How Do Higher Harmonics Affect the Performance of Asynchronous Motors?

In the previous tweets we talked about the operating characteristics of starting asynchronous motors, also known as induction motors. This involves the electromagnetic torque of the motor, which is generated by the fundamental magnetic field in the air gap and the rotor current induced by it. For any motor, in addition to the fundamental magnetic field in the air gap, there are also different degrees of high harmonic magnetic field, under the action of the high harmonic magnetic field, there is a corresponding harmonic torque is also called additional torque. The additional torque will interfere with the effect of electromagnetic torque to different degrees, which may cause the motor to have difficulty in starting or even not be able to start at all.

Due to the objectivity of the motor stator and rotor core teeth, the existence of the core slot, the winding distribution for the non-sinusoidal wave, as well as the degree of saturation of the motor magnetic circuit of the unevenness of the motor, resulting in the inevitability of the existence of high harmonics in the operation of the motor.

There are two kinds of torque generated by high harmonics, namely, asynchronous additive torque and synchronous additive torque, and both kinds of additive torque have adverse effects on the starting performance of the motor. Generally, the asynchronous additional torque will cause the motor to creep at a certain speed and fail to reach the rated speed; the synchronous additional torque, if it occurs at the moment of starting, may lead to the dead point of motor starting, even if the motor is started under no-load condition. Regarding this problem, it is found from time to time in the test state of the motor: for the same batch of the same specifications of the motor, despite the application of the same motor, from time to time, there are always a few motors that can not be started, and in this case, the usual practice is to rotate the rotor of the motor at an angle can be easily started.

Through theoretical analysis, we can find that the torque curve is superimposed with the torque generated by the high harmonics, which will cause a concave point on the curve, thus deteriorating the starting performance of the motor. How to reduce the influence of high harmonics and improve the motor starting performance? The more commonly used and very effective measures are: adjusting the motor stator and rotor slot with; increase the air gap of the motor stator; stator or rotor slant slot; using short pitch and integer slot winding.

The purpose of adjusting the slot fit of the motor stator and rotor is to reduce and eliminate the synchronous additive torque; increasing the stator-rotor air gap is to attenuate the higher harmonics through the occupancy relationship between the values of the operating wave and the harmonic permeability.

According to the actual production process, the use of slanting slots on the rotor can effectively control the value of synchronous additional torque. However, the power factor and maximum torque performance of the motor deteriorate to different degrees when the slant slot is used.