Interpoles of dc generetors

Direct current Machine -interpoles

1. Interpolesof Direct Current
Machines
By SuvraPattanayak

2. Concept of armature reaction
•In dc machine, unloaded condition magneto
motive force (mmf) acting in a dc generator is
that due to the field. However, when it loaded
the current flowing in the armature coils also
creates a powerful magnetomotiveforce that
distorts and weakens the flux coming from the
poles. This distortion and field weakening
takes place in both motors and generators.
The effect produced by the armature mmfis
called armature reaction.

5. Interpolesor Commutating poles
•To counter the effect of
armature reaction in
medium and large-
power dc machines, we
always place a set of
commutating
poles/Inter poles
between the main poles

6. Interpoles(continued)
•These narrow poles carry windings that are connected
in series with the armature.
•The number of turns on the windings is designed so
that the poles develop a magnetomotiveforce (mmfc)
equal and opposite to the magnetomotiveforce
(mmfa), of the armature.
•As the load current varies, the two magnetomotive
forces rise and fall together, exactly bucking each other
at all times. By nullifying the armature mmfin this way,
the flux in the LpaCebetween the main poles is always
zero and so we no longer have to shift the brushes.

7. Interpoles(continued)
•In practice, the mmfof the commutating poles is
made slightly greater than the armature mmf.
•This creates a small flux in the neutral zone,
which aids the commutation process.
•In the figure 1 shows how the commutating
poles of a 2-pole machine are connected. Clearly,
the direction of the current flowing through the
windings indicates that the mmfof the
commutating poles acts opposite to the mmfof
the armature and, therefore, neutralizes its
effect.

8. Why not other methods
•Other methods to compensates Armature
reaction are
1.High reluctance pole tips
2.Reduction of armatures flux
3.Compensating windings/pole face windings/
Thompson Ryan windings

9. Advantages of Interpoles(continued)
•Both high reluctance pole tips & reduction in
armature flux are mechanical techniques have
major disadvantages in that they do not inter act
the effects of high armature currents and MMF
due to heavy loads.
•Compensating Winding Disadvantages is winding
neutralizes the cross magnetizing effect due to
armature conductors only but not due to
interpolarregion. This winding is used in large
machine in which load is fluctuating.

10. Advantages of Interpoles(continued)
•Compensating winding and interpolesare
used for same purpose but the difference
between them is, interpolesproduce e.m.ffor
neutralizing reactance e.m.fwhereas
compensating winding produces an m.m.f
which opposes the m.m.fproduced by
conductors.

11. Disadvantages of interpoles
•However, the neutralization is restricted to the
narrow brush zone where commutation takes
place. The distorted flux distribution under
the main poles, unfortunately, remains the
same.

12. Discussion
•Another method of providing the necessary “reversing
field" is to fit special commutation poles to the frame
of the machine. These poles are placed midway
between the main poles, and are excited in such a
manner as to produce a commutation field of the
required polarity and strength.
•For a generator, to assist the reversal of the current
the polarity of the interpolemust obviously be the
same as that of the main pole in advance
•For a motor, on the other hand the converse will be
true.

13. Discussion
•Again, since the reactance e.m.f. increases in
direct proportion to the current, the reversing
field must increase in the same pro portion.
This involves the interpolewindings being
connected in series with the armature.

14. Discussion
•It will be noted that the reversing field required is of
opposite polarity to that which armature reaction tends to
establish. If, in fact, the interpoleswere not excited,
commutation would be seriously impaired.
•By arranging that the m.m.f. developed by the interpoleis
exactly equal and opposite to that of armature reaction,
commutation would take place in the magnetic neutral
position, and there would be no reversing field. In short, to
establish a reversing field under the interpoleshoe, it is
essential that the m.m.f. developed by the interpole
winding shall overpower that due to armature reaction, the
flux established being determined by the difference of the
magnitude of these two m.m.f.

15. Discussion
•The interpolewinding is local in its action. That is
to say, the winding develops an m.m.f. which
overcomes armature reaction in the commutating
zone only.
•Thus the distortion of the main field under the
pole shoes is not affected by the use of
interpoles. This is illustrated by Figure, which
gives the resulting field form for a loaded
interpolemachine and indicates that, in a
generator, the interpolesact as outposts of the
main poles.

17. More discussion on interpoles
•By the use of these series-wound interpolesit then become
possible to obtain satisfactory commutation with brushes in
a fixed position—the geometric neutral—under all
conditions of load.
•Further, in motors, satisfactory commutation becomes
possible with fixed brushes for both directions of rotation,
and over a considerable range of speed obtained by field
control.
•More important still, with interpoleit has been made
possible to construct machines, such as relatively high
voltage and high speed machines, which could not possibly
be built to operate satisfactorily without interpoles.

18. More discussion on interpoles
(continued)
•In fact, until interpoleswere used, the rated
output of d.e. machinery was quite often
proscribed solely from sparking Con siderations.
•With interpoles, temperature rise practically
alwaydetermines the rated output, so that it has
become possible to increase very considerably
the rated output of machines of gjven
dimensions, and to take full advantage of the
more intensive methods of ventilation now
commonly employed.

19. More discussion on interpoles
(continued)
•This vastly improved performance and, often, vastly
increased rating, is then obtained at the expense of
building, assembling, and fitting the interpolesand
their relatively powerful coils. The extra expense
moreover, is partly compensated by the fact that it is
not necessary with interpolemachines to work with
such strong field windings on the main poles as in non-
interpolemachines. In fact, the air gap can be reduced
to a figure not much in excess of the minimum
necessitated by mechanical reasons. The advantages
gained bvthe use of interpolesare so great that it is
standard practice always in employ interpolesexcept
for very small machines.

20. The END