Technological University of the Philippines Ayala Boulevard, Ermita, Manila, Philippines College of Engineering Electrical Engineering Department Electrical Machines 1 Laboratory Types of DC Generators Submitted by: RICOHERMOSO, ANGELINO CESAR SAMPAGA, MARK FROILAN SENORIN, MARC IVAN JOSHUA B. TAMBA, VINCENT B. Submitted to: ENGR. SEIGIE B. LLADONES, REE Date: May 1, 2021 Discussion Questions: 1. Draw the cross-sectional view of a four-pole dynamo and label all parts. 2. Discuss all the parts of the said dynamo and its function. The Dynamo consists of 3 major components: the stator, the armature, and the commutator. - The stator is a fixed structure that makes magnetic field. The main function of the stator is to produce the rotary magnetic field due to three-phase supply. - The armature is made of coiled copper windings which rotate inside the magnetic field made by the stator. When the windings move, they cut through the lines of magnetic field. This creates pulses of electric power. - The commutator is needed to produce direct current. In direct current power flows in only one direction through a wire, the problem is that the rotating armature in a dynamo reverses current each half turn, so the commutator is a rotary switch that disconnects the power during the reversed current part of the cycle. - Brushes, the brushes must conduct electricity as the keep contact with the rotating armature. With the help of the brushes, the current which is produced on the windings is passed on to the commutator and then to the external circuit. These wore out easily and they developed graphic blocks to do the same job. 3. Why are pole core usually made pf laminated instead of solid? Usually, the pole core and shoes are made of thin cast steel or wrought iron laminations which are riveted together under hydraulic pressure. The poles are laminated to reduce the Eddy Current loss. As the armature is a rotating part of the DC Generator or machine, the reversal of flux takes place in the core, hence hysteresis losses are produced. The silicon steel material is used for the construction of the core to reduce the hysteresis losses. The rotating armature cuts the magnetic field, due to which an emf is induced in it. This emf circulates the eddy current which results in Eddy Current loss. Thus, to reduce the loss the armature core is laminated with a stamping of about 0.3 to 0.5 mm thickness. Each lamination is insulated from the other by a coating of varnish. 4. In what way is the flux linking any coil of a generator armature made to vary? Describe the manner in which this variation of flux causes an EMF to be induced. How does this induced EMF vary with the speed? The flux? The number of turns in the coil? Flux Linkage Flux linkage is defined by multiplying the number of turns (N) of a coil to the flux (Ø), where flux (Ø) is equal to the product of the flux density (B) and area (A). It is the linking of the magnetic field with the conductors of a coil when a magnetic field passes through its loops. Flux linkage on coils of a generator armature is made to vary the flux density (B). The variation of flux density (B) causes changes in the flux (Ø) thus creating induced current or EMF in the coils. The induced EMF produced is directly proportional to speed, flux (Ø), and number of turns in the coil. The faster the change in flux density (B) the greater EMF induced. Same is true with the increase in flux (Ø) so as with the number of turns in the coil (N). 5. In the armature of a rotating machinery why is it usually more convenient to consider the EMF as being due to the cutting of flux by the conductors, rather than to the change in flux linkages in the armature coils? If the induced EMF is considered as being due to the cutting of the flux by individual conductors, how does the EMF vary with the length of conductor? Flux density? Velocity of conductor? Cutting of Flux Flux (Ø) is defined by the product of flux density (B) and area (A) multiplied by cosθ, θ being the angle difference between the direction of flux density (B) and the normal line. Flux cutting works by cutting or changing the magnetic fields around a conductor to produce EMF. It is more convenient than flux linking when it comes to armatures of rotating machinery for it is easier to do. To cut the flux we can simply put the coils at the center of the armature and magnets on its stator so when it spins the flux cutting is easily done, whereas to flux linkage in which changing the flux density (B) is necessary it would be a complex process. The EMF produced from cutting the flux varies proportionally with the length of the conductor, flux density, and velocity. The longer the conductor the more field could be cut in one motion, greater flux density would mean the conductor has cut a greater value of flux, and greater velocity means more rotation could be done in a second thus cutting more field lines. Discussion Questions: 1. Draw the schematic Diagram of each DC generator types (i.e. Separately Excited and Self-Excited) showing all its parts/components. 2. Discuss operations, applications, and cautions for every type of DC Generators. Separately Excited DC Generator The field magnets in these generators are energized by an external DC source, such as a battery. It is also referred to as field coils that can be activated by the generator. Self-Excited DC Generators Self-excited DC generators have field magnets that are energized by current supplied by the generator itself. Field coils are internally attached to the armature in these devices. Any flux is still present in the poles due to residual magnetism. EMF is generated when the armature is rotated. As a result, some induced current is produced. This small current flows through both the field coil and the load, increasing the pole flux. As the pole flux increases, more armature EMF is generated, causing the current through the field to increase even more. This increased field current increases armature EMF even further, and the cycle repeats until the excitation reaches the rated value. Self-excited generators can be categorized as: 1. Series Wound Generators 2. Shunt Wound Generators 3. Compound Wound Generators Series Wound Generator The field windings are connected in series with the armature conductors in these generators. The entire current flows into both the field coils and the load. Since series field windings carry maximum load current, they are equipped with a small number of thick wire turns. As a result, the electrical resistance of the series field winding is very low (nearly 0.5). Shunt Wound DC