What effect does the inverter have on the ordinary asynchronous motor during variable frequency speed regulation?
The speed-regulating motor is designed for AC speed regulation in terms of its original intention. However, the most direct reason for the rise of the frequency conversion speed regulation is the simple structure of the ordinary asynchronous motor, low cost and convenient speed regulation. If the frequency conversion speed regulation must be equipped with a special motor for frequency conversion, then there is a contradiction. The inherent simplicity, sturdiness and durability of the frequency conversion speed regulation are not gone?
Influence on the motor and its performance during variable frequency speed control Variable frequency speed control The voltage pulse output to the motor end is non-sinusoidal regardless of the control method. Therefore, the analysis of the running characteristics of ordinary asynchronous motors under non-sinusoidal waves is the effect on the motor during variable frequency speed regulation.
There are mainly the following aspects:
Motor Loss and Efficiency Motors operating under non-sinusoidal power supplies, in addition to the normal losses due to the fundamental, will also introduce 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.
1. The stator current damage in the stator windings causes the harmonic current to increase I2R. When the skin effect is ignored, the stator copper loss at non-sinusoidal current is proportional to the square of the rms current. If the number of stator phases is m1 and the stator resistance of each phase is R1, the total stator copper loss P1 is substituted into the above equation for the total stator current rms Irms including the fundamental current. The second term in the equation is obtained. Harmonic loss. It is found through experiments that due to the existence of harmonic current and the corresponding leakage flux, the saturation of the magnetic flux of the leakage flux is increased, and the excitation current is increased, so that the fundamental component of the current is also increased.
2, the rotor copper loss in the harmonic frequency, generally can be considered as the stator winding resistance is constant, but for the asynchronous motor rotor, its AC resistance is greatly increased due to the skin effect. Especially the deep-groove cage rotor is particularly serious. A synchronous motor or a reluctance motor under a sine wave power supply has a small harmonic potential due to the stator space. The losses caused in the rotor surface windings are negligible. When the synchronous motor is running under a non-sinusoidal power supply. The time harmonic magnetic potential induces the rotor harmonic current, just like an asynchronous motor operating at its fundamental synchronous speed.
The 5th harmonic magnetic potential of the reverse rotation and the 7th harmonic magnetic potential of the forward rotation will induce a rotor current 6 times the fundamental frequency. When the fundamental frequency is 50Hz, the rotor current frequency is 300Hz. Similarly, The 11th and 13th harmonics induce 12 times the fundamental frequency, ie 600HZ of rotor current. At these frequencies, the actual AC resistance of the rotor is much greater than the DC resistance. How much the rotor resistance actually increases depends on the conductor cross section and the geometry of the rotor slots in which the conductors are arranged. A typical copper conductor with an aspect ratio of about 4, the ratio of the AC resistance to the DC resistance is 1.56 at 50 Hz, the ratio is about 2.6 at 300 Hz, and the ratio is about 3.7 at 600 Hz. When the frequency is higher, the ratio is frequency. The square root increases proportionally.
3. The core loss in the harmonic iron loss motor is also increased due to the occurrence of harmonics in the power supply voltage; the harmonics of the stator current establish a time harmonic magnetomotive force between the air gaps. The total magnetic potential at any point in the air gap is the synthesis of the fundamental and time harmonic magnetic potentials. For a three-phase six-step voltage waveform, the peak 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 small. The stray loss due to the leakage flux at the end and the flux leakage at the chute will increase under the harmonic frequency. This must be considered when non-sinusoidal power supply: the leakage effect at the end is in the stator and rotor windings. Both exist, mainly the eddy current loss caused by the leakage flux entering the end plate. Due to the change of the phase difference between the stator magnetic potential and the rotor magnetic potential, the chute leakage flux is generated in the chute structure, and the magnetic potential is the largest at the end, which causes loss in the stator core and the teeth.
