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Differences between AC and DC Generators, taking a good look at Generators

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Explore the differences between AC and DC Generators and learn how to protect generator bearings. Plus read about using the correct carbon brush is a key component for outstanding motor life, …

Explore the differences between AC and DC Generators and learn how to protect generator bearings. Plus read about using the correct carbon brush is a key component for outstanding motor life, maximizing brush life, and reducing commutator wear.

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  • 1. The DC Motor/Generator Commutation Mystery Article Link // Posted on July 29, 2013 by David Manney Carbon Brushes from Morgan AM&T One small, yet vital piece of the DC electric motor puzzle is the carbon brush. Using the correct carbon brush is a key component for outstanding motor life, maximizing brush life, and reducing commutator wear. Morgan AM&T is a world-class manufacture of carbon brushes. In this technical paper, Richard D. Hall, Senior Design and Application Engineer, explores the relationship between brushes and the commutator of a DC electric motor and generator. One of the keys to meeting the specifications of a motor application is determining the correct brush grade. Commutation and Brushes When direct current motors and generators are manufactured, they are tested and adjusted at the factory to provide desired output characteristics and optimum commutation. Once these machines are put in service, the original adjustment of the machine may be disturbed when the components are changed or when work is done on the motors or generators. This can lead to excessive sparking at the brushes and possibly reduce brush life and increase commutator wear. In order to restore these machines to good operating condition, it may be necessary to adjust them in the field.
  • 2. DC Machine Basics Before discussing the adjustment of DC machines, it is import to understand how DC generators produce voltage and current, and how various machine components assist in the commutation process. A DC motor or generator is constructed of a magnetic circuit consisting of iron or steel parts and an air gap (g) (Figure 1). The air gap allows for movement between the rotating and stationary parts of the machine. Inside the machine are various windings made of electrical conductors that wrap around the iron parts of the motor or generator. These windings have some number of turns (N). IfDC voltage (e) is applied across a winding, a current (i) will flow through the winding, and this current will be determined by the voltage (e) and the resistance of the winding according to Ohm’s Law (Figure 2).
  • 3. The current flowing through this winding will magnetize the iron parts of the core and cause a magnetic flux (0) to flow through the iron parts and across the air gap. The amount of flux is proportional to the product of the number of turns (N) × the current (i). The relationship between the product of N × I (ampere turns) and flux is not linear (Figure 3). At low flux levels, the flux is proportional to the ampere-turns. The primary resistance to the flow of flux at these flux levels is the air gap in the machine. At higher flux levels, it becomes increasingly difficult to force more flux through the iron parts of the machine. This is called saturation of the iron. Here, it takes a large increase of ampere-turns to get only a small increase in flux. The relationship for flux and ampere-turns for aDC machine is shown by the saturation curve (Figure 3). If the current in a winding of a DC machine is established, and the flow of flux is established and something happens that causes a change in the flux or current in the winding, a voltage will be produced at the terminals of the winding (Figure 4). The voltage is proportional to the number of turns × the rate of change of flux (dø/dt). Notice the negative sign, which indicates this voltage is produced in such a way as to oppose the change in flux. Another way of writing this equation is using a property called inductance (L). A voltage will be produced that is proportional to the inductance × the rate of
  • 4. change of current (di/dt). Again the negative sign indicates that the voltage opposes the change in current; that is, it attempts to keep the current flowing in the same direction. If a conductor is passed through the magnetic flux in the air gap with a velocity (v), a voltage will be produced at the ends of the conductor (Figure 5). This voltage is called an E.M.F. or electromotive force. The voltage is proportional to the flux density (flux per unit area) × the length of the conductor (I) in the air gap × the velocity at which it is moved through the air gap. The voltage will increase with higher flux densities, longer lengths of conductors or movement at a higher velocity. If a circuit is completed so that current can flow through the conductor, it will take some force (F) to pull the conductor through the air gap (Figure 6). The amount of force it takes is equal to the flux density × the current in the conductor × the length of the conductor in the air gap. Higher flux densities, more current in the conductor or a longer length of conductor will increase the force required to pull that conductor through the air gap. Therefore, in a direct current generator, an output voltage will be produced that is proportional to the flux density (Β) × the length (I} × the velocity of the conductor (v). V=Β×l×v
