Energy-saving retrofit design for escalators

Abstract : This design utilizes a control combination of an MM440 frequency converter and a Siemens PLC to transform a traditional escalator into a speed control system, enabling it to achieve smooth start-up, energy-saving operation, and maintenance operation functions. During escalator startup, it avoids generating large starting currents; when no one is riding, the escalator switches from its rated operating speed to a low speed, saving energy, reducing mechanical wear, and clearly indicating the escalator's direction of travel for passengers; during escalator maintenance, the maintenance operation function ensures the accuracy of escalator inspections.

introduction

Consumers are the mainstream group in today's society. They have increasingly higher requirements for the shopping environment, atmosphere, and functions, and generally expect the emergence of "large-scale" and "mass-produced" commercial venues. As a result, escalators have emerged in department stores, airports, subways, train stations, and other places that drive continuous economic development.

In the past, escalator control commonly used traditional relay control, and many escalators still use this method today. This electrical control principle is relatively simple and intuitive. However, relay control circuits have many contacts, resulting in complex wiring, more frequent operational failures, and greater difficulty in troubleshooting. Furthermore, relay control is limited by circuit control limitations, making it difficult to implement many additional advanced functions. When the escalator is in operation, it operates at its rated capacity regardless of whether there are passengers, leading to high energy consumption, severe mechanical wear, and short service life. With the increasing demands for automation and safety performance in escalators, the traditional relay control system can no longer meet the requirements for high performance. Therefore, a stable, convenient, and energy-efficient control system combining PLC and frequency converter has become the development direction of escalator control technology.

1 Design Requirements

A pair of uprights are installed at both the top and bottom of the escalator. These uprights are used to mount and fix the infrared sensor switches and the escalator's up and down indicators. Additionally, a pair of key switches are installed on the upper uprights: one is a switch to switch between energy-saving and mains power modes, and the other is a switch to select the up, stop, and down modes (the traction machine is installed in the electrical shaft at the top of the escalator, so the switch is installed nearby). When the energy-saving/mains power switch is set to mains power, the escalator's start, stop, and up/down modes remain the same as before the modification. When the energy-saving/mains power switch is set to energy-saving, the escalator's built-in start switch and up/down switches are disabled. In this case, the escalator's running direction is selected and controlled by the up/stop/down switches mounted on the uprights. The escalator is started by the infrared sensor switch. When someone steps onto the escalator, the infrared sensor switch is triggered first. When the system detects the infrared sensor switch signal, the escalator is started and a timer begins. During the timer, if a second person steps onto the escalator, the system restarts the timer until the set time expires, at which point the escalator automatically enters low-speed operation.

1.1 Appearance Modification and Work Process

The exterior renovation of the escalator is shown in Figure 1:

The escalator workflow is shown in Figure 2:

1.1.2 How to implement:

The system mainly consists of the following parts: power supply, PLC, frequency converter, etc. The PLC is the core of the control system. The PLC determines the output of the high-speed operation command based on whether the input photoelectric signal is valid. The frequency converter controls the running speed of the escalator according to the high-speed operation command of the PLC, and completes the fast and slow and fast and slow stop cycle of the escalator.

The main control flowchart of the escalator is shown in Figure 3 below:

2 Hardware and Software Design

2.1 Hardware Configuration

The hardware primarily utilizes a Siemens S7-200CPU226 programmable controller, a Siemens MM440 frequency converter, and an LHI878 pyroelectric infrared human body sensor. The motor is a commonly used 4-pole Y225S-4 type three-phase AC induction motor. This control combination offers advantages such as powerful functionality and minimal modification work. The hardware configuration is shown in Figure 4.

Figure 4

2.2 Software Configuration

PLCs are one of the three pillars of industrial automation technology and have been widely used since their inception. Speed ​​regulation and control are frequently used aspects of industrial automation applications. Frequency converters (VDCs) possess high-efficiency drive performance and excellent control characteristics, achieving significant economic benefits in improving control quality, reducing maintenance costs, and saving energy. In these applications, the role of VDCs is irreplaceable by any other control equipment or device.

The PLC, acting as the main controller, and the frequency converter, acting as the execution and detection device (equipment or device), must cooperate to complete the control task. The PLC can control the frequency command signal of the frequency converter so that the frequency converter outputs the corresponding speed control curve to control the process parameters; the detection signals and other intelligent control signals on the frequency converter can also be connected to the PLC to complete the system alarm and speed control, such as controlling the start, stop, forward and reverse rotation of the motor through the frequency converter.

After the electrical system is assembled, a carefully crafted PLC control program is required to achieve optimal control. The first logic signal processed in the PLC control program is the safety circuit detection signal. This signal is used to determine whether a fault exists in the escalator's safety circuit and its location; it is a crucial guarantee for the safe operation of the escalator. Secondly, signals for escalator stop, ascending, descending, maintenance, and lubrication must be detected. Then, by processing these input signals, corresponding operating status displays and alarm signals are provided. All these control signals are generated by the PLC through the control program, processing various input signals. These programs must be coordinated and make reasonable judgments to achieve overall escalator control.

2..2.1

The PLC's I/O allocation is as follows:

2.2.2 Speed ​​control is achieved by writing a PLC ladder diagram program using Siemens STEP7 software. The ladder diagram is shown in Figure 5.

After the escalator starts, the PLC continuously monitors the input of electrical pulse signals from the photoelectric sensors to determine whether anyone needs to ride the escalator and controls the frequency converter to operate at normal speed or low speed. During normal speed operation, if no photoelectric pulse signal is detected, the system automatically switches to low speed operation after a 120-second delay (passengers moving from one end to the other). If the sensors installed at the escalator entrances detect passengers, the escalator speed immediately and smoothly increases to the rated speed. If passengers continue to enter the escalator, it will continue to operate normally at the rated speed. When the escalator is running at the rated speed, if no passengers are riding, the escalator automatically reduces its speed to approximately one-fifth of the rated speed.

