Indeed, a motor add frequency converter, it is possible to achieve variable frequency operation, but it is not a real variable frequency motor.
1.The influence of inverter on the motor, is mainly on 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 higher harmonics inside will cause the stator copper loss, rotor copper loss, iron loss and additional loss of the motor. The most notable is the copper loss of the rotor. These losses will cause additional heating of the motor, reduce the efficiency, and reduce the output power. The temperature rise of motor generally increases by 10%-20%.
2.The problem of Motor dielectric strength:
The carrier frequency of the inverter is from several to a dozen kilohertz, which makes the stator winding of the motor withstand a high voltage rise rate, it is equivalent to applying a steep shock voltage to the motor, and makes the inter-turn insulation of the motor bear a serious test.
3.Harmonic electromagnetic noise and vibration:
When the motor is powered by a inverter, 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 spatial harmonics of the electromagnetic part of the motor to form various electromagnetic exciting forces, thereby increasing noise. Since the operating frequency range of the motor is wide and the range of the rotational speed is large, the frequencies of various electromagnetic force waves are difficult to avoid the natural vibration frequency of each structural member of the motor.
4.Cooling problem at low speed:
When the frequency is low, the loss caused by the higher harmonics in the power supply is large; when the speed of the modified motor decreases, the cooling air volume decreases in proportion to the cube of the rotational speed, so that the heat of the motor does not dissipate and the temperature rises sharply. Increased, it is difficult to achieve constant torque output.
Main attention matters:
1>Type selection: for example, centrifugal blower and centrifugal pumps, FRENIC-VP should be chosen ; Constant torque or approximate constant torque load, such as the motor of air compressor, vacuum pump, refrigeration (heat) compressor, a general inverter or heavy load type inverter should be chosen. Some types of loads, such as: air compressors, refrigeration (heat) compressors, Roots blowers, etc., in selection of the model, according to the equipment parameters, the inverter rated power is amplified 1~2 files.
2>About cooling, it is necessary to pay special attention to the temperature rise of the motor for a long time and frequent start, so as to prevent the motor from burning because of bad heat dissipation.
3>Frequency conversion speed regulation is usually reduced to frequency. The centrifugal load type is: speed reduction (frequency), power reduction, and torque reduction.The constant torque class load is: speed reduction (frequency), power reduction, constant torque;
Keywords: vacuum pump ; liquid ring vacuum pump; roots pump; compressor，roots blower
Edited by: Annie Hou / Shanghai EVP Vacuum Technology Co., Ltd.