buck-boost transformers.......

They are used to raise or lower AC voltage, typically the line voltage. The concept is simple: A small transformer with a secondary near the change you need, and able to handle the current is wired so the primary is across the line, while the secondary is in series with one side. This is usually the line side. If the transformer is in phase it boosts the line voltage by whatever the transformer puts out under load, and if the phase is reversed, it bucks the voltage and drops the voltage by the same amount.

| They are used to raise or lower AC voltage, typically the line | voltage. The concept is simple: A small transformer with a secondary | near the change you need, and able to handle the current is wired so the | primary is across the line, while the secondary is in series with one | side. This is usually the line side. If the transformer is in phase it | boosts the line voltage by whatever the transformer puts out under load, | and if the phase is reversed, it bucks the voltage and drops the voltage | by the same amount.

It's also possible to wire other configurations for different voltages that do not involve the primary "across the line". If there are multiple secondaries (many have a pair of secondaries), then it is also possible to get yet other voltages with the primary wire between the two secondaries where one is wired for buck and the other for boost.

For example, with a pair of 12 volt secondaries, with a 120 volte primary, you can change 108 volts to 132 volts, or the reverse.

The reason buck-boost transformers are good is that it takes only a smaller transformer to achieve the voltage changes that would require much larger transformers for an isolation configuration. Buck-boost transformers wired for bucking or boosting the line voltage are wired as autotransformers, and so they do not isolate the system. They must be wired correctly on the grounded conductor.

Autotransformers (often made from two winding transformers).

They're autotransformers. In "buck" mode, they reduce the AC voltage by the tap ratio. In boost mode, they increase it by the tap ratio.

On Fri, 03 Apr 2009 21:57:39 -0600 Hope for the Heartless wrote: | In article | , | VIDHYA wrote: | |> what are buck-boost transformers and its operation?????? | | They're autotransformers. In "buck" mode, they reduce the AC voltage by | the tap ratio. In boost mode, they increase it by the tap ratio.

Some buck-boost transformers can be wired as isolation transfermers. Those that have TWO line voltage windings can do this. It reduces the capacity.

Suppose you have a BB transformer that has 2 120V windings and 2 12V windings. In the usual cases, the 2 120V windings can be wired in parallel to a 120V supply, or wired in series to a 240V supply. The 12V windings would then be wired in parallel (for 12V) or series (for 24V) to provide a boost to 132V or

144V or 252V or 264V, or the equivalent ratio for lower voltage supplies. Or these can be connected backwards for voltage drop.

To make an isolation transformer, the two 120V windings are kept separate. One of them is connected to the 120V supply while the other has one or two of the 12V windings in series for 132V or 144V. This can be reversed to step down an overvoltage supply.

Another configuration is to wire the 12V windings backwards. This reduces the voltage (and also reduces the capacity somewhat), or in reverse supply can step lower voltages up. The ratio is slightly different and is a choice for slight variations in voltage if needed. Both isolation and autotransformer methods exist for this, too.

Yet another configuration only works in autotransformer mode that can give yet another ratio. That involves the two 12V windings in series with each other, but the 120V windings (whether parallel for 120V purpose or series for

240V purpose) tapping to the center of the two 12V windings.

Buck boost transformers also exist with 16V and 24V windings (for 32V and

48V when in series). Also, 240V line voltage windings exist (480V in series).