I'm trying to justify not using a VFD to those wanting to use a VFD in an appplication where (3) 40 HP fans feed (3) 50MMBTU (ea.) burners. Typically, they run 80-90% of full load.
My arguement against the VFD (vs. simple damper operation) is the added complexity of having both VFD and damper operation available (in the event of VFD failure)and increased maintenance and spares cost.
Their arguement is energy efficiency, which I'm claiming will be minimal due to normal running at a high load compared to the above costs.
I'm told by others that an inverter driven motor running at full speed is less efficient than a motor with ATL starting. Is this a true statement?
In general, yes, a motor running at full speed on a VFD will probably be slightly less efficient than a motor running on a sine wave. The typical VFD output is far from a sine wave, so there are some additional losses in the motor.
The motor efficiencies on the nameplate or in the published data are all based on operating on a sine wave.
I would expect the installation of the drive to add around 4% to 6% losses compared to a motor running ATL.
A motor running full-speed with a VFD is always less efficient than a motor running full-speed using ATL starting. The VFD is another power component you are inserting with it's own losses that will lower the efficiency of the whole system (power line to shaft output).
