Design of inter poles

1. -: Topic :-
Design of inter poles

2.  Inter pole
•Inter pole are used for reduced the effect of
‘ARMATURE REACTION’

3.  Armature Reaction
- Armature Reaction is the effect of “Armature
field” on the “Main Field”.
-Armature field is the field which is produced by the
armature conductors due to current flowing through
them.
- Main field is the field which is produced by the
poles which is necessary for the operation.

4. N
S
Direction of rotation
Current going inside
Current coming outside
Main Field
Armature Field

5. o Magnetic flux density increases over one half of the
core and decreases over the other half.
o The flux wave is distorted and there is a shift in the
position of M.N.A.
o It causes the commutation problem.
 Effects Of Armature Reaction

6. • They are so effective that normally all DC compound
motors that are larger than 1/2 hp will utilize them. Since
the brushes do not arc. they will last longer and the
armature will not need to be cut down as often. The
interpoles also allow the armature to draw heavier currents
and carry larger shaft loads.
• When the interpoles are connected, they must be tested
carefully to determine their polarity so that it can be
matched with the main pole. If the polarity of the
interpoles does not match the main pole it is mounted
behind, it will cause the motor to overheat and may damage
the series winding.
 Design of inter pole

7. o As the compensating
windings are too
costly, inter-poles are
used to neutralize the
Cross-magnetizing
effect of armature
reaction.
o These are small poles
fixed to the yoke and
spaced in between the
main poles.
 Inter-poles:

8. • They are wound with
comparatively few heavy
gauge Cu wire turns and
are connected in series
with the armature so that
they carry full armature
current.
• Their polarity, in the case
of a generator, is the same
as that of the main pole
ahead in the direction of
rotation.
 Inter-poles:

9.  The tow function of interpoles :
• (i) As their polarity is the same as that of the main pole
ahead, they induce an emf in the coil (under commutation)
which helps the reversal of current.
• The emf induced by the interpoles is known as commutating or
reversing emf.
• The commutating emf neutralizes the reactance emf thereby
making commutation sparkless. With interpoles, sparkles
commutation can be obtained up to 20 to 30% overload with
fixed brush position. Infact, interpoles raise sparking limit of
a machine to almost the same value as heating limit.
• Hence, for a given output, an interpole machine can be made
smaller and, therefore, cheaper than a non-interpolar machine.
• As interpoles carry armature current, their commutating emf
is proportional to the armature current. This ensures
automatic neutralization of reactance voltage which is also
due to armature current.

10. • (ii) Another function of the interpoles is to neutralize the
cross-magnetising effect of armature reaction. Hence,
brushes are not to be shifted from the original position.
• OF as, represents the mmf due
to main poles. OA represents the
crossmagnetising mmf due to
armature. BC which represents
mmf due to interpoles, is
obviously in opposition to OA,
hence they cancel each other
out.
• This cancellation of
crossmagnetisation is automatic
and for all loads because both
are produced by the same
armature current.

11. N
N
S S
D.C. Generator
Main Pole
Inter-Pole

12. N
N
S S
D.C. Motor
Main Pole
Inter-Pole

13. N
S
N
S
Main Field
Field by inter poles
Armature Field
Main Field
Field by inter poles

14. Design of inter pole
• For the particularly design following things are found:
• Reactance voltage for (a) straight line commutation (b)
sinusoidal commutation.
• Mmf required for air gap and mmf required for iron parts.
• M.M.F. required for inter pole
• Number of turns of windings on interpoles.
• Compensating winding provision.

15. Details of inter pole
• Material-cost steel or parched from sheet steel.
• No special pole shoe needed.
• Current density in the inter pole winding between
2to4 A/mm2.
• The winding may consist of bare conductor made of
copper which are air-spaced for right construction.
• Length of inter pole is lesser than armature core
length. It may be 0.5 to 0.67 of armature core
length. It may equal to length of main pole.
• Interpole winding carry the same current passing in
armature i.e. Ia.

16. Use full relation for design purpose :
(1) M.M.F. required to overcome armature reaction
𝐴𝑇𝑎 =
𝐼 𝑧 𝑍
2𝑃
….. For non-compensating winding
= (1-𝛼)
𝐼 𝑧 𝑍
2𝑃
for provision compensating winding
Where, 𝐼𝑧 = current in each conductor
Z = Number of conductor
P = Number of pole
𝛼 = Pole arc/pole pitch
(2) M.M.F. required in the interpole air-gap to produced the
flux-density Bgip
= 800000*Kgip * Bgip* Igip
Where, Kgip= karter’s co-efficient for interpole gap
Bgip= Flux density in interpole gap
Igip = air-gap length under interpole

17. Use full relation for design purpose :
(3) M.M.F. required for inter pole 𝐴𝑇𝑖𝑝:
𝐴𝑇𝑖𝑝 = 800000*Kgip * Bgip* Igip + 𝐴𝑇𝑎
= 800000*Kgip * Bgip* Igip+
𝐼 𝑧 𝑍
2𝑃
This is for the machine with no compensating winding
𝐴𝑇𝑖𝑝 = 800000*Kgip * Bgip* Igip+ (1-𝛼)
𝐼 𝑧 𝑍
2𝑃
For machine with compensating winding turns can be found,
Tip =
𝐴𝑇 𝑖𝑝
𝐼 𝑎

18. The reactance voltage (Er) :
Performance co-efficient for slot
𝜆s = [
ℎ1
3𝑊𝑠
+
ℎ2
𝑊𝑠
+
2ℎ3
𝑊𝑠+𝑊𝑜
+
ℎ4
𝑊𝑜
]
Performance co-efficient for
tooth top
𝜆t=
𝑊 𝑖𝑝
6𝐼 𝑔𝑖𝑝
Where, 𝑊𝑖𝑝 = width of inter pole

19. Performance co-efficient for over hang
𝜆o=
𝐿 𝑓𝑟
𝐿
[0.23𝑙𝑜𝑔10
𝐿 𝑓𝑟
𝑏
+ 0.07]
Where, 𝐿 𝑓𝑟 = free length of overhang.
Total permeanceco-efficient
𝜆= 𝜆s + 𝜆t + 𝜇𝜆o
Effective value of leakage flux
= Iz * Zs * 𝜆 * 𝜇 * L
Iz * Zs= Mmf per slot

20. Average reactance voltage :
Erav = 𝐿 𝑑𝑡
𝑑𝑖
As per pichelmayer’s formula
Erav = 2 𝑇𝑐
2 L λ
2𝐼 𝑧
𝑡 𝑐
Where, Tc = number of turns in a coil
λ = specific permeance
tc= time of commutation