Induction Motors: What Impact Do High-Order Harmonics Have On Their Performance?

In previous tweets, we discussed the starting and operating characteristics of induction motors, also known as asynchronous motors. This involves the electromagnetic torque of the motor, which is generated by the interaction between 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, there are varying degrees of high-order harmonic magnetic fields in the air gap. Under the action of high-order harmonic magnetic fields, corresponding harmonic torques, also called additional torques, are produced. These additional torques will interfere with the effect of the electromagnetic torque to varying degrees, and in severe cases, lead to difficulty in starting the motor or even failure to start at all.

Due to the objective existence of teeth and slots in the stator and rotor cores of the motor, the non-sinusoidal distribution of windings, and the uneven saturation of the motor magnetic circuit, high-order harmonics are inevitable during motor operation.

There are two types of torques generated by high-order harmonics: asynchronous additional torque and synchronous additional torque. Both have adverse effects on the starting performance of the motor. Generally speaking, asynchronous additional torque will cause the motor to crawl at a certain speed and fail to reach the rated speed; if synchronous additional torque occurs at the moment of starting, it may lead to a dead point in motor starting, even when the motor is started under no-load conditions. Regarding this problem, it is occasionally found during motor testing: for motors of the same specification in the same batch, even if the same voltage is applied, individual motors sometimes fail to start. In this case, the common practice is to rotate the motor rotor by an angle to start it easily.

Through theoretical analysis, we can find that because the torque generated by high-order harmonics is superimposed on the torque curve, concave points will appear on the curve, thereby deteriorating the starting performance of the motor. How to reduce the impact of high-order harmonics and improve motor starting performance? Common and very effective measures include: adjusting the slot fit between the motor stator and rotor; increasing the air gap of the motor stator; skewing the stator or rotor slots; adopting short-pitch and integer-slot windings, etc.

The purpose of adjusting the slot fit between the motor stator and rotor is to reduce and eliminate synchronous additional torque; increasing the stator-rotor air gap weakens high-order harmonics through the proportional relationship between the working wave and harmonic magnetic conductance values.

According to the actual production and processing technology, skewed slots are more commonly used on the rotor, which can effectively control the value of synchronous additional torque. However, when skewed slots are adopted, the power factor and maximum torque performance of the motor will deteriorate to varying degrees.

In addition to the impact on motor starting performance, another adverse effect of high-order harmonics is the high-frequency electromagnetic noise of the motor. We have discussed high-frequency electromagnetic noise quite a lot in previous articles, and the measures to deal with such problems are similar.

For the slot fit between the stator and rotor of an induction motor, it must be based on a large number of design and test results. Many design documents provide some empirical fit relationships for selection, and of course, there are also some taboo fit relationships. We will set up a separate section to communicate the relevant detailed content with you.