How VFD for LV motor works?

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.