Variable frequency drives (VFDs) are widely used in the control of various mechanical equipment, including conveyors, cranes, extruders, and machine tools, highlighting their importance. To enhance understanding of VFDs, this article will introduce VFDs and their impact on motors. If you are interested in VFDs, please continue reading!
I. Frequency Converter
A frequency converter mainly consists of rectification (AC to DC), filtering, inversion (DC to AC), braking unit, drive unit, detection unit, and microprocessor unit. The frequency converter adjusts the output voltage and frequency by switching its internal IGBTs, providing the required power voltage according to the actual needs of the motor, thereby achieving energy saving and speed regulation. In addition, the frequency converter has many protection functions, such as overcurrent, overvoltage, and overload protection. With the continuous improvement of industrial automation, frequency converters have been widely used.
Compared to the development of frequency converters abroad, the application of frequency converters in my country started relatively late, only achieving widespread adoption in the late 1990s. The current state of frequency converter technology development in China can be summarized as follows: The overall technology of frequency converters is relatively backward, lagging significantly behind the advanced achievements of foreign countries in variable frequency speed control research; there is a lack of technology in the core components used in frequency converters; currently, almost no domestic manufacturers can produce the key power devices required for frequency converter production, resulting in our core technology being controlled by foreign countries and requiring imports; the main products are concentrated in low-voltage products and the low-to-mid-range market. Due to low product reliability and manufacturing processes, domestic frequency converter products are currently mainly targeted at the low-voltage market and markets with general performance requirements, while the high-performance, high-power market is mainly dominated by large foreign companies.
Since the beginning of the 21st century, domestically produced frequency converters have gradually risen to prominence and are now gradually seizing the high-end market. Shanghai and Shenzhen have become the forefront of the development of domestically produced frequency converters.
II. The impact of frequency converters on motors
1. Issues related to the efficiency and temperature rise of electric motors.
Regardless of the type of frequency converter, it will generate harmonic voltages and currents of varying degrees during operation, causing the motor to operate under non-sinusoidal voltage and current conditions.
Taking the commonly used sinusoidal PWM inverter as an example, its low-order harmonics are basically zero, and the remaining high-order harmonic components, which 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 losses in motors, with the most significant being rotor copper (aluminum) loss.
Because asynchronous motors rotate at a synchronous speed close to the fundamental frequency, high-order harmonic voltages will generate significant rotor losses when they cut the rotor bars with a large slip.
In addition, the additional copper losses caused by the skin effect must be considered. These losses will cause the motor to generate extra heat, reduce efficiency, and decrease output power. If a typical three-phase asynchronous motor is operated under non-sinusoidal power conditions from a frequency converter output, its temperature rise will generally increase by 10% to 20%.
2. Electric motor insulation strength issues
Currently, many small and medium-sized frequency converters use PWM control, with a carrier frequency of several thousand to tens of kilohertz. This causes the stator windings of the motor to be subjected to a very high voltage rise rate, which is equivalent to applying a very steep impulse voltage to the motor, subjecting the inter-turn insulation of the motor to a severe test.
In addition, the rectangular chopper impulse voltage generated by the PWM inverter is superimposed on the motor operating voltage, which poses a threat to the motor's insulation to ground. The insulation to ground will age faster under repeated high voltage impacts.
3. Harmonic electromagnetic noise and vibration
When a regular asynchronous motor is powered by a frequency converter, the vibration and noise caused by electromagnetic, mechanical, and ventilation factors become more complex.
The time harmonics contained in the frequency converter interfere with the inherent spatial harmonics of the electromagnetic components of the motor, forming various electromagnetic excitation forces. When the frequency of the electromagnetic force wave is the same as or close to the natural vibration frequency of the motor body, resonance will occur, thereby increasing noise.
Because electric motors have a wide operating frequency range and a large speed variation range, it is difficult for the frequencies of various electromagnetic force waves to avoid the natural vibration frequencies of the various components of the electric motor.
4. The motor's ability to adapt to frequent starting and braking.
Because of the use of frequency converters for power supply, motors can start at very low frequencies and voltages without inrush current, and can be quickly braked using various braking methods provided by the frequency converter, thus creating conditions for frequent starting and braking.
Therefore, the mechanical and electromagnetic systems of the electric motor are under the action of cyclic alternating forces, which causes fatigue and accelerated aging of the mechanical and insulation structures.
5. Cooling issues at low speeds
First, the impedance of asynchronous motors is not ideal. When the power supply frequency is low, the losses caused by high-order harmonics in the power supply are relatively large.
Secondly, when the speed of a conventional asynchronous motor decreases, the cooling airflow decreases proportionally to the cube of the speed, which leads to a deterioration in the low-speed cooling of the motor, a sharp increase in temperature, and difficulty in achieving constant torque output.