  • 5. Since the length of the conductor is determined when the machine is designed, and the velocity is related to the RPM of the generator which is constant, we see that the output voltage of a DC generator is proportional to the flux density (Β) which is related to the generator field current by the saturation curve. The output voltage of a direct current generator is controlled by adjusting its field current. Direct current motors work by the same laws as direct current generators (Figure 7). A voltage will be produced that is equal to the flux density × the length × the velocity. Since the length is predetermined and the velocity is related to the RPM, we see that the voltage is proportional to the flux density (Β) × the RPM. If this equation is rearranged to solve for the RPM, we find that RPM is proportional to the voltage applied to the motor divided by the flux. Since the flux density is determined by the field current, we see the RPM is proportional to the voltage divided by the field current. This means that the higher the voltage applied to the motor, the faster it will rotate. Also, if the motor field current is high, the motor will run slower. If the motor field current is low, the motor will run faster. On excavators, there is often a strong and weak field setting for the DC motors. This allows the motors to run faster for a given voltage in the weak field setting. An example of this would be paying out the drag motion on a dragline. If we look at the illustration on Figure 7, we see an armature with a core length (I) and a conductor passing through this armature at radius (R) from the center line of the shaft. Flux density (B) enters the armature in the cross-hatched area. If a current flow through the conductor in this armature and the armature rotates so as to pass the conductor through the magnetic flux, a force will be produced on this conductor that is proportional to the flux density × the current × the length of the armature iron. Torque is equal to force × radius. Since the length of the armature is predetermined at the time of machine design, we can see that, for a given machine, the torque is proportional to the flux density × the current in the armature. As the current in the DC motor increases, its output torque increases. Also, if the flux density is high, such as in the strong field setting, the torque will be high. If the flux density is low, such as when the field is weakened, the torque will be low. Looking at the speed and torque equations, we see that high field flux produces high torque and low-speed. Low field flux produces low torque and high-speed.
  • 6. The ability to control the speed and torque of these motors has made DC drives the choice for powering industry for many years. The next important consideration in DC machinery is the commutation process. Commutation is the reversal of current in the armature windings of a DC machine. Figure 8shows an example of a part of the DC machine showing a North and South main pole. As the armature passes under the main poles, the current in individual conductors reverses. If we look at this graphically, we see that we have positive current underneath the North pole and negative current underneath the South pole. During the time that the armature conductors pass between the North and South pole, the current must reverse. This change in current is known as Δ, and this occurs during a time period Δ. Recall that if we have a change in current in a winding of a motor, there will be a voltage produced (V) that is equal to minus the inductance × the rate of change in current di/dt. This can be represented as inductance × Δ divided by Δ. This voltage that is produced is called the reactance voltage. Its polarity is such that it would try to keep armature current flowing in the same direction, which is undesirable. Ideal or linear commutating is represented in Figure 8. Actual current reversal will be somewhat different. In order to help the current reverse, an additional component is added to DC machines. This part is called a commutating pole or interpole. It fits between the main poles and produces a flux that cuts the armature conductor (Figure 9).
  • 7. A voltage will be produced in the armature conductors that are proportional to the flux density × the length of the conductors × the velocity, which is the same as saying the voltage is proportional to the flux density × the length of the conductors × the RPM. We would like this voltage to be exactly equal to the reactance voltage, which is equal to minus L × Δ / Δ. If we set these voltages equal and realize that the length of the conductors is determined by the machine designer: and the • • • Velocity is constant for a generator Inductance is fixed by the machine design Change in time is determined by the speed of the machine Then we see that we would like to have a flux density produced by the commutating pole that is proportional to armature current. This is done by connecting the windings on the commutating pole in series with the armature. The commutating pole magnetic circuit should not be saturated so that flux is always proportional to ampere-turns. If we look at the flux distribution underneath the main poles (Figure 10), we see that there is a uniform distribution of flux when the main field is excited. If we look at the distribution of flux if there is current in the armature, but no excitation of the main fields, we see a flux distribution as shown in the center of Figure 10. If we have both main field excitation and current in the armature, the fluxes combine as shown at the bottom of Figure 10. The flow of current in the armature distorts the main pole flux and causes the flux density to be high at one pole tip of each pole. It also distorts the flux so that it may not be zero near the center line between the main poles. This tends to sustain armature current in the same direction, which is undesirable. The effect of armature current on the flux in the air gap is called armature reaction.
