The impact of variable frequency speed regulation on the motor

The impact of variable frequency speed regulation on the motor is the analysis of the operating characteristics of ordinary asynchronous motors under non-sinusoidal waves, because no matter what control method is used for variable frequency speed regulation, the voltage pulse output to the motor end is non-sinusoidal. It is specifically manifested in the following aspects:

In addition to the normal losses caused by the fundamental wave, motors running under non-sinusoidal power will also have many additional losses, mainly manifested in the increase of stator copper loss, rotor copper loss and iron loss, which will affect the efficiency of the motor.

If the high-order harmonic components in the stator voltage waveform are relatively low, as in the 6-step waveform, the harmonic iron loss will not increase by more than 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 not affected, so the total loss of the motor increases by less than 20%.

If the motor efficiency is 90% when the power supply is 50Hz sinusoidal, the motor efficiency will only decrease 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 increase significantly and may be greater than the fundamental loss.

That is, in the case of a 6-step wave power supply, a low leakage reactance reluctance motor may absorb a large harmonic current, thereby reducing the motor efficiency by 5% or more.

In this case, in order to operate satisfactorily, a 12-step wave inverter or a six-phase stator winding should be used. The harmonic current and harmonic loss of the motor are actually independent of the load, so the loss of time harmonics can actually be determined by comparing sinusoidal power supply and non-sinusoidal power supply under no-load conditions. This can be used to determine the approximate range of motor efficiency reduction of a certain type or structure.

The magnitude of the harmonic loss of the motor efficiency is obviously determined by the harmonic content of the applied voltage. The larger the harmonic content, the greater the motor loss and the lower the efficiency. However, most static inverters do not produce harmonics below the 5th order, and the amplitude of higher harmonics is smaller.

This waveform voltage does not seriously reduce the motor efficiency. Calculation and comparative tests on medium-capacity asynchronous motors show that their full-load effective current is about 4% higher than the fundamental value. If the skin effect is ignored, the motor copper loss 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 3 times on average 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 motor structure and the magnetic materials used.

The stator copper loss causes harmonic currents in the stator windings, which increases I2R.

When the skin effect is ignored, the stator copper loss under non-sinusoidal current is proportional to the square of the total current effective value. If the number of stator phases is m1, the stator resistance of each phase is and R1, then the total stator copper loss P1 is Substituting the total stator current effective value Irms including the fundamental current into the above formula, the second term in the formula represents the harmonic loss.

It is found through experiments that the existence of harmonic current and the corresponding leakage flux increase the magnetic circuit saturation of the leakage flux, thereby increasing the excitation current and thus increasing the fundamental component of the current.

The core loss in the harmonic core loss motor is also increased due to the presence of harmonics in the power supply voltage; the harmonics of the stator current establish a time harmonic magnetomotive force in the air gap. 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 value of the magnetic flux 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 stray losses caused by end leakage flux and skewed slot leakage flux will increase under the action of harmonic frequency, which must be considered when non-sinusoidal power supply: the end leakage flux effect exists in both stator and rotor windings, mainly eddy current loss caused by leakage flux entering the end plate. Due to the change in the phase difference between the stator magnetic potential and the rotor magnetic potential, skewed slot leakage flux is generated in the skewed slot structure, and its magnetic potential is the largest at the end, causing losses in the stator and rotor cores and teeth.

At the harmonic frequency, the resistance of the stator winding can generally be considered to be a constant. However, for the rotor of an asynchronous motor, its AC resistance is greatly increased due to the skin effect.

This is especially serious for cage rotors with deep slots. For synchronous motors or reluctance motors under sinusoidal power supply, the loss caused in the rotor surface winding can be ignored because the stator space harmonic magnetic potential is very small.

When a synchronous motor is operated on a non-sinusoidal supply, the time harmonic magnetic potentials induce rotor harmonic currents, just like an asynchronous motor running close to its fundamental synchronous speed.

The reverse rotating 5th harmonic magnetic potential and the forward rotating 7th harmonic magnetic potential will both induce a rotor current 6 times the fundamental frequency. When the fundamental frequency is 50Hz, the rotor current frequency is 300Hz.

Likewise, the 11th and 13th harmonics induce rotor currents at 12 times the fundamental frequency, 600 Hz. 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.

For a copper conductor with a typical length-to-width ratio of about 4, the ratio of AC resistance to DC resistance is 1.56 at 50 Hz, about 2.6 at 300 Hz, and about 3.7 at 600 Hz. At higher frequencies, this ratio increases in proportion to the square root of the frequency.