Selection of Variable Frequency Drives

The size of a variable frequency drive (VFD) should be based on motor voltage and current and not on the torque or power rating. The voltage and full load current information can be taken from motor nameplate or datasheet.

While the motor can be rated for more than one input voltage, the input voltage required for the particular application that is critical. VFDs are rated on design specific voltage range, hence, it is vital to assess the application for the amount of current it will draw and the speed at which it will be operating.

Calculations

The following equations help show the VFD power flow from AC to DC to AC to motor shaft power.

1. VFD Input Power (AC), P_IN
$P_IN = sqrt{3}*I_IN*V_IN*PF_IN$
Where:
P_IN = AC input power to the VFD
I_IN = AC input current to the VFD
V_IN = AC input voltage to the VFD
PF_IN = AC input power factor to the VFD

2. VFD DC Power (DC), P_DC

Where:
P_DC = VFD DC power
I_DC = VFD DC current
V_DC = VFD DC voltage

3. VFD Output Power (AC), P_OUT
$P_OUT = sqrt{3}*I_OUT*V_OUT*PF_OUT$
Where:
P_OUT = VFD AC output power
I_OUT = VFD AC output current
V_OUT = VFD AC output voltage
PF_OUT = VFD AC output power factor

4. Motor Shaft (Output) Power, P_shaft
$P_motor={P_shaft}/{eta_motor} = {sqrt{3}*V_motor*I_motor*PF_motor}/{eta_motor}$
Where:
P_motor = Motor input power
P_shaft = Motor output power
η_motor = Motor efficiency
V_motor = Motor terminal voltage
I_motor = Motor current
PF_motor = Motor power factor

Equations (2) and (3) from the above equations are related to the VFD.Hence the equations can be summarized as
$P_IN = {P_out}/{eta_VFD} = {P_shaft}/{{eta_VFD}*{eta_motor}} = {sqrt{3}*V_motor*I_motor*PF_motor}/{{eta_VFD}*{eta_motor}}$

Examples will be provided in the next article.