Hello bufordwon
The greater the applied current the greater the sensitivity will be.You have to weigh the need to find the problem ,if there is one,without destroying the rotor.
You can't kill the patient in the process.
As the previous post suggest the 10 to 20% voltage will suffice.As pete says ,if you can apply 50% without damage to the rotor,you will have better results.
The best approach is to try 10% and see what you get.If you find a fault at 10% you don't have to go any further.Rotor needs work.
I would be very surprised, if you get to 30% and still no indication of a problem.I would think that perhaps you don't have a rotor problem.If you have to go as high as 50%
than it becomes crucial that you monitar temperature rise.99% of rotor problems will be found at 20 to 30% rate.
GusD