theoretically, the mains is a sine wave AC voltage, and that is all you would be provided. However today we have many sources of pollution, such as spikes on the mains voltage, serious wave form distortion, and a small DC components on the mains. Each of those three can cause serious problems with equipment, but sometimes we get all three problem sources together. Even so when the mains appears clean on one moment, this can suddenly change, when solar panels all together begin to produce electricity, or when time has come to switch on machinery.
- Switched electronics for motor control. These devices basically switch the motor on and off very fast. The ratio between on and off will change the speed of the motor. Like when an electric motor is switched on only 10% of the sine wave, it runs at very low speed. In the end, this creates a heavy distortion of the sine wave.
- Switched power supplies. Some of those work really bad, switching on and off a rectification capacitor, with the mains frequency itself. Others have switching devices inside, which are so small, they fit into the base of a light bulb.
- Solar panels feeding electricity back into the mains, or course also switched. Why not.. It works so efficient, and low cost.
You can just say, any cheap mess, they make it, and it's in your house at 200 or more places. Then, you are not the only one polluting your mains. Also your neighbor makes sure, the mains is sufficiently polluted, before the cable is passed on to your house. And so does the whole street, or some factories near by.
All together, the days of a clean mains voltage are gone, but what does this do to the equipment?
Those of you who use "patent" tube testers, may have seen it, without realizing. Such testers use the mains voltage as power supply for the tube under test. Their designers long ago, knew only a sinusoidal mains. Have you ever noticed, a particular tube tests one day at 91% and the other day at 96%? You may have thought that's because tubes are "live" object. Well, the reason is another! This is is because the mains distortion is not the same every day, and not the same at every moment of the day.
Phase cutting electronics, such as triacs, cut off the same percentage from the negative half and from the positive half. But nothing is perfect, and at slightest imbalance, this can leave a residual DC voltage on the mains, by lifting off the neutral against ground. You can try this out yourself. Just let a normal incandescent bulb burn on half the power, by putting a power diode in series. You pull DC current now from an AC network, and so you are lifting up the neutral line, and that will give more mains hum on any mains transformer without air gap. That is not much, just a Volt or so, or less, but effects in equipment transformers is there, when they are not made for this. Just take a good old 100 Watt light bulb, with a 1N4007 diode in series, and measure the voltage of the neutral against ground, with and without the lamp-diode combination. This is a real good eye opener for everybody who is interested to see this for the first time.
But why do this with a light bulb only...? Phase cutting is always done, when there is a LOT of power involved. Like in washing machines or air conditioning motors, or heavy machinery of a company a few blocks away from your house. All of this makes the mains voltage dirty.
Here is another modern problem: For small equipment, there are circuits that rectify the mains voltage directly, and feed this into DC to DC converters. These have a charge pumping effect. So the mains is loaded via current "spikes" into a capacitor, and the sine wave gets distorted from this. Such are these typical miniature mains plug power supplies. Like the ones to charge a mobile phone. Don't worry about the phone, it's well shielded against it's own power supply mess. Just mains itself, it's not protected. n be found dozens of such devices.
Nowadays, there is always some device which lifts up the neutral (so not the ground) of the house hold mains. Specially when this is done by phase cutting electronics, the risk of a small DC component becomes realistic.
Please check the pdf file below, to read more about it. I see sometimes often people spend a fortune on Voodoo mains cables, with "specially treated" materials, and whatever. I understand people have a problem, and seek a way to get rid of all the noise and disturbances, but I don't think any "cryogenically treated" copper is going to do that for you.
One of the most convenient devices for this is something simple like a transformer. It has to be a good one though, NOT under dimensioned, and using linear magnetic materials. Any "Cheapo" will be the opposite. Those will saturate a tiny little bit at the voltage peaks of the mains. Though they will work, they rather distort the mains even further. So a worse wave shape comes out, as what went in.
The Lundahl company since many years has a line of mains cleaning transformers in the program, which have cores same as a choke, and using very linear magnetics, same as used to build their audio transformers. Using a core with air gap, gives a transformer the capability to withstand a small DC component. A very typical air gap product is a power supply choke of course, as it carries more DC than AC. However, an air gap can be extremely beneficial nowadays, to filter the mains voltage.
The most recommended mains cleaning transformer is LL1662. This transformer can also increase or reduce the mains voltage is needed. So when you have 238.. 248 Volts all of the time, you may want to wire it for 5% voltage reduction, and of course get the filtering function still.
Please click on these pictures to see my recommended way to connect LL1662 .
These are just two examples, 230 to 230, and 240 to 230, but almost any configuration can be
made from unbalanced to balanced.