Efficiency of Motor

Motor losses (the invert of efficiency) can be separated into two basic categories, fixed losses and load dependent losses. The fixed losses, those associated with just making that hunk of iron and copper into a motor in the first place, don't vary with loading, and are the ones you need to concern yourself with because the load related losses will be the same either way. So as a percentage of total losses, the fixed losses appear to increase as load is decreased, but that is an increase in the percentage, not the amount, of losses. So when we say that a motor is 90% efficient at full load and 85% eff. at half load, the total amount of losses has gone down, but the fixed losses have remained the same which skews the percentage. Since the fixed losses are rarely more than 30-50% of the TOTAL losses at full load, there is that to consider in terms of cost of operation.

The thing that changes with using a VFD, is that the VFD is theoretically maintaining only the amount of power output from the motor as is necessary to operate it with the load connected. So from a losses standpoint, you are ALWAYS running the motor at full load, therefore full efficiency. It's not really true because there are other factors involved in the basic use of VFDs in motors that adds back in some new losses and those gets worse as speed gets very low. But in general, and especially with centrifugal (quadratic) loads like pumps and fans, the motor efficiency will remain relatively constant within the working speeds of the machine.

So how does that relate? In your application you have a 110kW motor on a load that requires 85kW. The VFD will lower the speed to where the motor output is only 85kW, so in essence the motor will be running at full load for that speed / flow and therefore it will be maintaining it's full load efficiency. If you did NOT have the VFD and were running a 110kW motor on an 85kW load, you would be running that motor at reduced efficiency. So in essence the VFD is "fixing" that problem. In addition, were the VFD not there, the only way to reduce the output of the fan would be to reduce the air flow by restricting it, which adds in ADDITIONAL losses not related to the motor. THAT is where the VFD is gaining overall throughput efficiency in the system.

Current Nema High Efficiency motors in this size will be over 90% efficient. Possibly as high as 96%.

A motor is rated on the shaft power, not the input power, so if your load is 85kw then the motor rated at 85 kw is sufficient, but won't tolerate long term low level overloads, so I agree with the oversized frame selection.

Be sure you have adequate cooling for the speed you are running. In the frame size you have I suspect you will have separate vent blower cooled to give you rated torque from zero to rated speed.

Most energy efficient inverter duty motors will be good for momentary 200% OL capability. The caveat is that most VFD's are sized at 100% of a nema sized motor size, so if you want to use the OL capacity be sure to select a VFD with sufficient capacity, and it may be up to 200% of the motor frame size. The VFD sizing is a game manufacturers play. "A" has a VFD rated 100kw for $x, no O/L capacity. "B" has a VFD for $y rated 100kw but has 150% O/L for 60 seconds. Be careful, there is much marketing hype.