  • 8. In order to cancel the effect of armature reaction, one additional winding is placed in large DC machines. This winding is called pole face winding (Figure 11). The pole face winding consists of large conductors placed through the face of the main pole pieces. They are connected in a manner that opposes the flux from current in the armature conductors. The pole face winding cancels the effect of armature reaction, reduces the bar to bar voltage at the commutator bars and improves some output characteristics of the machine, notably speed stability in DC motors. The pole face winding is connected in series with the armature. In order for a machine to commutate properly, it is necessary to adjust the amount of flux from the commutating fields. Since the amount of flux is proportional to the number of turns × the current, and the number of turns and current are predetermined by the machine design and load, some method must be provided to adjust the amount of flux. This can be done by changing the air gap. There are two air gaps in the magnetic circuit for the commutating fields (Figure 12). One air gap is between the commutating pole tip and the armature. This gap is called the “front gap”. Increasing the front gap reduces the amount of flux for given ampere turns in the commutating field winding. It also changes the distribution of flux over the armature surface. Since it is desirable to have a certain flux distribution, large changes in the “front gap” are not recommended. A second “air gap” is provided between the back of the commutating pole and the machine frame. This gap is called the “back gap”. It consists of non-magnetic shims, usually aluminum or brass. By adjusting the quantity of
  • 9. non-magnetic and magnetic shims in this area, the amount of flux can be adjusted. The order of the shims is important as well as the quantity of magnetic and non-magnetic shims. The correct order for GE machines is thin steel shims next to the frame, thin aluminum shims, thick aluminum shims and thick steel shims next to the commutating pole. (Other motor manufacturers may use a different shim arrangement.) Another adjustment that can be made in DC motors and generators is the position of the brushes on the commutator surface. The brush arms are connected to a large ring which can be moved on larger machines. When properly positioned, the brushes will contact commutator segments that are connected to armature coils that are passing through the commutating zone where armature current is reversing (Figure 13). Since the relationship between the position of armature conductors and commutator bars varies slightly from one armature to the next, it is important to check brush position when a new armature is installed. If brush position or the amount of flux from the commutating poles is incorrect, the current in the armature windings will not reverse properly, and sparking will result at the brushes causing reduced brush life and deteriorating commutator surface conditions. The brush position and commutating field strength are adjusted at the factory by a method called the black band method of commutation adjustment. This method requires large and specialized equipment that is not practical for use in the field. Other methods of checking machine adjustment are necessary for use in the field. Machine Adjustment in the Field Increased sparking levels at the brushes, rapid brush wear or burning or etching of commutator bars are signs that commutation is not occurring properly in a DC motor or generator. If this happens, it is necessary to determine why the machine is not working properly. Also, if any major component of the machine is changed or the machine has been disassembled, it may be necessary to check machine adjustment.
