1. Inverter Concept and Working Principle
(1) VVVF is an abbreviation for Variable Voltage and Variable Frequency.
(2) CVCF is an abbreviation for Constant Voltage and Constant Frequency.
The AC power supply used in various countries, whether for homes or factories, is typically 200V/60Hz (50Hz) or 100V/60Hz (50Hz) in both voltage and frequency. A device that converts AC power with a fixed voltage and frequency into AC power with a variable voltage or frequency is usually called a "frequency converter ." To generate variable voltage and frequency, this device first converts three-phase or single-phase AC power into direct current (DC). Then, it converts the DC power back into three-phase or single-phase AC power. The device that performs this conversion is called a "frequency converter."
Inverters are also used in home appliances. These appliances include not only motors (such as air conditioners) but also products like fluorescent lamps. Inverters used for motor control can change both voltage and frequency. However, inverters used for fluorescent lamps are primarily used to regulate the frequency of the power supply. Devices used in automobiles that generate alternating current from batteries (DC) are also sold under the name "inverter." The working principle of inverters is widely applied in various fields. For example, in computer power supplies, inverters are used to suppress reverse voltage, frequency fluctuations, and momentary power outages.
2. Why can the rotational speed of an electric motor be changed freely?
(1) r/min Motor rotation speed unit: number of rotations per minute, which can also be expressed as rpm. For example: 4-pole motor 60Hz 1,800 [r/min], 4-pole motor 50Hz 1,500 [r/min], the rotation speed of the motor is proportional to the frequency.
The motor referred to in this article is an induction AC motor, which is the most common type of motor used in industry. The rotational speed of an induction AC motor (hereinafter simply referred to as the motor) depends approximately on the number of poles and the frequency. The number of poles is fixed. Because the number of poles is not a continuous value (it is a multiple of 2, e.g., 2, 4, 6 poles), it is not suitable to adjust the motor speed by changing the number of pole pairs. Furthermore, the frequency is the electrical signal supplied to the motor, so this value can be adjusted externally before being supplied to the motor, thus allowing for free control of the motor's rotational speed. Therefore, a frequency converter, designed for frequency control, is the preferred device for motor speed regulation.
n = 60f/p, where n is the synchronous speed, f is the power supply frequency, and p is the number of motor poles. Changing both frequency and voltage is the optimal method for motor control. Changing only the frequency will burn out the motor. This problem is particularly pronounced when the frequency is reduced. To prevent motor burnout, the inverter must simultaneously change the voltage while changing the frequency. For example, to halve the motor's rotational speed, the inverter's output frequency must be changed from 60Hz to 30Hz, which requires changing the output voltage from 200V to approximately 100V.
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