21 Jun 12 1:44
I have a question regarding the overall efficiency of VFD Driven motors. Normally the VFD efficiency is around 96% and motor efficiency around 94% (around 30kW range).
When we made a measurement for a motor used in pumping system, the overall efficiency fell much below the expected, and as a result the contractor is held responsible for the lower efficiecy assuming the lower pump efficiency. The pumps are KSB, manufactured in Germany and factory tested and confirmed to operate at declared efficiency.
Therefore i feel the lower efficiency is due to the VFD / or measurement method
Test arrangement is as indicated below,
* Pump flow is measured with calibrated electromagnetic flow meter at the pump delivery.
* Pressure is measured using calibrated pressure gauges across the pump.
* Power analyzer connected to the input of the VFD.
Inut power is measured and the VFD efficiency of 96% and motor efficiency of 94% is used for the calculation of overall efficiency.
As the input current to the VFD is not sinusoidal, can the current transformer detect the correct current for calculating the power? Is there any other way of doing this test?
Has anyone of you come across a similar situation?