There's a word called mystery, and the mystery stems from a lack of understanding. Once you understand something, it loses its mystery. Today, Ms. Can will share some insights about load loss with you. If you have some knowledge of motors, you probably won't find them so mysterious.
The additional losses of asynchronous motors under load are usually not calculated in detail. Standards in many countries generally stipulate that the additional losses under load account for 0.5 % of the motor's output (generator) or input (motor) power. Of course, this figure is very rough. In small asynchronous motors using pressure casting aluminum technology, the additional losses under load generally account for about 2-3% of the output power, and in some cases even as high as 4-5% or more. This not only seriously affects the motor's operating economy and starting performance, but may also cause excessively high coil temperature rise. Therefore, for many years, how to accurately calculate and reduce the additional losses of squirrel-cage asynchronous motors under load has been a focus of attention.
Additional load losses of squirrel-cage rotor asynchronous motor
The additional losses of a squirrel-cage induction motor under load mainly consist of the following components:
●Additional losses caused by the leakage magnetic field of the stator winding in the metal components within the winding and near the winding ends.
●Additional losses caused by the magnetic field generated by the stator magnetomotive force harmonics inducing current in the squirrel-cage rotor windings.
● Surface losses caused by the magnetic field generated by stator magnetomotive force harmonics on the rotor core surface. Due to the demagnetizing effect of the induced current in the squirrel-cage rotor windings, only a small amount of harmonic magnetic field can penetrate deep into the rotor teeth, so the pulsation losses generated by these harmonics in the teeth can be ignored. The additional losses generated by rotor magnetomotive force harmonics in the stator core are relatively small and can usually be ignored as well.
●Losses caused by leakage current in cast aluminum rotors without slot insulation.
In the above description, the additional losses generated by the leakage magnetic field of the stator winding in the winding and in the metal components near the winding ends are produced by the fundamental frequency current, and are therefore also called fundamental frequency additional (stray) losses. All other losses are generated by high-frequency currents, and are therefore also called high-frequency additional (stray) losses.
Loss analysis under skewed groove conditions
In the case of skewed slots, if the conductor bars are well insulated, the losses generated by the stator phase band harmonic magnetomotive force in the squirrel cage winding can still be approximately calculated according to equation (1), but it must be multiplied by K, where K is the skew coefficient of the squirrel cage rotor winding for the vth harmonic. Assuming the rotor slot is skewed by one stator tooth pitch, the combined potential induced by the stator magnetomotive force tooth harmonic over the entire conductor bar length is close to zero.
p2v=(4m12W12Kdpv2K2v4R2vI12)/Z2……………………(1)
In formula (1):
I1—Stator phase current;
R2v — AC resistance of the rotor bars (corresponding to the relevant harmonic frequency);
Kdpv—Stator winding coefficient for the v-th harmonic;
K2v — the winding coefficient of the hypothetical rotor winding for the v-th harmonic.
However, in the case of skewed slots, if there is no good insulation between the conductor bars and the core, the potential induced by the stator magnetomotive force harmonics in the rotor squirrel-cage windings will form a "lateral" current between adjacent conductor bars through the silicon steel sheets of the core. The "lateral" current formed between adjacent "half" conductor bars will generate additional losses. The magnitude of this loss is related to the frequency and magnetic flux density of the harmonic magnetic field, but is mainly determined by the contact resistance Rc between the conductor bar and the core. However, the processing factors affecting the contact resistance Rc are difficult to control, making it difficult to determine a general formula for calculating Rc. As a result, there is currently no mature and simple method to calculate this loss.
Measures to reduce additional losses when an asynchronous motor is under load
Although the additional losses under load account for a small portion of the input power of each asynchronous motor, the total electrical energy consumed by these losses is still considerable due to the wide range and large number of squirrel-cage asynchronous motors used. This is especially significant for the high-efficiency motors that are currently being promoted. In recent years, research on how to reduce these losses has been conducted both domestically and internationally.
For small and medium-sized asynchronous motors, high-frequency losses account for a larger proportion of the additional losses under load, while fundamental frequency losses generally account for a smaller proportion. The following measures can be taken to reduce high-frequency additional losses.
● Adopt various stator winding types with lower harmonic content. For example, double-layer short-pitch distributed windings can generally be used; in small asynchronous motors, single-double layer windings may be used instead of single-layer windings; Δ-Y hybrid connection windings are used (these windings have lower phase harmonic content).
● Adopting a near-groove fit.
● Adopt inclined slots, and at the same time pay attention to improving the rotor aluminum casting process or adopting other processes (such as replacing pressure casting with low-pressure casting, but the former has a lower productivity) to increase the contact resistance between the conductor bar and the core.
Additional losses under DC motor load
The additional losses of DC motors under load are generally small and are usually not calculated in detail. For motors without compensation windings, it is generally taken as 1% of the output (generator) or input (motor) power; for motors with compensation windings, it is generally 0.5 % respectively.
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