Harmonic Induction Torques and their Elimination

Harmonic induction torques are undesirable torques that occur in electric machines, particularly in induction motors. These torques result from the interaction between the stator and rotor magnetic fields, causing fluctuations in the torque output and potentially leading to mechanical vibrations and noise. Therefore, it is important to eliminate or minimize these torques to ensure smooth and efficient operation of the motor.

Options for Eliminating Harmonic Induction Torques

a) Integral Slot Winding: Integral slot winding is a technique used in the design of electric machines, where the number of stator slots is chosen such that the number of slots per pole per phase is an integer. This helps in reducing the occurrence of harmonic torques by ensuring that the stator winding is distributed evenly around the stator circumference. By having an integral slot winding, the magnetic field produced by the stator winding is more sinusoidal, reducing the occurrence of harmonics and hence minimizing the harmonic induction torques. However, integral slot winding alone may not completely eliminate these torques.

b) Skewing: Skewing is another technique employed to reduce harmonic torques in electric machines. In skewing, the rotor or stator core is skewed or twisted slightly with respect to the axis of rotation. This helps in breaking the symmetry of the magnetic field and reduces the interaction between the stator and rotor magnetic fields. Skewing effectively minimizes the occurrence of harmonic torques and contributes to the smooth operation of the motor. However, it may add complexity to the manufacturing process and increase the cost of the motor.

The Correct Option: Chording

c) Chording: Chording is the correct option for eliminating harmonic induction torques. Chording involves modifying the shape of the stator or rotor slots by introducing small indentations or cutouts. These modifications disrupt the symmetry of the magnetic field distribution, reducing the interaction between the stator and rotor magnetic fields. As a result, the occurrence of harmonic torques is minimized, leading to smoother and more efficient motor operation. Chording is a relatively simple and cost-effective technique compared to integral slot winding and skewing, making it an attractive option for eliminating harmonic induction torques.

d) None of these: This option is incorrect as chording is indeed an effective technique for eliminating harmonic induction torques.

In conclusion, chording is the simplest way to eliminate harmonic induction torques in electric machines. This technique involves introducing small modifications to the shape of the stator or rotor slots, disrupting the symmetry of the magnetic field and reducing the interaction between the stator and rotor magnetic fields. By implementing chording, the occurrence of harmonic torques is minimized, resulting in smoother and more efficient motor operation.