Hi. Just had a 150kW (old type) WEG motor fail due to rotor bars collapsing. Bars are copper section welded at ends. Bars have burnt through laminations at the ends in places. Debris has cut windings. Motor is soft started and drives a high inertial load. Soft start is AB. Start time is 20 secs. Is this just old motor problems or has the high rotor currents flowing for protracted periods caused the rotor bars to overheat The second failure is a 90kW weg motor. In this case the rotor assembly has shifted along the shaft and hit the endbell. Motor is vertically mounted. My beleif this would be a warranty case but motor is 10 yrs old.

Hello arcserv

When you start a motor, there is a period of time when the motor is accelerating the load from zero speed to full speed. During this time, there are high slip losses dissipated in the rotor. This will cause heating of the rotor bars.
Provided that the start current is high enough, the motor will have plenty of acceleration torque and spin freely to full speed. The total power dissipated in the rotor is equal to the full speed kinetic energy of the driven load. A high inertia results in a high power dissipation. If the start current is too low, the motor will struggle to accelerate the load and the power dissipation will be increased.

Motors have a maximum load inertia that they can start. This is related to the maximum energy that can be dissipated in the rotor. This can also be rated as maximum start time or max Locked Rotor time.

If the motor is inappropriate for the load inertia, or the soft starter not set up correctly, you will have the rotor problems that you describe.

Best regards,

Hi Mark
yes i appreciate your comments. Motor has been in this application for many years and is the appropriate size. The soft start has been set so as to accelorate the motor as quickly as possible and within the starting characteristics of locked rotor start time. This motor has been rewound once and before i changed the soft start it had an overload which was incorrectly sized ie set too high, possibly it has recieved a thrashing. Soft start is AB SMC flex.