Analysis of the impact on the motor during variable frequency speed regulation

The speed regulating motor is a kind of motor that changes the speed of the motor by changing the number of stages, voltage, current and frequency of the motor to make the motor reach higher performance.

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?

The effect on the motor and its performance during variable frequency speed control Variable frequency speed regulation 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:

Loss and efficiency of the motor The motor operating under a non-sinusoidal power supply, in addition to the normal losses caused by the fundamental wave, 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.

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 magnetic saturation 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.

Both the 5th harmonic magnetic potential of the reverse rotation and the 7th harmonic magnetic potential of the forward rotation will induce a rotor current of 6 times the fundamental frequency, and the rotor current frequency is 300 Hz at a fundamental frequency of 50 Hz. 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 having an aspect ratio of about 4 has an AC resistance to DC resistance ratio of 1.56 at 50 Hz, a ratio of about 2.6 at 300 Hz, and a ratio of 3.7 at 600 Hz. At higher frequencies, this ratio increases in proportion to the square root of the frequency.

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 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.

4, motor efficiency Harmonic loss is significantly determined by the harmonic content of the applied voltage. The harmonic component is large, the motor loss is increased, and the efficiency is lowered. However, most static inverters do not produce harmonics below 5, while the magnitude of higher harmonics is smaller. The voltage of this waveform is not critical to the efficiency of the motor. Calculations and comparison tests on medium-capacity asynchronous motors have shown that their full-load effective current increases by approximately 4% from the fundamental value. If the skin effect is ignored, the copper loss of the motor is proportional to the square of the total effective current, and the harmonic copper loss is 8% of the fundamental loss. Considering that the rotor resistance can be increased by an average of three times due to the skin effect, the harmonic copper loss of the motor should be 24% of the fundamental loss. If the copper loss accounts for 50% of the total motor loss, the harmonic copper loss increases the loss of the entire motor by 12%. The increase in iron loss is difficult to calculate because it is affected by the structure of the motor and the magnetic material used.

If the higher harmonic components in the stator voltage waveform are relatively low, as in the 6-step wave, the harmonic iron loss increase does not exceed 10%. If the iron loss and stray loss account for 40% of the total motor loss, the harmonic loss accounts for only 4% of the total motor loss. Friction loss and windage loss are unaffected, so the total loss of the motor increases by less than 20%. If the efficiency of the motor is 90% at 50Hz sinusoidal power supply, the motor efficiency is only reduced by 1% to 2% due to the presence of harmonics. If the harmonic component of the applied voltage waveform is significantly larger than the harmonic component of the 6-step wave, the harmonic loss of the motor will increase greatly and may be greater than the fundamental loss.

In the case of a 6-step wave power supply, a low-leakage reluctance motor can absorb a large harmonic current, thereby reducing the efficiency of the motor by 5% or more. In this case, in order to operate satisfactorily, a 12-step wave inverter or a six-phase stator winding is used. The harmonic current and harmonic losses of the motor are virtually independent of the load, so the loss of time harmonics can actually be determined by comparing the sinusoidal supply with the non-sinusoidal supply under no-load conditions. This is used to determine the approximate range of motor efficiency degradation for a certain type or structure.