Common D. C Tachogenerator Principle The D. The D. C tachogenerator is shown in below figure. The armature of the D. C Tachogenerator is kept in the permanent magnetic field.
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PDF Version Question 1 Generators used in battery-charging systems must be regulated so as to not overcharge the battery ies they are connected to. The fundamental principle upon which their operation is based is called negative feedback: where a system takes action to oppose any change in a certain variable. In this case, the variable is generator output voltage. Explain how the relay works to prevent the generator from overcharging the battery with excessive voltage.
Reveal answer If the battery voltage becomes excessive, the relay opens and de-energizes the field winding. When the voltages sags back down to an acceptable level, the relay re-closes and re-energizes the field winding so that the generator can begin generating voltage again.
Notes: The circuit drawn here is very similar to real generator regulator circuits used in American automobiles before the advent of inexpensive, reliable semiconductor circuits. I show it here not just for historical background, but also to demonstrate how relatively crude circuits are still able to perform certain tasks reasonably well.
A simple system like this provides a good way to gently introduce students to this vital concept. Question 2 A mechanic has an idea for upgrading the electrical system in an automobile originally designed for 6 volt operation. He wants to upgrade the 6 volt headlights, starter motor, battery, etc, to 12 volts, but wishes to retain the original 6-volt generator and regulator. Reveal answer So long as the generator is capable of outputting 12 volts, this system will work!
Challenge question: identify factors that may prevent the generator from outputting enough voltage with the regulator connected as shown in the last diagram. The regulator circuit senses only 6 volts, but the generator outputs 12 volts. Fundamentally, the focus of this question is negative feedback and one of its many practical applications in electrical engineering. This idea actually came from one of the readers of my textbook series Lessons In Electric Circuits.
He was trying to upgrade a vehicle from 12 volts to 24 volts, but the principle is the same. An important difference in his plan was that he was still planning on having some volt loads in the vehicle dashboard gauges, starter solenoid, etc.
It is a bit more complex than the system shown in the question, due to the two different load banks. Question 3 If an electric current is passed through this wire, which direction will the wire be pushed by the interaction of the magnetic fields? Is this an example of an electric motor or an electric generator? The wire will be pushed up in this motor example. Notes: A visual aid to understanding the interaction of the two magnetic fields is a diagram showing the lines of flux emanating from the permanent magnets, against the circular lines of flux around the wire.
Ask those students who came across similar illustrations in their research to draw a picture of this on the board in front of the class, for those who have not seen it.
Question 4 If this wire between the magnet poles is moved in an upward direction, what polarity of voltage will the meter indicate? Describe the factors influencing the magnitude of the voltage induced by motion, and determine whether this is an example of an electric motor or an electric generator.
The voltmeter will indicate a negative voltage in this generator example. Notes: Ask your students to explain their answers regarding factors that influence voltage magnitude. Where did they obtain their information? Are there any mathematical formulae relating these factors to induced voltage? Question 5 If this wire between the magnet poles is moved in an upward direction, and the wire ends are connected to a resistive load, which way will current go through the wire?
We know that current moving through a wire will create a magnetic field, and that this magnetic field will produce a reaction force against the static magnetic fields coming from the two permanent magnets.
Which direction will this reaction force push the current-carrying wire? Follow-up question: What does this phenomenon indicate to us about the ease of moving a generator mechanism under load, versus unloaded? What effect does placing an electrical load on the output terminals of a generator have on the mechanical effort needed to turn the generator?
Notes: If you happen to have a large, permanent magnet DC motor available in your classroom, you may easily demonstrate this principle for your students. Just have them spin the shaft of the motor generator with their hands, with the power terminals open versus shorted together.
Your students will notice a huge difference in the ease of turning between these two states. In what way does a PMMC meter movement resemble an electric generator? How does shorting the terminals together help to protect against damage from physical vibration during shipping? Ask your students to describe what factors influence the magnitude of this reaction force. Question 6 Determine the polarity of induced voltage between the ends of this wire loop, as it is rotated between the two magnets: Challenge question: if a resistor were connected between the ends of this wire loop, would it be direct current DC , or alternating current AC?
Notes: Note that the two wire ends switch polarity as the loop rotates. Ask your students to explain why the polarities are as they are. Reveal answer Follow-up question: does the polarity measured at the two carbon brushes ever reverse? Explain your answer. What, specifically, must be changed in order to increase this rate-of-change over time?
