Chapter 1: A Historical Introduction
AC/DC Book of Knowledge
AC/DC Book of Knowledge
Depending where you are travelling in the world, the mains voltage available from the wall plate will be 50Hz or 60Hz AC (Alternating Current) with a nominal voltage of around 120VAC or 230VAC. Unless you are plugging in a hair dryer, kettle or a lamp, you will probably need an adaptor to convert the high voltage AC supply down to a low voltage DC (Direct Current) to be useful, for example to charge your phone or power your laptop. Considering that all electronic equipment runs natively on DC power, you might think why is the mains power always AC? And while we are on the subject, who chose 50/60Hz or 120V/230VAC as the “correct” numbers for the mains supply anyway?
Back in the nineteenth century, when public power distribution networks were first being developed, the choice was much wider. Both AC and DC mains supplies were offered, with the standard AC frequency ranging from as low as 16⅔Hz up to as high as 133Hz. Electronic appliances had not yet been invented, so the most common use of electricity was for lighting or heating, both of which worked equally well with either AC or DC supplies, so the AC frequency was not so important. The most common value was 42Hz and in America, Edison patented DC power distribution and heavily promoted it as being as safe as and more reliable than AC1.
To a certain extent, this was true, as early electrical generators were less than reliable and the banks of batteries both stabilized the output voltage and bridged any short duration generator faults with the DC supply. This was not the case with AC generators which needed very good speed regulators to maintain the correct output voltage with changes in demand and had no back-up supply possibility in the event of a generator fault.
AC eventually won over DC distributed networks for three main reasons: the simplicity of the first AC generators which led to a rapid improvement in reliability, the ease in which the voltage could be changed up or down using transformers and the advantages of multiple-pole alternators to reduce the rotation speed of more powerful generators. The simple electrical generators used at the time converted mechanical energy into electrical energy by rotating a magnet within coils of wire (figure 1.1). Note that there are no moving electrical contacts.
Fig. 1.1: Principle of operation of an alternator
The arrangement shown in figure 1.1 is more commonly called an alternator because the current flows alternately in one direction and then in the other as the magnet spins. If a non-alternating output is desired, then a mechanical switch called a commutator is needed to reverse the connections every half cycle:
Fig. 1.2: Commutator Action
In this arrangement, the coil of wire rates within a fixed magnetic field instead of rotating a magnet within the coils of wire, but the generating effect is the same. The commutator switching action is typically performed by a split slip ring on the shaft of the generator which reverses the connections every half turn:
Fig. 1.3: Split slip ring commutator
As the power distribution network developed and the demand increased, the current flowing through the commutator brushes increased and made the DC system more unreliable than the simple AC generators used at the time which needed no slip rings.
The second reason for the demise of the DC power transmission scheme was the increasing losses as more and more houses were connected to the system. The power losses in a cable with resistance R are proportional to the square of the current , I , flowing through it (i²R loss), so if the voltage can be doubled to halve the current (Power = VI), then the same power cable can carry the current four times further. This principle applies to both DC and AC power transmission, but it was much easier to use transformers to step up the AC supply voltage for long-distance transmission and to step it back down again at the far end again using transformers. Edison tried to compete with his DC system by using generator sets (a DC motor connected to a dynamo to step up or down the supply voltage) but although a low voltage DC motor for the step-up part was easy to make, a high-voltage DC motor for the corresponding step-down part was not so reliable and the system broke down often. In the end, even Edison abandoned the DC distribution concept and changed to alternating current power distribution.
Although most mains power sockets are single phase, electrical AC power stations generate three phases at 120° from each other. The advantage of this is that 3x120° = 360°. In other words, the phases cancel out when connected to a common point.
Fig. 1.4: Three phase waveform. The sum of all three phases added together is always zero.
This means that unlike in DC power distribution where the current flows equally in the positive and negative cables which therefore both need to be equally massive, an AC power distribution grid can be made with three heavy duty phase cables and a light gauge neutral wire which is only needed to carry any imbalance current if the loads on the three phases are not exactly equal. If you look at an electricity pylon, you can see the thick power cables suspended from the cross beams with a single, thinner cable running across the tops of the pylons. This is the neutral return wire. The earth (or ground) connection is for safety only. It carries no current in normal conditions. If a current flows from any phase to earth then it is due to a fault and a protective device (fuse or residual current trip) should cut off the power.
