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VFD(Variable Frequency Drives)controls the speed of the motor using Power Electronic Equipments(IGBT) in order to reduce losses and get accurate output. The main formula works here is N(S) = 120F/p. N(s) - Speed in RPM, F-Frequency, P-No. Of poles. Hope this brief answer works.
First VFD is meaning Variable Frequency Drive, so the basic control element here is frequency.
At Staring conditions.
the internal motor current equals to ((V-E)/(R+X)) so at motor starting the frequency will be low value and E is equal to zero, X=2*3.14 F*L so it’s value will be small, so the amount of current at starting will be high as illustrated in the above equation. VFD at starting will apply a low voltage may be 30 %of rated voltage and at the same time keep V/F constant value so the starting will be smooth in speed due to low voltage applied and with low staring current because of V/F is constant value.
At Operating conditions.
Change in frequency will change the motor speed from that equation N(S) = 120F/p. N(s) .and keeping V/F constant in the range of rated motor frequency to save the motor consumption power in the safe limit.
You can change the motor frequency by many input method for VFD
· Variable resistance fixed on VFD.
· Control voltage signal -+ 10 volt.
· Current signal.
