The difference between variable frequency motors and ordinary motors

In many cases, many mechanical equipment requires variable frequency speed regulation of motors. Some companies choose variable frequency speed regulation motors for speed regulation, while others directly choose the most common three-phase asynchronous motors for speed regulation. What are the differences between variable frequency motors and regular motors? Let's take a look at three major aspects together.



1、 Ordinary asynchronous motors are designed based on constant frequency and voltage, and cannot fully meet the requirements of variable frequency speed regulation. The following is the impact of frequency converters on motors



1. The efficiency and temperature rise of electric motors


Regardless of the form of frequency converter, it generates varying degrees of harmonic voltage and current during operation, causing the motor to operate under non sinusoidal voltage and current. Refusing to provide information, taking the widely used sine wave PWM frequency converter as an example, its low order harmonics are basically zero, and the remaining high order harmonic components that are about twice the carrier frequency are 2u+1 (u is the modulation ratio).


High order harmonics can cause an increase in stator copper loss, rotor copper (aluminum) loss, iron loss, and additional loss of the motor, with the most significant being rotor copper (aluminum) loss. Because asynchronous motors rotate at synchronous speeds close to the fundamental frequency, high harmonic voltages can cause significant rotor losses when cutting the rotor bars with a large slip. In addition, it is also necessary to consider the additional copper consumption caused by the skin effect. These losses will cause the motor to generate additional heat, reduce efficiency, and reduce output power. For example, if an ordinary three-phase asynchronous motor is operated on a non sinusoidal power supply output by a frequency converter, its temperature rise generally increases by 10% -20%.




2. Insulation strength issues of electric motors


At present, many small and medium-sized frequency converters adopt PWM control method. His carrier frequency is about several thousand to ten thousand kilohertz, which makes the stator winding of the motor withstand a high voltage rise rate, equivalent to applying a steep impulse voltage to the motor, making the inter turn insulation of the motor withstand a more severe test. In addition, the rectangular chopping impulse voltage generated by PWM frequency converters superimposed on the operating voltage of the motor poses a threat to the ground insulation of the motor, which will accelerate aging under repeated high-voltage impacts.


3. Harmonic electromagnetic noise and vibration


When an ordinary asynchronous motor is powered by a frequency converter, it will make the vibration and noise caused by electromagnetic, mechanical, ventilation and other factors more complex. The various time harmonics contained in the variable frequency power supply interfere with the inherent spatial harmonics of the electromagnetic part of the motor, forming various electromagnetic excitation forces. When the frequency of electromagnetic force waves is consistent or close to the natural vibration frequency of the motor body, resonance phenomenon will occur, thereby increasing noise. Due to the wide operating frequency range and large speed variation range of the electric motor, it is difficult to avoid the natural vibration frequency of various components of the electric motor due to the frequency of various electromagnetic force waves.


4. Adaptability of electric motors to frequent starting and braking


Due to the use of a frequency converter for power supply, the motor can be started at a very low frequency and voltage without impulse current, and various braking methods provided by the frequency converter can be used for rapid braking, creating conditions for frequent starting and braking. Therefore, the mechanical and electromagnetic systems of the motor are under the action of cyclic alternating forces, causing fatigue and accelerated aging problems to the mechanical and insulation structures.


5. Cooling issues at low speeds


Firstly, the impedance of asynchronous motors is not ideal, and when the power frequency is low, the losses caused by higher harmonics in the power supply are relatively large. Secondly, when the speed of an ordinary asynchronous motor decreases again, the cooling air volume decreases proportionally to the cubic power of the speed, resulting in a deterioration of the low-speed cooling condition of the motor and a sharp increase in temperature rise, making it difficult to achieve constant torque output.