The effect of frequency conversion on the three-phase asynchronous motor is the analysis of the operating characteristics of ordinary asynchronous motors under non-sinusoidal waves, because the voltage pulses output to the motor end are non-sinusoidal regardless of the control method used by variable frequency speed regulation. The editor of Dalan Motor gives you a specific analysis:
In addition to the normal loss generated by the fundamental wave, the motor running under a non-sinusoidal power supply will also have many additional losses, mainly manifested in the increase of stator copper loss, rotor copper loss and iron loss, which affects the efficiency of the motor.
If the higher harmonic components in the stator voltage waveform are relatively low, if iron losses and stray losses account for 40% of the total motor losses, then harmonic losses only account for 4% of the total motor losses. Friction loss and wind resistance loss are not affected, so the total loss increase of the motor is less than 20%.
If the efficiency of the motor in a 50Hz sinusoidal power supply is 90%, the efficiency of the motor is only reduced by 1% to 2% due to the presence of harmonics. If the harmonic component of the applied voltage waveform is significantly greater than the harmonic component of the 6-step wave, the harmonic loss of the motor will greatly increase, and may be greater than the fundamental loss.
In the case of a 6-step wave power supply, a low-leakage reluctance motor may absorb a large harmonic current, thereby reducing the efficiency of the motor by 5% or more.
The harmonic current of stator copper loss in stator winding makes I2R and increase.
When ignoring the skin effect, the stator copper loss under non-sinusoidal current is proportional to the square of the effective value of the total current. If the number of stator phases is m1 and the stator resistance of each phase is R1, then the total stator copper loss P1 is the effective value of the total stator current including fundamental wave current Irms substituted into the above formula. .
For a three-phase 6-step voltage waveform, the peak value of the magnetic density in the air gap is about 10% larger than the fundamental value, but the increase in iron loss caused by the time harmonic flux is very small.
The copper loss of the rotor is generally considered to be a constant resistance of the stator winding at the harmonic frequency, but for the rotor of the asynchronous motor, its AC resistance is greatly increased due to the skin effect.
In particular, the deep-groove cage rotor is particularly serious. Synchronous motor or reluctance motor under sine wave power supply, due to the stator space harmonic magnetic potential is very small. The losses caused in the rotor surface windings are negligible.
----- Responsible Editor: Dalan Oil Pump Motor 02-Procurement Consultant
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