I was reading about 87Hz technique. I saw we can configure the drive for a motor 400Y/230D as 400V /87hz. I know v/hz ratio should be kept constant and when we configure drive as 400V and 87 Hz and wire it delta this ratio is still constant. But there will be 1.7 times more voltage applied to motor windings. Is it ok for winding. Is there no risk of damaging insulation ? For example Can we apply 800V and 174 Hz to a motor without damaging assuming that we can source it?
Motor winding insulation is generally adequate for any distribution or connection of the individual windings. The technique that is described is commonly used without consulting motor manufacturers to verify that it is ok. Motor bearings are also selected conservatively. Some standards require motors to be capable of some speed above the mains frequency synchronous speed. Friction windage and iron losses need to be considered. The motor should be advertised by the motor manufacturer to be "VFD rated." The manufacturer literature should be studied to determine what "VFD rated" covers. There is not a standard that specifically defines that, but there are standards that cover aspects of VFD powered motors.
It would probably be a good idea to check with the motor manufacturer before using this technique with a 2-pole motor. In a situation where a 4-pole motor does not have enough starting torque after converting from fixed speed to VFD operation, switching to a 6-pole motor and setting the VFD up for 87 or 90 Hz operation may solve the problem.