Generators The field windings are paired in parallel with the armature conductors in these DC generators. The voltage in the field winding of a shunt-wound generator is the same as the voltage across the terminal. When IL is at its highest, the efficient power across the load is at its highest. As a result, the shunt field current must be kept as low as possible. The resistance of the shunt field winding is usually held high (100) for this reason, and a large number of turns are used to achieve the desired EMF. Compound Wound DC Generator The output voltage of series wound generators is proportional to the load current. The output voltage of shunt-wound generators is inversely proportional to the load current. The drawbacks of both forms of generators can be solved by combining them. Compound wound DC generator refers to this configuration of windings. Both series and shunt field winding are used in compound wound generators. One winding is connected to the armature in order, while the other is connected to the armature in parallel. Short shunt compound-wound generators and long shunt compound-wound generators are two types of DC generators. Short-Shunt Compound Wound DC Generator Only the shunt field winding is in parallel with the armature winding in short shunt compound wound DC generators. Long Shunt Compound Wound DC Generator The shunt field winding is in parallel with both the series field and the armature winding in long shunt compound wound DC generators. 3. Explain in detail how a shunt generator “build-up”. What limits the voltage to which a generator can build up? Conditions to build up voltage in shunt generator: 1. There should be a residual magnetic field in the shunt winding. 2. Shunt winding and armature winding should be oriented in such a way that the flux produced by each aids the other. 3. Critical winding resistance should be present in the shunt winding. The residual flux in the field winding induces a slight voltage in the armature when it is rotated. The induced voltage in the armature produces a flux, which will aid(add) to the field flux, increasing the net flux even further. This process will continue until the terminal voltage is reached. When the terminal voltage is reached, the winding becomes exhausted, and there is no further increase in flux, as well as a constant voltage. The generator's terminal voltage is limited to a steady-state value due to magnetic saturation in the pole faces. 4. What is meant by critical field resistance? Give four causes, each of which may prevent the building up of generator. What tests and remedies should be used for each cause? Critical Field Resistance A shunt DC generator's critical field resistance is referred to as critical field resistance. The voltage generated across the armature in a DC shunt generator, Va, is proportional to the flux operating across it. The flux in a DC generator is proportional to the field current, If. The critical field resistance is the maximum field circuit resistance that the shunt generator can excite (at a given speed). Only if the field circuit resistance is less than the critical field resistance can the shunt generator generate voltage. It's a tangent to the generator's open-circuit characteristics (at a given speed). Give four causes, each of which may prevent the building up of generator. 1. No residual magnetism in the field - When there is no field in the field poles, the rotating armature will not induce any voltage. As a result, no voltage will cause any field current, and therefore no voltage will accumulate. 2. Open field relation - Since there is no current in the field, the field will not increase over time. As a result, armature voltage will not rise. 3. Field relation reversed - In this case, the armature's slight induced voltage would often try to oppose the field's path, resulting in no voltage rise. 4. Field circuit resistance is too high - a high field resistance would prevent the appropriate field current from flowing, lowering the armature field voltage. Test and remedies Often it's only because the computer hasn't been used in a while and has been knocked around. If this is the case, a trained and skilled person will possibly only use a 9V or 12V battery to flash the field windings. A static flash might work, but it might have to be done while the computer is running. If you don't know what you're doing, don't try it at home because you could die from electric shock. The laminations may become rusted and start to separate over time. The magnetic field splits, and this unit would almost certainly never have enough residual. The computer should probably be discarded at this stage, but there are a few workarounds if you really want to keep it. 5. Why are commutating poles connected in Series with the armature? Why they have an unusually long air-gap? Why are commutating poles connected in Series with the armature? -In DC motors, interpoles are used to counteract the effects of the armature reactance and the machine's self-induction. Interpoles are 90 degrees electrically from the main poles of most shunt and compound DC motors over one-half horsepower. With good performance, some motor designs only use one interpole. The armature is wired in series with the interpoles. The interpole windings pass any changes in armature current caused by armature reactance, loading, or selfinductance. This cancels the effect by creating a shifting magnetic field that is equal to and opposite that of the armature. Why they have an unusually long air-gap? - The permanent magnet is the only source of magnetic flux through the air-gap when no current is flowing in the windings. From the N end-cap through the stator poles, the magnet flux crosses the air-gap, flows axially through the body of the stator, and returns to the magnet by crossing the air-gap to the S end-cap. If there was no offset between the two sets of rotor teeth, there would be a heavy periodic alignment torque when the rotor was rotated, and we would achieve a stable equilibrium position any time a set of stator teeth was in line with the rotor teeth. However, there is an offset, which almost eliminates the alignment torque caused by the magnet. In practice, a slight ‘detent' torque remains, which can be felt if the shaft is turned while the motor is turned off: the detent torque helps to keep the motor in its phase positions. This is also very useful: for example, when the power is turned off, it is normally enough to keep the rotor stationary, allowing the motor to be left without fear of being inadvertently nudged into a new location.
