THE D.C. GENERATOR
D. C. MACHINE CONSTRUCTION
The principal features of the machine will be described under (a) the field system o r stator and (b) the armature or rotor.
I N
F I E L D
SYMBOLS USED FOR THE MACHINE
S E R I E S F LD
Fig 79
FIELD
This includes the magnet arrangement comprising the poles interpoles are essentially part the armature electrical circuit and will be mentioned later under this latter heading.
arc cores of machine and are usually fitted with pole-shoes which across air-gaps in the conductors
is of system, forms the
a n d carry flux from and to
THE D.C. GENERATOR 159 the poles. The diagram (Fig 80) shows typical ways in which the
cnn hc
The poles and yoke can be constructed from cast steel or fabricated from mild-steel sheet which cut and rolled
Fig
160 REED'S ELECTROTECHNOLOGY but the series coils may be self-supporting. The diagram (Fig shows a typical cross-section of construction; the insulated from each other to minimise induced circulating currents, called 'eddy currents', and if clamping bolts are used to pass through the core as shown in the diagram (Fig then be insulated. Modern techniques use stamped lamina-
w h i c h pressed onto to the
plates being screwed onto for smaller machines.
larger designs a 'spider' is employed which allows ventilation and keeps the iron required to a minimum.
number of conductors, their size, shape, etc, are decided design requirements for machine. The
( F i g of locating
THE D.C. GENERATOR 161
0 1
0 1 CONSTRUCTION
MORE
Fig 82
coil sides in position. In sketch, a wedge made
or is but slots
with a closing piece of fibre and 'binders', made from tensile steel wire, are frequently used. For small machines, mainly motors: as for vacuum-cleaners, cabin-fans, etc, the armature windings consist of enamelled or cotton-covered wire placed in position by hand winding. Semi-enclosed slots are used with a fibre insert closing the slots.
The method of arranging the conductors to form a closed winding will be considered in greater detail after the machine construction has been dealt with.
Fig 83
SHAFT. This is made the best forged mild steel and is designed so that it will not deflect unduly when running up to its maximum speed.
COMMUTATOR. This consists of copper segments, insulated from each other by mica. The thickness of the top of a segment may be up and the segments may be mounted on but insulated from a sleeve, which is secured to the shaft, and are clamped by an end-ring which can be bolted o r screwed as shown in the diagram (Fig 84). Insulated rings, made from micanite are used to insulate the segments from the stee!
soldered the The mica must be between
segments.
Fig 84
Any one brush is pressed onto the commutator by means of the pressure arm and is connected to the holder by means of a pig-tail of braided copper wire which is moulded into the brush. One o r more brush-holders may be carried o n an insulated which is mounted the brush rocker-ring.
The brush is arranged to be clamped firmly once the brush position has been set. Brushes of modern machines are always of moulded carbon and graphite, the grade of hard- ness being chosen to s u i t the running condition;. The diagram (Fig 85) shows a typical arrangement.
THE D.C. GENERATOR 1 63
For are of
the ball or roller type. Advantages are (i) its axial length is shorter than that of journal type (ii) after initial packing with grease, service for a long period can be obtained. Journal bear- ings, ie sleeve types, give quieter running and are frequently preferred for marine work,. since they resist 'transmitted' vibration troubles better. They are usually of the 'ring-oiler' pattern. The steel shaft runs in a brass or cast-iron sleeve lined with white metal. For small and medium size machines, the bearings are. carried in the end shields, but for large machines, the bearings are carried in separate pedestals.
D.C. ARMATURE WINDING ARRANGEMENTS
The simplest winding possible would be built up from turn coils of span equal to one 'pole pitch', ie a two-pole machine. For a four-pole machine the coil would still be one pole pitch but now 90 mechanical degrees. This is illustrated by the diagram (Fig 86).
I PITCH
I P I T H
MECHANICAL
Fig 86
In it is not usual to make the span equal to one pole pitch exactly and many small machines have an odd number of slots. Each slot carries two coil sides, ie it contains more one conductor. D.C. windings are usually of the 2-layer type, a
side lying at the bottom of the and another at the top.
Sometimes more than 2, such as 4 , 6 or even 8 coil sides may be contained in slot since it may not be practicable to have too many slots. There are two basic methods of connecting up the conductors on an armature after they have been
either single or multi-turn coils, and the complete winding falls into one of two distinct types namely (a) a wave winding or (b) a lap winding.
(a) The WAVE two-circuit Winding. This winding results in there always being two paths in parallel irrespective of the
number of poles of the machine. Two sets of brushes only a r e necessary but it is usual to fit as many sets of brushes as the machine has poles. The diagram (Fig 87a) shows the essential layout.
