The stated voltage value of an AC supply, say that known for the voltage of the domestic
supply, 240V in the UK, is 0.707 times the peak value. This is known as the "effective"
or "RMS" voltage (root mean squared), "effective" because it is the voltage that
produces the same heating effect in a resistor as would a DC supply of the same value.
In a very few cases the average value is also used. This is arrived at by taking
instantaneous voltage values at numerous points on the waveform, summing them and
then calculating their average. This results is a value of 0.636 times the peak value.
I have included this for completeness just in case you should come across it. However,
in all common applications it is the RMS value that is given.
Applying an AC voltage to a load results in a similar current waveform, again its
quoted value will be the RMS value, that is 0.707 times its peak value.
Taking resistive first, being the easiest, here the current waveform will follow
faithfully that of the voltage(A), known as being in phase. In this case, calculating
the relationship between current, voltage, resistance and power will be the same
as for a DC circuit, that is
I = ——— or V = I x R or R = ———
and Power = V x I watts
Unfortunately, beyond this point the calculations become much more complex and will
be beyond what most will need to understand. However, even if you find what follows
difficult to comprehend do read on as there is a very important conclusion regarding
power consumed that is probably not what most will anticipate.
As the title says the content of these pages only cover the subject at a basic level.
They should though help with fault finding and designing simple control systems.
Also, understanding other web sites and magazine articles having an electrical content.
If you would like to go beyond that I list on the last page some web sites and books
that may be of help.
Elsewhere on the site I consider how various components function when connected into
a circuit supplied from a DC source. Whilst not essential, I think it is preferable
that you read those pages before studying these relating to AC supplies.
Sketch 1 shows how an AC supply voltage varies with time, both in magnitude and polarity.
Applying such a voltage to a load will result in a similar current waveform. What
then is the relationship between the peak voltage and the quoted voltage of a supply.
Unfortunately, in terms of it being easy to understand, the current waveform can
occur, simultaneously(A), before(B), or after(C) the voltage, Sk 2. The factor that
determines this relationship is the type of load, resistive, capacitive or inductive.