When the motor is regulated by a frequency converter, it will generate noise and vibration, which is caused by the high-order harmonic components in the output waveform of the frequency converter. As the motor's operating frequency changes, the fundamental component and high-order harmonic components change over a wide range, which may cause resonance with various parts of the motor.
When a frequency converter is used to drive an electric motor, the output voltage and current contain high-order harmonic components, and the high-order harmonic flux in the air gap increases, so the noise increases.
Characteristics of electromagnetic noise
The low-order harmonic components in the inverter output resonate with the rotor's natural mechanical frequency, and the noise near the rotor's natural frequency increases. The high-order harmonic components in the inverter output resonate with the core housing, bearing frame, etc., and the noise near the natural frequencies of these components increases. The harsh noise generated when the inverter drives the motor is related to the switching frequency of the PWM control, especially in the low-frequency area. The suppression method is to set an AC reactor on the inverter output side. If there is a margin for electromagnetic torque, U/F can be set smaller. When the noise is more serious in the lower frequency band when using special motors, check the resonance with the natural frequency of the shaft system (including load).
When the inverter is working, the magnetic field caused by the high-order harmonics in the output waveform generates electromagnetic vibration force on many mechanical parts. The frequency of the vibration force is always close to or overlaps with the natural frequency of these mechanical parts, resulting in resonance. The high-order harmonics that have a great impact on vibration are mainly low-order harmonic components, which have a greater impact in PAM mode and square wave PWM mode. However, when the sine wave PWM mode is used, the low-order harmonic components are small and the impact becomes smaller. The method to reduce or eliminate vibration is to set an AC reactor on the output side of the inverter to absorb the high-order harmonic current components in the inverter output current. When using a PAM mode or square wave PWM mode inverter, a sine wave PWM mode inverter can be used instead to reduce the pulsating torque.
The causes of motor vibration can be divided into two types: electromagnetic and mechanical:
1) The vibration caused by electromagnetic reasons is manifested as: the resonance of lower harmonic components and the rotor increases the vibration components near the natural frequency. Vibration occurs due to the influence of the pulsating torque generated by the harmonics, especially when the frequency of the pulsating torque is consistent with the natural frequency of the shaft system torsion formed by the motor rotor and the load, resonance will occur.
2) The vibration caused by mechanical reasons is manifested as: when there is an overhanging weight on the motor shaft and the natural frequency of the shaft system is reduced, if the motor runs at high speed, the full rotation frequency is close to the natural frequency of the shaft system, and the vibration is aggravated. The residual imbalance of the rotor causes the centrifugal force to increase in proportion to the square of the speed, so when the inverter drives the motor to run at high speed, the vibration increases.
Main methods of heat dissipation
The inverter is an electronic device, so temperature has a great impact on its life. The ambient temperature of general inverters is generally required to be -10~+50℃. If the operating temperature of the inverter can be reduced, the service life of the inverter will be extended and the performance will be stable. The heating of the inverter is caused by internal losses, mainly in the main circuit, which accounts for about 98% of the total loss, and the control circuit accounts for 2%. In order to ensure the normal and reliable operation of the inverter, the inverter must be cooled.
The main methods are:
1) Use fan cooling: The built-in fan of the inverter can take away the heat inside the inverter cabinet.
2) Use a separate inverter room with an air conditioner installed inside to keep the temperature between +15~+20℃.
3) For machines with low power, the inverter's own metal heat sink is also used for natural heat dissipation.
The inverter heating described above refers to the loss of the inverter in normal operation within the rated range. When the inverter is abnormally operated (such as overcurrent, overvoltage, overload, etc.), the loss must be avoided through normal selection.
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