Is there a difference in construction of motors for variable speed drives and DOL?

 

no difference in b/w the construction

 

To be more precise:

You can --and often do-- connect a standard induction AC motor to a VFD. But the VFD does not provide a sinusoidal output signal to the motor (it uses SCRs or IGBTs to generate a variable-frequency sinus-looking signal from a DC voltage) and it includes many harmonics that can proove harmful to a standard motor (over the long term), tending to overheat it especially at low-frequency runs.
So the industry provides what is known as inverter-duty motors, with special insulation and more air-cooling capabilities, for pumps and fan applications.

Have a look at the following document, you might find it interesting:
http://www.pdhengineer.com/Course F...PDF Files/Electrical/Inverter Duty Motors.pdf

Hope this helps,
Daniel Chartier