Patent ID: 8106615
Filing Date: 2012-01-31
Classification: F04B

Abstract:
1. A pump jack pump-off control method in which a pump jack is driven by an inverter having a power supply of a variable voltage and a variable frequency using an ac electric motor, the ac electric motor is protected against overload, and a pump-off condition of the pump jack is detected according to an average value or effective value of a secondary current of the ac electric motor during a down stroke period in each cycle of the pump jack or according to a delay time from each down stroke reference point to the maximum value of the secondary current of the ac electric motor, the method comprising the steps of: on detecting the pump-off condition, reducing a speed of the pump jack by an amount equivalent to a previously set speed; on detecting the pump-off condition in the reduced speed as well, reducing the pump jack speed sequentially step by step down to a previously set lowest speed; and during an operation of the pump jack with a speed thereof being reduced or during an operation of the pump jack at the lowest speed, switching the pump jack into an intermittent operation thereof according to an overload warning signal supplied from the ac electric motor.