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As an important product used in many fields, single-phase VFD not only has special and key performance parameters, but also has advantages of many aspects. For professionals, single-phase VFD can play a role of protection and achieve an outstanding effect on controlling voltage. It’s widely used in low power fans, water pumps, axial flow fans and single-phase capacitance start-up asynchronous motor.
Variable frequency drive can change power factor. Internal filter capacitor of VFD reduces the reactive power loss and increases active power of power grid, thus improving the power factor. Appropriate reactors can be connected in series at the input side and output side of VFD to absorb harmonics and increase the impedance of power supply or load, so as to achieve the goal of suppressing harmonic and reducing electromagnetic radiation in the transmission process. By suppressing harmonic current, the power factor could be improved form previous (0.5-0.6) to (0.75-0.85). VFD has a good effect in improving the power factor.
When a VFD controls a motor and does not need to switch between the power supply, an output contactor is not needed to be connected between the variable frequency drive and motor. The main reason is that: if an output contactor is connected, the motor may be started directly when the output frequency of VFD is high. As a result, a large starting current would be generated, which may lead to trip of VFD.
We are often asked “can the motor with a rated frequency of 60Hz operate at 50Hz (vice versa)?” When the motor is operated at a frequency different from the one on the nameplate, the following problems may occur.
Synchronization speed of AC motor depends on two factors: pole number of motor and frequency of motor's input power. For a specific motor, the pole number can’t be changed because it’s determined by the physical structure of motor. The frequency of input power is usually determined by the power distribution system in the country where the motor operates. Commonest frequencies are 50 Hz and 60 Hz.
If the hardware are not damaged, the reason for unable to raise the frequency is generally that maximum output torque of VFD is smaller than that of load lifting, namely the variable frequency drive is poor in driving loads.
In theory, the longer the acceleration time is, the stronger lifting capacity with load will be. If the acceleration time is set too short, some VFDs display over-current or overload, overheat alarm, while some would not display, only the VFD stays in a specific frequency band.

The rotating speed of inductor type AC motor (hereinafter referred to as motor) is determined by the number of pole and frequency. Working principle of the motor determines that the number of pole is fixed and invariable. As the pole number is not a continuous number (times of 2, such as 2, 4, 6), it’s improper to regulate the motor speed by changing the pole number.
In addition, the frequency can be adjusted outside the motor and then supplied to the motor, so that the motor’s rotation speed can be freely controlled. Therefore, variable frequency drive with a purpose of frequency control is a preferred equipment for motor speed control.
Input current of the VFD is related to the power required by the motor, the supply voltage, the efficiency of variable frequency drive and the power factor. Among, the power factor of VFD changes along with the impedance of power supply, low impedance leads to low power factor and high impedance leads to high power factor. As the load of VFD is a motor, the input and output power factors are different, the power factor at output end after using the VFD is higher than that of the input end.
Selection of V/f type includes the maximum frequency, basic frequency and torque type. Maximum frequency refers to the highest frequency that the VFD & motor system can operate. As the maximum frequency of variable frequency drive may be higher, when the allowable maximum frequency of motor is lower than the maximum frequency of the VFD, it should be set according to the requirements of the motor and its load.
To avoid electromagnetic interference, shielding line shall be used for input and output control lines of VFD. The shielding line shall not be wrongly connected, otherwise opposite effects would be achieved. It may also be protected with a steel pipe or equipped with a wave filter, so as to reduce the carrier frequency. We find that if the switch pipe of converter’s switching power supply is a field effect transistor, its interference will be greater.
VFD tripping occurs at starting. It’s a serious phenomenon of overcurrent, inspections shall be made:
Whether the working machine is stuck, whether the load is short circuited, checking whether the grounded is short circuited with megger, whether the power module of the VFD is damaged, starting torque of the motor is too small, and the drive system can’t be operated.
Variable frequency drive often encounters various problems in debugging and application, and overvoltage phenomenon is the most common.
Once the overvoltage occurs, over-voltage protection function of the variable frequency drive will acts to stop the operation of VFD, so as to prevent internal circuit damage. As a result, the equipment would be unable to work normally. Therefore, measures must be taken to eliminate overvoltage and prevent the occurrence of faults. Because the applications of VFD and motor are different, the causes of overvoltage are also different, countermeasures should be taken according to the specific situation correspondingly.
Input reactors are used to suppress surge voltage and surge current, protect the variable frequency drive and extend its service life. Since the VFD adopts the frequency conversion method to adjust the speed, high-order harmonics are often generated during the speed regulation to affect the normal use of the equipment. To this end, adding a reactor at the input can improve the power factor of the VFD and suppress harmonic currents, filtering out harmonic voltages and harmonic currents.
Output reactor is used to extend the effective transmission distance of the VFD. It can suppress the instantaneous high voltage generated when the IGBT is switched, reduce the noise and eddy current loss of the motor, and protect the power switching device inside the VFD.
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Variable Frequency Drives
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