EFFECTS OF HARMONICS ON ROTATING MACHINES

Harmonics has effect on Rotating Machines in various ways, e.g.:

1. Harmonic losses: Harmonic voltages or currents increase losses in the stator windings, rotor circuit, and stator and rotor lamination. Normally the losses in the stator and rotor conductors of A.C. machines are greater than those associated with the D.C. resistances due to the presence of eddy current and skin effect in ac circuit. Harmonics increase both copper loss and core loss. This results in overheating and reduction of the efficiency of a machine.

2. Harmonic torque: Harmonic currents present in the stator of an AC. machine produce induction motoring action (i.e. positive harmonic slips Sn), which gives rise to shaft torques in the same direction as the harmonic field velocities in such a way that all positive sequence harmonic will develop shaft torques aiding shaft rotation whereas negative sequence harmonics will have the opposite effect.

3. Speed torque characteristics: Each harmonic component contributes to the magnetic force. The presence of harmonics has effect on speed/torque characteristics.

4. Cogging: Cogging is the failure of an induction motor to run up to normal speed due to a stable operating point occurring at a lower frequency.

5. High capacitive current: The presence of harmonics increases capacitive current through the stray capacitance in ASD-fed electric motors which is one of the reasons for their failure.

6. Voltage stress on insulation: Harmonic voltages increase the stress on insulating materials.