  • 10. WARNING: Working around rotating electrical machinery can cause serious or fatal injury due to electrical shock hazards or contact with rotating parts. Contact the original equipment manufacturer’s service engineers for performing adjustments on electrical motors and generators. These people have the necessary training and information available for properly adjusting DC machines. Before adjusting a motor or generator, it is important to determine that the components are of good integrity and that the machine is properly assembled. There are two methods that can be used to check the integrity of main coils and commutating coils. The first of these methods is called the “DC drop” method. To measure DC drops, a steady state current is passed through the main coil or commutating coil windings and the voltage drop at individual coils is measured. If there are shorted turns in an individual coil, its DC voltage drop will be low. DC drops are easy to measure and can usually be done without disconnecting the machine. The DC drop method is somewhat limited in that it may not show shorted turns if the short is minor or only a few turns are shorted. A second method of checking for shorted turns is the AC drop method. To perform AC drop tests on main field coils, two or more coils are connected in series and an AC voltage is applied to the coils. Since these coils have a very high inductance that will limit the current, it is possible to connect them directly across a 120 volt AC line. The voltage is measured at each coil using a standard voltmeter. If there are shorted turns in a coil, the shorted turns will act as a shorted secondary of a transformer and will reduce the impedance of the coil. The coil with shorted turns will have a lower voltage drop than the good coils. Commutating field coils have a much lower impedance and so need a high current/low voltage source for AC excitation. It has been found that a standard pistol-grip soldering gun can be used as a current source for measuring AC drops on commutating coils. Two or more coils are connected in series for these measurements. The AC voltages on main field and commutating field coils should agree within 15%. The armature may be in place or removed when making these measurements, but the frame must not be split or erroneous data will result. Another important factor for good commutation is uniform brush spacing around the commutator. On pedestal type machines, individual brush arms are adjustable. Using a paper tape, such as an adding machine tape, it is possible to measure the space between individual brush boxes around the commutator surface. All brush studs should be spaced within 3/64″ [0.047" or 12 mm]. Brush boxes should be spaced .070″ to .080″ [1.8 mm to 2 mm] above the commutator surface. Also, a uniform air gap between the armature core and the poles is important. On pedestal machines, the frames are independently adjustable from the bearings (i.e. armature). It is possible to have nonuniform or tapered air gaps. Air gaps should be equal within .007″ [0.18 mm] for the main poles or the commutating poles. Note: The main poles will have a different air gap than the commutating poles.
  • 11. The spacing between main and commutating pole tips is also important. If this spacing varies more than 1/8″ [0.125" or 3.2 mm] the commutating field flux will not pass through the proper armature conductors. The commutator surface should have no more than .003″ [0.08 mm] run out and no more than .0002″ [6 microns] variation between two adjacent commutator bars. An easy way to check this is by using a device that consists of a linear voltage transducer with a mating power supply and a strip chart recorder. Finally, it is import that the electrical connections in all the windings of the machine are tight and corrosion free and vibration should be within the limits shown in the motor or generator instruction book. The first adjustment of machines that is made in the field is the brush position. This is done by a technique called the pencil volt neutral test. A special template is made that fits around the brush on one brush path. The template has a series of small holes drilled through it and by measuring the voltage from the commutator surface to the brush stud through these holes, the proper brush position can be determined. The machine is operated at approximately 100 volts no-load for this test. There are a number of alternate static or operating tests that can be used to set brush position. Once proper brush position has been established, the next check is to determine if the proper flux is being produced by the commutating fields. This is done by a method called lead-trail voltage. The voltage is measured at the leading and trailing edge of the brush with the generator operating at full load and low voltage. This can be done by using the template and pencil probe or by using a special insulated brush. Carbon Brushes Carbon brushes provide the electrical contact between the stationary and rotating parts of DC machines. The brushes carry load current into the rotating parts and aid in the commutation process. By controlling the ingredients that go into the base carbon of the brush, certain properties of the brush can be controlled. The base carbons are manufactured in various grades which we call the electrographitic family of brushes. This type of brush is used in most DC machines. They get their name from the manufacturing processes where the carbon is graphitized in high temperature
  • 12. electric furnaces. These grades are generically shown as A through E on Figure 14. The strength of the base carbon is increasing as we move from grade A through E. Generally the lower strength carbons have higher resistance. The commutating ability of the brush is a combination of the lower strength or modulus of the brush and the lower density which allows it to ride the commutator better along with the higher resistance which reduces the circulating current in the brush face. In general, life increases as the strength increases due to the ability of the brush to resist mechanical wear. However, the high strength brushes do not have the commutating ability of the lower strength materials, and if a brush is selected that does not have sufficient commutating ability for a particular machine, its life will actually be reduced due to electrical wear. After the base carbon is manufactured, it is usually treated with some organic or inorganic materials. Very few modern machines use brushes that are not treated. The treatments added to the brush improve its characteristics as follows: • • • • • • • • Improve Brush Life Improve Filming Provide Low Humidity Protection Allow High Temperature Operation Prevent Copper Drag in High Humidity Operation Allow Operation in Contaminated Atmospheres Minimize Commutator Wear Reduce Friction Specific treatments are used in specific applications, and the development of new treatments is an ongoing process. Figure 15 shows how brush material properties can affect circulating currents in the brush face. There is always some uncompensated reactance voltage VR in the armature coils. This voltage will cause current to flow through the armature coil and through the brush face. An equation is shown for the voltage drops around this circuit. The first voltage drop is the circulating current × the resistance of the armature conductor. There is a voltage drop as the current passes from the commutator segment to the brush, which is called the “contact drop”. It is shown as VCD Next, we have a voltage drop as the current flows through the brush faces, shown as i × RB.
