Generators and Alternators

Safety is always critical when it comes to electrical systems, and electrical generators/alternators are no exception. According to the Electrical Safety Foundation International (EFSI), “Electrical accidents cause as many as 165,380 electrical fires, 910 deaths, and 7,000 injuries each year.” To give some perspective, as little as 20 milliamperes can cause a loss of voluntary local muscle control. Anything above 20 milliamperes can cause severe muscular contractions, ventricular fibrillation, and/or changes in heart rhythm. A 10kW, 240 volt generator set generates 41,600 milliamperes.

Electrical generating systems can be either rotating or static. Rotating electromechanical machines use mechanical energy to convert into electrical energy. This would be a typical diesel or gas powered generator set. Static electric generator systems are designed to convert energy resulting from a chemical reaction into electrical energy. 1 example of a static device would be a solar powered electrical generating system.

Rotating electrical generator systems can be 2 different types; alternating current (AC) types, or direct current (DC) types. An alternating current generator is typically called an alternator, while a direct current generator should be called a dynamo. Regardless of whether it is a DC or AC generator, there are always 3 things that are necessary to produce electricity; a wire, a magnetic field, and relative motion between the wire and magnetic field. Wires should be made of a highly conductive material such as copper or aluminum. In order to have a magnetic field, there must be a minimum of 2 magnetized poles, one south pole and one north pole. The area between these 2 poles is where the wire must pass in order to produce voltage. The invisible lines between these poles are called flux lines. If the wires do not pass through the flux lines, there is no voltage produced. It is important to keep in mind that the voltage can be increased on a generator by increasing the number of wire loops (coils), the speed of the relative motion, or the strength of the magnetic field. The type of configuration described above is known as a basic 2 pole revolving armature AC generator. An armature can be defined as any winding into which an AC voltage is induced, and it can be stationary or rotational. In this example, as the wire loops spin across the flux lines between to 2 poles, an electrical sine wave is generated. As the wire loops spin 360 degrees, this completes 1 cycle of the sine wave.

One easy example that can help people visualize what a 2 pole configuration is, would be to take the planet earth as an example. Earth has both a north and a south pole, and lines of flux that run from pole to pole. In February of 1996, a team of Italian and American scientists sent out a space satellite that let out almost 12 miles of tether wire to try and produce a current and voltage. After several miles had been laid out, the voltage that ran across the wire was so great that the wire snapped off and floated out into space. The largest generator on our planet is the planet itself.  

Most diesel generators today are 4 pole, which means that there are now 2 sets of magnetized poles, however in this case the poles will be set north, south, north, south, rotating 90 degrees for each pole. With a 4 pole generator, there is also a sine wave created, however for a full 360 degree rotation, there are 2 cycles, as 2 sets of poles have been passed. The frequency of the generated AC voltage is a function of the number of field poles, and speed of the generator’s shaft rotation. The number of magnetic field poles divided by 2 will give the number of cycles generated with each complete revolution of the generator’s shaft. Frequency is measure in Hertz (Hz) and in the United States 60 Hz is the standard. During operation, a generator has no control over the frequency of the generated voltage sine wave. The frequency is a function of the design of the generator, and the speed of the engine which is using mechanical energy to rotate the armature. To find the number of rotations per minute, you can use the following equation; RPM=(120 x Frequency) / (Number of Poles). In this case the equation would be RPM=(120 x 60) / (4), which equals 1800 rpm. This is a typical number for diesel generators in the United States.

There are 2 basic functional components of rotating electric generators: generator armatures, and generator fields. We have already mentioned armatures above, as a winding of an electric generator into which an AC voltage is induced or generated. Therefore, any armature winding should be considered an AC winding. The armature winding is an assembly of wire coils inserted into slots ln a laminated steel core. These coils are then grouped together and connected to create the winding. The laminated steel core is made up of a group of discs that are stacked together closely as the coils are inserted. The reason the laminated steel core is not 1 solid piece, is to prevent overheating. If it were 1 solid piece, there could be the possibility of a meltdown. By stacking the laminated steel discs together, the circulating current is reduced significantly and the heat can be managed by pumping air across the discs.

In order to make coils, bunches of enamel coated wires, typically called magnet wires are put together and used in the armature. The enamel acts as an insulator, which helps prevent over heating between the wires in each coil. These wires are typically made up of copper, which can have problems with overheating or electric shorts. In order to prevent damage to the generator set, it is important to perform insulation testing, and plot the decay of the insulation within the alternator to understand the longevity of the alternator. This type of testing is done with a device called a Megohmeter. Be sure to consult the manufacturer’s suggestions for insulation testing, as each manufacturer will give specific instructions on how to test the windings without damaging the alternator. Another device, called a low resistance Ohmeter, can test for short detection. Low resistance Ohmeter’s work by passing a large current through the low resistance of the windings. Be sure to use caution when using either the Megohmeter or low resistance Ohmeter, as both generate large amounts of voltage and current. Only trained personnel should be allowed to perform such tests.