SEGMENT
Fig 87
(b) The LAP multi-circuit Winding. This winding results in as many paths in parallel as the machine has poles. There are a s many sets of brushes as the machine has poles. The diagram (Fig 87b) also shows the essential layout.
In building up a winding i t is essential to connect coil elements in such a manner that induced in the conductors add, in much the same way as cells are connected in series so that their add to give the required battery voltage. Thus conductor X i s in series conductor Y which occupies relatively the same position as X but is under a pole of reversed polarity. The coil element so formed by conductors X Y should then be connected series with a similarly placed coil element under a pair of poles so that the voltage for a parallel path of the armature can be attained in this manner. For a wave winding the connection can be readily seen from the diagram already introduced and for a lap winding the same rule is followed.
coil under of poles con-
nected in series before the winding progresses to connect u p the of
The example, follows, illustrate both simple and wave A small armature is to be designed to have turn coils-one turn comprising 2 conductors. There are to be 8 coils. There will be I segment to a coil, ie 8
segments. If only 2 coil-sides to be accommodated in slot then must slots and if a four-pole
is to will will
pitch 3 pitch of
D.C. GENERATOR 165
slots by the number of sides
should be under the influence of the correct field poles, the
winding
pitch must be as as possible cqunl to pols pitch. Thus the winding pitch would also be equal to 2 o r a coil should embrace 2 teeth.The LAP winding is considered first, being suitable for this Now for such a winding, the connecting up of the conductors is such that the winding progresses round the armature by being pitched alternatively forwards and backwards.
For our example, if (Fig 88) is considered, it will be seen that conductor No 1 is connected to No 6 which is spaced 2 teeth away. No 6 is then connected to No 3 and so on. The winding thus progresses by 1 slot until it is closed by all the slots having been occupied and conductor No 15 being connected to No through N o 4.
Fig 88
If now, for our example, a WAVE winding required then a preliminary examination would show that this could not be achieved. If the winding started at No proceeded to No 6 and then on through Nos 9 and 14 it would close back onto con- ductor No It is obvious that an armature with 8 slots would not be suitable for such a winding and one of 7 or 9 slots should be considered. A nine-slot armature winding would give a winding pitch of length slightly less than the true pole pitch length and is considered as suitable. Consider now the diagram (Fig 89). Here conductor No 1 is connected to No 6 as before which in turn is connected to Nos 9, 14, 17 and then to No 4, ie
Fig 89
the winding passes into the slot beyond that at which the start was made. The winding, thus does not close immediately and if the connecting-up proceeds as described, it will be seen that the winding will progress four times round the armature before the is made at the starting slot by conductor No 1 1 being joined to No 1 through N o This then would be a suitable winding but 9 coils would be used with 9 armature slots and 9 commutator
More details o n armature windings will be found in a book dealing more fully with the practical subject since machine design and armature winding is specialists' work. However, it is of interest to find where the brushes are to be placed on the and one accepted way is to draw out the 'equivalent ring' winding,
E Q U IV AL E N T K ING The diagrams (Figs 88 and 89)
conductors On
the armature diagram, current flow is assumed in the
this flow to
induced the conductors under a N pole, direction of the current is assumed from the bottom of the page to the top, then for those under a pole, it would be from the top of the page to the bottom. If next, the winding is drawn out a ring winding. as shown at the bottom of each figure and the conductors are correctly
I N o I to 6 and onto
so o n .
THE D.C. GENERATOR 167 the position between the poles. Thus a +ve point is at the
6-9 o r 14-1 7, there in effect being no potential in loop 9 and 14. A brush could be placed at either commutator 4 o r 8 to maintain uniformity, be placed at these points and connected as to form the +ve termina!
of the machine. Similarly - ve points occur at the junctions of and 13 or the actual joints 2-5 Brushes may be placed at these points, there being no current (induced in loop 5.
10. As for the +ve terminal, brushes are placed on segment Nos 2 and 6 and to form the - ve terminal. The also been shown that there are two fundamental ways of winding an armature (a) with a lap winding or with a wave winding.
Interpoles o r commutating-poles, termed poles, d o not any function' of the main poles and are to be disregarded for the rule just enunciated:
THE E.M.F. EQUATION
Consider the diagram (Fig 90) and the factors for a machine below. A simple expression for the composite
is now deduced and it is stressed that this is of the utmost importance. I t must be memorised and the student should be capable of proving it from first principles.
Let = the speed of the machine in P = the num- ber of poles. = the (webers). Z = the number of armature conductors. A the number of parallel paths of the armature winding.
Fig 90
N .