Test Motors, through its predictive maintenance tool TestIM, allows to detect when the useful life of a component, in this case of an air-conditioning turbine, has ended, and allows to know when its replacement is necessary. It is a centrifugal air-conditioning turbine powered by a 4 kW three-phase asynchronous motor and started to be monitored at the end of August.
While still learning, the TestIM SmartSoft system detects a high probability of failure in the motor rotor. Specifically, the system detects an important presence of frequencies typical of said fault. The rotor of an induction motor consists of a normally active part of aluminum casting and a passive one of sheet metal to favor the passage of the magnetic field. The active part must be constant in section to facilitate the passage of current and not generate hot spots. The presence of air pores during the casting or breaking of bars during operation, hinder the passage of current and generate hot spots.
Causes of the fault
The reason for the presence of pores in the rods or in the short-circuit rings of the motor rotor is due to defects in the manufacturing process, in this case, the aluminum casting process. The aluminum casting processes for the electric motor rotor are usually injection molding.
If a suitable pressure profile is not produced, or the quantity of material is not correct or the process of cooling the mold is not the pre-established, for whatever reason, the pores may originate.
Possible effects on the machine:
The presence of a fault in the rotor, does not usually cause a catastrophic effect on the motor. What it causes, are hot spots that increase the temperature in the rotor. Precisely, the rotor is the hottest part of the engine because it has the worst cooling, and that heating is difficult to detect. This increase in temperature can affect the insulation of the coil of the stator, reducing the useful life of the same and therefore, favoring the appearance of some type short circuit, which would be catastrophic for the motor and consequently for the machine.
After commenting with the client, and seeing that the fault indication persists in the same state from September to November, it is decided to replace the engine in early December, with a new one of the same characteristics.
Once the motor has been dismantled, and to check the presence of these high internal resistance points of the rotor, a tomography is made on the rotor. It consists of an x-ray scanning, in sections of thickness of 0.5 mm, perpendicular to the axis, to detect the presence of pores or fissures in the aluminum of the rotor. For this purpose, the rotor is longitudinally split in two halves.
The results clearly show the presence of pores with diameters greater than 3 and 4 mm in both the short side rings of the rotor and the rotor bars. The maximum permitted diameter is 1mm, per VDG standard P201 D5 / 1 for die-cast aluminum.