3.12: Direct Direct Current Current Generators/MotorsGenerators/Motors
3.12.1
3.12.1 : : TheoryTheory
A loop of wire rotated in a magnetic field has a continuously changing flux through it and so long as the rotation continues, an induced voltage will be maintained in the wire. The magnitude of this induced voltage depends on the rate at which the flux changes. This principle forms the basis of any rotating electrical generator, (AC or DC). The method by which the generator electricity is actually connected into the external circuit will determine the ultimate generator function. This method will be Commutator (DC generator) and Slip Rings (AC generator), with ‘collection’ provided by carbon brushes. A generator converts mechanical energy into electrical energy. It does this by
producing relative motion between loops of wire and magnetic flux so that an induced voltage is set up in the loops of wire.
A Simple dc Generator A Simple dc Generator
The simplest form of dc generator is shown and consists of a single loop of wire able to rotate freely between the poles of a permanent magnet. Connection is made from the loop to the external circuit (or ’load’ ) by carbon brushes pressing on a commutator, which is connected to the ends of the loop and rotates with it.
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Production of Direct Current Production of Direct Current
Direct current can be obtained in the external circuit by substituting a form of automatic reversing switch, known as a ‘COMMUTATORCOMMUTATOR’’, for the slip rings. The commutator automatically reverses the connection between the loop and the external circuit as the voltage in the loop reverses, thus maintaining the direction of current in the load, as shown.
Production of DC by Commutator Action Production of DC by Commutator Action
Each end of the loop is connected to a segment of the commutator and the load is connected to the loop by brushes on opposite sides of the commutator.
As the loop rotates, an alternating voltage is induced in it, but, because of the action of the commutator, a ‘rippled dc’‘rippled dc’ is produced as opposed to a genuine ac waveform.
Because the commutator rotates with the loop, the brushes bear on opposite segments of it during each half cycle.
This results in the left hand brush always being in contact with the segment that is positive, with the change-over taking place at the instant when the voltage induced in the loop is zero.
The current in the external circuit is therefore always in the same direction and is called a UNI-DIRECTIONALUNI-DIRECTIONAL current. It is also the first step towards
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EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
The voltage at the brushes, and therefore the current in the external circuit of a simple example single loop dc generator, falls to zero twice during each complete revolution. As has already been mentioned, this variation of dc is called ‘ripple’‘ripple’ and can be reduced by the addition of more loops as shown. Remember, an operational generator will not return to zero after switch-on until it is switched ‘off’.
Multi-Loop dc Generator Multi-Loop dc Generator
As the number of loops is increased, the variation between maximum and minimum values of voltage is reduced and the output voltage of the generator approaches a steady dc value, as can be seen.
Output Waveform of a Multi
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
It should also be noted that the number of segments on the commutator is increased in direct proportion to the number of loops;
There are;
Two segments for one loop.
Four segments for two loops.
Eight segments for four loops.
The loops are not just loops of wire but are made up like coils and so the construction of them can be a big determining factor in the output obtained.
The voltage induced in a single-turn loop is quite small, and although an increase in the number of loops does not increase the maximum value of generated voltage, an increase in the number of turns in each loop will. Within narrow limits, the output voltage of a dc generator is determined by the product of the number of turns per loop, the total flux pair of poles in the machine and the speed of rotation of the armature.
Whether it is an ac or dc generator, they are identical as far as the method of generating voltage in the rotating loop is concerned. However, if the current is taken from the loop by slip rings, it is an alternating current and if it is
collected by a commutator, it is direct current.
The variation in the output of a dc generator is reduced to a very small amount by having a large number of loops and a commutator with a correspondingly large number of segments. The construction is such that each loop is connected between adjacent segments, the end of one loop being connected to the same segment as the beginning of the next loop, as shown.
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EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals
Connection of Multiple Loops with
Connection of Multiple Loops with Commutator Segments and ResultantCommutator Segments and Resultant Output
Output
Loop A is connected between segments 1 and 2, loop B between segments 2 and 3 and so on. With this arrangement, the emf induced in each loop will reach its maximum value when the emf in the preceding loop is already decreasing, and that in the succeeding loop is still increasing. Thus, the emf in
loop ‘E’ is at maximum.
loop ‘F’ is decreasing.
loop ‘D’ increasing.
The voltage at the brushes equals the sum of the emf induced in the loops connected in series between the brushes.
Loops ‘ A, ‘B’ and ‘C’ are in series between the brushes on the right.
Loops ‘D’, ‘E’ and ‘F’ with the brushes on the left.
The two branches are parallel with each other.
The graph shows the resultant voltage between the brushes. Only three loops need to be considered as the arrangement is symmetrical and both branches (A, B and C and D, E and F) give the same voltage at the instant shown. As the number of loops is increased, the ripple in the brush voltage becomes smaller and the magnitude of the dc output voltage increases
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Electrical Fundamentals Module 3 EASA Part 66 – C/009 Book 3
Notes: Notes:
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EASA Part 66 – C/009 Book 3 Module 3 Electrical Fundamentals