Constant Voltage Stabilizer - CVT - Explained

[madbullram]

We had an useful Chennai meet. Based on that I had been talking to Alpha Power corp for a nice write-up on various equipment. Here is the first one on CVT.

Constant Voltage Transformer (Magnetic Voltage Stabilizer)
are separating transformers according to EN 61558/ VDE 0570; equipped with some special characteristics.

They generate within the permissible operation range a stabilized output voltage with an open-loop circuit only by vectorial addition or subtraction of partial voltages.

Fluctuations of a 50 cps input voltage up to +10/-20% are reduced in 40...60 ms to about +/- 1%; presupposing a constant loading with power-factor cos ??1 .

Constant Voltage transformers regenerate the sinusoidal shape of distorted mains voltages, suppress glitches, attenuate high- frequency voltages in both directions, and shunt-out short-breaks of up to 10 ms.

Constant Voltage Transformers are short-circuit proof and require no maintenance ; neither radio interference nor additional harmonics occur within the permissible operation range.

Constant Voltage Transformers regenerate, clear up and stabilize the peration voltage of appliances for

• Data processing
• Test equipment
• Measuring and control engineering
• Film and photo
• Optical illumination systems
• Audio equipment , etc.

What Constant Voltage Transformers can not do:

• Constant Voltage transformers are not able to regenerate the voltage at the end of long dead- end lines when conductor cross section and system impedance are too high.
• A reliable stabilization is only possible when lines are so dimensioned, that the voltage drop between feeding and consumers point even by inrush reaches no more than 4% of the rated voltage.
• Constant Voltage Transformers are short- circuit proof and therefore, they are not able to accept high inrush currents. Depending on transformer size, the output voltage collapses to zero with currents of approx. 1.3 ...1.5 times of the rated current. Inrush currents are only accepted with Constant Voltage Transformers dimensioned for at least 90% of the inrush power.
• Constant Voltage Transformers cannot regenerate frequency deviations or stabilize the rated frequency.

Credits: Alpha Power Corporation

 

[madbullram]

FAQs discussed in the meet
Whats the maximum a CVT can go to?
1 KVa

Whats best CVT or Servo?
It depends on the application. Though CVT gives constant voltage, the current is not pure. For audio/visual equipments, its beneficial to invest in Servo. CVT costs lesser than Servo. So this needs to be decided individually

Does it makes any noise?
No, since CVT has no moving part (like a Servo stab) it makes no noise and maintenance free

pls feel free to add more.

 

[anooj30299301]

Ram,

A query - my servo is air cooled and is dead silent when driving my AMP to high decibels. Will post pictures tommorrow to this thread.

It makes a rumbling sound at times. -- hope that is when voltage fluctuations happen

So no moving parts right?

tx

 

[cmsajith]

Ram,

A query - my servo is air cooled and is dead silent when driving my AMP to high decibels. Will post pictures tommorrow to this thread.

It makes a rumbling sound at times. -- hope that is when voltage fluctuations happen

So no moving parts right?

tx

If it is a servo, definitely it will have a servo motor and will make a small sound while the motor is functioning (voltage correction).

 

[madbullram]

Ram,

A query - my servo is air cooled and is dead silent when driving my AMP to high decibels. Will post pictures tommorrow to this thread.

It makes a rumbling sound at times. -- hope that is when voltage fluctuations happen

So no moving parts right?

tx

In fact, we asked the same question during our meet. Like I posted above, a Servo Stabilizer has moving parts and does make minimal noise unlike a CVT (will post write-up on servo soon). If it makes a rumbling noise, it was suggested to get the service engineer to tune it as servo needs maintenance now and then to remove deposits etc.

 

[srinisundar]

Ram,

A query - my servo is air cooled and is dead silent when driving my AMP to high decibels. Will post pictures tommorrow to this thread.

It makes a rumbling sound at times. -- hope that is when voltage fluctuations happen

So no moving parts right?

tx

I think the engineer has answered the carbon brush inside to be cleaned to remove the rumbling noise... Am I right Ram?

 

[madbullram]

I think the engineer has answered the carbon brush inside to be cleaned to remove the rumbling noise... Am I right Ram?

On the money sir

 

[cmsajith]

In fact, we asked the same question during our meet. Like I posted above, a Servo Stabilizer has moving parts and does make minimal noise unlike a CVT (will post write-up on servo soon). If it makes a rumbling noise, it was suggested to get the service engineer to tune it as servo needs maintenance now and then to remove deposits etc.

Ok, for more info. i will try to post some inner pics of my Vertex servo stab.

 

[Kamal]

Thanks for the initiative, MBR.
A small reqest-is it poss to translate such descriptions into plain, ordinary ,easy to understand language for non techies like me?
E.g, terms like vectoral, constant loading, dead end lines, inrush power leave me scratching my head!

 

[madbullram]

LOL...yes Kamal, I have asked them for the same as even I didnt understand. If our in house experts can shed some light, it will be great

 

[cmsajith]

I think the engineer has answered the carbon brush inside to be cleaned to remove the rumbling noise... Am I right Ram?

