hb003 Constant Voltage Transformer (CVT) background data |
The Constant Voltage Transformer effect was discovered during the 1930's in the USA by Joseph Sola a German born engineer.
The industrial use of ferroresonant transformers goes back to early 1940’s. Through the last 5 decades a series of applications has been found for products based on the technique. In each case the CVT has some feature which made it the most reliable and cost effective solution to the problem. These characteristics continue to make the CVT one of the most cost effective ac power conditioners available.
Although different manufacturers use varying techniques the Advance CVT is normally based on a single transformer rather than an arrangement of transformer and separate filters. This lends itself to one of the most important aspects of the CVT its inherent reliability. Ignoring nuts bolts and other small components the unit consists of 3 or 4 windings and a special capacitor. With good manufacturing technique only the capacitor fails and a considerable time and effort goes into making this as unlikely as is practical.
The second major characteristics that the CVT is almost indestructible. It can be completely and continuously short circuited in use either at switch—on or from full load and the unit will be unaffected.
As can be seen from the output curve the CVT output characteristic is such that the published specification may be set at an arbitrary distance from the knee. This is important when comparing product from different sources.In situations where it is correctly installed the CVT is unaffected by low input voltage but will present a low impedance to very high voltage surges ensuring that in-line fusing or circuit breakers are opened before any damage occurs.
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The graph of input current with voltage at fixed load shows this. It also indicates the normal operating ‘dip’ point. This means that the unit is self protecting to its supply and the critical load connected to it. The CVT provides the most effective buffer available to near—direct lightning strikes on power lines. The CVT also has the ability to provide usable output from low lines when operated below nominal power rating. |
The third characteristic which makes the CVT suitable for more recent applications with computers is that there is limited energy storage in the resonating circuit. This means that the CVT is able to ‘fill-in’ small gaps in the power waveform (up to about 10 mS—half a cycle in a normal situation).
The output voltage waveform will not be a perfect sinewave but certainly one that will satisfactorily drive modern computer based equipment. This waveform remains the same even if the input voltage is very severely non-sinusoidal. |
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There is a limit to the dynamic range afforded by most designs but all CVT’s exhibit the ability to provide a stable output voltage curve from a varying source. Although usually specified over a + / - 15% range the CVT does not just ‘stop’ outside this voltage range.
This is practically limited to a ± 25% input swing for a usable output unless very special design precautions are taken. This voltage stabilisation is a continuous operation on a cycle to cycle basis. Power supplies in modern computers will accommodate variations slightly larger than the planned ± 6% of most electricity supplies so it is usually straightforward to get the power supply input inside the necessary band using a CVT.
Another major feature of the CVT is that it inherently absorbs high energy voltage spikes on the input. In addition it is practical with careful constructional techniques to provide an effective barrier against lower energy ‘fast’ spikes which cause data corruption in computers and computer-based equipment.
On the negative side the CVT is a large and heavy transformer with operating efficiencies around 90% at full load. It has a substantial magnetic field and also produces a 50 or 60 Hz ‘hum’ dependent on unit size. The CVT is also frequency sensitive and will normally work at either 50 or 60 Hz.
We feel these features are more than offset by the knowledge that a properly selected and installed unit will usually cure mains problems on sensitive equipment. There is still a limited number of applications for specially wound CVT’s with output waveforms which are essentially ‘square’ rather than sinusoidal for use in simple power supplies or heating/lighting arrangements.
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