Frequently asked questions about VFD

1. What is a variable frequency drive?
The AC drive is a power control device that uses the on-off function of the power semiconductor device to convert the power frequency power supply to another frequency.

2. What is the difference between PWM and PAM?
PWM is the abbreviation of Pulse Width Modulation, which changes the pulse width of the pulse train according to a certain rule to adjust the output and waveform.
PAM is the abbreviation of Pulse Amplitude Modulation in English. It is a modulation method that changes the pulse amplitude of the pulse train according to a certain rule to adjust the output value and waveform.

3. What is the difference between voltage type and current type?
The main circuit of the inverter can be roughly divided into two categories: the voltage type is an inverter that converts the DC of the voltage source into an AC, and the filter of the DC circuit is a capacitor; the current type is an inverter that converts the DC of the current source into an AC. Its DC circuit filter stone inductance.

4. Why does the voltage of the inverter change proportionally to the current?
The torque of the asynchronous motor is generated by the interaction between the magnetic flux of the motor and the current flowing in the rotor. At the rated frequency, if the voltage is constant and the frequency is only reduced, the magnetic flux will be too large, and the magnetic circuit will be saturated. will burn out the motor. Therefore, the frequency and voltage should be changed proportionally, that is, the output voltage of the frequency converter should be controlled while changing the frequency, so as to keep the magnetic flux of the motor constant and avoid the occurrence of magnetic field weakening and magnetic saturation. This control method is mostly used in energy-saving inverters for fans and pumps.

5. When the motor is driven by a commercial frequency power supply, the current increases when the voltage drops, for inverter drive, if the voltage also drops when the frequency drops, does the current increase?
When the frequency drops (low speed), if the same power is output, the current increases, but under the condition of constant torque, the current is almost unchanged.

6. What is the starting current and starting torque of the motor when the frequency converter is used?
The frequency converter is used to operate, and the frequency and voltage are increased correspondingly with the acceleration of the motor, and the starting current is limited to below 150% of the rated current (depending on the model, it is 125%~200%). When starting directly with a power frequency power supply, the starting current is 6~7 times, so mechanical and electrical shocks will occur. Using frequency converter drive can start smoothly (starting time becomes longer). The starting current is 1.2~1.5 times of the rated current, and the starting torque is 70%~120% of the rated torque; for inverters with automatic torque enhancement function, the starting torque is above 100%, which can start with full load.

7. What does V/f mode mean?
When the frequency drops, the voltage V also drops proportionally. This question has been explained in answer 4. The proportional relationship between V and f is determined in advance considering the characteristics of the motor. Usually, there are several characteristics stored in the storage device (ROM) of the controller, which can be selected with a switch or a dial.

8. When V and f are changed proportionally, how does the torque of the motor change?

When the frequency drops, the voltage is completely reduced proportionally, so the torque generated at low speed tends to decrease because the AC impedance becomes smaller and the DC resistance remains unchanged. Therefore, given V/f at low frequency, the output voltage should be increased to obtain a certain starting torque. This kind of compensation is called enhanced starting. It can be realized by various methods, such as automatic method, selection of V/f mode or adjustment of potentiometer.

For more information, please consult us, a professional variable frequency drive factory, manufacturer and supplier.