31-07-2012, 04:35 PM
DC MOTOR
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PRINCIPLE
An electric motor is a machine which converts electric energy into mechanical energy.its action is based on the principle that when a current carrying conductor is placed in a magnetic field it experiences a mechanical force whose direction is given by fleming’s left hand rule.
When its field magnets are excited & its armature conductors are supplied with current from the supply mains, they experiences a force tending to rotate the armature. Armature conductors under n pole are assumed to carry current downwards & those under S poles, to carry current upwards. By applying fleming left hand rule, the direction of the force on each conductor can be found.
CONSTRUCTION
A single turn rectangular copper coil ABCD rotating about its own axis in a magnetic field provided by either permanent magnet it or electromagnets. The two ends of the coil are joined to two slip rings which are insulated from each other & from the central shaft. Two collecting brushes press against the slip rings. Their function is to collect the current induced in the coil & to convey it to the external low resistance.
WORKING
The coil to be rotating in clockwise direction. As the coil assumes successive position in the field, the flux link with it changes. Hence, emf is induced in it which is proportional to the rate of change of flux linkages. When the plane of the coil is at right angles to lines of flux when it is in position 1, then the flux linked with the coil is maximum but rate of change of flux linkages is minimum.
It is so because in this position, the coil sides do not cut or shear the flux, rather they side along them they move parallel to them. Hence, there is no induced emf in the coil.
It consist of following essential parts:
• Magnetic frame or yoke
• Pole cores & pole shoes
• Pole coils or field coils
• Armature core
• Armature winding or conductors
• Commutator
• Brushes & bearings
The outer frame or yoke serves double purpose:
(i) It provides mechanical support for the poles &acts as a protecting cover for the whole machine.
(ii) It carries the magnetic flux produced by the poles.
POLE CORES &POLE SHOES
The field magnets consist of pole cores &pole shoes. The pole shoes serve two purposes
(i) they spread out the flux in the air gap & also, being of larger cross-section, reduce the reluctance of the magnetic path (ii) they support the exciting coils.
There are two main types of pole construction.
(a) The pole core itself may be a solid piece made out of either cast iron or cast steel but the pole shoe is laminated & is fastened to the pole face by means of counter sunk screws.
(b) In modern design, the complete pole cores & pole shoes are built of thin lamination of annealed steel which are riveted together under hydraulic pressure.
ARMATURE CORE
It houses the armature conductors or coils & causes them to rotate & hence cut the magnetic flux of the field magnets. Its function is to provide a path of very low reluctance to the flux through the armature from N pole to S pole.