GB1432268A - Stator current vector control in rotary-field electrical machine - Google Patents

Abstract

1432268 Control of A.C. motors SIEMENS AG 6 July 1973 [26 July 1972] 32356/73 Heading H2J A method of generating in a rotary-field electrical machine, a stator current vector V with a desired effective position comprising the steps of generating a stator current vector which alternately occupies a first position on one side of said desired effective position then a second position on the other side of the first desired position. Fig. 1 (not shown) relates to a threephase bridge of thyristors with a commutating bridge of six thyristors supplied from the output of a rectifier and driving a three-phase asynchronous motor. A trigger circuit as described in Specification 1,290,962 provides triggering pulses for the main and commutating thyristors at appropriate times so that the stator current vector can rotate in steps of 60 degrees in response to control signals S in the form of cos B and sin B derived from a static two-phase generator 7 (Fig. 4). This generator is not described in detail but may comprise two series-connected integrators each preceded by a multiplier, the output of the second integrator being fed back to the input of the first and which generates signals the frequency of which is a function of the input voltage B. A signal in the form of cos B* and sin B*, representing the desired stator current vector V is fed into a vector rotator VD where it is compared with the output of the two-phase generator 7, the resulting difference signals being passed to a divider 8 so as to provide a tangent function from which a function generator 9 provides a signal which is a function of the angle. This is fed to a proportional plus integral controller 10, positive outputs of which are passed by a diode 11 and negative outputs being substituted by a constant feed B determining minimum frequency. Where high speed operation of the motor is required, electronic change over switch 14 is set to its upper position and the rotating field of the motor steps at a constant desired speed. If the signals V are such as to require low speed or stopping of the motor, a signal is derived by an angular velocity measuring device 12 and a limit value device 13 so as to set electronic change over switch 14 to its lower position. This allows a maximum speed signal # max to provide rotation of the stator current vector at a speed too great for the rotor to follow between angles G1 and G2 (Fig. 3). When the error between V and S is sufficiently small as determined by a preset signal D and a limit signalling device 16, mixer 15 overrides the signal # max and the stator current vector is stepped relatively slowly from a first position, clockwise of V (Fig. 3) to a second position anticlockwise of V. The effective position V is determined by the relative duration for which the stator current vector remains in the first and second positions. Alternatively, the angular range which is symmetrical about vector V may be determined by an amplifier 19 responsive either to the difference B*-B or to the sine of the difference. Fig. 5 (not shown) relates to a pair of multipliers and differentiators for use in block 12 of Fig. 4.