Subsynchronous cascade drives are typically used for large compressor and fan drives. As such drives are usually started by means of a liquid starter, subsynchronous cascade drives are normally considered to be unproblematic with respect to torsional behaviour during start-up. During commissioning of a 5500 HP exhaust fan drive with subsynchronous cascade, severe mechanical damage occurred on the shaft string. This paper presents the investigations and simulations carried out in order to find the cause of the damage and the possible solutions. The investigations showed that backfiring on the cascade during the start-up sequence has been the initial cause for an excitation of the shaft string. Due to the fact that the shaft string has been excited near its first torsional resonance, excessive mechanical torques were developed. These torques were so high that the coupling hub broke in two pieces and the shaft was twisted. The present study confirms that a torsional analysis must be carried out in an early project stage on all drive systems with higher power rating. This will reduce the risk for such type of damage and is the basis for a trouble-free commissioning and a safe operation of such drives.
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