Insulation fault in one direction only

Copper oxide will behave like a rectifier. We used to use copper oxide rectifiers in precision instruments like AVO meters.

Sounds like you have big lump of copper oxide somewhere creating a one way path to your DC megger.

By "direction", are you talking about the relative application of positive and negative voltages, or are you referring to the rotation direction of the motor? The copper oxide explanation is legitimate if the former is true, but if you're speaking of having an insulation fault that is apparent only when the motor is being run in, say, clockwise rotation, I'd start looking for wear and loose mechanical component problems (brushes moving in holders, rotor moving slightly if plain bearings are worn, etc. that could differ with direction of applied loading). I've seen similar problems in MUCH smaller motors, but have no experience with large ones such as yours. And I'm assuming that your motor is reversed, rather than that there is a mechanical system serving that purpose, but again, that is outside my experience. Providing more information improves your chances of getting good advice.

Not enough information.

Measured what?? at what point?? what with?? what readings??What direction??On supply lines??From starter?? At motor??Brushes down??Brushes up??

Please supply information.

How are you supplying dc to motor in for/rev through seperate scr, diode banks does for/reverse share same power supply

Insulation resistance is measured normally with Megger (that checks resistance in mega-ohms at higher voltage like 500 or 1000 or even 5000V). I do not know whether you used a megger or simple Digital multimeter. DMM applies only 1 or 3 V (max. 9V) to the circuit while used for checking high resistance. It is possible that some diode is present in the path and your are using a simple digital multimeter for insulation checking, which is not correct method. Use Megger to check Insulation but take care to isolate the motor from all types of electronics otherwise the megger will damage all electronics (this because electronic components have low PIV characteristics).