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DC generator theory
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Questions
Question 1
If an electric current is passed through this wire, which direction will the wire be pushed (by the interaction of the magnetic fields)?
N Magnet S N Magnet S
wire
Is this an example of an electric motor or an electric generator? file 00382
Question 3
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?
N Magnet S N Magnet S
wire
motion
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? file 00807
Question 4
Determine the polarity of induced voltage between the ends of this wire loop, as it is rotated between the two magnets:
N Magnet S N Magnet S
N Magnet S N Magnet S
N Magnet S N Magnet S
sequence over time
N Magnet S N Magnet S
N Magnet S N Magnet S
file 00808
Question 9
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:
Generator Gen Battery
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:
Generator Gen Battery
Reverse current
cutout relay
series coil
shunt coil
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. file 00804
Question 10
A shunt-wound generator has an electromagnet ”field” winding providing the stationary magnetic field in which the armature rotates:
Field Armature
A
To DC
Ammeter
Circuit
breaker
load
Generator
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? file 00812
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. 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...?
Field Armature
A
To DC
Ammeter
Circuit
breaker
load
Generator
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. file 00814
Question 14
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:
Generator
Battery
(6 volts)
Regulator
(6 volts)
Mtr
Fuse 6-volt loads
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:
Generator
Battery
Regulator
(6 volts)
Mtr
12-volt loads
(6 volts)
Battery
(6 volts)
Fuse
Explain how this system is supposed to work. Do you think the mechanic’s plan is practical, or are there any problems with it? file 01022
Answers
Answer 1
The wire will be pushed up in this motor example.
Answer 2
The voltmeter will indicate a negative voltage in this generator example.
Answer 3
The reaction force will be directly opposed to the direction of motion, as described by Lenz’s Law.
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?
Answer 5
N Magnet S N Magnet S
Follow-up question: does the polarity measured at the two carbon brushes ever reverse? Or, to phrase the question another way, if a resistor were connected between the two brush contacts, would it ”see” direct current (DC) or alternating current (AC)? Explain your answer.
Answer 6
The ”neutral plane” is that point of rotation where a rotating armature winding has no induced voltage in it, due to dφ dt being equal to zero. In a simple, two-pole machine, the neutral plane is perpendicular to the centerline of the field poles:
N S
Neutral plane
Answer 7
Increase the dφ dt rate of change, or increase the number of turns in the armature winding.
Answer 8
The most common method of generator voltage control is adjustment of field winding excitation.
Answer 9
If a reverse current goes through the series coil, the magnetic field produced will ”buck” the magnetic field produced by the shunt coil, thus weakening the total magnetic field strength pulling at the armature of the relay.
Answer 10
Usually, there is enough residual magnetism left in the field poles to initiate some generator action when turned.
Challenge question: what we could do if the generator’s field poles ever totally lost their residual magnetism? How could the generator ever be started?
Answer 11
At a certain amount of field winding current, the generator’s field poles saturate, preventing further increases in magnetic flux.
Answer 12
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.
Answer 13
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.
Challenge question: what would we have to change in this circuit to alter the generator’s voltage regulation set-point (the ”target” voltage at which the generator’s output is supposed to be regulated)?
Answer 14
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.
Notes 9
A ”reverse current cutout” relay ingeniously exploits reversible magnetic polarities to close or open a contact under the proper conditions. 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.
Notes 10
Back in the days when generators were common in automotive electrical systems, this used to be a fairly common problem. However, generators could be ”flashed” so as to re-establish this residual magnetic field once again.
Notes 11
This question provides a great opportunity to review the concept of magnetic ”saturation,” as well as introduce the engineering concept of positive feedback.
Notes 12
Being that brush and commutator wear is the main reason AC motors and generators are favored over DC, any idea that may potentially reduce the ”wear and tear” on DC motor or generator brushes is worth considering. However, the idea proposed in this question will never work. This is not necessarily an easy question to answer, as it tests the students’ comprehension of generator theory. The hint given in the question (”consider a permanent-magnet generator”) is intended to force students to simplify the problem, by considering a working generator design that only has one winding (the armature). 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.
Notes 13
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. ”Negative feedback” is one of the fundamental principles of electronics and electrical engineering. A simple system like this provides a good way to gently introduce students to this vital concept.
Notes 14
In this question, we see a foreshadowing of op-amp theory, with the regulator’s negative feedback applied to what is essentially a voltage divider (two equal-voltage batteries being charged by the generator). 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. The depth to which you discuss this concept will vary according to the students’ readiness, but it is something you should at least mention during discussion on this question. 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 12-volt loads in the vehicle (dashboard gauges, starter solenoid, etc.), with the full 24 volts supplying only the high-power loads (such as the starter motor itself):
Generator
Battery
Regulator
Battery^ Mtr
(12 volts)
(12 volts)
(12 volts)
24-volt loads
12-volt load
Fuse
As a challenge for your students, ask them how well they think this system would work. It is a bit more complex than the system shown in the question, due to the two different load banks.
