3phase 480v 200 hp well motor, operated via soft start controller. During the monthly back up genset test, when the transfer switch changes from generator back to utility, the motor shuts off. It then goes into a pre-set lock out, so that the motor shaft can "backspin"..with out damage.After 2 minutes, the motor starts again.
I've seen this happen on other back up systems, with big motors. The transition is "seamless" to computers and other machines, but with big motors they turn off and have to be reset.
Would changing the delay from genset to utility solve this?..or can it be?
My guess is that it is open transition (break before make), and the two sources are not synchronized. Since during re-transfer back to utility, both sources are available, the re-transfer happens very quickly and the motor ends up with a tremendous inrush due to out of phase transfer. That may be a cause of starter lock-out due to a protective safety setting in the starter. Some transfer switches have a delayed transfer or an in-phase monitor, where either the field in the motor is permitted to decay prior to transfer or the transfer is only permitted when the "good" source is in phase with the load (decaying field).
#55807 - 09/07/0506:20 AMRe: transition from Genset to Utility
There is information that says for motors 50 hp and larger, there is a chance of breaking the shaft if the load it transferred from the generator to the utility with an open transition transfer switch. You need the coast down time or a closed transition transfer switch. Don
#55808 - 09/07/0507:59 AMRe: transition from Genset to Utility
At another well site, 1 well motor same size as above, along with 2 " booster pumps", are backed up via the same type of system. Same brand transfer switch, same programming on the plc.
BUT...the starters are VERY old, Clark Controllers. ( i'd say about 50 years old) And when they retransfer, none of the motors, even the 200hp motor gets bumped offline. I think the starters on the old system are not sensitive to the voltage fluctuation.
Im in the process of trying to dig up info on the soft start Westinghouse controllers, to see what can be done.
#55809 - 09/07/0501:54 PMRe: transition from Genset to Utility
Open transition switching causes very high current transients. These transients can be in the order of 10 - 20 times the rated current of the motor. Torque is proportional to current squared, so the torque transients are even worse. These transients are commonly the cuase of mechanical damage to the motor and driven load. - doesn't break every time, but the life is shortened. If you eliminate the transients, the gear will last longer. You need to have an OFF time long enough for the rotor field to decay significantly. This requires a delay in the order of a second or two. It is not important that the motor slows to zero speed, just that the rotor field has almost disappeared.