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hb001 Constant Voltage Transformer (CVT)
introduction the perfect sine wave
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Outstanding Spike and Electrical Noise protection:
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The very best power protection comes from a special type of transformer
known as a Constant Voltage Transformer (CVT). Providing unparalleled
reliability and conditioning performance, spikes and electrical noise
are neutralised with attenuation as high as 75dB.
In addition the input (or primary) and output (or secondary) transformer
windings are physically separated. Known as Galvanic isolation this separation
ensures that there is no direct connection between the mains supply and
load.
A CVT therefore provides an impenetrable barrier to spikes and high frequency
electrical noise. This barrier also works in reverse mode to prevent a
'noisy' load from polluting the mains supply itself.
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How does it work?
Although simple in concept the CVT is very difficult to explain, leading
to some electronics experts to describe it as magic. Effectively the aim
is to keep the Iron core of the secondary saturated, which keeps the voltage
on the output winding constant.
The primary winding needs to be unsaturated to prevent unacceptably
high losses.
This effect is achieved in two ways.
Firstly the two Magnetic Circuits are separated but inter-linked allowing
the transfer of energy from Primary to Secondary.
Secondly the secondary circuit has a deliberate inductance introduced
and is connected to a resonating capacitor. This LC circuit is tuned to
resonate at the desired transformer frequency.
The consequence is that the secondary part of the transformer runs saturated
and the output voltage is constant.
Superior Sag, Surge and Brownout protection:
Mains voltage sags and surges are automatically corrected by a CVT.
When faced with an extreme surge voltage such as a local lightning strike
the power conditioner will present a low impedance to the mains to protect both
itself and any connected loads.
Automatic Sine-wave generation:
using ferro resonant transformer technology means that each power conditioner
will always generate a pure stable sine-wave even when fed from a polluted mains
or square-wave supply.
Perfect Switched Mode Power Supply (SMPS) driver:
Ferro resonant transformer technology provides waveform shoulder-lifting -
the CVT is the kindest way to drive a switched mode power supply. Input surges
are reduced prolonging life and conduction times are lengthened. In addition
the CVT provides harmonic buffering and improved reservoir capacitor hold up
for the inevitable supply micro breaks which occur with grid protection switching.
Constant Voltage Transformers don't care about the ambient
Unlike some sensitive electronic stabilisers a CVT works in a very
large range of conditions. Standard units can work at permanent temperatures
of 40 deg C with 50 deg C not a problem for short periods, alternately a special
design can cope with 70 deg C. CVT's will operate at minus 40 deg C although
the heat generated by their operation means the transformer is soon warm to
the touch.
Overload protection
The CVT is designed to supply an overload of 150% at around 200% overload the
output waveform collpases to near 0V, WITH NO HARM TO THE CVT, direct short
circuiting for long periods of time is not a problem for the CVT, as soon as
the short is removed it will carry on supplying useable power.
Oscilloscope hold-up picture
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ride-through picture
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Something for nothing: so long as at least 30% of the normal supply voltage
is present the suitably selected CVT can provide adequate power for your
critical load.
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Common mode noise attenuation
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Series mode noise attenuation
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Regulation
The output stays within 5% from zero to full load, if a tighter regulation
is required trade-offs can be made. At 50% load the regulation is about
3%. Between zero to light loading (3%) is the greatest drop in output,
so a permanent light load improves the regulation further. A Full to Zero
load event may cause a momentary variation of 8% but the CVT settles in
a couple of cycles.
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Output within 5% for zero to full load
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Outstanding output regulation at low loads or high input voltages
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Stabilisation
The output stays within 3% for an input swing of 15% at half load 3%
output can be achieved for an input swing of approximately 30%
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Output within 3% for nominal input +/-15% Even wider input swings at
below nominal loads
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Regulation outside of specification
The combined stability and regulation is 8% again improvements can be
made by reducing the loading
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Output within 8% for zero to full load and nominal input +/-20%
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Power Factor
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All units present a power factor to the supply which varies with output
load.
The CVT will drive a wide range of power factor loads (+/-0.75)
Small changes in the output voltage will be found in comparison to the
setting with a resisitive load.
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Output changes with frequency of input
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A 1% frequency change produces a 1.5% change in output voltage
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Phase shift
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There is a smal phase shift across the CVT varying with load
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part of the AEL technical handbook return to hb000 Handbook
Index
