Energy saving system for larger three phase induction motors

An energy-saving system for feeding an electric induction motor acting as a load, said motor having a rated voltage, and being operable from a 3-phase voltage power line, and in conjunction with feeding conductors for said motor, said motor including an iron core defining a hysteresis curve, said hysteresis curve having a linear portion, and a knee adjoining said linear portion, comprising in combination

• power-delivery means operating in a magnetically linear region and being fed from said 3-phase voltage power line for applying variable feeding voltages to said induction motor in a range which includes the range above said rated voltage,
• sensing means coupled to said induction motor for generating a signal indicative of the derivative of active power consumed by said induction motor with respect to the voltage of at least one phase provided to said induction motor,
• means for limiting said feeding voltages so that the magnitude of the applied voltaged produces a flux density up to just below the knee of said hysteresis curve and up to near the end of said linear portion of said hysteresis curve, and
• control means coupled to said sensing means and to said power-delivery means for regulating at least one of said feeding voltages so that the derivative of said active power with respect to the voltage of said at least one phase is always substantially zero, said control means including variable transformer operable voltage raising and voltage lowering means fed from one phse of said 3-phase power line, said means for limiting said feeding voltages to the end of the linear portion of said hysteresis curve being operable so as to disable said voltage raising means from operating beyond a safe limit, whereby for a given mechanical power output the power consumption of said induction motor is optimized, while concurrently losses in said feeding conductors are being minimized, and the exciting current of said motor is held within a safe level, so that said motor is operable at said flux density up to just below the knee and up to near the end of said linear portion of said hysteresis curve,
• wherein said sensing means comprise
• sampling means for sampling said active power consumed by said motor and by said feeding lines, and for sampling the feeding voltage of at least one phase at a sampling frequency sufficiently high to detect any changes in average power and mean voltage,
• active power-to-time derivative measuring means, and feeding voltage-to-time derivative measuring means, said derivative measuring means being coupled to said induction motor, and
• logic circuitry including divider means connected to said derivative measuring means for obtaining a signal proportional to the derivative of said active power with respect to said feeding voltage of said at least one phase,
• a digital-to-analog converter postcoupled to said divider means, and wherein said signal proportional to the derivative of said active power with respect to said at least one feeding voltage includes positively-going and negatively-going signals, and a reversible servomotor, and wherein said control means includes first signal passing means passing only said positively-going signals, and second signal passign means passing only said negatively-going signals, said signal passing means being connected to an output of said digital-to-analog converter, said reversible servomotor being actuatable by said signal passing means, and wherein said power delivery means includes variable voltage producing means actuated by said servomotor,
• wherein said variable voltage producing means include said varible transformer fed from said one phase of said 3-phase power line, and wherein said control means includes master control means regulating the voltage of said one phase, and providing a reference signal, and two slave controlmeans including second and third variable transformers, respectively, controlled by said master controlmeans, and regulating the voltages of each of the remaining phases of said 3-phase power supply line in dependence of said reference signal of said master control means.