The sound is because of the rough surface made of copper winding (which has the variable current) on which the brush is moving. The sound can be because of the wear and tear of the brush, cleaning will not make any difference, it has to be replaced if it is over. This is the only part in a servo which needs replacement as part of the maintenance and needs replacement only once in 5/6 years depending on the degree of the voltage fluctuation in your area. The brush is similar (only for functional comparison) to the brush used in Mixer-Grinder which needs replacement once it is over.

 

[murali_n]

Came across this in wikipedia,

AC voltage stabilizers:

A voltage stabilizer is a type of household mains regulator which uses a continuously variable autotransformer to maintain an AC output that is as close to the standard or normal mains voltage as possible, under conditions of fluctuation. It uses a servomechanism (or negative feedback) to control the position of the tap (or wiper) of the autotransformer, usually with a motor. An increase in the mains voltage causes the output to increase, which in turn causes the tap (or wiper) to move in the direction that reduces the output towards the nominal voltage.

An alternative method is the use of a type of saturating transformer called a ferroresonant transformer or constant-voltage transformer. These transformers use a tank circuit composed of a high-voltage resonant winding and a capacitor to produce a nearly constant average output with a varying input. The ferroresonant approach is attractive due to its lack of active components, relying on the square loop saturation characteristics of the tank circuit to absorb variations in average input voltage. Older designs of ferroresonant transformers had an output with high harmonic content, leading to a distorted output waveform. Modern devices are used to construct a perfect sinewave. The ferroresonant action is a flux limiter rather than a voltage regulator, but with a fixed supply frequency it can maintain an almost constant average output voltage even as the input voltage varies widely.

The ferroresonant transformers, which are also known as Constant Voltage Transformers (CVTs) or ferros, are also good surge suppressors, as they provide high isolation and inherent shortcircuit protections.

A ferroresonant transformer can operate with an input voltage range 40% or more of the nominal voltage.

Output power factor remains in the range of 0.96 or higher from half to full load.

Because it regenerates an output voltage waveform, output distortion, which is typically less than 4%, is independent of any input voltage distortion, including notching.

Efficiency at full load is typically in the range of 89% to 93%. However, at low loads, efficiency can drop below 60% and no load losses can be as high as 20%. The current-limiting capability also becomes a handicap when a CVT is used in an application with moderate to high inrush current like motors, transformers or magnets. In this case, the CVT has to be sized to accommodate the peak current, thus forcing it to run at low loads and poor efficiency.

Minimum maintenance is required. Transformers and capacitors can be very reliable. Some units have included redundant capacitors to allow several capacitors to fail between inspections without any noticeable effect on the device's performance.

Output voltage varies about 1.2% for every 1% change in supply frequency. For example, a 2-Hz change in generator frequency, which is very large, results in an output voltage change of only 4%, which has little effect for most loads.

It accepts 100% single-phase switch-mode power supply loading without any requirement for derating, including all neutral components.

Input current distortion remains less than 8% THD even when supplying nonlinear loads with more than 100% current THD.

Drawbacks of CVTs (constant voltage transformers) are their larger size, audible humming sound, and high heat generation.

N.Murali

 

[gannyboy]

Having decided to go for a servo stabiliser for the HT system , Ram ..was just wondering if the capacity required shd better be decided by the servo engineer..anyone else in the fellowship going for it??!

 

[Thad E Ginathom]

I really wish I could have stayed for the presentation, a I have lots of confusions in this area.

How does the CVT compare to the commonly-seen "Stabilizer"?

 

[madbullram]

@Ganny: I would suggest getting a technician to measure your actual voltage caclculation and also check for anomolies. Alpha has offered to do this free for our chennai members. Contact them. You can also reach vertex etc who are other manufacturers.

I am getting one, so guys can come over once its done at my place.

@Thad: The other stabilizers also do the same function. But what i saw was that the components used in the custom made stabs like Alpha, Vertex, Audire etc are much better and uses pure copper. This gives us peace of mind that it really performs.

 

[gannyboy]

@ram- great !!...will check with alpha.

 

[murali_n]

hai Thad,

A common stabilizer called static stab, has a transformer with multiple tap windings at their secondary(output). For a given input voltage level the values at the output taps will have values above and below the input voltage. So whenever the input varies i.e either increases or decreases , the corresponding tap (corresponding to the desired set value) is connected to the output(load) through an arrangement of relays controlled by a control circuit. Thus the output voltage is controlled within limits, for the fluctuations in the input voltage.

N.Murali.

 

[Thad E Ginathom]

Does that mean that the voltage of an ordinary household stabilizer will be jumping?

My hifi (and my wife's laptop) is connected to a small (APC Back-UPS500) UPS, which I was hoping would regulate the voltage as well as provide backup for short outages. However, the amplifier usually trips off on a power cut. It is more sensitive than a computer! Other components remain on.

Whilst I don't want to spend a lot of money on a stabiliser, I doubt that I'll be able to afford the same quality amplifier again, so protection is important.

 

[madbullram]

Thad: I am no expert, but an UPS is not going to keep the voltage fluctuations at bay. The fluctuations in voltage is the main culprit isnt it?

 

[Thad E Ginathom]

Yes, sorry... the UPS is supposed to have "AVR", voltage regulation.