Importance of Core Testing AC Motors

As regulations on carbon emissions come into force, and more attention is focused on concerns about global warming and energy costs, more attention is being placed by industry on the efficiencies of AC Induction motors.

Although their basic design hasn’t changed for over a hundred years (as after all you can’t change the laws of physics), improvements in construction methods and materials has made them smaller and, more importantly today in a “green” conscious world, more energy efficient.

It has been said within industry that “You cannot rewind high efficiency motors”. This is a myth that is simply not true. You certainly can, but only if you have the right equipment, procedures and fully trained engineers working to the best industry practices. Here at Fletcher Moorland we do. We measure the core loss before any rewind work is carried and afterwards to compare and check for any deterioration. We the put this data in our comprehensive repair and failure analysis report. You can then be assured that your motor is as efficient as the day it was made.

High efficiency is gained by reducing the inherent losses. In an AC motor there are many sources of losses. However losses from the core are a major source of energy loss, about 25% of total losses in the motor. Significantly core losses still exist at low loads or with no load because they occur due to the alternating magnetic field in the core which is always present.

So why do we get losses in the core ?  

Well to explain we need to describe what happens in an AC motor. The core is made from individual thin discs of high grade steel. Steel is used because of its excellent magnetic properties (its high permeability). These discs are pressed to form a hole in the centre that has a regular arrangement of teeth around its inner circumference. Each disc then has an insulated layer applied to both sides. These discs are called the “laminations”. Many layers of these shaped discs are stacked one on top of the other to form a long laminated hollow tube (the rotor runs inside the hollow centre section).  This laminated stack is known as the Motor Core