  • Operate the CVT in floating output mode

In floating output mode, the CVT output becomes shock-proof, improving safety for sensitive and critical applications.

12. Effect of Load Power Factor

The output voltage of the CVT may slightly vary from the nameplate value depending on the power factor of the connected load.

This is a normal characteristic of CVTs and should be considered while selecting the correct rating for the application.

13. Applications

Constant Voltage Transformers and Servo Stabilizers are widely used in applications where stable and regulated voltage is critical, such as:

  • Telemetry equipment and monitoring systems

  • SCADA (Supervisory Control & Data Acquisition) systems

  • CNC machines

  • Computers and data processing equipment

  • Colour photography laboratories

  • PA systems

  • Telecommunications, Telex, TV, VCR

  • Electronic typewriters and fax machines

  • EPABX systems

  • Desktop publishing and pre-press systems

  • Biomedical equipment such as ultrasound machines

  • Centralized monitoring systems

  • Computerized medical equipment

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