Why does the shaft or rotor of an electric motor turn blue?

Most readers are familiar with the term bluing, which is a process that involves heating steel or steel parts to a suitable temperature in air, steam, or chemicals to form a blue or black oxide film on their surface.

For motor components, the blueing process is often not due to a special blueing treatment, but rather a physical phenomenon that occurs during the motor’s manufacturing or operation.

There is no blueing factor during the machining of the motor shaft itself. However, blueing may occur during the assembly of the motor shaft with other components, or during other subsequent machining processes.

The shaft and rotor core of a squirrel-cage motor have a typical interference fit relationship. When the shaft and rotor core are assembled in a hot state after the rotor core is cast, the heat of the cast aluminum rotor is enough to cause the motor shaft to show a local blueing phenomenon (the mating position between the rotor core and the shaft, extending to different degrees at both ends), and some may even show a blackening state.

Under normal circumstances, whether during the production process or during the operation of the motor, the surface of the motor rotor is the original color of the metal after machining. However, some faulty motor rotors may exhibit a partial or complete blue discoloration. From the physical phenomenon of metal blue discoloration, this is due to the problem caused by excessively high temperature during motor operation.

Generally, if a part of the rotor of a motor turns blue, it is caused by casting defects in the rotor itself, such as broken or thin bars. During the testing of such motors, a serious imbalance in stall current will be observed, and in severe cases, aluminum leakage may occur in the rotor. If the entire rotor of the motor is in a blue state, one possibility is that the stator winding temperature rise is too high. At this time, the stator winding may have shown a state of insulation aging similar to overload. Of course, thinner rotor aluminum bars can also cause similar problems, but this possibility is relatively small.

In summary, blueing of motor components is an abnormal phenomenon caused by unstable processing technology or potential quality problems in the motor. Motor manufacturers and users should accumulate experience in this area and take measures to control and improve it.