Abstract: This paper introduces the application of PLC timers in the protection of dual-speed elevator terminal stations. This method can improve the safety and reliability of elevators.
A PLC is a digital computing electronic system designed specifically for industrial applications. It stores instructions for performing logical operations, sequential control, timing, counting, and arithmetic operations, and controls various types of machinery or production processes through digital or template-based inputs and outputs.
PLCs have been widely adopted in the elevator industry due to their advantages such as reliable operation, simple programming, ease of use, and short design and debugging cycles. According to incomplete statistics, of the 4100 elevators in use in Tianjin, 562 are controlled by PLCs, with AC dual-speed elevators primarily using C-series PLCs manufactured by OMRON Corporation of Japan. Based on on-site safety inspections, the elevator drive system uses AC contactors, with the motor stator windings connected in series with impedance switching via contactors for acceleration and deceleration control. Terminal safety protection employs mechanical forced speed-changing switches, limit switches, and extreme limit switches. While this design meets operational requirements, it is susceptible to malfunctions such as contactor contact sticking, spring failure, inability to reset contacts, electrical component malfunctions, and switch mechanical damage, potentially leading to "overshooting" or "bottoming out" accidents. To prevent such malfunctions, in addition to improving construction quality and component quality, we can utilize the underutilized timers and relays in the PLC, leveraging its fault diagnosis capabilities to implement terminal protection through programming, thus compensating for the aforementioned deficiencies and improving the elevator's safety and reliability.
As shown in Figure 1, the application of PLC timers in terminal station protection is described below.
When the elevator is moving upwards, the upward direction relay SFJ0015, the express auxiliary contactor KF, the express running contactor KY0501, the door lock relay MSJ, and the upward contactor SX0502 are all energized and engaged, the brakes are released, and the elevator moves upwards. When the car encounters the upper forced speed change switch SQH0016, the PLC internal latching relay KRRP1001 is energized and engaged, and timers Tim10 and Tim11 begin timing. The timing duration can be set according to the terminal floor distance and elevator speed. After the upper forced speed change switch is activated, the elevator switches from express to slow speed operation. Under normal circumstances, the elevator should stop when leveling at a floor. If the car continues to move upwards without stopping, when the Tim10 set value decreases to zero, its normally closed contact opens, the slow contactor MY0503 and the upward contactor SX0502 are de-energized, and the elevator stops running. After the elevator car encounters the forced speed change switch SQH0016, it fails to switch to slow speed operation for some reason, and the fast speed contactor KY0501 fails to release. When the Tim11 setpoint is reduced to zero, its normally closed contact opens, de-energizing both the fast speed contactor KY0501 and the upward contactor SX0502J, causing the elevator to stop. Therefore, regardless of whether it is running at slow or fast speed, as long as the forced speed change switch SQH issues a signal, the elevator can be stopped using Tim10 and Tim11, regardless of whether other protection switches at the terminal station are activated, thus making the elevator terminal station protection more reliable.
When the elevator needs to descend, once the selection command is received, the descending direction relay XFJ0014 is energized, its normally open contact closes, the latching relay keep is reset, and both Tim10 and Tim11 are de-energized, their normally closed contacts close, preparing for the elevator to descend normally. The protection principle of the lower station is similar to that of the upper station and will not be repeated.
This method has no impact on the normal operation of the elevator, and its operation has been satisfactory. Since the instructions and functions of different PLC systems are not entirely the same, the specific approach must be considered, but the general meanings are essentially the same.
