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My problem is that I have several motors on cooling tower fans that drive the fans through a gear reducer (5.5:1). During the summer months we need the additional cooling of the fans, during the winter months the fans ore off. The problem comes during the winter months because the fans turn slowly from the rising heat. The gear reducer bearings are lubricated with oil slingers and the slow rotation causes the upper bearings to run without oil thus causing premature bearing failure. Four of the motors are 30 hp and one is 100 hp, all are induction motors started with across-the-line 3 phase starters @480vac.
My plan is to apply a small DC current to two of the phases when the motors are stopped to act as a holding brake. My concern is that the motors will never come to a complete stop on their own and the regenerative effect will blow my DC circuit. My plans are to start a 10 minute timer from a starter auxillary contact that will energize a curcuit consisting of a 1kv full wave bridge, a load resistor, and a grounded shunt resistor. the purpose of the load resistor is to limit the current to the motor windings, the purpose of the shunt resistor is to limit the back emf from the regenerative effect.
Has anyone seen such a circuit in action? Are there other considerations that I have missed? Keep in mind that this is a very wet application and there are chlorine based chemicals present so any type of mechanical brake is out of the question.
My plan is to apply a small DC current to two of the phases when the motors are stopped to act as a holding brake. My concern is that the motors will never come to a complete stop on their own and the regenerative effect will blow my DC circuit. My plans are to start a 10 minute timer from a starter auxillary contact that will energize a curcuit consisting of a 1kv full wave bridge, a load resistor, and a grounded shunt resistor. the purpose of the load resistor is to limit the current to the motor windings, the purpose of the shunt resistor is to limit the back emf from the regenerative effect.
Has anyone seen such a circuit in action? Are there other considerations that I have missed? Keep in mind that this is a very wet application and there are chlorine based chemicals present so any type of mechanical brake is out of the question.