With the rapid development of power electronic technology and new semiconductor devices, the AC speed regulation technology has been continuously improved and improved, and the gradually improved frequency converter has been widely used in AC motors with its good output waveform and excellent performance-price ratio. For example: large electric motors and medium and small roller table motors for steel rolling in steel mills, traction motors for railways and urban rail transit, elevator motors, hoisting motors for container lifting equipment, motors for pumps and fans, compressors, and household appliances Motors, etc. have successively used VFD electric motors such as the explosion proof vfd, and have achieved good results. So why can't ordinary motors be used as variable frequency motors?
Ordinary motors are designed according to constant frequency and constant voltage, and cannot fully meet the requirements of frequency converter speed regulation, so they cannot be used as vfd frequency converter motors.
The influence of the frequency converter on the motor is mainly on the efficiency and temperature rise of the motor: the frequency converter can generate different levels of harmonic voltage and current during operation, so that the motor runs under non-sinusoidal voltage and current, and the high-order harmonics inside will cause The stator copper loss, rotor copper loss, iron loss and additional loss of the motor increase. The most significant is the rotor copper loss. These losses will cause the motor to generate additional heat, reduce the efficiency, and reduce the output power. The temperature rise of ordinary motors is generally increased by 10% to 20%, so it cannot be used as a variable frequency motor.
The problem of the insulation strength of the motor: the carrier frequency of the inverter is from several thousand to more than ten kilohertz, so that the stator winding of the motor has to withstand a high voltage rise rate, which is equivalent to applying a large impulse voltage to the motor to insulate the turns of the motor. It can withstand more severe tests, so ordinary motors cannot be used as the vfd compatible motor.
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 problem at low speed: when the frequency of the power supply is low, the loss caused by the higher harmonics in the power supply is large; when the speed of the variable frequency motor decreases, the cooling air volume decreases in proportion to the cube of the speed, resulting in the heat of the motor If it cannot be distributed, the temperature rise increases sharply, and it is difficult to achieve constant torque output, so ordinary motors cannot be used as variable frequency motors.