Constructional Features of D.C Machines

Question

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Answer 1

35.1 Goals of the lesson 

In this lesson, important constructional features of a D.C machine are presented along with a discussion on D.C armature winding. Key Words: Field winding, armature winding, commutator segments & brush arrangement. After going through this section students will have clear ideas about the followings: • The function of commutator & brush in a D.C Machine. • Double layer winding. • Coil span & commutator pitch. • Lap & wave winding and number of armature parallel paths. 35.2 Introduction As pointed out earlier, D.C machines were first developed and used extensively in spite of its complexities in the construction. The generated voltage in a coil when rotated relative to a magnetic field, is inherently alternating in nature. To convert this A.C voltage into a D.C voltage we therefore need a unit after the coil terminals. This unit comprises of a number commutator segments attached to the shaft of the rotor and a pair of suitably placed stationary carbon brushes touching the commutator segments. Commutator segments together with the fixed brushes do the necessary rectification from A.C to D.C and hence sometimes called mechanical rectifier. 

35.3 Constructional Features

Figure 35.1 shows a sectional view of a 4-pole D.C machine. The length of the machine is perpendicular to the paper. Stator has got 4 numbers of projected poles with coils wound over it. These coils may be connected in series in order that consecutive poles produce opposite polarities (i.e., N-S-N-S) when excited from a source. Double layer lap or wave windings are generally used for armature. Essentially all the armature coils are connected in series forming a closed armature circuit. However as the coils are distributed, the resultant voltage acting in the closed path is zero thereby ensuring no circulating current in the armature. The junctions of two consecutive coils are terminated on to the commutator segments. Stationary carbon brushes are placed physically under the center of the stator poles touching the rotating commutator segments. 

Now let us examine how a D.C voltage is obtained across the brushes (armature terminals). Let us fix our attention to a particular position in space. Whichever conductor is present there right now, will have some definite induced voltage in it (dictated by e = blv). In course of rotation of the armature newer conductors will occupy this position in space. No matter which conductor comes to that particular position at any given point of time, it will have same voltage induced in it. This is true for all the positions although the magnitude and polarity of the voltages in different position may be different. The polarity of the voltage is opposite for conductor positions under north or south pole. Remembering that all the conductors are connected in series and brushes are suitably placed for obtaining maximum voltage, the magnitude of the voltage across the brushes will remain constant. To understand the action of the commutator segments and brushes clearly, let us refer to the following figures (35.3 and 35.4) where a simple d.c machine working as generator are shown with armature occupying various positions. Armature has got a single rectangular coil with sides 1 and 2 shown in detail in figure (35.2). The two terminals 1 and 2 of the coil are firmly joined to commutator segments C1 and C2 respectively. Commutator segments C1 and C2, made of copper are insulated by mica insulation shown by lines between C1 and C2 and rotate along with the armature. 

B1 and B2 are stationary carbon brushes are placed over the rotating commutator in such a way that they always make electrical contact with the commutator segments. It is from the two brushes, two terminals are taken out and called the armature terminals. Brushes are kept in brush holders with a spring arrangement. Spring tension is so optimally adjusted that brushes make good contact with the commutator segments C1 and C2 and at the same time allows the rotor to move freely. Free end of conductors 1 and 2 are respectively terminated on C1 and C2. In other words any point on C1 represents free end of the conductor 1. Similarly any point on C2 represents free end of conductor 2. However, fixed brushes B1 and B2 make periodically contact with both C1 and C2 as rotor rotates. For clarity, field coils are not shown in the figure. Let us assume that the polarity of the projected stator poles are N and S. Let the armature be driven at a constant angular speed of ω in the ccw direction. Start counting time from the instant when the plane of the coil is vertical i.e., along the reference line. Position of the armature at this instant is shown in figure 35.3(i). There cannot be any induced voltage in conductors 1 and 2 at this position as no flux density component is available perpendicular to the tangential velocity of conductors 1 and 2. 

Thus polarity of B1 remains +ve as before and that of B2 remains –ve unaltered. Before going further you must understand very clearly the following: 1. Polarity of voltage across C1 and C2 will periodically reverse. This is because any point on C1 always means free end of conductor 1 and any point on C2 always means free end of conductor 2. In other words V_C1C2 will be alternating in nature. 2. Polarity of voltage across B1 and B2 will not change with time – in the present case, B1 always remains +ve and B2 always – ve. Thus VB 1B 2 always remains unidirectional. 3. A particular brush is not associated with a fixed conductor but it makes contact with different conductors when they come at some fixed position in space. In this simple machine, any conductor coming between 0 < ωt < 180° in space will be connected always to B1. Although, the voltage VB 1B 2 is always +ve (i.e., unidirectional), its magnitude does not remain constant, since e = Blv and value of B is not constant under a pole. If B is sinusoidally distributed with B = Bmax sin θ [figure (35.5)], then variation of V_C1C2 and V_{B 1B 2} are as shown in figures (35.6 and 35.7). 

The brush voltage (or armature voltage) obtained from this simple generator having a single turn in the armature, is unidirectional no doubt but the magnitude of the voltage is not constant with time. Therefore, to improve the quality of the voltage similar to the nature of a battery voltage, a single coil in the armature with two commutator segments will not do. In fact, a practical d.c machine armature will have large number of slots housing many coils along with a large number of commutator segments. All the coils are connected in series forming a closed circuit. However, no circulating current result as the net emf acting in the closed circuit is zero. Each coil ends are terminated on two commutator segments. Armature windings may be of different types (namely lap and wave), depending on which coil ends are terminated on specific commutator segments. For example, when ends of a coil are terminated on two consecutive segments, lap connected armature winding is obtained. On the other hand, if the ends of a coil are terminated on segments which are apart by approximately two pole pitch, a wave connected armature winding results. It can be shown that in armature, across the brushes there exists parallel paths denoted by a. Number of parallel paths (a) in case of lap winding is equal to the number poles (P) of the machine while a = 2 in case of wave winding. We shall discuss along with diagrams Simple lap and wave windings in the following sections. To know more about d.c machine armature windings, one may refer to any standard book on Electrical Machine Design. It may be emphasized, that to analyse the performance of a d.c machine one should at least be aware of the fact that: 

 Number of parallel paths in armature, a = P for LAP winding. 

and a = 2 for WAVE winding.

Version 2 EE IIT, Kharagpur