DC Generator Theory
DC Electric Circuits
Generators used in battery-charging systems must be regulated so as to not overcharge the battery(ies) they are connected to. Here is a crude, relay-based voltage regulator for a DC generator:
Simple electromechanical relay circuits such as this one were very common in automotive electrical systems during the 1950’s, 1960’s, and 1970’s. 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.
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. Shown here is the original 6-volt electrical system:
The mechanic’s plan is to replace all the 6-volt loads with 12-volt loads, and use two 6-volt batteries connected in series, with the original (6-volt) regulator sensing voltage across only one of those batteries:
Explain how this system is supposed to work. Do you think the mechanic’s plan is practical, or are there any problems with it?
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?
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? How does the direction of this force relate to the direction of the wire’s motion? Does this phenomenon relate to any principle of electromagnetism you’ve learned so far?
If the ends of a wire loop are attached to two half-circular metal strips, arranged so that the two strips almost form a complete circle, and those strips are contacted by two “brushes” which connect to opposite poles of a battery, what polarity of voltage will be measured as the loop is rotated counter-clockwise?
DC generators will act as DC motors if connected to a DC power source and not spun at a sufficient speed. This is a problem in DC power systems, as the generator will act as a load, drawing energy from the battery, when the engine or other “prime mover” device stops moving. This simple generator/battery circuit, for example, would not be practical for this reason:
Back in the days when automobiles used DC generators to charge their batteries, a special relay called the reverse current cutout relay was necessary to prevent battery discharge through the generator whenever the engine was shut off:
When the generator is spun fast enough, it generates enough voltage to energize the shunt coil with enough current to close the relay contact. 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.
A shunt-wound generator has an electromagnet “field” winding providing the stationary magnetic field in which the armature rotates:
Like all electromagnets, the magnetic field strength produced is in direct proportion to the amount of current through the wire coil. 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-22 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?
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. For a given armature speed, what prevents the output voltage from “running away” to infinite levels, since the output voltage energizes the field winding, which leads to greater field flux, which leads to greater output voltage, which leads to greater field flux, which leads to . . . ?
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.
Suppose a generator is mechanically coupled to an internal combustion engine in an automobile, for the purpose of charging the starting battery. In order that the battery not be over-charged by the generator, there must be some way of controlling the generator’s output voltage over a wide range of engine speeds.
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? Express your answer in relation to Faraday’s Law of electromagnetic induction.
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.
Couldn’t the generator be re-designed so that the field conducted most of the current, with the armature only conducting a small amount? 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.
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