Everything, particularly when it comes to the differences between alternators and generators.
If you ask an automotive technician or mechanic to name the device that produces electrical energy in a vehicle, he would answer that it is the alternator. But if you asked the same person to name the device that produces electrical energy in a portable DC generator set that is mounted in the vehicle, he would answer that it is a generator — even when it is used to charge the same battery as done by the vehicle's alternator. Why the different terminology? Does the amount of electrical output power have anything to do with this? Batteries are charged by direct current (DC), whereas household electric power is alternating current (AC). Does the different type of current relate to the terminology in use? We can answer these questions if we know a few definitions and facts about generating electricity.
An electrical generator is defined as a device that converts mechanical energy into electrical energy. No distinction is made about the electrical energy being AC or DC. It could be either type of current. There is no distinction about how much electric power is produced. "Generator" therefore is a fairly universal term for a device that generates electricity.
The term accurately describes the device on an automotive engine that typically has a belt-driven shaft, and that spinning shaft somehow causes electrical energy to appear at the electrical output terminals of the device. This particular generator then powers the vehicle's electrical system and its batteries. So this would seem to be a DC generator, despite the fact that most people in the automotive industry call it an alternator.
An alternator is defined in the electrical industry as an electrical generator for producing alternating current. This definition makes the alternator a particular type of generator, specifically an AC generator. AC is unsuitable for charging a battery that happens to be a source of DC power. So why would anyone call the generator that charges a vehicle battery an alternator? Before we get completely confused, we need to know something very basic about the process of generating electrical energy.
Theories of Motion
Mechanical power involves motion. When a conductor is being moved in the presence of a changing or moving magnetic field, it creates a voltage across the conductor. The voltage induces an electric current flow in the conductor. The changing magnetic field cannot perpetually increase toward infinity, nor can it perpetually decrease to zero, so the mechanical motion serves to either switch the magnetic field repeatedly between on and off, or to reverse the magnetic field repeatedly between north and south. This process of magnetic field-switching typically takes place several times during each rotation of the generator shaft. The nature of this process causes the electric current in the conductor to reverse direction in response to the changes of magnetic field and thus creates alternating current, or AC.
When this phenomenon was discovered about 200 years ago, there was absolutely no use for AC. Batteries were known to store and deliver electrical energy, so DC was seen as the only usable form of electric power. The first generators had to solve the problem of converting the AC that the generator produced into DC. This was done with the invention of the commutator. The commutator is a series of segments or bars formed around one end of the rotor shaft with each segment connected to a rotor winding (see Photo 1). An even number of carbon brushes, usually two, rides atop the commutator segments (see Photo 2). As the rotor turns, the carbon brushes align repeatedly with the next adjacent winding segments to conduct current through the brush and winding circuit. In this way the rotor current is made unidirectional via the commutator process as the generator shaft turns. The direction of the current may be reversed by reversing the direction the shaft turns.
This is the normal way DC generators and motors are built. When the shaft is turned by mechanical power, then DC electric power appears at the output terminals of the machine and we call it a generator. Conversely, when DC electric power is applied to the terminals of the machine, then the shaft turns and we call it an electric motor.
DC power was used when the United States began commercial electrification about 120 years ago. It was convenient to have battery-storage backup for the DC power; the battery also helped accommodate load surges. But power distribution problems ultimately made DC impractical. DC generators are designed for their specific output voltages, and DC power cannot be converted easily by a transformer to different voltages as can be done with AC. This means that DC generators need to be located near the user to avoid line voltage drop if the power is carried over long distances. The 120 volts in use meant a 600-kw generator would have a 5,000-amp output with cables that would be nearly 2 inches in diameter — very heavy and hard to handle. Generator current losses rise exponentially with higher current to become the limiting factor in large generator designs. This limitation gives a distinct advantage to the use of higher-voltage generators that use a lower output current to produce equivalent power with a smaller generator package size.
But a high-voltage DC system would need to be converted down to the user voltage level. The DC voltage conversion would require converting DC to AC, stepping the AC down through a transformer, and then converting the resulting AC back to DC. One hundred years ago, the process of converting DC to AC normally was done by using a DC motor to drive an AC motor; the AC motor then would be used to drive a DC generator. The motor windings were designed to meet the appropriate voltage step-up or step-down requirement. These motor/generator combinations would be very large for high-power conversion, and had significant mechanical losses in addition to the normal winding losses.
Tesla and Edison
The DC generators of 100 years ago were mostly powered by steam engines. The space needed for a 500-hp steam engine would occupy the space of a typical two-bedroom house. It would require a coal room that could hold tons of coal, a chute, firebox, boiler, controls, water reservoir, condenser, steam engine and, of course, a shift of personnel who would monitor, maintain and service the equipment. The DC generator, in comparison, was just a small component of the steam plant. Very few cities were as fortunate as Buffalo, N.Y. — adjacent to Niagara Falls — which could tap into a water chute to drive a water turbine that could power a generator. As commercial electrification expanded, the need to build numerous local steam plants to power the DC generators was a costly requirement. Such limitations kept electrification localized and mainly urban.
