The requirement of 40lb lift is for DC as well as permanent yokes. DC produces a constant-direction, constant-strength field whereas AC produces a 50 Hz reversing direction and fluctuating-strength field. Have you ever tried lifting MORE than 10lb with an AC yoke, very difficult. Even the 10lb lift needs a well-maintained, flat-footed and deftly handled AC yoke to persistently achieve the lift.
The major reason for this difference is based on the expected or required depth of penetration; AC has a maximum of 2mm penetration for surface testing whilst DC is expected to penetrate up to 8mm for surface and sub-surface testing.
Lifting capacity of yokes are 40 lbs for DC as well as permanent yoke.
Permanent yoke shall have 40 lbs since we are not using any current with it. So fully charged Permanent yoke lift 40 lbs. Due course of time its lifting capacity will drop down in that case you have to send it for re charging. These are used for normally sub surface discontinuities. But for AC as Mr. Nigel explained in one second AC changing its polarity for fifty times. So there is no chance for creating hard magnetic field. and it can lift 10 lbs. Its again gives us better results for surface discontinuities. AC producing magnetic field with sine wave current.
better mobility for particles and lighter penetration in material.