To cope with the effect of the power electronics and the pulse thereof, it is usual to wind stator with specially varnished wire from supplier. This is often referred as triple coated as opposed to double coated (Standard offering). Don't be confused by number of coatings terminology. The wire has multiple coats applied. The term double and triple are generic to the industry. In effect the number of pin holes per m length of wire is reduced and the insulation (Enamel coating) on the wire has a higher value for "triple" coated versus "double coated. You will need the connection diagram and due to increase in diameter the slot fill factor will increase slightly.
An induction motor sees inrush current during a line start. As the motor begins to rotate, this current declines - which ultimately produces the speed-torque profile for the machine.
On a variable frequency drive (VFD), the current is controlled by the power electronics. This means the speed-torque curve looks more like a constant slope as both frequency and voltage are increased (keeping the ratio between them constant).
To successfully accelerate on a VFD, the motor torque (constant slope, right?) has to be greater than the load torque required at ALL speed points up to operating speed. Can't stay too long at slow speeds, either, since the cooling is not matching the heat generation in the windings.
In general - individual strands will have more insulation to withstand the spikes produced by the VFD pulses. You'll also need more insulation between phase leads, particularly if you have a low turn count. You probably need to reduce copper cross-section, but it really depends on slot fill.
Increase slot liners by one to 3 overall. (Assume there are two currently. I am also assuming we are talking about a mush wound machine not formed coil.)But make them the same overall thickness as 2 liners. That then will improve insulation but have no effect on slot fill. The extra enamel should not be enough to cause a problem with hand winding. If you can rewind a perfect lap so much the better. Ideally the slot liners should be 1/3. 1/3, 1/3 a sandwich of Nomex, Polyester Film and Nomex. a total of three such liners will be more than adequate and not affect heat transfer much if any. A 12 pulse rectifier is better than a six pulse to give truer wave form and less heating.
The motor cooling ventilator of normal induction motor is on a motor shaft, rated for cooling with rated speed. There can be overheating problems in case a motor operate long time with lower speed, in could be not enough cooling. So 1) stop operating with speed lower than 30% of rated speed 2) we use thermal model (microprocessor based motor protection) with amendment for 30% 100% of VFD controlled rated speed. The practice says if your motor operates near most time near 80% of rated speed, you'll not take a problem.