allowed to declerate down to the initial low limit speed w1 and upon achieving that speed is again accelerated up to the high limit speed w2 again the time required for such acceleration is measured this being the time between points t 2 and t 3 the two time periods that is t 3 t 2 and t 1 t o are compared and if significantly different the basket is caused to decelerate again to the initial low limit w1 where it is again accelerated up to the high limit w2 and the time required for acceleration measured the new time period t 5 t 4 is compared with the most recent measured time period that is t 3 t 2 and if there is a significant difference the same steps would be repeated in the illustration of fig3 the two time periods t 5 t 4 and t 3 t 2 are substantially identical thus indicating that the acceleration time is the same and therefore indicating that the inertia of the load is the same this is known because the torque applied by the motor during each acceleration step is held constant the basket is then caused to accelerate up to a new low limit speed w3 as an initial point from where it is accelerated up to a new high limit speed w4 and the time required for such acceleration t 7 t.