Demonstrative Video
Simple Loop Generator and Function of Commutator
Consider only one coil AB placed in the strong magnetic field.
The two ends of the coil are joined to slip rings
Two brushes rest on these slip rings.
When coil is rotated in counter clockwise direction at \(\omega\) radian per second, \(\phi\) is cut by the coil and an emf is induced in it.
The induced emf is alternating and the current flowing through the external resistance is also alternating i.e., at second instant current flows in external resistance from M to L, whereas, at fourth instant it flows from L to M.
Commutator Action
Now, consider that the two ends of the coil are connected to only one slip ring split into two parts (segment) i.e., \(A^{''}\) and \(B^{''}\).
Each part is insulated from the other by a mica layer.
Two brushes rest on these parts of the ring
In this case when the coil is rotated is counter clockwise direction at \(\omega\), \(\phi\) is cut by the coil and an emf is induced in it.
\(|\text{EMF}|\) induced in the coil at various instants will remain the same.
However, the flow of current in the external resistor or circuit will become unidirectional i.e., at second instant the flow of current in the external resistor is from M to L as well as the flow of current in the external resistor is from M to L in the fourth instant
AC is converted into unidirectional current in the external circuit with the help of a split ring (i.e., commutator)
In an actual machine, there are number of coils connected to the number of segments of the ring called commutator.
The emf or current delivered by these coils to the external load.
The actual flow of current flowing in the external load is shown by the firm line which fluctuates slightly.
The number of coils placed on the armature is even much more than this and a pure direct current is obtained at the output.
Thus, in actual machine working as a generator, the function of commutator is to convert the alternating current produced in the armature into direct current in the external circuit.
Connections of Armature Coils with Commutator Segments and Location of Brushes
Consider armature with four coils 1,2,3 and 4 equally spaced in slots
The number of commutator segments is equal to number of coils.
When armature is rotated clockwise, direction of induced emf and hence the current in coil sides \(3^{\prime}, 1,4^{\prime}, 2\) is downward and in coil sides \(1^{\prime}, 3,2^{\prime}, 4\) is upward.
The coils should be connected in such a way that the emf induced in the two sides of the same coil be added up
The coil sides \(1-4^{\prime}, 1^{\prime}-2,2^{\prime}-3\) and \(3^{\prime}-4\) are connected to the commutator segments namely 1,2,3 and 4
The next point of discussion is to determine how many brush sets are required and where these are to be placed with respect to the poles.
The brushes are to collect or deliver the current and are placed at such a position that sparking is minimum at the brushes.
The rules to be followed for determining the position of brushes are discussed now:
(i) Place the brush at each meeting point or each separating point of two emf’s.
The brushes at the meeting point are of positive polarity and those at the separating point are of negative polarity.
At segment 1 , current is separated towards conductor 1 and \(4^{\prime}\), hence it is a separating point.
Whereas, current is coming towards segment 3 from conductor \(2^{\prime}\) and 3 , hence it is a meeting point.
So, commutator segment 1 is the position of negative brush and commutator segment 3 of the positive brush.
(ii) Brushes are generally equally spaced and placed directly opposite to the pole centres
This brings out a very important point, namely, that in certain positions of the commutator the brushes will be actually short circuiting the coils connected to the segments with which they are in contact
In this case, two coils are short circuited, and as such the width of brushes must be greater than the thickness of the mica insulation between segments.
However, for representation, the general convention is to place the brushes at geometrical neutral plane (G.N.P.).
All conductors above the brush axis carry current in one direction and all conductors below the brush axis carry current in the opposite direction.
This convention represents that the brushes are placed at the coil or coils in which the voltage induced is zero.
The actual position of the brushes is shown and also show the position of coil sides and the direction of currents after the coils 2 and 4 undergo commutation.
The negative brush short circuits the segments 1 and 4 whereas positive brush short circuit the segments 2 and 3.
The coils 4 and 2 are short circuited by the brushes and hence no emf should induce in these coils.
Armature winding forms a closed circuit and consists of two parallel paths.
When a coil under goes commutation no emf is induced in it since it passes through magnetic neutral axis (MNA), and the coil is short-circuited by the brushes, hence no sparking will take place.