Harmonic effects on Induction Motors

Harmonics is currently the most difficult topic to understood, but is already causing lots of Electrical equipment failures in your plant.

If in your plant number of motors burning are more, contact undersigned at [email protected], we will help you out with whether you should go for harmonics analysis or not and if yes than we can do the harmonic study for you.

Below are their effects on induction motor.

Harmonics distortion raises the losses in AC induction motors in a similar way as in transformers and cause increased heating, due to additional copper losses and iron losses (eddy current and hysteresis losses) in the stator winding, rotor circuit and rotor laminations. These losses are further compounded by skin effect, especially at frequencies above 300 Hz. Leakage magnetic fields caused by harmonic currents in the stator and rotor end winding's produce additional stray frequency eddy current dependent losses. Substantial iron losses can also be produced in induction motors with skewed rotors due to high-frequency-induced currents and rapid flux changes (i.e., due to hysteresis) in the stator and rotor.

Excessive heating can degrade the bearing lubrication and result in bearing collapse. Harmonic currents also can result in bearing currents, which can be however prevented by the use of an insulated bearing, a very common practice used in AC variable frequency drive-fed AC motors. Overheating imposes significant limits on the effective life of an induction motor. For every 10°C rise in temperature above rated temperature, the life of motor insulation may be reduced by as much as 50%. Squirrel cage rotors can normally withstand higher temperature levels compared to wound rotors. The motor windings, especially if insulation is class B or below, are also susceptible to damage due high levels of dv/dt (i.e., rate of rise of voltage) such as those attributed to line notching and associated ringing due to the flow of harmonic currents.

Harmonic sequence components also adversely affect induction motors. Positive sequence components (i.e., 7th, 13th, 19th ...) will assist torque production, whereas the negative sequence components (5th, 11th, 17th ...) will act against the direction of rotation resulting in torque pulsations. Zero sequence components (i.e., triplen harmonics) are stationary and do not rotate, therefore, any harmonic energy associated with them is dissipated as heat. The magnitude of torque pulsations generated due to these harmonic sequence components can be significant and cause shaft torsional vibration problems