At one of our onshore plants, there are two 100 HP motor driven effluent pumps. These are started with ATS (Auto Transformer Starter) with 60%,75%,90% tappings. During starting session the motor is connected through these tappings successively and the final contactor is closed. In one occasion the auto transformer in one of the starter burnt out. On inspection,it was found the timer for cutting out one of the tappings was defective and the auto transformer remained in the circuit causing over heating and subsequent failure. To avoid such mishaps, one indicator is provided in the starters which will glow when the main contactor connects the motor to the main. Operators are advised to watch this indication and asked to stop the unit if it does not glow within 8 seconds, the normal time taken for the main contactor to close. Also in one of the starters only the 60% tapping is used on experimental basis which can reduce the probability of failure.
In another occasion, it is found that the motor is tripping even before the completion of the starting session. There is a latching contactor in the starter which remains energized till the motor is stopped. This contactor is found releasing immediately after closing and the start fails. The circuit was checked thoroughly but found all components are healthy. Interestingly, this is happening in both starters. Finally the problem was traced to the gas fuel regulators of the gen-set catering power to the motors. The regulators could not supply the required fuel gas at the starting and the gen-set becomes sluggish and its voltage drops below the minimum voltage required by the coil of the contactor to hold the moving contacts and breaks the control circuit and the motor stops. The regulators were repaired/replaced and the problem is rectified.
In another occasion, it is found that the motor is tripping even before the completion of the starting session. There is a latching contactor in the starter which remains energized till the motor is stopped. This contactor is found releasing immediately after closing and the start fails. The circuit was checked thoroughly but found all components are healthy. Interestingly, this is happening in both starters. Finally the problem was traced to the gas fuel regulators of the gen-set catering power to the motors. The regulators could not supply the required fuel gas at the starting and the gen-set becomes sluggish and its voltage drops below the minimum voltage required by the coil of the contactor to hold the moving contacts and breaks the control circuit and the motor stops. The regulators were repaired/replaced and the problem is rectified.
