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Generator Operation | Concept of Electric Generator Working and Expression | Electric Machine

Generator Working and Expression :

 Operation of Machine as a Generator (Conversion of Mechanical Energy into Electric Energy)

A coil is placed in a constant stationary magnetic field. Let it be rotated in the clockwise direction at an angular velocity of Z radians per second by some outside driving mechanical torque Tm. The coil sides cut the magnetic field (or the flux linking with coil changes) and emf (e) is induced in the coil. The direction of induced emf can be determined by applying Fleming’s right-hand rule or Lenz’s Law and is marked in Fig. 1 (a). The coil is connected to an external load resistor R, therefore current (i) flows through the coil and external load resistor. The direction of flow of current in the coil is marked in Fig. 1 (a) and (b). When current flows through the coil conductors, they produce their own magnetic field. The direction of this rotor field is marked by arrowhead Fr . The rotor field Fr tries to come in line with the main field Fm and an electromagnetic torque Te is produced in the opposite direction to that of the rotation.


Figure :1 (a) Coil Rotated in a magnetic field 

If the coil circuit is not closed, no current would flow through the coil and hence no electromagnetic torque will be developed (i.e., Te = 0), under such a condition the opposition is only due to frictional torque (neglecting iron losses). Therefore, the mechanical torque Tm applied must be sufficient to overcome the frictional torque. It may be noted that frictional torque always acts in opposite direction to the direction of rotation.

When the load resistance is connected, current flows and electromagnetic torque is produced in opposite direction to that of mechanical torque. Under this condition, the mechanical torque Tm must be sufficient to overcome the electromagnetic torque and frictional torque (iron losses neglected).                                                                                                Figure: 1(b)

where,
 w Tm = Mechanical input power.
w Tf = Power losses due to friction.
w Te = Mechanical power developed in the rotor which is converted into electrical power.
When conductors move perpendicular to the magnetic field

Induced emf in the coil, e = 2 B l v                                                                                                                                                                                                                                                                                                     
or                                   e i = 2 B i l v = 2 F v                                              [∵ F = B i l]
                                            

Thus, we conclude that out of the input power (Z Tm) only ZTe is the mechanical power which is converted into electrical power (e i). After subtracting the copper losses (i2r), the electrical power available at the load is only i2R. This is how the conversion of power takes place in an electrical machine working as a generator. The power flow diagram (neglecting iron losses) for the generator 






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