ARMATURE REACTION:

We know that an emf is induced in the coils of armature when there is a change in magnetic flux associated with it.So when there is no load connected to the armature, the current flowing through the armature winding is zero.Therefore the net magnetic flux of the system is created by the field system.

When there is a load connected to the armature winding the current flowing through the armature winding is non-zero value.So since there is a current flow through the winding,it causes generation of magnetic field and hence magnetic flux.Now the net magnetic flux get reduced since the magnetic flux created by armature winding either opposes or crosses perpendicularly to that produced by field system.

Any dc machine has two types of planes known as Magnetic Neutral Plane(MNP) and Geometric Neutral Plane(GNP).

The position of MNP and GNP are important on case of dc machine.GNP is always peerpendicular to the line connecting the centres of magnetic poles.Whereas MNP is always perpendicular to the resultant magnetic flux or mmf.

So when the load connected to armature winding is zero, resultant mmf is from north pole to south pole.So the MNP and GNP coincides.

But when a load is connected to armature winding,the net mmf rotates in a direction along the rotation of armature and hence there is a shift in MNP.Now MNP and GNP never coincides.

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DC Machines| DC motor | DC generator

A machine that converts mechanical power to electrical power is known as a generator.The basic principle of a generator whether ac or dc is Faraday’s laws of electromagnetic induction.

The law states that whenever the magnetic flux linked with a conductor changes, an emf is induced in it and the direction of induced emf is given by Lenz law.

Learn about construction of dc machine

For a dc generator the direction of induced emf is given by Flemmings Right Hand rule.

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Construction of DC Machine

CONSTRUCTIONAL DETAILS OF A DC MACHINE

In this section, we will be talking about the main parts used in dc machines.

• ·         An armature consisting of a number of conductors suitably placed and connected so as to form a closed loop. Generally armatures are the rotating part of a machine and is termed as rotor.An armature is a cylindrical body rotating in space between the north and south pole comprising of slots,teeth,core and windings inside slots.
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• ·         A field system to produce the required magnetic field.It is usually the stationary part in a machine and is termed as stator. It consist of pole body,pole shoe and field windings.
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• ·         The stator and rotor are separated by a small air gap which forms the main criteria for determining the efficiency of a machine.
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• ·         A commutator consisting of a large number of segments which are well insulated from each other.
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• ·         Commutating poles or interpoles which are inter-posed between the main poles to ensure sparkless operation of the brushes at the commutator under the loaded conditions of machines.
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• ·         Yoke which acts as a mechanical support and as a path for the magnetic flux to flow.
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• ·         Brushes to collect the current from the rotating commutator or to lead current towards it in case of dc motor.
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• ·         Bearing-to support the rotating parts and to allow smooth motion with little friction .
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• ·         Arrangement of cooling-To keep the machine protected from heating up.

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