I have an application where I could use a variable transformer, but I would need at least 40 amps output at 0-120 VAC. I can find variable transformers that meet these specs., but they are heavy. Is there any way I could build an AC amplifier that could handle 40 amps at 120 VAC? This setup would go into a test box, that is why the heavy variable transformer is a last resort. I am just looking for some direction on possible alternatives. Any thoughts or help would be appreciated!

 

Do the math. 40 amps at 120V is 4.8KW.
New house wiring is either 60A or 100A total to your house. Each circuit can deliver only 15A max. You would not get 40A from the wall unless you are in an industrial setting.

 

My house, and many around me, have 150 to 200 amp service @ 240V.
I commonly run 60 amp circuits for electric furnaces in residences.
40 amps can be had in a residence.
A small, light, variable transformer than can adjust 40 amps @ 60 Hz can not be had.
A high speed switcher could probably be made to be a little bit smaller and lighter than a 60Hz transformer, but it's not easy or cheap.

 

Wow! As they say, it's always bigger in America!

 

This application is in an industrial setting. I could get by with 20 amps and cover about 80% of my needs. 40 amps would be better. I agree a light small variable transformer can not be had. The ones I have found are heavy and this application would be in a test box, so light is good.

 

Dear rainy, I sincerely suggest you avoid the weeks of work (involved in designing and building a high speed switcher) that is mostly about how to keep it from smoking, when you can just buy a dependable Variac and put a big handle on the box. Probably because I'm lazy. However, you can do as you like and find a lot of help here.

 

#12, after about 5 hours of research I agree, big handle and lug her around. It seems I would be re-inventing the wheel. Thanks for all replies

 

A 40A 0-120V Powerstat is about 50lbs. Is that too heavy for you?

 

Mr. Chips, yeah that is heavier than I would like, but that is exactly what I am currently looking at.

 

What are you doing that requires such a wide voltage range? Do you actually require the full 0 - 120 VAC range in your setup? I know that's what you stated in the original post, but is it necessary?

If you actually can get along with a smaller adjustable voltage range around 120 volts, you can dramatically reduce the size, weight, and cost of your setup by using a smaller transformer in boost/buck configuration to add/subtract from the line voltage. You still require a 40 amp secondary winding but the power rating (and therefore, size and cost) of the boost/buck transformer is reduced roughly in the ratio of the (secondary voltage/120V). Thus, if you actual requirements are, say, 60 to 120 volts, you can get by with a power transformer and a Variac with half the power rating. 90 to 120 volts, 1/4 the power rating, etc.

Additionally, if you do really need the full 0 to 120 volt range but don't need continuous voltage sweep (i.e., could stop and flip a switch at 60 volts), you could use the half-power equipment in conventional configuration for 0 to 60 volts and in line-bucking configruation for 120 to 60 volts.

Using this approach, you still need a 0 - 120VAC Variac, but its size and cost is also reduced by the same proportion as the main transformer.

Be careful about cooling if you put the equipment inside a box.

awright