GB802737A - Improvements in and relating to dynamic braking systems - Google Patents

Abstract

802,737. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. May 28, 1956 [May 26, 1955], No. 15255/55. Class 38 (3). When an induction motor is operating at full speed its rotor resistance controller is permitted to move at a high rate until it reaches a point providing a slightly higher value of rotor resistance than that affording maximum dynamic braking torque, this point corresponding with position A on potentiometer 6. Accordingly, a contact member 5, which is mechanically coupled to the control member for the rotor resistance, moves at a high rate to the point A. A further reduction in the value of the rotor resistance at this rate would lead to unstable braking operation, so that, when the contact 5 reaches the point B, a relay 3 is energized and contacts 31 close to make the circuit of relay 311 for reducing the rate of change of resistance in the rotor circuit. The relay 3 is subjected to the difference of voltages derived respectively from the potentiometer 6 and a tachometer generator 1, which is driven by the induction motor and is connected to rectifier 2. The relay becomes energized when the speed-responsive voltage exceeds the voltage related to the position of the rotor controller. If the induction motor is running at a speed lower than full speed; the tachometer voltage is less and the change to a lower rate of rotor resistance decrease occurs later than in the conditions just described. In another arrangement, a tachometer generator 1, driven by the induction motor, is connected to voltage-sensitive relays 11, 12, which close their contacts when the tachometer speed exceeds 60 per cent and 30 per cent of full speed, respectively. Conductive strips 14, 15 and 16 are mechanically coupled to the moving element of the rotor resistance and are arranged to close contacts 17, 18 and 19 in succession. If the induction motor is operating at a speed above 60 per cent of full speed, then contacts 11<SP>1</SP>, 12<SP>1</SP>, are closed and relay 13 is energized to reduce the rate of removal of rotor resistance when strip 14 closes contacts 17, the rotor resistance still being at a high value. If the motor is operating at 30-60 per cent of full speed, contact 12<SP>1</SP> only is closed so that the change in rate occurs when strip 15 closes contacts 18. When the induction motor is operating at a low speed, the relay 13 is not energized until strip 16 closes contacts 19.