  • 13. And finally, there is one more voltage drop as the current passes back from the brush into the commutator segment, shown as VCD. When this equation is solved for current, we see that the term for the resistance of the brush is in the denominator. If higher resistivity brushes are used, the denominator becomes larger and the circulating current is reduced. Also, higher resistance brushes have a higher contact drop. This increases the term VCD which is subtracted from VR in the numerator. This also tends to reduce the circulating current in the brush face. Therefore, higher resistivity brushes are used for difficult-to commutate machines. The resistance of the brush material cannot be increased indiscriminately. The load current must also pass through the carbon, and higher resistance materials can cause higher losses and higher brush temperatures. Another way to limit the circulating current in the brush face without increasing the resistivity of the carbon is to use a multi-wafer brush construction. Figure 16 shows the voltage drops through a brush of this type. An extra term is added for the voltage drop between the wafers shown as i × RW When the equation is solved for current, this term is in the denominator. Using a split brush increases the resistance around the loop and reduces the circulating current in the brush face. Split brushes will also follow slight imperfections in the commutator surface more easily. Summary A number of factors can affect the commutation of DC motors and generators. Some of these are related to the DC machine design, symmetry, or adjustment. Some are related to the brush design and materials. A properly assembled and adjusted machine, using a suitable brush, should have relatively low levels of sparking and good commutator and brush life. A Better Way to Protect Generator Bearings Article Link // Posted on February 28, 2013 by David Manney As we’ve talked about repeatedly (here, here, and here among others), a good way to shorten the life of a bearing is current running along the shaft. In a rather long article, Windpower Engineering & Development features our good friends at AEGIS giving a solution to protecting generator bearings. To sum it up, a local representative for a manufacturer of bearing protection rings called on the wind farm and obtained permission for up-tower testing.
  • 14. This schematic shows the grounded AEGIS ring mounted on the drive end of the generator Results revealed the bearings were being “fried” by generator shaft currents much like those seen on ac motors controlled by PWM inverters (variable frequency drives) in industrial HVAC, pumping, and processing equipment. When reliability engineers pointed out similarities and identified shaft currents as the suspected problem, they were allowed to install a conductive-microfiber bearing protection ring intended to protect wind turbine rotor bearings. Problems at the wind farm unfolded this way: The generator bearings in one wind turbine failed only 11 months after the unit was brought on-line in May 2006. The wind farm operator replaced the bearings and slip rings, only to see the new bearings fail five months later. Damage to a generator’s bearing race occurs for the same reason EDMs work: An electric current through the balls chisels away at the race metal. This time, in addition to replacing a set of insulated (ceramic-coated) bearings and slip rings on both ends of the generator, the owner decided to try a conductive microfiber bearing-protection ring and shaft collar with a high conductive surface on the drive end.
