It may be essential for certain critical process drives to have an autostarting feature, to re-accelerate them after a momentary main power failure. This is required to save the process and the downtime, and prevent re-switching of these drives on a rapid restoration of power. This scheme can also be useful for critical processes where a restart of a drive may take a long time due to its torque characteristics or process requirements and resulting in a long downtime. Such a scheme can achieve faster stabilization of the process by retaining the drive in motion, and picking it up quickly on a rapid restoration of power, as in polypropylene plants and gas crackers. A paper mill, for instance, would require the whole length of paper to be removed from its drying cylinders if the mill is to be re-started after a shutdown.
This is a waste of paper, in addition to a longer downtime. Re-acceleration can be achieved by introducing an OFFdelay timer T1, into the control circuits of all these drives with a time setting of, say, 0-60 seconds, so that the contactors of the critical drives restart automatically after restoration of power within the set period. The time setting, however, has to be such that the drives are still in motion at speeds so that the process can be restored. Figure illustrates a typical scheme.
This can also be achieved by placing a capacitor across the operating coil of the main contactor which can provide a time delay up to 1 4 seconds by holding the contactor. Contactors with built-in capacitors are available, and are called OFF-delay release contactors. Such a short-duration hold-on feature may be adequate when the system is required to hold without a trip against momentary heavy voltage dips, arising from system disturbances or simultaneous switching of large motors or during an auto-manual bus transfer.
Re-acceleration may be restricted to only critical drives to avoid a high switching inrush current on resumption of power.
This can be achieved by grouping all these drives in two groups, one with a delay of T1 and the other through an ON-delay timer T2. Thus extending a progressive time delay so that each subsequent group of drives accelerates only after the previous group is switched, to avoid simultaneous switching of all motors.