The timing starts when the sensor outputs an output. If less than 2 minutes have passed, the timing restarts with the next output, continuing until the timing is complete.

Figure 5

2.2.3

To check the operating status, press the up/down check button. The escalator will run at 1/2 of its rated speed, as shown in Figure 6.

Figure 6

2.3 Energy Saving Analysis

Currently, conventional escalators operate at their rated speed even when unloaded, resulting in high energy consumption, significant mechanical wear, and a short lifespan. For example, the Jilin Guomao Shopping Center has 16 escalators, each with a power consumption of 8kW, consuming approximately 70kWh per day, and each escalator is unloaded for about 5 hours daily. If the escalators stopped or slowed down when unloaded, electricity consumption would be significantly reduced. Changing the escalator operation mode from continuous constant speed operation to normal constant speed operation when passengers are using the escalator, and slowing down or stopping when unloaded, would achieve energy savings. After consulting numerous resources, I believe this approach is entirely feasible.

After research and testing, a frequency converter was installed in the escalator's electrical control circuit. Variable frequency speed control ensures smooth start-up and energy-efficient operation. When no one is using the escalator, it switches from its rated speed to a lower speed, saving energy and reducing mechanical wear. When a passenger approaches, the escalator starts and runs at normal speed; after the passenger leaves, it slows down or stops, waiting for the next passenger. If passengers continue to arrive, the escalator runs continuously at normal speed until the last passenger leaves.

Most escalators currently manufactured lack speed control functionality, and their mechanical components are driven by three-phase asynchronous motors via gearboxes. According to electrical engineering theory, the formula for the rotational speed of an AC motor is: (5.1)

In the formula: f is the frequency of the stator power supply/voltage regulator; p is the number of pole pairs; n is the rotational speed; s is the slip rate.

① Changing the number of pole pairs p of a motor can change the motor speed; this is the speed control method used in AC dual-speed motors.

②Speed ​​regulation is achieved by adjusting the stator winding voltage to change the slip s. This is the speed regulation method used in AC speed-regulating motors.

③ Changing the stator power supply frequency f can also achieve speed regulation, but f cannot exceed the rated frequency of the motor. As a constant torque load, the elevator needs to maintain the maximum torque during speed regulation, according to the torque formula: (5.2)

In the formula: Cm is the motor constant; I is the rotor current; φ is the air gap flux of the motor; cosφ is the rotor power factor, which must be kept constant. Also, according to the voltage formula:

(5.3)

In the formula: U is the stator voltage; f is the stator voltage frequency; W is the number of turns of the stator winding; k is the motor constant. U/f must be kept constant. According to the motor drive principle, the ratio of speeds is equal to the ratio of frequencies, the ratio of voltages, and the ratio of power.

Right now

(5.4)

In other words, the frequency converter must have both voltage transformation and frequency conversion functions, which is the basic control principle of the variable frequency elevator.

2.4 Economic Benefit Analysis:

Converting existing constant-speed escalators to variable-speed escalators, while keeping the motor unchanged, has proven to be the most effective energy-saving method by installing a frequency converter. The frequency converter changes the power supply frequency to achieve speed control.

For example: The escalator motor is a 2-pole single-speed motor. According to formula (4.1), when the slip rate does not change significantly, n is basically proportional to f. When the escalator is at a constant speed, f = 50Hz, and the motor speed is:

n = 60 × 50 / 2 = 1500, which means 1500 revolutions per minute.

When the escalator is moving slowly, f=20Hz, motor speed:

n = 60 × 20 / 2 = 600, which means 600 revolutions per minute.

When the rated voltage of the motor is 380V and the frequency is 50Hz, after the frequency is changed by the frequency converter to 40% of the original frequency, i.e., 20Hz, the voltage supplied to the motor becomes:

U1 = (380V × 20Hz) / 50Hz = 152V

According to formula (3.4), P1 = 0.4P0

Therefore, the power consumed by the motor is 40% of the original power.

Based on the above calculations, if an 8kW electric motor runs for 11 hours a day and the cost of electricity is 1 yuan per kilowatt-hour, then its daily electricity cost would be:

1×11×8×0.8=70.4 yuan.

After installing the frequency converter, if the daily slow-speed operation time is 50% of the day, then the daily electricity cost for slow operation is:

1×5.5×8×0.4×0.8=14.08 yuan.

The daily electricity consumption after installing the frequency converter is: 70.4/2 + 14.08 = 49.28 yuan.

The daily electricity savings after installing the frequency converter are: 70.4 - 49.28 = 21.12 yuan.

The actual electricity savings, measured by the electricity meter, average about 20 kWh per day.

It is evident that the equipment cost of escalator energy-saving retrofits is relatively small compared to the costs of electricity savings and mechanical wear and tear, resulting in a considerable return on investment.

3. Conclusion

Escalators are widely used in hotels, subways, train stations, office buildings, and other similar locations. However, due to the specific nature of their use, some escalators often run idle. In today's energy-scarce society, operating escalators in an idle state is a huge waste and causes unnecessary wear and fatigue damage to escalator components (such as motors, gearboxes, and handrails). Therefore, energy-saving retrofitting of escalators is crucial. On the hardware side, a control system combining pyroelectric infrared sensors, programmable logic controllers (PLCs), and frequency converters offers advantages such as powerful functionality and minimal retrofitting workload.

This escalator energy-saving control system features high portability, simple structure, ease of use, low power consumption, and high reliability, saving approximately 30% to 50% of electricity and has broad application prospects.

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