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Bypassing inter poles in a DC
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What are inter poles ? As the name suggests, these are poles which
are in between something. These are magnetic poles placed in between main
magnetic poles in a DC motor. Normally these are present only in bigger
motors. Look at the sketch below...
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What is the purpose of these poles? These poles are used for reducing
Armature Reaction in a DC motor. Armature Reaction is nothing but an
undesirable effect seen in DC motors when the armature starts carrying
current. Armature current in a DC motor is supposed to produce a magnetic
flux, which reacts with the magnetic flux produced by Field winding, to
create a mechanical torque. For optimum creation of torque, these two fluxes
should be at Right Angles to each other. In practice, when the armature
starts carrying current, it has a tendency to distort the flux produced by
the field winding. This distortion results in reducing the 90 degree angle
between the two fluxes. This is called ‘shifting of the neutral axis’ of the
main field. Neutral axis is the axis which is perpendicular to the North –
South axis of the main field. When the neutral axis is shifted by a few
degrees, its effect is that the carbon brushes which are supposed to be along
the neutral axis, no longer fall on
the neutral axis. This results in sparking between the commutator segments
and the brushes. Sparking takes place because the brushes which short two
consecutive segments no longer have zero voltage across them. This voltage
causes sparking. In order to nullify this ill effect of
the armature current, Inter poles are placed between main poles to counter
the distortion of main filed. For making the compensation effective at all
levels of armature current, the inter poles are connected in series with
armature. Thus, the magnetic nullification effect produced by the inter poles
is always proportional to the distorting effect. Thus sparking is avoided at
all levels of armature current. Polarities of Inter poles should be made
dependent on the direction of rotation of the motor. Effect of Inter poles is
adverse in other direction. Therefore, if there is reversal of armature is
involved, it should reverse the inter pole connections also. If during trouble shooting it is seen
that the inter pole windings are short to body they can be removed
temporarily till the shut down is planned and motor is spared for repairs.
This is normally possible in medium sized motors.
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Motor okay? Check
Electronic parts
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About the author
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