The effect of variable frequency speed regulation on the motor is the analysis of the operating characteristics of ordinary asynchronous motor under non-sinusoidal wave, no matter what control method is adopted, the voltage pulse output to the motor terminal is non-sinusoidal. This is manifested in the following ways:

In addition to the normal loss caused by fundamental wave, there are many additional losses in the non-sinusoidal motor, mainly in the increase of stator copper loss, rotor copper loss and iron loss, which affect the efficiency of the motor.

If the higher harmonic component of the stator voltage waveform is relatively low, the harmonic iron loss will not increase by more than 10% , as in the 6-step wave. If the iron loss and stray loss account for 40% of the total loss, the harmonic loss accounts for only 4% of the total loss. Friction loss and wind resistance loss are not affected, so the total loss of the motor increased by less than 20% .

If the motor at 50Hz sinusoidal power efficiency of 90% , because of the existence of harmonic motor efficiency only reduce 1% -2% . If the harmonic component of the applied voltage waveform is obviously larger than that of the 6-step waveform, the harmonic loss of the motor will increase greatly and may be larger than the fundamental loss.

Even on a 6-step power supply, a low-leakage reactance reluctance motor may absorb a large harmonic current, reducing the motor's efficiency by 5% or more.

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

The harmonic loss of motor efficiency in Xiaoming depends on the harmonic content of the applied voltage. The harmonic component is big, the motor loss increases, the efficiency reduces. However, most static inverters do not produce harmonics of less than 5 orders, and the amplitude of higher order harmonics is smaller.

This waveform of voltage on the motor efficiency is not serious. The calculation and comparison tests of medium capacity induction motor show that its full-load effective current is about 4% higher than the fundamental current. If 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 3 times on average due to 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 loss of the motor, the harmonic copper loss increases the loss of the whole motor by 12% . The increase in iron loss is difficult to calculate because it is influenced by the structure of the motor and the magnetic material used.

The harmonic current in stator winding caused by stator copper loss increases I2R.

When the skin effect is ignored, the stator copper loss under non-sinusoidal current is proportional to the square of the effective value of the total current. If the stator phase number is M 1 and the stator resistance per phase is R 1, then the total stator copper loss p 1 is the total stator current effective value Irms including the fundamental

Through the experiment, it is found that the saturation of magnetic circuit increases due to the existence of harmonic current and corresponding leakage flux, so the excitation current increases, and the fundamental component of current also increases.

The core loss of the harmonic iron loss motor is also increased by the occurrence of harmonics in the supply voltage, and the harmonics of the stator current establish time harmonic emfs in the air gap. The total magnetic potential at any point in the air gap is a combination of fundamental and time harmonic magnetic potentials.

For a three-phase 6-step voltage waveform, the peak magnetic density in the air gap is about 10% larger than the fundamental value, but the increase of iron loss caused by time harmonic flux is very small.

The stray loss caused by end leakage flux and slotted leakage flux will increase under the action of harmonic frequency, which must be considered in non-sinusoidal power supply: end leakage flux effect exists in both stator and rotor windings, the eddy current loss is mainly caused by the leakage flux entering the end plate. Because of the phase difference between the stator and rotor magnetic potential, the slotted leakage flux is produced in the slotted structure.

Under harmonic frequency, the resistance of stator windings is considered to be constant, but for the rotor of induction motor, its AC resistance is greatly increased because of skin effect.

In particular, the deep groove of the cage rotor is particularly serious. Synchronous motor or reluctance motor under sinusoidal power supply, because the stator space harmonic magnetic potential is very small. The losses caused in the surface winding of the rotor are negligible.

When the synchronous motor is running under non-sinusoidal power supply. The time-harmonic magnetic potential induces a harmonic current in the rotor, just as it does in an asynchronous motor operating near its fundamental synchronous speed.

Both the 5th harmonic magnetic potential of reverse rotation and the 7th harmonic magnetic potential of forward rotation will induce a rotor current of 6 times the fundamental frequency, which is 300Hz when the fundamental frequency is 50Hz.

Similarly, the 11th and 13th harmonics induced a rotor current of 12 times the fundamental frequency, 600HZ. 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 slot in which the conductors are arranged.

A typical copper conductor with a length-to-width ratio of about 4 has an ac-to-dc resistance ratio of 1.56 at 50 Hz, a ratio of about 2.6 at 300 Hz, and about 3.7 at 600 Hz. At higher frequencies, the ratio increases proportionally with the square root of the frequency.

Source: internet (for study only, intrusion and deletion)