The following simplified diagram illustrates this arrangement when applied to whole streets in a town.
Fig. 1.5: Diagrammatic representation of a three-phase power distribution system. The neutral wire will carry no current if the load on each phase is balanced.
Why three phases and not two? Well, two-phase power distribution is still used in some parts of the USA (2x120VAC at 180° so that 240VAC equipment for heavier loads such as ovens and washing machines could be used on a 120V system), but the big advantage of an odd number of phases is for use with AC motors. It does not matter where the rotor sits, a three-phase motor will always start up in the same direction and as the load is equally balanced on all three phases, a neutral wire is not required (L1, L2, L3 and Earth). An AC motor with an even number of phases could either not start if the rotor was exactly in line with the poles, or worse, start up in the wrong direction. Additionally, a two phase system delivers power at twice the fundamental frequency and this pulsating supply must be smoothed out by the inertia of the motor, making a two phase motor larger and heavier than a 3-phase motor of the same power.
Fig. 1.6: Principle of operation of a three-phase motor. As each phase peaks, the rotor is pulled around to line up with that set of windings. The rotor then follows the rotating magnetic field.
But the final nail in the coffin for DC distribution was the popularity of electrical lighting. As more and more houses, public buildings and streets switched from gas lighting to electric lamps, the demand for electrical power increased rapidly. The lower cabling cost of three-phase transmission compared to DC became the deciding factor when raising the investment needed to electrify whole towns (a 3-phase system uses 50% more copper than a 2-phase system, but delivers three times the power).
More powerful and larger generators were manufactured to meet this demand. These generators were very heavy and the slower a very massive generator rotor can rotate, the less stress on the bearings and framework. This is why there were originally so many different AC frequencies used: a smaller generator spinning at 2500 RPM created a 42Hz output, while a larger one spinning at 1000 RPM created a 16⅔Hz output (note that “a nice whole number” of revolutions per minute (RPM) was more often used, an indication that mechanical engineers built the alternators, not electrical engineers. 16⅔Hz is still used by the railways because if a commutator is fitted to both the stator and rotor windings, an electric motor will run with either DC or AC at this low frequency). However, while an incandescent filament may not flicker much at 42Hz, at 16⅔Hz it was disturbingly visible. The AC flicker was even more pronounced with ...
Back in the nineteenth century, when public power distribution networks were first being developed, the choice was much wider. Both AC and DC mains supplies were offered, with the standard AC frequency ranging from as low as 16⅔Hz up to as high as 133Hz. Electronic appliances had not yet been invented, so the most common use of electricity was for lighting or heating, both of which worked equally well with either AC or DC supplies, so the AC frequency was not so important. The most common value was 42Hz and in America, Edison patented DC power distribution and heavily promoted it as being as safe as and more reliable than AC1.
To a certain extent, this was true, as early electrical generators were less than reliable and the banks of batteries both stabilized the output voltage and bridged any short duration generator faults with the DC supply. This was not the case with AC generators which needed very good speed regulators to maintain the correct output voltage with changes in demand and had no back-up supply possibility in the event of a generator fault.
AC eventually won over DC distributed networks for three main reasons: the simplicity of the first AC generators which led to a rapid improvement in reliability, the ease in which the voltage could be changed up or down using transformers and the advantages of multiple-pole alternators to reduce the rotation speed of more powerful generators. The simple electrical generators used at the time converted mechanical energy into electrical energy by rotating a magnet within coils of wire (figure 1.1). Note that there are no moving electrical contacts.
Fig. 1.1: Principle of operation of an alternator
The arrangement shown in figure 1.1 is more commonly called an alternator because the current flows alternately in one direction and then in the other as the magnet spins. If a non-alternating output is desired, then a mechanical switch called a commutator is needed to reverse the connections every half cycle:
Fig. 1.2: Commutator Action
In this arrangement, the coil of wire rates within a fixed magnetic field instead of rotating a magnet within the coils of wire, but the generating effect is the same. The commutator switching action is typically performed by a split slip ring on the shaft of the generator which reverses the connections every half turn:
Fig. 1.3: Split slip ring commutator
As the power distribution network developed and the demand increased, the current flowing through the commutator brushes increased and made the DC system more unreliable than the simple AC generators used at the time which needed no slip rings.