Nikola Tesla was employed by Thomas Edison to improve generators and other electrical devices. Tesla promoted the use of an alternator for commercial power. It was much more reliable than a DC generator because it did not use maintenance-dependent carbon brushes. The AC power could be converted easily to higher or lower voltages through transformers as needed. High-voltage alternators could efficiently produce megawatts of power to convey through those 2-inch-diameter power lines used to distribute DC power for a short distance. High voltage could be distributed over long distances to receiving substations that used transformers to convert the power to the local distribution voltage. The local distribution voltage was again converted by a transformer to the user's voltage. The transformer at each stage was matched to supply the output voltage needed.
This AC system overcame the shortcomings of the DC system by providing efficient power distribution over long distances, as well as providing the user with a reliable voltage level for lamps and motors. Lamps would illuminate on either AC or DC, and lighting was by far the main demand for electric power one hundred years ago. Electric motors at that time were less common, but both AC and DC motors were available.
Tesla determined that AC had all of the power distribution advantages over DC, but Edison had too much invested in his electric power grid and his DC motors to consider such a dramatic change. Tesla soon quit working for Edison and found another supporter in George Westinghouse, who was willing to financed the AC distribution system that uses high-voltage generators and multiple transformers. The Tesla and Westinghouse AC system was used to provide electrification to less-urban areas of the country. The efficiency and reliability of AC eventually caused Edison to adopt it also, and it gradually replaced the older DC systems in use. After the novelty of AC wore off, the term generator replaced alternator as the device that generates the electric power in commercial electric-power systems.
In the 1950s the semiconductor industry developed diodes that may be used to rectify AC into DC. Diodes are a much simpler alternative to the commutator for converting AC electric power into DC. In 1959, Chrysler introduced the "alternator" to their cars, despite the fact that the electrical output is the diode-rectified DC and not AC, as the term alternator would imply. Chrysler coined the alternator terminology to distinguish their product from conventional generators. The Chrysler alternator was indeed distinctive. It had a large diameter but short body made from mostly shiny and open-frame aluminum. It also was lighter in weight than the old DC generators that looked like clunky starter motors.
Electrically, Chrysler had imbedded the field coil into a claw-pole rotor that induced electric power in the stator (see Photo 4). The stator output was AC, and diodes were used to rectify the AC into DC output in place of the mechanical commutation used in the generators. The rotating field coil meant carbon brushes remained in use, except in this case the carbon brushes conducted the rotor field current and not the output current, as does a DC generator. The fact that the field current is only about 5% of the output current means that the brushes lasted much longer than the old generator brushes that conducted the full generator output current. The Chrysler claw-pole design uses reversing magnetic fields to produce electric power. To this day, most automotive generators, commonly called alternators, employ a similar design.
Commutator brushes generate arcing, which is a source of prominent electromagnetic interference. Higher brush current causes more arcing and more powerful interference. The claw-pole alternator field may be fixed in position, with an additional air gap in the magnetic circuit to eliminate the use of carbon brushes. These are called "brushless" alternators. Other brushless alternators include the homopolar design (see Photo 3) that employs a switching (on/off/on) magnetic field rather than the claw pole's reversing magnetic field (north/south/north). All of these automotive alternators use diode rectification to produce the DC output. The popular terminology for these generators continues to be alternator whenever they are found in an automotive application. That is nothing more than a legacy from 1959 Chrysler marketing.
Today's Power Sources
Today's automotive industry is active with all sorts of new electrical technology. There are electric vehicles with new types of batteries and hybrid vehicles of various types that include an integrated starter generator (ISG) fitted to an engine flywheel. Various mechanically driven items are being replaced with electric-powered devices that contribute to a growing need for much more vehicle electric power. Permanent magnet generators and high-voltage generators are being used in hybrid vehicles. These are all called generators and not alternators. None of these new generators employ the claw-pole design that relates to the 1959 era.
The next generation of vehicles is more likely to use the term "generator" to define its electric power source. The military never adopted the alternator terminology and continues to use "generator" to define the dynamic electric energy source. The original name for a generator was "dynamoelectric machine" or dynamo for short. This was a suitable term at the dawn of the electric age because the same machine could be used as a generator when it was driven by a steam engine, or it could be used as an electric motor if it was powered by a large battery. The term dynamo has been out of vogue for about 80 years and it is unlikely to return. The term "alternator" probably would have vanished with "dynamo" had not Chrysler brought it back in a somewhat misapplied way as a marketing tool.
Now that we know about the origins of alternator and generator terminology, what is the right term in fire apparatus and automotive usage? The right term to use is" generator," with a caveat about popular usage. A generator by definition may have either AC or DC output, whereas an alternator has, from the original electrical industry definition, an AC output. It was advertising and marketing from 50 years ago that brought alternator into popular use to describe automotive generators that have DC output.
As always, know your audience. If your audience is familiar with the term "alternator," keep using the term to avoid confusion. But with the present dynamic state of automotive electrical technology, you may soon need to explain the facts as revealed. If your audience is not familiar with the term "alternator," they may not be familiar with the term "generator" either. For them and for general usage, stick with the term "generator." Generators are going to be around for a long time to come.
James Becker is an application engineer for C.E. Niehoff.
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