DC Machines- Construction

DC generator and motor have identical construction.

Generator

Field winding – DC SupplyArmature rotates inside the magnetic field .EMF is induced in the armature conductors

MotorField winding – DC Supply.Armature winding – DC supply.Torque is produced in the armature and it

starts rotating.

Main parts

Field windingArmature CoreArmature windingCommutatorCarbon brushe

Commutator – Converts AC to DC and vice-versa

brushes

• Field System For creating uniform magnetic field within

which the armature rotates.Electromagnets or permanent magnets are

used.Field magnet consists of:1. Yoke or frame2. Pole cores3. Pole shoes4. Magnetising coil or field winding.

• Yoke - Acts as frame of the machine and carries the magnetic flux produced by the poles.

• Pole core – Used to carry coils carrying filed current

• Pole shoe – Spreads out the flux over the armature periphery more uniformly.

• Magnetising coils- To provide number of ampere-turns of excitation.

Armature

• Rotating part. Its purpose is to rotate the conductors in the uniform magnetic field.

• It consists of insulated wires wound around an iron and are arranged so that electric currents are induced in these wires.

• Armature core is made from high permeability silicon- steel stampings with varnish as insulation.

Armature slots

Commutator

• Material – High conductivity hard drawn copper.

• It provides the electrical connections between the rotating armature coils and the stationery armature circuit.

• It will reverse the connections to the external circuit at the instant of each reversal of current in the armature coils.

Brushes

Function – To collect current from the commutator and supply it to the external load circuit.

Carbon, graphite, copper or metal graphite.

Air Gap

• The air gap in a motor is very important for a motor to spin.

• If there is no air gap , armature that will not rotate.

• You need clearance between the moving parts of the electric motor, or other wise it will not move at all.

Armature windings

• The armature winding of dc motor is attached to the rotor, or the rotating part of the machine.

• The armature core are provided with slots made of the same material as the core to which the armature winding made with several turns of copper wire distributed uniformly over the entire periphery of the cor

Armature winding types

• Basically armature winding of a dc machine is wound by one of the two methods, lap winding or wave winding.

• The difference between these two is due to the end connections and commutator connections of the conductor.

• In lap winding, the finishing end of the coil is connected to a commutator segment and starting end of the following coil.

• In wave winding, a conductor under one pole is connected at the back to a conductor which occupies an almost corresponding position under the next pole which is of opposite polarity.

• Number of parallel paths.

• In a lap winding, the number of parallel paths, a, is always equal to the number of poles, p, and also to the number of brushes.

• Lap winding has many parallel paths; thus it is more suited to high-current, low-voltage dc

machines.

• In wave windings, the number of parallel paths, a, is always two (2), and there may be two or more brush positions.

• Because wave windings have only two parallel paths, this winding is more suited for high voltage, low-current dc machines.