Adjust Mains Voltage Using a Transformer

[fluorescent]

In this video Big Clive shows us how to adjust mains voltage up or down using a regular 12v lighting transformer. I did not know you could do this and it's VERY useful to know it can be done using a cheap & widely available AC transformer, which you may already have.

A good example for us lighting folk would be if you have a rare magnetic ballast but it's rated for a 220v or 230v supply & your voltage is much higher (typical UK voltage is around 245v), you can now use that ballast!

Doesn't have to be a 12v transformer either, should work just the same with, say, a 24v transformer.

https://youtu.be/ZEFVNFyG9po

[lasagafield]

I saw the video not too long ago but for some reason I didn't post it.
Is a brilliant little trick.

[fluorescent]

It is indeed, I've been a sparky about 14 years & didn't know you could use transformers in this way. Every day's a school day.

I'm currently looking for a 24v tranny to lower the voltage from ~250 to ~226 for my rusty revo BC fitting, seeing as I believe the unlabeled ballast is actually a 220v rated item! Will report how it goes when I eventually around to it (should be sometime in the next 5 years or so lol)...

Rusty Revo® http://80.229.24.59:9232/gallery/displa ... p?pid=6416

[FrontSideBus]

A neat little trick.

I’ve got an old ballast somewhere that has taps for 200-250v in 10v increments! I guess baack then in some areas it was a bit hit or miss what you got. Frequency is listed as cycles per second instead of hertz so that give you an idea how old it is lol.

[Ash]

It still is, especially for large lighting-only installs

The power company over here accounts for appx 10V drop (on each phase) from the LV terminals of the transformer to the customer's premises, so they aim to get 240V for a nominal 230V. In light to no load condition, he would indeed be getting near 240V

With several 10's kVA being the smallest available transformer from the power company, a small road junction with 10 150W SON lamps in the mid of nowhere would be getting 250V or more... (and in the day, photocells getting allmost the fullest no load voltage from the transformer, which might be a few more V higher)

On the other end there are places with long line of poles along a road, with wiring sized to the minimum permissible cross section for the current, the luminaires at the end would be getting well below 230V

For UK's 240V nominal (atleast in the past ?), shift all voltages i named another 10V up

[BC5-80]

Before harmonisation to 230V, our voltage always used to be 250V or more. I measured 254V many a time. Nowadays I measure anything between 220V and 235V.

[fluorescent]

Before harmonisation to 230V, our voltage always used to be 250V or more. I measured 254V many a time. Nowadays I measure anything between 220V and 235V.

That's actually quite low and makes me wonder if you have some supply issues. The supposed "230v" we now have here in the UK is a myth. In reality, all that happened was that the tolerances were written so that our normal 240-250v was within range of our European neighbors.

[BC5-80]

Hmmm, interesting. I'll try to remember to measure it again tonight when I get home. I must admit I haven't minded it being lower for all these years. Everything works just as well, but presumably less stressed.

Not that I use incandescent bulbs now, but I remember back in the 70s when 240V bulbs lasted no time at all on >250V.

[Ash]

There are few distinct cases when lower voltage is not good, that i can think of



Induction motors that power mechanical things that are hard to get moving from a halt, compressors that start against existing gas pressure (as in when they restart after a momentary brownout) and such, have harder starts on lower voltage

The problem is that the low voltage limits the starting current available to them, and current translates into torque. They have less torque to push with, so the starting takes longer time, so the motors get stressed for longer

The low voltages may be result of wrong transformer tap choice (so the voltage is lower, but the line is still fairly low impedance), or of high line impedance, as in you are away from the transformer, or fed with thin cable and such. In the latter case, your voltage will drop lower the more current you draw. So, while normally you get say 220V, when the motor starts and draws few 10's Amps (for a medium motor), at this moment your voltage might drop into the <<200V area - your motor will be starting at that voltage, not at 220V

I'd guess that as long as the motor still starts fairly quicky, this is not as much of threat for 3 phase induction motors, since the only stress they get is by overcurrent and nothing is going to heat up within the time it takes to start. The risk group is probably 1 phase motors that use all sorts of quirks to start, like start coils (wound with fairly thin wire, that are not meant to get powered for any longer than a few seconds at a time), start relays, starting capacitors (many of them are not meant to handle stress for any longer than what a "normal" start takes), and such



If the low voltage is result of line impedance, in some extreme cases it can get to the point when it interferes with safe operation of circuit breakers

Lets say your ring breaker is C32. This breaker is suppposed to let you draw 32A indefinitely. At above 32A it will trip. For moderate overcurrent (overload) by a bimetal that takes time to heat up, and for big overcurrent (short circuit) by an electromagnet that trips it immediately (within 1 AC cycle, 1/50th of a second). For C type breakers, the electromagnet kicks in between 5x..10x overcurrent, so between 160..320A (each individual breaker is different). Let's say you happened to have a "high" breaker, that will only trip immediately at around 320A or higher

Let's say your line impedance is 1 Ohm. It is a bit high but can be expected for a house located somewhat away from the transformer, and having an individual line without neighbors (so with thin conductors, rated only for the load of one house)

Let's say your no load line voltage is good looking 250V

Normally you won't see much of a problem. At light loads your line voltage is excellent. At 10A you get 250 - 10*1 = 240V. At 20A its 230V. At full 32A load its 218V - still not causing any visible signs of a problem

Now you happened to get a short..

250V / 1 Ohm = 250A

The breaker won't react by electromagnet, but by the bimetal. At so high overcurrent It will heat up and trip within a couple seconds, but it means that instead of a single loud bang, you'll have a whole 2 seconds of shooting sparks



Higher line impedance will mean generally more small brownouts from starting of appliances (you will notice the lights glitch when motors are started or electronics with inrush current are plugged in). This is normally not a big problem..

But there is one case that differs. If you have 3 phases, or if you are on the same long line with a neighbor who happened to be on a different phase of the supply. Since you share the neutral wire, whatever voltage drop one of you adds by drawing current, the other one will see as extra voltage (out of phase by 120deg, so for each 1V your neighbor drops, you'll see as +0.5V to your voltage)

When your neighbor draws some current and you draw little, you'll have some overvoltage. When he draws an inrush current (starts a vacuum), you'll get a few 10's V spike above the line voltage you see. This could blow an aging GLS. If your neighbor have a short circuit, he could blow up some of your electronics..