The main difference between the motor powered by frequency conversion power supply and the motor powered by power frequency sine wave is that on the one hand, it operates in a wide frequency range from low frequency to high frequency, and on the other hand, the power waveform is non-sinusoidal. Through Fourier series analysis of the voltage waveform, the power supply waveform contains more than 2N harmonics in addition to the fundamental wave component (control wave) (the number of modulation waves contained in each half of the control wave is N). When the SPWM AC converter outputs power and applies it to the motor, the current waveform on the motor will appear as a sine wave with superimposed harmonics. The harmonic current will generate a pulsating magnetic flux component in the magnetic circuit of the asynchronous motor, and the pulsating magnetic flux component is superimposed on the main magnetic flux, so that the main magnetic flux contains a pulsating magnetic flux component. The pulsating magnetic flux component also makes the magnetic circuit tend to be saturated, which has the following effects on the operation of the motor:
1.Pulsating magnetic flux is generated
Losses increase and efficiency decreases. Because the output of the variable frequency power supply contains a large number of high order harmonics, these harmonics will produce the corresponding copper and iron consumption, reducing the operating efficiency. Even the SPWM sinusoidal pulse width technology, which is widely used at present, only inhibits the low harmonics and reduces the motor’s pulsating torque, thus extending the stable operation range of the motor at low speed. And the higher harmonics not only did not decrease, but increased. In general, compared with the power frequency sine power supply, the efficiency is reduced by 1% to 3%, and the power factor is reduced by 4% to 10%, so the harmonic loss of the motor under the frequency conversion power supply is a big problem.
b) Generate electromagnetic vibration and noise. Due to the existence of a series of high order harmonics, electromagnetic vibration and noise will also be generated. How to reduce vibration and noise is already a problem for sine wave powered motors. For the motor powered by the inverter, the problem becomes more complicated due to the non-sinusoidal nature of the power supply.
c) Low frequency pulsating torque occurs at low speed. Harmonic magnetomotive force and rotor harmonic current synthesis, resulting in constant harmonic electromagnetic torque and alternating harmonic electromagnetic torque, alternating harmonic electromagnetic torque will make the motor pulsation, thus affecting the low speed stable operation. Even if the SPWM modulation mode is used, compared with the power frequency sine power supply, there will still be a certain degree of low-order harmonics, which will produce pulsating torque at low speed and affect the stable operation of the motor at low speed.
2.Generate impulse voltage and axial voltage (current) to insulation
a) Surge voltage occurs. When the motor is running, the applied voltage is often superimposed with the surge voltage generated when the components in the frequency conversion device are commutated, and sometimes the surge voltage is high, resulting in repeated electrical shock to the coil and damage to the insulation.
b) Generate axial voltage and axial current. The generation of shaft voltage is mainly due to the existence of magnetic circuit imbalance and electrostatic induction phenomenon, which is not serious in ordinary motors, but it is more prominent in motors powered by variable frequency power supply. If the shaft voltage is too high, the lubrication state of the oil film between the shaft and the bearing will be damaged, and the service life of the bearing will be shortened.
c) Heat dissipation affects the heat dissipation effect when running at low speed. Due to the large speed regulation range of variable frequency motor, it often runs at low speed at low frequency. At this time, because the speed is very low, the cooling air provided by the self-fan cooling method used by the ordinary motor is insufficient, and the heat dissipation effect is reduced, and independent fan cooling must be used.
Mechanical influence is prone to resonance, in general, any mechanical device will produce resonance phenomenon. However, the motor running at constant power frequency and speed should avoid resonance with the mechanical natural frequency of the electrical frequency response of 50Hz. When the motor is operated with frequency conversion, the operating frequency has a wide range, and each component has its own natural frequency, which is easy to make it resonate at a certain frequency.
Post time: Feb-25-2025