As mentioned by Mr. Joe Buckley, it is normal to have some vibration if the yoke is being used in AC mode. But in DC mode it should not give any vibration.
In AC mode the noise resulting from the vibration is just a hum and perceptible to the operator but barely audible to the people in the vicinity.
Again in AC mode, if either or both the poles of the yoke are not making full contact with the part, the vibration and the noise may be much the higher. Though the vibration and noise are manageable it is not a good sign as you may not be getting enough field into the part for proper testing. If the contact cannot be improved due to difficult geometry you have to use a field indicator to verify the technique.
In Ac mode if you are testing on a thin plate/part (loosely held ) the vibration and noise may be very much on the higher side but the testing itself may be Ok.
P V SASTRY
14:38 Jan-09-2012 Dent Consultant, NDE Manager and Level III/3, Consultant, Canada, Joined Nov 1998 186
AC is an alternating current. The poles alternate from negative to positive in most parts of the world 50 times a second and some 60 Hz, and in turn the poles alternate from North to South. When then magnet is energized the magnet pulls. When the polarity is switching from the North to the South the magnetic field goes through a neutral point and does not pull. So you are feeling the magnet pulling down and neutralizing at 50 Hz.
09:41 Jan-10-2012 P V SASTRY R & D, NDT tecniques metallurgy , TAKEN VRS FROM THE POSITION OF SR. DEPUTY GENERAL MANAGER BHEL CORPORATE R&D, India, Joined Jan 2003 117
Yes I know the pit fall and any way thanks for reminding.
In lab we have experimented with known defects ( wire cut notches and a few natural cracks in turbine blades) with different contact positions of the poles and in most positions the defects formation was there though not the same degree. Off course each time we demagnetized as the turbine blades particularly retain lot of magnetism.
P V SASTRY
13:15 Jan-10-2012 Nick Welland Consultant, Quality and NDT , Self-employed, Australia, Joined Oct 1999 35