【Daily Question 20090316】Why does an inverter need to change voltage as well as frequency?

The induced electromotive force in a motor is essentially equal to the terminal voltage of the motor, and it is proportional to the product of frequency and flux. When the frequency decreases while the terminal voltage remains constant, it inevitably leads to an increase in flux; this increase in flux causes magnetic saturation of the motor. As the frequency increases (above the supply frequency), the flux will decrease, resulting in underexcitation of the motor. Either over-excitation or under-excitation is bad for the motor. Therefore, it is necessary to maintain the motor flux constant. This is the principle behind the need to adjust the voltage accordingly when changing the frequency. Learn* online

Answer: The load of an inverter is generally an inductive load, and the impedance of an inductive load is proportional to the frequency. When the frequency increases, the voltage must be increased accordingly in order to maintain a constant excitation current, thereby ensuring that the motor has sufficient torque.

After the frequency is reduced, if the voltage is not decreased accordingly, the voltage/frequency ratio becomes too high, causing the motor to be over-excited and the core to enter the saturation region, preventing it from operating properly

Variable frequency speed control achieves speed regulation by changing the frequency of power supplied to the motor’s stator windings. To maintain the maximum torque of the motor during speed regulation, it is necessary to keep the magnetic flux of the motor constant; therefore, the supply voltage to the stator must also be adjusted accordingly. A frequency converter is a device that adjusts both the frequency (Variable Frequency) and the voltage (Variable Voltage), hence it is abbreviated as VVVF. According to the analysis of electrical theory, torque is proportional to the flux (at its maximum value); when the rotor parameters remain constant, torque is proportional to the square of the supply voltage. The working principle of an inverter is to convert the mains electricity (380V, 50Hz) into smooth direct current using a rectifier, and then use a three-phase inverter composed of semiconductor devices (GTO, GTR, or IGBT) to transform this direct current into alternating current with variable voltage and frequency. Thanks to the sine wave pulse width modulation (SPWM) method programmed by a microprocessor, the output waveform is made to resemble a sine wave, which is used to drive asynchronous motors and achieve stepless speed control. The two transformations mentioned above can be simplified to the AC-DC-AC (alternating current–direct current–alternating current) conversion method. By using an inverter, it is possible to achieve automatic and smooth acceleration or deceleration in response to changes in the motor load, thereby maintaining the inherent characteristic of asynchronous motors, namely a low slip rate. This approach offers advantages such as high efficiency, a wide range of operation, high precision, and the ability for stepless speed control, which makes it very suitable for equipment such as water pumps and fans.

First, when the frequency remains constant and the input power on the stator side of the motor or the output power on the rotor side changes, the operating condition of the motor will adjust automatically. 1. When the supply voltage increases (an increase within the allowable range that does not cause the magnetic circuit to reach saturation), the input power increases. Through the magnetic field, this increased power is transmitted to the rotor, resulting in an increase in the rotor’s output power. 2. When the output power on the rotor side changes, the load torque increases, and the power required by the rotor rises; to counteract this, the magnetic flux is reduced by increasing the rotor current, thereby drawing more power from the power supply. Second, special situations during frequency conversion. The special feature of variable frequency speed control for asynchronous motors is that the regulation takes place on the stator side. When only the frequency is adjusted, the input power to the stator side increases rather than decreases, whereas the output power of the rotor decreases (when the frequency is reduced), resulting in a severe imbalance between input and output power. This further leads to a severe accumulation of magnetic field energy that drives energy transfer. It causes saturation of the magnetic circuit and distortion of the excitation current waveform; the more severe the saturation of the magnetic circuit, the greater the distortion. Therefore, in variable-frequency speed control systems, it is necessary to address the issue of how to maintain a constant magnetic flux in the motor. To maintain a constant flux, it is necessary to keep the ratio of the stator back emf to that frequency constant at any given frequency. The magnitude of the back emf is approximately equal to the supply voltage; therefore, during the inverter process, the frequency must be changed simultaneously along with the voltage, in order to keep this ratio constant and thus maintain a constant flux. That’s why inverters are also commonly denoted as VVVF. The purpose is to maintain the magnetic flux constant.