The second reason for the demise of the DC power transmission scheme was the increasing losses as more and more houses were connected to the system. The power losses in a cable with resistance R are proportional to the square of the current , I , flowing through it (i²R loss), so if the voltage can be doubled to halve the current (Power = VI), then the same power cable can carry the current four times further. This principle applies to both DC and AC power transmission, but it was much easier to use transformers to step up the AC supply voltage for long-distance transmission and to step it back down again at the far end again using transformers. Edison tried to compete with his DC system by using generator sets (a DC motor connected to a dynamo to step up or down the supply voltage) but although a low voltage DC motor for the step-up part was easy to make, a high-voltage DC motor for the corresponding step-down part was not so reliable and the system broke down often. In the end, even Edison abandoned the DC distribution concept and changed to alternating current power distribution.
Although most mains power sockets are single phase, electrical AC power stations generate three phases at 120° from each other. The advantage of this is that 3x120° = 360°. In other words, the phases cancel out when connected to a common point.
Fig. 1.4: Three phase waveform. The sum of all three phases added together is always zero.
This means that unlike in DC power distribution where the current flows equally in the positive and negative cables which therefore both need to be equally massive, an AC power distribution grid can be made with three heavy duty phase cables and a light gauge neutral wire which is only needed to carry any imbalance current if the loads on the three phases are not exactly equal. If you look at an electricity pylon, you can see the thick power cables suspended from the cross beams with a single, thinner cable running across the tops of the pylons. This is the neutral return wire. The earth (or ground) connection is for safety only. It carries no current in normal conditions. If a current flows from any phase to earth then it is due to a fault and a protective device (fuse or residual current trip) should cut off the power.
The following simplified diagram illustrates this arrangement when applied to whole streets in a town.
Fig. 1.5: Diagrammatic representation of a three-phase power distribution system. The neutral wire will carry no current if the load on each phase is balanced.
Why three phases and not two? Well, two-phase power distribution is still used in some parts of the USA (2x120VAC at 180° so that 240VAC equipment for heavier loads such as ovens and washing machines could be used on a 120V system), but the big advantage of an odd number of phases is for use with AC motors. It does not matter where the rotor sits, a three-phase motor will always start up in the same direction and as the load is equally balanced on all three phases, a neutral wire is not required (L1, L2, L3 and Earth). An AC motor with an even number of phases could either not start if the rotor was exactly in line with the poles, or worse, start up in the wrong direction. Additionally, a two phase system delivers power at twice the fundamental frequency and this pulsating supply must be smoothed out by the inertia of the motor, making a two phase motor larger and heavier than a 3-phase motor of the same power.
Fig. 1.6: Principle of operation of a three-phase motor. As each phase peaks, the rotor is pulled around to line up with that set of windings. The rotor then follows the rotating magnetic field.
But the final nail in the coffin for DC distribution was the popularity of electrical lighting. As more and more houses, public buildings and streets switched from gas lighting to electric lamps, the demand for electrical power increased rapidly. The lower cabling cost of three-phase transmission compared to DC became the deciding factor when raising the investment needed to electrify whole towns (a 3-phase system uses 50% more copper than a 2-phase system, but delivers three times the power).
More powerful and larger generators were manufactured to meet this demand. These generators were very heavy and the slower a very massive generator rotor can rotate, the less stress on the bearings and framework. This is why there were originally so many different AC frequencies used: a smaller generator spinning at 2500 RPM created a 42Hz output, while a larger one spinning at 1000 RPM created a 16⅔Hz output (note that “a nice whole number” of revolutions per minute (RPM) was more often used, an indication that mechanical engineers built the alternators, not electrical engineers. 16⅔Hz is still used by the railways because if a commutator is fitted to both the stator and rotor windings, an electric motor will run with either DC or AC at this low frequency). However, while an incandescent filament may not flicker much at 42Hz, at 16⅔Hz it was disturbingly visible. The AC flicker was even more pronounced with ...
