How to Select Variable Frequency Drive Reactor?
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, highorder 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.
Here are some things about how to choose a reactor.
Selection of Rated AC Current
Rated AC current is the longterm working current for the reactor design from the heating perspective, enough higher harmonic components should be considered at the same time. Namely, the actual current flowing through the output reactor is the output current of the VFD motor load.
Voltage Drop
Voltage drop refers to the actual voltage drop at both ends of reactor coil corresponding to the actual rated current at 50Hz. Generally, the voltage drop is 4V~8V.
Selection of Inductance
Reactor’s rated inductance is also an important parameter! If the inductance is not selected properly, it will directly affect the change of voltage drop under rated current, thus causing faults. The inductance depends on the crosssectional area of reactor core, the turn number of coil and the adjustment of air gap.
Selection of output reactor inductance is determined according to the length of cable in the rated frequency range. Then, the crosssection areas of iron core and conductor are selected according to the actual rated current of the motor under corresponding inductance requirement, the actual voltage drop can be thereof calculated.
Rated Current, Inductance and Cable length
For an ideal reactor, its inductance shall remain unchanged at rated AC or below current, its inductance decreases gradually with the current increase.
When the rated current is more than 2 times, the inductance decreases to 0.6 times of the rated inductance.
When the rated current is more than 2.5 times, the inductance decreases to 0.5 times of the rated inductance.
When the rated current is more than 4 times, the inductance decreases to 0.35 times of the rated inductance.
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.
Here are some things about how to choose a reactor.
Selection of Rated AC Current
Rated AC current is the longterm working current for the reactor design from the heating perspective, enough higher harmonic components should be considered at the same time. Namely, the actual current flowing through the output reactor is the output current of the VFD motor load.
Voltage Drop
Voltage drop refers to the actual voltage drop at both ends of reactor coil corresponding to the actual rated current at 50Hz. Generally, the voltage drop is 4V~8V.
Selection of Inductance
Reactor’s rated inductance is also an important parameter! If the inductance is not selected properly, it will directly affect the change of voltage drop under rated current, thus causing faults. The inductance depends on the crosssectional area of reactor core, the turn number of coil and the adjustment of air gap.
Selection of output reactor inductance is determined according to the length of cable in the rated frequency range. Then, the crosssection areas of iron core and conductor are selected according to the actual rated current of the motor under corresponding inductance requirement, the actual voltage drop can be thereof calculated.
Rated Current, Inductance and Cable length
Cable Length (meter) 
Rated Output Current (A) 
Inductance Value (μH) 
300 
100 
46 
200 
23 

250 
16 

300 
13 

600 
100 
92 
200 
46 

250 
34 

300 
27 
For an ideal reactor, its inductance shall remain unchanged at rated AC or below current, its inductance decreases gradually with the current increase.
When the rated current is more than 2 times, the inductance decreases to 0.6 times of the rated inductance.
When the rated current is more than 2.5 times, the inductance decreases to 0.5 times of the rated inductance.
When the rated current is more than 4 times, the inductance decreases to 0.35 times of the rated inductance.
Variable Frequency Drives
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