Stator Windings Tightness Check
The tightness of the stator wedges in the slot should be checked on a regular basis. The effective methods are wedge tapping and ultrasonic detection. The stator wedge is struck by a blunt object. The tightness of the wedge in the slot will determine the type of sound “ring.” A tight wedge will give a dull sound, and a slack wedge a hollow sound. A ring between these extremes indicates that the wedge will become loose in the future.
A measuring instrument using an ultrasonic technique can also be used to determine the tightness of the stator wedges. This portable equipment uses a vibrator, accelerometer, and force gauge to excite the wedge. It identifies the natural vibration resonance and assesses the tightness of the wedge assembly.
Stator Winding Side Clearance Check
This test is done to ensure a tight fit between conductor bars and the slot sides. A feeler gauge is inserted to determine the tightness.
Core Laminations Tightness Check (Knife Test)
This test involves inserting a standard winder’s knife blade (maximum thickness of 0.25 mm) between laminations at several locations in the core. If the blade penetrates more than 6 mm, then the core is soft. It should be retightened by packing.
Visual Techniques
If there is indication of insulation aging or a fault, the machine should be visually inspected. Flashlights and magnifying glasses are normally used. Small mirrors can be used for examining the inside edges of retaining rings. Boroscopes are used sometimes to gain access to stator core laminations or conductors. Magnetic strips are swept along the internal surfaces to pick up small magnetic fragments.