GB1467041A - Control system for induction motor - Google Patents

Abstract

1467041 Control of A.C. motors GENERAL ELECTRIC CO 19 Feb 1974 [20 Feb 1973 9 May 1973] 7576/74 Heading H2J A three-phase reversible squirrel-cage induction motor 10 is operated as a motor or generator in conditions of non-saturation of the magnetic circuit of its core, with acceleration or deceleration at maximum torque, by a control circuit comprising a drive unit (or frequency changer) 40 supplying the motor with a sinusoidal alternating voltage of frequency and amplitude determined by signals # s , E which, during motor operation in a normal range, are the same as signals # SC , E C supplied from a control signal generator (50), Fig. 6 (not shown), and signal limiters 150, 80, which define the normal range between limits # SA , # SD for the signal w SC and between an upper limit E L (determined by a voltage limiter 70) and a lower limit O (dependent on # SD ) for the signal E c , all these limits being dependent on the actual speed # r of the motor sensed by a tachometer 20 and on signals #p and (-)#p, respectively, corresponding to the slip frequency during acceleration and deceleration providing the maximum torque without core saturation. Signals #p, # r and (-)#p, # r are summed at junctions 301, 311 to provide the limits # SA , # SD for limiter 150, whose output signal # s is applied to unit 40, limiter 70 and a subtraction junction 60, which also receives the signal # r to produce a signal # of actual slip frequency applied to limiter 70. The output E L of limiter 70 controls limiter 80 and is a function of the synchronous frequency signal # s and the actual slip frequency signal #, equations for E L and the magnetizing current I M being derived from the design parameters of the motor to relate the slip frequency #p for maximum torque T M with the current I M . Thus a family of torque/slip frequency characteristics may be obtained for various values of I M up to that corresponding to the onset of saturation, Fig. 2 (not shown), the value of E L also depending on the selected value of I M . The torque/slip frequency characteristic for motor or generator operation with maximum magnetizing current I M is discussed, Fig. 3 (not shown). Also described are the circuit details of limiter 70, Figs. 4 and 5 (not shown), and limiters 80, 150, Figs. 7 and 8 (not shown).