Patent GB-1309135-A (GB)
Title: Variable voltage drive system
IPC: B66B1/30
Filed: 1970-06-12
Abstract: 1309135 Automatic speed control WESTINGHOUSE ELECTRIC CORP 12 June 1970 [30 June 1969] 28555/70 Heading G3R [Also in Division H2] A lift motor 1 is supplied from a Ward Leonard generator 3 whose field is excited by SCR amplifier 17 associated with an A.C. source comprising transformers TR2, T3. The input terminals of the amplifier 17 receive an error signal representing the difference between a reference signal developed across potentiometer PP and a speedresponsive signal from tachometer generator 77. When the car is travelling at full speed and the potentiometer PP fails as an open circuit, a resistor 31 attenuates the tachometer signal so that a moderate signal is applied to the amplifier 17 tending to reduce the speed of the car. If the tachometer generator is open-circuited during full-speed operation, the motor speed tends to increase. The tachometer generator is shunted, however, by the armatures of the motor 1, generator 3, and the resistor R36 to provide a feedback signal which brings the car to a slow speed. When the car is started from a floor, full potential is applied to the amplifier 17 through the closure of contacts CL-5 and the opening of contacts CL-6 in the power supply circuit. Contacts CL-7 open to disconnect resistor R30 shunting potentiometer PP. The rectifier 75 is energized, but no signal is supplied by the potentiometer PP at this time. Energization of bias circuit 37 applies a voltage across resistor R25 which introduces a signal to amplifier 17 for upward travel of the car. Pattern generator 22 displaces slider 73 to accelerate the lift, as a function of the distance the car has travelled, until the motor armature voltage causes Zener diode ZD1 to break down whereupon feedback through resistor R34 and capacitor C7 tends to dampen changes in the car speed. If a stop signal is  generated before the car has completely accelerated, contacts 34X-2 open but the bias signal is not removed instantaneously due to energy stored in the circuit 37. Tap 73 is moved towards the neutral position, capacitor C6 dampening sudden reduction of speed. The charge stored in capacitor C7 reverse biases the diode ZD2 and opposes sudden reduction in the input signal to the amplifier 17. Door-opening is initiated, and when the car reaches a floor-level, plate 67 interrupts the circuit of a photo-electric detector 65 to remove the power supplies from the rectifier 75 and bias circuit 37. Contacts B2 complete a feedback connection to force the motor current down to zero. When no stop signal is generated when the car approaches the upper terminal, cam 69 opens switch USD to remove the signals from the pattern potentiometer and the bias circuit. Closure of contacts TSD-1 applies a voltage across resistor R25 tending to cause the car to continue travelling upwards. The A.C. voltage applied to the amplifier 17 is reduced so that the car proceeds at a fixed speed until limit switch UL is actuated, and the car remains at the upper terminal until the motor-generator set is shut down and restarted. The A. C. potential applied to the amplifier 17 is also reduced in the event of over-speed, the pattern signal being reduced by the shunt resistor R30. Opening of the doors is delayed under these circumstances. A hand control pattern circuit 41 positions the car at points intermediate the landings.