Constructional Features of DC Machines - 1 | Basic Electrical Technology - Electrical Engineering (EE) PDF Download

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.

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.  

It is interesting to note that the coil is short circuited via commutator segment C2, brush B1, commutator segment C1 and brush B2 at ωt = 0 position.  This short circuiting does not however produce circulating current in absence of any voltage.  Let the coil moves by some angle, say 45° as in figure 35.3(ii).  Since conductor 2 is under the influence of N pole, polarity of the induced voltage in it will be
.  Similarly conductor 1 being under the influence of the S pole, polarity of the induced voltage in it will be ⊗.  Therefore across B1 and B2 we will get a voltage with B1 being +ve and B2 being -ve.  The polarity of the voltage in conductors 2 and 1 does not change so long 2 remains under N pole (which automatically means 1 under S pole).  Figures 35.3(i) to 35.3(iv) show some selected positions of the coil corresponding to ωt = 0°, ωt = 45°, ωt = 90°, ωt = 135° and ωt = 180°.  After this conductor 2 comes under S pole and conductor 1 under N pole.  Therefore polarity of voltage in conductor 1 is  while polarity of voltage in conductor 2 is ⊗.  B1 now makes contact with C1 and B2 makes contact with C2.

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 V_B1B2 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 V_{B 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 = B_max sin θ [figure (35.5)], then variation of V_{C 1C 2} 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. 

D.C machine Armature Winding 

Armature winding of a D.C machine is always closed and of double layer type.  Closed winding essentially means that all the coils are connected in series forming a closed circuit.  The junctions of the consecutive coils are terminated on copper bars called commutator segments.  Each commutator segment is insulated from the adjacent segments by mica insulation.  For reasonable understanding of armature winding, let us first get acquainted with the following terminologies.

• A coil has two coil sides occupying two distinct specified slots.  Generally two maximize induced voltage in a coil, the spacing between them should be close to 180° electrical.  This essentially means if at a given time one coil side is under the center of the north pole, the other coil side should be under the center of the south pole.
• Coil span is nothing but the spacing between the two coil sides of a coil.  The spacing is expressed in terms of number of slots between the sides.  If S be the total number of slots and P be the total number of poles then coil span is S/P.  For 20 slots, 4 poles winding, coil span is 5.  Let the slots be numbered serially as 1, 2,…, 20.  If one coil side is placed in slot number 3, the other coil side of the coil must occupy slot number 8 (= 3 + 5). 
• A Double layer winding means that each slot will house two coil sides (obviously belonging to two different coils).  Physically one coil side is placed in the lower portion of the slot while the other is placed above it.  It is because of this reason such an arrangement of the winding is called a double layer winding.  In the n th slot, coil side in the upper deck is numbered as n and the coil side in the lower deck is numbered as n'.  In the 5th slot upper coil side is numbered as 5 and the lower coil side is numbered 5'.  In the winding diagram, upper coil side is shown with firm line while the lower coil side is shown with dashed line.

Remembering that a coil has two coil sides, for a double layer winding total number of coils must be equal to the total number of slots.

• Numbering a coil:  A coil is so shaped, that when it is placed in appropriate slots, one coil side will be in the upper deck and the other side will be in the lower deck.  Suppose S = 20 and P = 4, then coil span is 5.  Let the upper coil side of this coil be placed in slot number 6, the other coil side must be in the lower deck of slot number 11.  The coil should now be identified as (5 - 11').  In other words coil sides of a coil are numbered depending on the slot numbers in which these are placed.  A typical single turn and multi turn coils are shown in figure 35.8

Figure 35.11: Single turn & Multi turn euil

• On a Commutator segment two coil sides (belonging to two different coils) terminate.  2S being the total number of coil sides, number of commutator segments must be equal to S, number of slots.  Commutator segments can also be numbered as 1,2,…,20 in order to identify them clearly.
• Commutator pitch:  As told earlier, the free ends of the coil sides of a coil (say, 6 – 11’) are to be terminated on to two specific commutator segments. The separation of coil sides of a coil in terms of number of commutator segments is called the commutator pitch, yc.  In fact the value of yc decides the types of winding (lap or wave) which will result.  For example, in case of lap winding y_c = 1.

Armature winding:  General procedure  

1. Type of winding (lap or wave), total number of slots S and total number of poles P will be given.
2. Calculate coil span (≈ S/P).
3. Calculate commutator pitch y_c.  For lap winding y_c = ±1 and for wave winding 
4. We have to complete the windings showing the positions of coil sides in slots, interconnection of the coils through commutator segments using appropriate numbering of slots, coil sides and commutator segments.
5. Finally to decide and place the stationary brushes on the correct commutator segments.