The influence of the frequency converter on the motor is mainly in the efficiency and temperature rise of the motor.
The inverter can generate different levels of harmonic voltage and current during operation, so that the motor runs under non-sinusoidal voltage and current. , The most significant is the rotor copper loss, these losses will make the motor extra heat, reduce the efficiency, reduce the output power, and the temperature rise of ordinary motors generally increases by 10%-20%.
Dielectric strength of electric motors
The carrier frequency of the frequency converter ranges from several thousand to ten kilohertz, so that the stator winding of the motor has to withstand a high voltage rise rate, which is equivalent to applying a steep impulse voltage to the motor, which makes the inter-turn insulation of the motor withstand a more serious test. .
03 Harmonic electromagnetic noise and vibration
When an ordinary motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors will become more complicated. The harmonics contained in the variable frequency power supply interfere with the inherent space harmonics of the electromagnetic part of the motor to form various electromagnetic excitation forces, thereby increasing the noise. Due to the wide operating frequency range of the motor and the wide range of rotational speed variation, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequency of each structural member of the motor.
Cooling problems at low rpm
When the frequency of the power supply is low, the loss caused by the high-order harmonics in the power supply is large; secondly, when the speed of the motor decreases, the cooling air volume decreases in proportion to the cube of the speed, resulting in the heat of the motor not being dissipated and the temperature rising sharply. increase, it is difficult to achieve constant torque output.
In view of the above situation, the frequency conversion motor adopts the following design:
①Reduce the stator and rotor resistance as much as possible, and reduce the copper loss of the fundamental wave to make up for the increase of copper loss caused by higher harmonics.
②The main magnetic field is not saturated. One is to consider that higher harmonics will deepen the saturation of the magnetic circuit, and the other is to consider that the output voltage of the inverter can be appropriately increased in order to increase the output torque at low frequencies.
③ The structural design is mainly to improve the insulation level; the vibration and noise of the motor are fully considered; the cooling method adopts forced air cooling, that is, the main motor cooling fan adopts an independent motor drive mode, and the function of the forced cooling fan is to ensure that the motor is at a low temperature. Cooling at RPM.
④The coil distributed capacitance of the variable frequency motor is smaller, and the resistance of the silicon steel sheet is larger, so that the influence of high-frequency pulses on the motor is small, and the inductance filtering effect of the motor is better.
⑤The ordinary motor, that is, the power frequency motor, only needs to consider the starting process and the working conditions of one point of the power frequency, and then design the motor; while the variable frequency motor needs to consider the starting process and the working conditions of all points within the frequency conversion range, and then design the motor.
⑥In order to adapt to the PWM width modulated wave analog sinusoidal alternating current output by the inverter, which contains a lot of harmonics, the function of the specially made variable frequency motor can actually be understood as a reactor plus an ordinary motor.
