Inverter Duty motor

To me the term "Inverter Duty" motor is a Black Max or Blue Max Marathon motor or equal designed to run on variable frequency drive (VFD) power. 1000:1 or 2000:1 CT or VT turn down.

"Inverter Rated" motor could be any motor with minimal turn down ratio. 2:1CT or 10:1VT
Ex: Standard motor running half speed @ nameplate amps may not last 3.5 years due to minimum air flow from fan. I've seen this many times where the motor does not trip on overload yet insulation breaks down and fails. However, if same motor is using 1/2-3/4 amperage. It may last 3.5 years or longer.

Inverter duty is for the manufacturer to determine. There is no uniformity. The other issue though, is that while there's a rule of thumb that a standard TEFC motor can turn down to 50% of nominal speed, how much of the time it spends there will affect motor life. A motor that spends most of its life at 50% speed won't last long, inverter duty or non-inverter duty. If you expect to be running that way (or slower), an external fan should be fitted.

There are at least three issues with variable frequency drive service: 1: harmonics can create high voltage spikes that break down insulation, 2: harmonics cause extra heating in the windings, causing extra cooling demand, and 3: running the motor slower than full speed reduces the regular fan speed, on a TEFC or open motor, and thus its ability to cool itself.
2 and 3 conspire to cook the windings. When the windings are sufficiently cooked, it makes it easier for 1 to finish the motor off.

Of course, you have to modify this analysis for a non-fan cooled motor such as a submersible pump, but you still have potential problems, and have to defer to the manufacturer, who is the only one with the complete thermal analysis.

This is why you need a complete engineering analysis, and not a cookbook full of rules of thumb.

The expected life for API pump as per API guide line is 20 years. In a similar fashion i want to know how life span of inverter duty motor.

Say for example an inverter duty motor of 4 pole running with ratio 2:1
Running Hours : 24 x 365 = 8760
Load : 90%
Motor Rating : 5 HP

Now running at above speed, you need to take care of heating by using cooling fan or anything, spikes generated by VFD. or Voltage fluctuation ( nil ) or using very high grade insulation in motor etc.