Effect of Harmonics on Performance of 3-phase Induction Motor

Effect of Harmonics on Performance of 3-phase Induction Motor

The induction motor performance is affected by the harmonics in the time variation of the impressed voltage. But its effect on the performance of the motor is not predominant hence it is not considered here.

The torque-slip characteristics as shown in Fig is obtained when the space distribution of flux wave along the air gap periphery is sinusoidal. But the air gap flux is not purely sinusoidal as it contains odd harmonics (5^th, 7^th, 11^th etc). Hence at low speeds, the torque-slip characteristic is not smooth. The distribution of stator winding and variation of air gap reluctance due to stator and rotor slots are main causes of air gap flux harmonics.

       The harmonics caused due to variation of air gap reluctance are called tooth or slot harmonics . Due to these harmonics produced in air gap flux, unwanted torque are developed along with vibration and noise.

Each space harmonic can be considered to produces its own asynchronous torque. The induction motor can be considered as equivalent to number of induction motors in series having poles equal to number of harmonics multiplied by number of poles. The torque produced by fundamental component and the harmonic are shown in the Fig.

                     Fig. 2   Presence of harmonics

1.  Crawling

Sometimes, squirrel cage induction motors exhibits a tendency to run at very slow speeds (as low as one-seventh of their synchronous speed). This phenomenon is called as crawling of an induction motor.

This action is due to the fact that, flux wave produced by a stator winding is not purely sine wave. Instead, it is a complex wave consisting a fundamental wave and odd harmonics like 3rd, 5th, 7th etc. The fundamental wave revolves synchronously at synchronous speed Ns whereas 3rd, 5th, 7th harmonics may rotate in forward or backward direction at Ns/3, Ns/5, Ns/7 speeds respectively. Hence, harmonic torques are also developed in addition with fundamental torque.

Due to crawling there is much higher stator current accompanied by noise and vibration.

The torque obtained from induction motor here is called synchronous torque.

       When two harmonic fluxes of same order one because of stator and the rotor because of rotor interact with each other at one particular speed and produces harmonic synchronous torque just like that produced in synchronous motor. These torques are caused by tooth harmonics. The stable operation at synchronous speed caused by slot harmonics is called synchronous crawling which is associated with vibration and noise.

 

2. Cogging (Magnetic Locking Or Teeth Locking)

Sometimes, the rotor of a squirrel cage induction motor refuses to start at all, particularly if the supply voltage is low. This happens especially when number of rotor teeth is equal to number of stator teeth, because of magnetic locking between the stator teeth and the rotor teeth. When the rotor teeth and stator teeth face each other, the reluctance of the magnetic path is minimum, that is why the rotor tends to remain fixed. This phenomenon is called cogging or magnetic locking of induction motor.

Conclusion

The cogging and crawling is not predominately in slip ring induction motor as these motors are started with higher starting torques with external resistance in rotor circuit.

The crawling effect can be reduced by taking proper care during the design. Still if crawling is observed then it can be overcome by applying a sudden external torque to the driven load in the direction of rotor. If there is reduction of in supply voltage then torque also decreases (Tα V₁²). Hence asynchronous crawling may observed which is absent under rated voltage conditions. Thus asynchronous torques can not be avoided but can be reduced by proper choice of coil span and by skewing the stator or rotor slots.

Key Point : The synchronous harmonics torques can be totally eliminated by proper combination of stator and rotor slots.