The output current from the VFD to the motor has two components:
1. The real current that produces torque or useful work (watts).
2. The reactive current that produces magnetic field in motor (vars).
The total output current (Ioutputvfd) from the drive will be the square root of the sum of squares of both the real current (Ir) and reactive current (Iq).
The real current (Ir) is supplied by the electric grid and the reactive current (Iq) is supplied by the DC bus capacitor inside the VFD-See figure. It can be seen from the equations above that the output current of the VFD can be larger than the input current. For smaller size drives and motors the difference in current magnitude may not be noticeable, however for larger drives it will be more pronounced.
It should be kept in mind that the output current from a VFD is very noisy.
You could get different readings depending on the type of meter you are using to measure.
A meter with a ‘VFD mode’ or “Low pass filter mode’ will give more accurate results.
The best place to measure the VFD output current is the drive display itself.
The drive electronics will be able to electronically process the noisy output current waveform and discern the magnitude of the desired current frequency component.
Released for: Schneider Electric Egypt and North East Africa
