55 KW Siemens Motor Rotor Crack - Reason and Effect

Most shafts are either aluminum or copper, each has advantages and disadvantages with aluminum shafts developing porosity, also the application of the motor can play a major factor in shaft failure, whether it is start and stop operation or continuous or variable load, and many other factors...

"Broken or cracked rotor bars will develop more severe high resistance connections then porosity. These high resistance

connections require the current to increase in the nearby bars to supply the torque required for start-up and operation of

the motor. During start-up, very high temperatures will develop around the crack or open bar causing potential damage

to the rotor core as well as the stator insulation. Thermal expansion of the copper or aluminum can change the severity

of the defect. The electrical effect of the broken or cracked rotor bar is an increase in the rotor circuit impedance and,

therefore, the stator impedance as well."

http://www.pdma.com/pdfs/Articles/Rotortest.pdf

Regarding the rotor crack-

I observed a 30kW motor fitted with a machine, runs normal for No Load and when tried to put load ( it is a instantaneous load ) in the machine , the Ampere shoots to very high ( infinity ) as the motor become stand still. When I reported this to the manufacturer , they tested the motor and said its because of rotor crack. It was a new motor and replaced under warranty. But due to this my starter ( star-delta ) and switch fuse unit got diamaged.