-: Topic :-

Design of inter poles

Inter pole

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

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.

NS

Direction of rotation

Current going insideCurrent coming outside

Main FieldArmature Field

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

• 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

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:

• 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:

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.

• (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.

N

N

S SN

NS

S

D.C. Generator

Main Pole

Inter-Pole

N

N

S S

N

N

S

S

D.C. Motor

Main Pole

Inter-Pole

N

S

NS

Main Field

Field by inter poles

Armature Field

Main Field

Field by inter poles

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.

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.

Use full relation for design purpose :(1) M.M.F. required to overcome armature reaction ….. For non-compensating winding = (1-) for provision compensating winding Where, = current in each conductor Z = Number of conductor P = Number of pole = 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 gapBgip= Flux density in interpole gapIgip = air-gap length under interpole

Use full relation for design purpose :

(3) M.M.F. required for inter pole = 800000*Kgip * Bgip* Igip + = 800000*Kgip * Bgip* Igip+ This is for the machine with no compensating winding = 800000*Kgip * Bgip* Igip+ (1-) For machine with compensating winding turns can be found,Tip =

The reactance voltage (Er) :Performance co-efficient for slot s =

Performance co-efficient fortooth top t=Where, = width of inter pole

Performance co-efficient for over hang=Where, = free length of overhang.

Total permeanceco-efficient= sEffective value of leakage flux= Iz * Zs * * * LIz * Zs= Mmf per slot

Average reactance voltage :

Erav = As per pichelmayer’s formulaErav =

Where, Tc = number of turns in a coil= specific permeancetc= time of commutation

THANK YOU!