  • 15. Three months later the crew used a probe and oscilloscope to measure shaft voltage on the generator with and without the new bearing-protection ring and collar engaged. Data from the field tests show the conductive-microfiber bearing protection ring and collar reduced shaft voltage by an average of 84.5%. The bearing protection ring and collar were on the drive end of the shaft, and the standard carbon block brushes were on the non-drive end. For the Series 2 readings, the bearing protection ring was disengaged and the shaft collar removed, leaving the carbon block brushes on the non-drive-end as the only shaft-current mitigation path. If not diverted, these stray currents discharge to ground through the generator’s bearings, causing pitting and fluting (just as electrical discharge machining would) that result in premature bearing failure and catastrophic turbine failure. To guard against electrical damage to bearings, stray currents must be diverted from the bearings by mitigation devices such as insulation, special current filters, an alternate path to ground, or some combination of these. Nonconductive ceramic bearings, often called hybrid bearings because the balls are ceramic but the rest of the unit (including the race wall) is metal, can divert damaging currents but leave attached equipment open to damage of its own. Damage to a generator’s bearing race occurs for the same reason EDMs work: An electric current through the balls chisels away at the race metal. It uses principles of ionization to boost the electron-transfer rate and promote the efficient discharge of the high frequency shaft currents induced by many wind turbine generators. The fibers significantly reduce voltage buildup on the generator shaft by conducting instantaneous currents of many tens of amperes and discharging from tens to thousands of volts with MHz frequencies. A close up of the bearing protection ring shows many microfibers.
  • 16. The ring is especially suitable for use at high frequencies because its fibers tend to compensate for variations in the roughness of the shaft surface, or microscopic misalignment of the ring and shaft, or both. Furthermore, unlike carbon block brushes, microfibers on the bearing protection ring are not adversely affected by oil, grease, dust, moisture, or other contaminants. In addition, the bearing protection ring safely diverts shaft current at frequencies up to 13.5 MHz and discharges up to 3,000 volts (peak). Results from a series of tests of a turbine with the bearing protection ring installed show an average generator shaft voltage of 6.41 V (peak-to-peak). The difference between these averages, 34.94 V, indicates that the bearing protection ring and collar successfully divert about 84.5% of the damaging current that remains on the turbine’s generator shaft when carbon block brushes at the non-drive end are the only form of bearing protection. Furthermore, the voltage wave form with the AEGIS ring and collar engaged was a smooth wave without detectable discharge to the bearings, while the wave form without the ring and collar showed a bearing-current-discharge pattern with voltage peaks an average of 6.5 times higher. As I said at the beginning, it’s a long article, but a very good one. Worth your time. Direct Information about DC Generators Article Link // Posted on February 21, 2013 by David Manney If you want to know more about electrical engineering or you’re just curious how all the things in your home operate, there’s plenty of information available on the topic. Alternating current generators and DC Generators both have their own pros and cons. Most people have only a basic understanding of circuits and how electricity works but even that is better than knowing nothing at all. If you’re one of those who haven’t a clue how your coffee maker, computer of television even turn on, then this is an article you need to read. First, let’s start by giving a little course on how electricity travels through a circuit.
  • 17. DC Generator Repair DC Generators are used anywhere an immediate response is needed from electrical appliances. While direct current is admittedly fast and responsive, there are some unique problems with this setup. First and foremost, if the circuit is broken for any reason, there is no other way for the electricity to travel through it. This is worlds apart from alternating current machines which give electricity several different paths to the same, inevitable end.AC generators are usually the better choice among the two, but sometimes using the more direct variety of current delivery is advisable. For instance, the seamless flow of power from solar collectors to batteries is best done on a direct current. If energy gets the chance, it will dissipate and flow naturally from where there is more to where there is less. When collecting energy and storing it for later use, giving the energy more pathways to follow means more of it will be lost on the way to those batteries. DC Generators are perfect for solar plants, wind plants, and other places where electricity is manufactured. Really, anywhere energy is being stored in batteries is a great place to set up a generator working on the direct current principle. Besides these very practical applications, when you need a machine to keep working no matter what, you should be looking to install the DC variety of generator. Machines working deep under the ground or up in the sky where access to the land is difficult will all benefit from a stronger, surer source of power. Mining rigs, underground elevators, and airplanes all rely on DC Generators to keep things working smoothly, even when a sudden fuel shortage or other issue threatens to stop these machines in their tracks. It’s a good thing, too; nobody wants to get hit by a plane falling from the sky. When the size of your generator is important, consider the DC varieties are typically significantly smaller than AC generators. Because the energy only needs to travel over a single-line, the whole machine can be made smaller and fit into a lesser space. The more compact size also means these
