Every day more and more motors operate through a VFD. It would be good to see what the practice of rewinding such motors. In addition to using special wires, which are important differences in rewind "ordinary" and "inverter-duty" motors?
Standard motor drived by VHF = Premature Failure
The inverter duty motors most whitstand the fast rise time pulses and capacitive voltage potencial,the insulation and wire most be special in order to avoid P.D at lower voltages.
ID Winding features are:
Wire coated with a high dielectric strenght film: To avoid the fast rise time pulses caused by the VHF operation.
Phase and slot Insulation increase: To avoid an insulation breakdown by PD at lower voltages
Thermal calass increased(H): To whistand the overheat when the motor operates below rated frequency.Insulated Bearings: To avoid shaft currentsVPI treatment and lacing reinforcement :
To increases the mechanical strenght,due to the winding will be experiment more vibrations.
Some manufacturers use 2 or 3 VPI cycles.
If motors operating below 30 Hz you will find external cooling like in DC motors.
Attached you will find a good article.
Regards
http://files.engineering.com/getfile.aspx?folder=290d8884-515c-4754-a5b8-18 From the view of a motor rewinder the most important consideration is the winding insulation. This is because of the peak voltages and the increased heating caused by the square wave voltage of a pulse width modulated VFD. - The magnet wire should have 1600V peak insulation rating instead of the standard 1000V peak rating. - All winding materials should have minimum Class F insulation rating (magnet wire, slot liners, wedges, varnish, ties...everything). Class H may be required for some types of VFD specific motors. Check the nameplate for 'insulation class' and/or 'temperature rise' information. - Phase paper should always be used. - Slot liners and top/bottom coil-slot separators should always be used. The best practice for rewinding random wound motors is to wind all motors using Class F, 1600V, VFD rated insulation materials. If you try to keep both types of materials on hand (ie, Class B/1000V and Class F/1600V), then the cost for material inventory will be twice as much. The other problem with having both types of materials on hand is the risk of accidentally substituting the wrong materials into a VFD winding. Overall, it is less expensive and more reliable to use the same high quality materials and winding practices on all windings.
