One of the main reasons for using an alternating current a.c. supply is that we can easily change the voltage levels up or down by using a transformer which would be more difficult and expensive to do if using direct current d.c
For efficient transmission the generated a.c. supply coming from the power stations is transformed up to 132,000V or more, and supplied throughout the country along the national grid power lines.
When it reaches the consumer, the electrical supply is then transformed down and is available as either single-phase 230v a.c. or three-phase 400v a.c.
Electrical equipment can make use of this alternating current supply, and for domestic installations due to the small amount of electrical equipment used a single-phase 230v supply is normally adequate.
However, in larger installations such as in large commercial (supermarkets, office blocks, shopping centres etc) or an industrial installation a larger ‘3-phase’ supply is usually required as more equipment is used some of which such as motors may require a 3-phase supply in order to work correctly.
For this reason, some knowledge of alternating current production and its use is useful for all practicing electricians.
Producing a single-phase alternating current supply.
This method but on a massive scale is still how we still produce electricity mechanically today.
Around 1830 Michael Faraday discovered that when a coil of wire is rotated inside a magnetic field electricity is produced as the coil cuts through the lines of magnetic flux a voltage is induced in the coil.
This induced voltage within the coil of wire follows a mathematical law known as the ‘sinusoidal law’ which produces a sine wave as the voltage rises to a peak in one direction of flow, and then subsides to zero, where it reverses, and descends to its peak in the opposite direction, before returning back to zero, and so on repeating this cycle on every revolution.
If the coil is rotated at 50 revolutions per second this cycle would repeat itself 50 times every second, which is where we get in the United Kingdom electricity generated at a frequency of 50 cycles per second or better known as a frequency of 50 Hertz ‘Hz’ alternating current.
The time taken to complete each individual cycle can easily be calculated using T = 1 ÷ f where f is the frequency of the supply giving us 1 ÷ 50 = 0.02 seconds which means that on a single-phase supply the voltage completes one full cycle, from 0v up to +’Peak’v and down to 0v then to -‘Peak’v and back up to 0 once every 0.02 seconds.
Let’s take a look at an a.c sinewave produced over one full cycle and the three voltage components produced…
The Peak voltage or maximum: is the greatest ‘or highest’ value reached by the generated waveform.
The Average: is the average over one half-cycle of the values as they change from zero up to a maximum and back down to zero. Note the maximum cannot be calculated over a full cycle as the values would include both + and – values which when added together would equal zero. Also, for any sinusoidal waveform the average value is equal to 0.637 of the maximum ‘peak’ value.
r.m.s value: This is the value of an a.c. voltage which produces the same heating effect across a resistance as the same value of a d.c. voltage and is the square root of the mean of the individual squared values. For any sinusoidal waveform the r.m.s. value is equal to 0.7071 of the peak or maximum value.
Note: When talking about (U0 which is nominal line voltage to earth ‘see symbols used in Part 2 of BS7671’) i.e 230v a.c we are in fact using r.m.s values and this is done in order to be able to correctly calculate electrical power and current values.
Producing a 3-phase alternating current supply
If using the same generator instead of using just one coil of wire, we placed two separate coils of wire (that are free to spin) within our magnetic field and placed them at 90 degrees to each other.
As both coils rotated, we would now produce two alternating current supplies with voltage on the second one starting, rising and falling etc one quarter of a turn after the first, or 90 degrees apart from the first.
The next step which gives us our 3-phase supply is to add another coil of wire to our generator and in order to separate them evenly we place each coil now at one-third of a turn after the previous one. So, each coil is now 120 degrees apart (which is 360 ÷ 3) from the next or previous one.
As all three coils rotated, we would now produce three individual alternating current supplies ‘phases’ with the voltage on each one starting, rising and falling etc one third of a turn after the previous, or 120 degrees apart.
If the RMS value on the voltage sine wave is 230V then the potential difference (voltage) between any of the other two phases is 400V.
This is because in a 3-phase system where all three phase voltages are the same (230 Vrms L – N) and they are 120 Deg apart then the voltage between any of the phases is equal to the √ 3 x (V L – N) or 1.73 x 230 = 397.9 V (400 V)
In summary:
A 3-phase supply consists of 3 single phase supplies (L1, L2, L3 and Neutral) with each supply starting, rising and falling etc one third of a turn after the previous, or 120 degrees apart.
With a L – N voltage of each phase being 230V and a L – L voltage between any of the phases of 400V.
Smaller installations such as domestic installations can be supplied using a single phase 230v L – N supply as there is not a high demand for power due to the small amount of electrical equipment to be run.
Bigger installations such as larger commercial or industrial have lots of equipment that can create a high demand for power, and this is best distributed over the three separate L – N 230V single-phases. Plus, they are also more likely to have equipment that requires a 3-phase 400V (L-L) supply in order to work correctly such as motors on machinery.
Another useful quirk of a 3-phase system is that the current flowing in the neutral of the supply is not always equal to the value of the combined current flowing in the Line conductors (L1, L2 and L3). This is due the line conductors being out of sync with each other (ie. 120 degrees apart) and the current flowing back through each line conductor as they reach opposite points along their sine wave cycle.
In fact, if the current in L1, L2 and L3 is the same this is called a balanced load and there will be no current flowing in the neutral conductor. A condition which should be aimed for within 3-phase installations as this can reduce the consumers electricity bills.
The additional risk of higher electric shocks currents is also something to be considered with 3-phase systems as you risk coming into contact with 400V as well as 230V.
This is Part Five of our Electrical Foundation Series
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