  • 18. generators weigh less, something which can be a big deal when two hundred pounds is one hundred pounds too much. More than anything, the convenience in repair work makes direct current generators the clear winner over the kind which operates on an alternating current principle. If you don’t know why, keep reading. Since there is only one electrical line running through a DC Generators, there is only the singular line to investigate if the machine breaks down. Repairs are easy, really easy; when there is only one place where anything could have possibly gone wrong, fixing a machine becomes a walk in the park. This is very different from AC generators which could have a short anywhere on any part of the circuit which would cause the whole structure to stop working. A single point on a straight line is much easier to find than the same, one point in a cluster of interspersed cords. Differences Between Generators and Inverters Article Link // Posted on July 10, 2012 by David Manney Exploring the differences between generators and inverters: Generator vs Inverter We all know about electric generators as they are the devices that produce electricity in power plants whether thermal or hydroelectric. They convert thermal or kinetic and potential energy of water and convert it into electric energy that is distributed to homes through transmission lines. But we have become so used to this electric supply that we are irritated whenever there are power outages. To have constant uninterrupted supply in times of power outages, two devices that are commonly used at homes, and these are generators and inverters. There are many differences in these two devices and it is prudent to know about them if you are going to the market to buy one of them. Generator A generator is a device that converts mechanical energy provided by the engine into electricity. It requires a fuel source such as kerosene, diesel, or petroleum to run this engine. Generators come in all shapes and sizes and their capacities also range from a mere 500 watts to many kilowatts so one can run all appliances at home with the help of a generator. But given rising prices of fossil fuels, maintaining a generator has become problematical these days. In any case, starting of a generator requires pulling a cord that is not easy for ladies at home and most of the generator sets are thus found installed at commercial premises where there is a man deputed to run the generator in the case of a power outage. One must keep the fuel always at disposal so that he can run his appliances with generator for a long time. Generators can operate for long durations and have high capacities to run even air conditioners.
  • 19. Inverter An inverter is a device that makes use of the electricity that is being supplied to your home by converting it into DC to charge a battery that is supplied along with the device in the case of a power outage, the same battery becomes a power source and the DC electricity from it is converted in AC before supplying it to household appliances. Inverter works on its own and there is no need to start it like a generator. The only problem is that it needs wiring to be done and you decide which appliances to run with the energy of an inverter in times of outage. As an inverter needs electricity to keep on charging all the time, it can supply only as much energy that is stored inside the battery and it is useless afterwards. For places where power cuts are of long durations, inverters need the back up of generators. Inverters have typically smaller capacities than generators but these days, expensive inverter systems with many batteries in conjunction are being used to supply power to even air conditioners in times of power outage. Summary • • • • • • • There is literally no time gap in the onset of power, once there is a power outage, in case of inverter, whereas starting a generator takes considerable time. Inverters are soundless, whereas even silent generators make a lot of noise Generators require a power source (kerosene, diesel or petroleum) to run, whereas an inverter charges the battery with the electricity itself. Generators require an effort to start, whereas inverters start on their own, once power is gone. Generators are available in high capacities, whereas inverters are available in lower capacities Inverters require installation and wiring, whereas one can start generator right out of the box Generators prove advantageous in places with long power cuts, whereas inverters are more convenient in places with short power cuts Differences Between Electric Motors and Generators Article Link // Posted on July 9, 2012 by David Manney Electric Motor vs Generator Electricity has become an inseparable part of our life; more or less our whole lifestyle is based on the electrical equipment. Energy is converted from many forms to the form of electrical energy, to power up all these devices. The electric motor is a device which converts mechanical energy into electrical energy. On the other hand, devices are used to transform electrical energy into mechanical as required. The motor is the device that performs this function.
