Regarding knowledge about frequency converters, and their impact on motors, it's necessary to understand the issues of motor efficiency and temperature rise, motor insulation strength, and the effects of harmonic electromagnetic noise and vibration. Additionally, there's the issue of cooling at low speeds. Let's take a closer look.
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.
