15 Replies Latest reply on Jul 2, 2014 1:41 PM by caengineer

# How to convert the AC to AC from high to low without using transformers!

Hi all!

Is there any way to lower the high voltage AC lets say in a range of 300-660 volt to a range of 120-277 volt without using transformers?

The power electronic devices clamp the shape of SIN wave which are not good for my purpose!

Thanks!

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Sure you *Could* do it.  But it would be comparatively costly and complex.  The way to get from AC to AC without a transformer is to first convert the AC into DC using a bridge rectifier, then smooth it into a DC Bus using electrolytic capacitors.  Then you would have a high voltage DC bus (in your case 420-925VDC), then this would be used to feed a full H-Bridge inverter circuit which would synthesize a sine wave using a microcontroller (or PWM implemented in hardware) of the appropriate amplitude which would then get you back to AC true sine wave.  Depending on the voltage drop, you will probably need inductors to reduce switching losses and for filtering purposes, but this would be a transformer-less solution.  The advantage is you could synthesize any output voltage or frequency sine wave you wanted up to the DC bus voltage.  However, it would be much less expensive, more reliable, and less complex to just use a transformer.

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Depending on the current and load requirements, you could either use a simple dropper resistor or may a powerless series capacitive dropper.

• ###### Re: How to convert the AC to AC from high to low without using transformers!

What is the problem with a transformer.?

How is your 300-660 volts arranged, as most 400 v systems are between phases, with it still being 230v to neutral.

mark

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Unless of course its a delta

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Unless of course its a delta

Yes true delta doesn't have a neutral

but each phase is still only 230v above the neutral (well here in NZ).

It's only 400v between the phases ....

So unless our OP is trying to get 120-277 volts 3 phase, he probably already has a lower voltage.

mark

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Yes, his initial email stated that he was trying to convert 300-660V to 120-277, which to me said that most likely OP is trying to run single phase equipment from a 3 phase source.  If the source is a WYE, you are correct in stating that you can go from phase to neutral, and in the case of 480V 3phase input, you would get 277V single phase.  If his source is a delta, then phase to phase is only option, and I think everyone here agrees, a transformer is the best solution (most other solutions would be equally as large, costly, and in most cases more complex).  ie a rotary phase converter will do the job too (and it isnt a transformer)

• ###### Re: How to convert the AC to AC from high to low without using transformers!

As Mark pointed out using a Transformer you still get a sine wave.

Electronically you can go from AC-DC using a bridge rectifier and smoothing then using some BIG AR\$E transistors such as IGBTs under microprocessor control you can create AC or what ever arbitrary waveform you like at the load end.

This is how many large AC motor speed controllers work and is well known, considering the energies involved are you  actually going to implement this?

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Henri, What is your target application ?

• ###### Re: How to convert the AC to AC from high to low without using transformers!

John, qmiteam, mcb1,g0,

Thank you all for your replies and suggestions.  Actually using or not using transformer is not a first priority. The major challenge is design of a smart circuit to take care of low and high voltage at the same time. Meaning that from 90 volt to 660 volt AC should be passed through the board.

@ John ==> It is an electricity meter.

We have an AC/DC converter of 12 volt output and 90-305 volt AC input .... using a AC/AC converter for the input VS remove the AC/DC and build a new AC/DC with a wider voltage range is aimed in this project.

@ qm ==> as I mentioned before, now using or not using it is not a major concern. I need to use a component like a comparator to differentiate between 90 and 660 volt maybe!

• ###### Re: How to convert the AC to AC from high to low without using transformers!

For an electricity meter (space constrained, low current application), the best solution would be to redesign the AC/DC converter section for the wider input range.  Typically the lowest cost topology is a non isolated buck converter for powering these.  There are many reference designs available:

ie, this is a wide input design good to 500VAC (will also operate down at 90v), no comparator required

http://www.ti.com/lit/an/slua721/slua721.pdf

There are plenty of reference designs designed for 3phase that extend to 660vac.

• ###### Re: How to convert the AC to AC from high to low without using transformers!

No actually it is said that the maximum output is 10 vDC. And max of 500 vAC. um... this is close but not the exact what I need!

• ###### Re: How to convert the AC to AC from high to low without using transformers!

Also the frequency is not 60 HZ

• ###### Re: How to convert the AC to AC from high to low without using transformers!

A Buck converter is a DC-DC converter.  The AC input gets rectified into DC at the beginning ( in the example I referenced, to handle high voltage inputs only 1/4 wave rectification is used, this keeps power component costs lower as ultimate DC voltage will be lower than if it was rectified full wave), the first inductor in the example circuit I referenced blocks the rest of the AC component.  So AC line frequency is not terribly relevant except for the inductor design.  Since your final application (electric meter) really only needs DC input, there is no advantage to AC-AC conversion.  The first step is to get the input AC voltage to high voltage DC, and use a buck converter (DC-DC) to step down the voltage.  Output voltage and current capability will depend on buck converter design.  As long as isolation is not needed, this is the simplest/lowest cost solution to get high voltage AC to low voltage DC.  I did not design the circuit for you, I merely provided an example of a reference design that you could modify to suit your needs.

• ###### Re: How to convert the AC to AC from high to low without using transformers!

qmiteam,

As the following picture depicts, I might change the voltage to 660 volt and can get 12 vdc. It is worth it to investigate on such design. Bravo! (June 2014 --- very fresh!)