  • 20. More about Electric Generator The fundamental principle behind the operation of any electrical generator is Faraday’s law of electromagnetic induction. Idea stated by this principle is that, when there is a change of the magnetic field across a conductor (a wire for example), electrons are forced to move in a direction perpendicular to the direction of the magnetic field. This results in generating a pressure of electrons in the conductor (electromotive force), which results in a flow of electrons in one direction. To be more technical, a time rate of change in magnetic flux across a conductor induces an electromotive force in a conductor and its direction is given by Fleming’s right hand rule. This phenomenon is used largely to produce electricity. To achieve this change in magnetic flux across a conducting wire, magnets and the conducting wires are moved relatively, such that flux varies based on the position. By increasing the number of wires, you can increase the resulting electromotive force; therefore, wires are wound into a coil, containing a large number of turnings. Setting either the magnetic field or the coil in rotational motion, while the other is stationary, allows continuous flux variation. The rotating part of the generator is called a Rotor, and the stationary part is called a stator. The emf generating part of the generator is referred to as the Armature, while the magnetic field is simply known as Field. Armature can be used as either the stator or the rotor while the field component is the other. Increasing the field strength also allows increasing the induced emf. Since permanent magnets cannot provide the intensity needed to optimize the power production from the generator, electromagnets are used. A lower current is flowing through this field circuit than the armature circuit and lower current pass through the slip rings, which keep the electrical connectivity in the rotor. As a result, most of the AC generators have the field winding on the rotor and the stator as the armature winding.
  • 21. More about the Electric Motor The principle used in motors is another aspect of the principle of induction. The law states if a charge is moving in a magnetic field, a force acts on the charge in a direction perpendicular to both the velocity of the charge and the magnetic field. The same principle applies for a flow of charge, is a current and the conductor carrying the current. The direction of this force is given by Fleming’s right hand rule. The simple result of this phenomenon is that if a current flows in a conductor in a magnetic field the conductor moves. All the induction motors are working on this principle. As like the generator, the motor also has a rotor and a stator where a shaft attached to the rotor delivers the mechanical energy. The number of turnings of the coils and the strength of the magnetic field affects the system in the same way. A quick summary: What is the difference between Electric Motor and Electric Generator? • • • Generator converts mechanical energy to electrical energy, while motor converts mechanical energy to electrical energy. In a generator, shaft attached to the rotor is driven by a mechanical force and electric current is produced in the armature windings, while the shaft of a motor is driven by the magnetic forces developed between the armature and field; current has to be supplied to the armature winding. Motors (generally a moving charge in a magnetic field) obey the Fleming`s left hand rule, while the generator obeys Fleming’s left hand rule.
  • 22. The difference between four-quadrant motor control and two-quadrant operation Article Link // Posted on March 22, 2013 by David Manney To understand the difference between two-quadrant and four-quadrant motor control, first one should know what are 2Q and 4Q controls. What are two-quadrant operations? A two-quadrant motor operation is where speed is on the horizontal axis and torque is on the vertical axis. A simple example for two-quadrant drive is a DC motor in single quadrant operation. Due to an additional switch, the motor reverses and then due to this inversion, the motor operates in two quadrants. However, if we do not want to reverse the operation or if it is unnecessary, then the drive can work in two quadrants by adding regenerative braking. What is a four-quadrant motor control? Basing the concepts of a motor, depend on the four distinct areas of operation on operation direction. Two of the quadrants represent the torque application motion direction and the other two have torque applied on the opposite direction. Motors have two types of energy conversions that take place, electrical to mechanical and mechanical to electrical. These allow the motor to operate in all four quadrants through regeneration. The use of four-quadrant motor control is beneficial since DC motors can run better than other machines. This helps in saving the production costs and reducing the long-term profit losses.
  • 23. Finally, the differences between two-quadrant and four-quadrant motor operations are as follows: • • • • • • Not every motor operates on both two-quadrant and four-quadrant controls; there are some motors that operate on two-quadrant while some that are designed to work specifically on four-quadrant. Four-quadrant motors are more efficient than two-quadrant and even more energy saving. In two-quadrant motors, there is only forward and reverse motoring while in four-quadrant motors, there is forward and reverse regeneration. As there is regeneration in four-quadrant motors, thus energy conversion takes place as generator converts mechanical energy into electrical but this is not done in two-quadrant. Two-quadrant operation is a conventional type of operation in motors but four-quadrant means, it is for specific purposes and not for regular functions. In two-quadrant operation, which one needs an external switch to reverse the operation while in four-quadrant operation, no external switching is required. These are some of the differences in both the quadrant operation. Both operations need to be in different areas of designing and functioning. Based on the field of deployment, the pros and cons for each of them allow analyzing and choices can be made.