MOTORS – convert electrical energy to mechanical energy
Motors and generators are the most frequent used electrical machines.
❖ Generators action can take place when and only when, there is a relative motion between conducting wires and magnetic lines of force.
❖ Electric motor is in operation when it is supplied with electrical energy and develops torque, that is, a tendency to produce rotation.
❖ In DC generator, the armature winding is mechanically rotated through the stationary magnetic fields created by the electromagnets or permanent magnets.
❖ In AC generator, the electromagnets or permanent magnets and their accompanying magnetic fields are rotated with respect to the stationary armature windings.
Classification of single phase motors
1. Shaded pole
Classification of Poly phase Motors
1. Induction Motor
2. Commutator (Schrage Motor)
Commutation on DC Motor
The function of the commutator and the brushes in DC motor and generator is to act as an inverter, that is to change direct current to alternating current.
DC Generator and DC Motor
A dc generator and a dc motor are identical in structure and are interchangeable. Any dc motor can be used as a generator, and any dc generator can be used as motor with only a minor change in connections. When a conductor (wires of the armature) cuts the lines of force (magnetic field of the stator), a voltage is generated in that conductor (wires of the armature). There are three factors or rules that govern the amount of voltage generated: 1. The number of lines of force being cut.
2. The speed at which the conductors are cutting the lines of force. 3. The number of conductors cutting the lines of force.
A dc motor is a machine that, when supplied with electric current, can be used for such mechanical work as driving pumps and running machine tools.
CLASSIFICATION OF DC MOTORS
1. Shunt motor – a comparatively high resistance field winding of many turns of fine wire is connected in parallel with the armature. From no load to full load, the speed of shunt motor will not change much, which means that it has a good speed regulation.
Shunt motor controller:
❖ Over speed control – voltage control connected in series to control the speed of the motor.
Full voltage applied to field and armature will produce slowest speed at full load. Reduce field voltage reduces current flow and reduces lines of force, thus, less back EMF generated in the armature then more current to flow in the armature, increasing its magnetic power and so it will speed up. More speed and current will increase the motor horsepower.
❖ Under speed control – a controller placed in series with the armature to control the voltage.
Less than full voltage applied to the armature will decrease the motor speed. The difference between back EMF and applied voltage to the armature is smaller. The effect of the applied voltage to the armature become smaller and because of this less current will flow through it and the torque will decrease.
The load will slow the armature, fewer lines of force being cut, back EMF decreases, more current to flow in the armature and increasing the torque until the sped again stabilized.
2. Series motor – an extremely low resistance field winding of very few turns of heavy wire is connected in series with the armature. It has high starting torque and a variable speed characteristic.
❖ As the speed of the series motor increases, the back EMF also increases, but the current flow decreases. The current flows through the...