An Oriented Strand Board (OSB) mill was experiencing multiple failures on motors located on their strander application; the copper bar rotor and windings were failing due to repeated and long reduced voltage starts. The application utilized an autotransformer starter to start each strander motor at 65% voltage. The motors were spending a full 60 seconds at reduced voltage, and there were no lockouts on the motor protection relay to prevent too many starts. When a motor is deprived of voltage, full torque cannot be achieved, and extended periods in this condition lead to rotor heating and damage.
After inspection, IPS suggested the customer install three 2000HP, 4160V, Toshiba Variable Frequency Drives (VFD’s) with Synchronous Transfer. Using a VFD allowed the motor to develop 100% of motor torque at starting and eliminated inrush current. A controlled start of the load with no rotor heating was achieved. When the motor was at 100% speed, it was then transferred to utility power removing the VFD losses and allowing a motor management relay to manage the motor profile. This kept the VFD from experiencing intermittent current spikes that come from the nature of the operation, leading to reduced downtime and damage to the VFD.