Step Up and Step Down supply voltage

[9fingers]

On a recent purchase of a machine imported from the US I had a big motor (3hp) designed for 240v ac and 60Hz (the American mains frequency) In UK we have a 50Hz supply and so the motor needs a reduced voltage in proportion to the frequency change. So I needed 200v. Some motors are designed to be able to cope with a frequency change and no voltage change but cheaper motors don't have that design margin and my motor ran far too hot on 240v.

The tool for the job is a transformer.



One way is to have a transformer designed simply for 240v input and 200 v output. This would be a custom design and be very expensive. Using this method, the transformer needs to handle the full power of the motor, making it big and heavy - approximately the same size again as the motor.

However there is another way to step the voltage down (or up) as needed.
I was talking to a friend about this and whilst he had heard of it - he was unsure how it works so I thought I would write it up here. I hope it is of interest.

Here is our incoming 240v mains supply.



and graphically this is a sine wave voltage like this



Now we add a transformer but one that produces the DIFFERENCE in the voltages that we need ie 40v. It still needs to deliver the current needed by the load but because the voltage is lower, the size, weight and cost is lower and in most cases will be a stock item you can buy.



and graphically



If the two waveforms are added together like this



the result is the Green sine wave which is 280volts

The circuit is below



If we connect the output of the transformer the other way round we get a reduction in the voltage



This is because the 40v is inverted or 180 degrees out of phase. Graphically the peaks of the blue line now line up with the troughs of the mains waveform and so the voltages subtract.



You might see our American cousins refer to this method as "buck" (step down) or boost (step up)

Hope this might be of interest.

Bob

 

[kirkpoore1]

OK, I think I get it now. I'm used to thinking of transformers working on both legs of the power supply (assuming 1ph), but with this one one of your power lines goes straight to the wall, and the other is effectively in series through the transformer. (Of course, that may be the wrong way to think of it, but what the heck, as long as it works and I don't have to figure it out...)


Thanks, Bob.

Kirk

 

[Robert]

So what you have done is turn a normal transformer into an autotransformer by joining the windings together..

Don't think I'd have thought of that but it is obvious it would work now you have said it.

I've used just the tapped mains side to change voltage in the past and not connected the secondary.

Nice idea and excellent explanation

 

[RogerS]

Nice idea, tanks, Bob.

The only caveat I would make is to write in very BIG LETTERS...

this is connected directly to the mains

as some folk tend to think of transformers as isolating devices - clearly not always the case.

 

[9fingers]

Robert, whilst it is similar to an auto transformer there is one important difference.
Normally an auto transformer will have the whole winding made from the same gauge of wire and hence the current through the active part of the transformer is limited by that. The chain is only as strong as the weakest link.

In the scheme I have shown the current carrying capacity of the primary and secondary are in inverse proportion to their voltages and so the secondary used to provide the step up or down is made from thicker wire and a relatively small transformer can do a large job.

A 500VA transformer about 140mm diameter and 45mm thick, in combination with a mains supply can provide a 2.5KVA supply enough for a 3hp motor.

There are lots of transformer trick you can play. Winding on extra turns of relatively light gauge wire(easy with a toroidal transformer) and putting these in series - in or out of phase with the primary, can trim the secondary voltage by +/- a few percent.
I have a car battery charger I made nearly 40 years ago with 5 output settings ( two trim primary windings, in or out of phase or not used at all) working on this principle. It means a low current switch can be used to vary a high current output with virtually 100% efficiency - ie no waste heat to get rid of.

Bob