AC to AC regulator

[alphacat]

Hello.

I read on Wiki how an AC to AC regulator works.
One way is to use an auto-transformer with a negative feedback which controls the position of the tap of the auto-transfomer.

I wanted to ask 2 questions please.

1. How does the regulator know what is the average voltage that it should maintain at the output?

2. How does the regulator prevent from harmonics to pass from input to output and from output to input?

Thank you very much.

 

[schmitt trigger]

1) this is a simple window comparator. If your voltage is below the lower limit, it selects a higher tap. If it is above, it is selects a lower tap. If the voltage is within the window, it does nothing.

2) Unless you add some filtering, the regulator -per se- will pass the harmonic content. It will actually add a small amount, due to the transformer's magnetizing current.

 

[alphacat]

Thank you very much.

1) this is a simple window comparator. If your voltage is below the lower limit, it selects a higher tap. If it is above, it is selects a lower tap. If the voltage is within the window, it does nothing.

What I dont get is how do you compare AC voltages?
Moreover, how does the regulator determine the lower and upper limits of that window?

2) Unless you add some filtering, the regulator -per se- will pass the harmonic content. It will actually add a small amount, due to the transformer's magnetizing current.

Oh I see.
What means are usually used in such cases?

 

[schmitt trigger]

In a few words....

You sample the input voltage. Either by using a stepdown transformer or a voltage divider. Rectify the resulting low voltage sinewave, and use a comparator like the LM339. Of course, the comparator requires a stable reference voltage.

For the harmonics, you need to use LC filters. They are usually quite bulky. Most cases they are tuned to filter the 3rd and 5th harmonics, which are usually the most troublesome.

 

[alphacat]

Thank you very much again!

In a few words....

You sample the input voltage. Either by using a stepdown transformer or a voltage divider. Rectify the resulting low voltage sinewave, and use a comparator like the LM339. Of course, the comparator requires a stable reference voltage.

I understand.
One clarification please.
The reference voltage that you mentioned needs to be set in advance?
Cause every line has different RMS value.
For example, in Europe, some lines have nominal value of 220Vrms, and some lines have nominal value 240Vrms.
doesnt the value of the reference voltage depend on the nominal value of that particular line?

For the harmonics, you need to use LC filters. They are usually quite bulky. Most cases they are tuned to filter the 3rd and 5th harmonics, which are usually the most troublesome.

Got it.
Thanks

 

[schmitt trigger]

No, the reference value depends on how much you divide the line voltage.
Let's say for example, you want to regulate 220 volts +/-10 volts. This means 210 to 230 volts.
Now, in a sinewave the peak is 1.414 higher than your RMS, or roughly 296 to 324 volts.

If you use a 30:1 voltage divider, your window is 9.87 to 10.8 volts.
With the example above, you can calculate for other voltages.

 

[alphacat]

Thanks a lot.

If you use only one winding (auto-transformer), do you have a way to deal with voltage sag?
(A single winding cannot increase the input voltage, just decrease it).

 

[schmitt trigger]

A single winding autotransformer can either increase or decrease the voltage (but not both) depending on how you connect it.
Autotransformers for AC regulators usually have several taps.

 

[alphacat]

Thanks for everything