There are 2 types of generator fields; permanent magnet fields, and electromagnetic fields. One of the key differences between these 2 types is that electromagnetic fields can be turned off, and the magnetic field can be changed. As the name suggests, a permanent magnetic field cannot be turned off. Generator fields supply the magnetism we need for the armature. Just like the armature, the electromagnetic field and the permanent magnet field can be either stationary or rotational. Electromagnetic field windings consist of magnet wire wound around the poles of a laminated steel core.

An amortissuer is a winding consisting of bars of a conductor material such as copper or aluminum that is set in slots in each of the pole faces of the field core. These windings are located on the rotors of the generator. Amortissuer windings are often called damper windings, and their purpose is to dampen generator rotor oscillations caused by certain parts of the entire generator set, such as piston engines. The bars of each pole are shorted together, either by shorting end laminations or shorting rings.

One way voltage can be controlled is by manipulating the magnetic field strength of a generator. Remember that this cannot be done to a permanent magnet generator field, only an electromagnetic generator field. The device that is used to manipulate the voltage does so by creating a direct current (DC) source of electricity, this device is called an exciter. Excitation is defined as when a direct current passes through an electromagnet’s windings. Exciters can also be rotating or static. Static exciters typically receive their power from the AC output of the main alternator of the generator, but do not have any moving components. Rotating exciters can be belt driven, and are often mounted on top of the main AC alternator of the generator. In order to change the AC current coming from the alternator, into DC current, the exciter uses a device called a commutator. A commutator is a device consisting of an arrangement of copper bars connect to the coil ends of a DC generator (the exciter). The commutator mechanically converts the incoming AC current into DC current.

Within the main alternator of a rotating electric generator, we now know there must be an armature, and a magnetic field. There are 2 basic types of alternators; either a revolving armature, or a revolving magnetic field. Revolving armature generators are usually reserved for smaller generator sets that only generate 10kW or less. It is much more common to find revolving field alternators in the larger generator sets, especially diesel powered generator sets. The purpose of this type of alternator configuration (revolving field) is to supply DC current to the field, which creates the large magnetic field necessary for larger generator sets. Of the revolving field alternators, there are 2 common types of connection configurations; brush-type, and brushless-type. Brush type were more popular in the past, and although they are still fairly common, they require more maintenance. Brush types consist of slip rings that are on the rotor, and deliver DC current to the magnetic field. Today, brushless types of alternators are much more commonplace. Brushless types of alternators use a device called a diode to deliver DC current to the magnetic field. Diodes allow current to flow in one direction, but block the other. As we know, AC current flows in both directions, and DC current flows in one direction. This means when AC current flows through a diode, it is converted to DC current. While this method of converting AC current to DC current is quite effective, diodes can fail occasionally. They fail by either shorting out, or they open up, and break the current flow completely.

The electrical characteristics of an alternator are greatly based on the design of the windings within the armature. Manufacturers often keep exciter designs and field designs the same, but the windings designs will differ greatly, in order to manufacture generator sets with different voltage outputs. As mentioned above, armature windings are the assembly of coils grouped together, and inserted along a laminated steel core, which consists of multiple steel discs stacked together. One way to visualize this is to think of a stack of quarters that make up a sleeve of metal. There are 2 methods of connecting coils in an armature; in series, or in parallel. A coil connection in series is when the end lead of a coil is tied to the start lead of the next coil. In such a connection, the voltage across the complete series of coils is equal to the sum of the voltages in each coil. However, the current capacity (amperes) of the entire group of coils in series is limited to the coil having the least current capacity. When coils are connected in parallel, all of the start leads of the coils are tied together, and all of end leads are tied together. In parallel connections are different not only the design, but by the amount of voltage that can be generated. The voltage in parallel connections of coils are equal to that of a single coil. However, the current of an in parallel connection of coils is equal to the sum of all the current of each coil.

With few exceptions, alternating current electrical distribution systems throughout both the United States and the rest of the world are in either single or three phases. Modern AC current generator sets are designed to produce either single, three phase, or a combination of both. In a three phase modern armature design, there are 3 major separate groups of coils, each set apart by 120 degrees, or one-third of a cycle. Three phase diesel generator sets are very common in today’s onsite power industry. 3 phase electrical generator sets produce 3 sine waves that cross the 0 threshold 120 degrees apart. This design allows for a more efficient way to produce electricity.