Which real-world variables are changeable after the generator has been manufactured, and which are not? This connects the generator with the battery, and charging current flows through the series coil, creating even more magnetic attraction to hold the relay contact closed.
If the battery reaches a full charge and does not draw any more charging current from the generator, the relay will still remain closed because the shunt coil is still energized. However, the relay contact will open if the generator ever begins to act as a load to the battery, drawing any current from it.
Explain why this happens. Although DC generators are no longer used in the majority of automobile electrical systems AC alternators using bridge rectifiers to convert AC to DC are used instead, with the rectifier circuit naturally preventing reverse current , this application provides an excellent opportunity to explore an application of relay technology in the context of generator control. But when the generator is sitting still, its output voltage is zero, and therefore there will be no current through the field winding to energize it and produce a magnetic field for the armature to rotate through.
This causes a problem, since the armature will not have any voltage induced in its windings until it is rotating and it has a stationary magnetic field from the field winding to rotate through. It seems like we have a catch situation here: the generator cannot output a voltage until its field winding is energized, but its field winding will not be energized until the generator armature outputs some voltage.
How can this generator ever begin to output voltage, given this predicament? Reveal answer Usually, there is enough residual magnetism left in the field poles to initiate some generator action when turned. How could the generator ever be started?
Notes: Back in the days when generators were common in automotive electrical systems, this used to be a fairly common problem. Question 11 In a shunt-wound DC generator, the output voltage is determined by the rotational speed of the armature and the density of the stationary magnetic field flux. Obviously, there must be some inherent limit to this otherwise vicious cycle. Otherwise, the output voltage of a shunt-wound DC generator would be completely unstable. What relation does the neutral plane have with regard to brush positioning?
Question 13 Suppose a generator is mechanically coupled to an internal combustion engine in an automobile, for the purpose of charging the starting battery. How is this regulation of generator output voltage typically achieved?
What variable within the generator may be most easily adjusted to maintain a nearly constant output voltage? The most common method of generator voltage control is adjustment of field winding excitation. Notes: Although adjustable field winding excitation is the most popular form of generator output voltage control, it is not the only means.
Challenge your students with inventing other means of charge control for the battery in this automotive electrical system, besides field winding excitation control. What else can we do to the generator, or to the circuit it is within, to achieve charge control for the battery?
Question 14 In most high-power DC generator and motor designs, the wire used to make the field winding is much thinner gauge than the wire used to make the armature winding. This indicates the relative magnitude of current through these respective windings, with the armature coils conducting much more current than the field coils. That the armature conducts more current than the field is no small matter, because all current through the armature must be conducted through the brushes and commutator bars.
The more current these components have to carry, the shorter their life, all other factors being equal. This way, the brushes and commutator bars would only have to carry a fraction of their normal current, making them less expensive and longer-lived.
Explain why this is impossible to do. Hint: consider the design of a permanent-magnet generator. It is impossible for the field winding to conduct more current than the armature in a functioning DC generator, because the armature has to be the source of electrical power, while the field is only a load. However, the idea proposed in this question will never work.
By simplifying the problem in this way, students should see that the armature winding has to carry the bulk of the current in a DC generator.
DC Generator Theory
PDF Version Question 1 Generators used in battery-charging systems must be regulated so as to not overcharge the battery ies they are connected to. The fundamental principle upon which their operation is based is called negative feedback: where a system takes action to oppose any change in a certain variable. In this case, the variable is generator output voltage. Explain how the relay works to prevent the generator from overcharging the battery with excessive voltage. Reveal answer If the battery voltage becomes excessive, the relay opens and de-energizes the field winding.
D.C Tachogenerator Principle
Morn And i am starting it from the scratch. The below mention circuit is used for measuring the speed of the rotor by considering the amplitude of the induced voltage. Figure 2 Tachogenerator equivalent circuit. The nonlinear relationship obtains between the output voltage and input speed when the rotor rotates at high speed. Electrical Tachometer The magnitude of the induces emf depends on the flux link with the conductor and the speed of the shaft. The commutator converts the alternating current of the armature coil to the direct current with the help of the brushes.
Construction and working of DC tachogenerator
Nikodal The commutator converts the alternating current of the armature coil to the direct current with the help of the brushes. The magnitude of the induces emf depends on the flux link with the conductor and the speed of the shaft. Published under the terms and tachogenetator of the Design Science License. The equivalent circuit of a tachogenerator is also the dual of an electric motor and is shown in Figure Permanent magnet, armature, commutator, brushes, variable resistor, and the moving coil voltmeter are the main parts of the DC tachometer generator.