Variable Frequency Speed-regulation System of Elevator
Using PLC Technology
Abstract-In this paper, the system of variable frequency speed-regulation based on PLC was proposed. Firstly, the overall structure of control system was determined. Inverter and programmable logic controller were chosen to complete variable frequency speed-regulation. Secondly, parameters of inverter were set by develop system hardware. PLC drive selection and 110 points distribution were completed; rotary encoder was connected with PLC interface. Finally, the software flow charts and ladder programs were designed by analysis of elevator system on the basis of software design methods.
Keywords-variable frequency speed-regulation; program; logic controller; elevator control system
I. INTRODUCTION
Human power was used as simple lifting equipment before Christ in china, until now hand-gin still was used as lifting device for extraction wells water in some rural of northern areas. So China is one of the earliest countries of the world in the aspect of elevator prototype [1]. In 1854, Elisa Graves Otis from Americans had shown his invention at the World Expo to the world in the Crystal Palace from New York, which was the first safety elevator in history [2]. Since then, elevator has been widely used in the world.
In modem life and economic activity, elevator has become a sign of the city. Elevator has been used as essential vertical transportation equipment, especially in high-rise building. The control systems of relays were used in the conventional elevator, whose control functions and signal processing were realized by hardware. Most of electrical components were common control electrical appliances [3]. Such elevator has been produced with mature technology in the past years, which has been formed series of products. With the rapid development of social economy and improvement of life quality, the shortcomings from traditional elevator controlled by original DC converter and relay had been more and more obvious and exposed many problems: (I) the traditional elevator control system have high failure rate that were mainly due to numerous contacts, complexity of wiring circuit. In addition, electrical contacts were easy to bum out, which could result in poor contact. (2) Electrical controller and hardware wiring based on common method were difficult to achieve a more complex control function, that is, why the functions of system control were difficult to increase. (3)Electromagnetic agencies and contact action were so slow, mechanical and electromagnetic have large inertia. Therefore, the control precision of system was difficult to rise. (4)The traditional elevator control systems have some shortcomings, such as large structure, high energy consumption, serious mechanical noise and so on. (5) Due to line complex and high failure rate of control system, large maintenance workload and high cost were required. It was shown that we should execute a comprehensive transformation to the traditional elevator control system.
II. INVERTER PARAMETERS DESIGN
Elevators were used as vertical transport tool, which belong to potential energy load and require frequent start and stop [4]. With the change of the passenger capacities, up or down transformation, the motor has minimum load. When elevator was run in no load up-going or heavy load down-going, motor was used in the state of power generation. Similarly, the motor has maximum load when elevator was run in heavy load up-going or no load down-going, motor was
used in the state of power. Therefore, the motor was required to run in the four quadrants of axis [5]. The change of motor work states were realized by contactors in the traction motor of traditional elevator. Speed control of motor was completed by series resistance or series reactance in the stator circuit of two-speed asynchronous motor, which can not satisfy the comfort of passengers. Traditional relay control modes were replaced by PLC control method, which could gradually transited from DC speed adjusting system to AC variable frequency speed adjusting system in the modes of motor drag [6]. A. Inverter Type Select
Inverter is the voltage frequency converter, which could convert AC with fixed requency to adjustable frequency voltage AC to supply the motor operation. Considered control requirements and reliability factors from elevator, ANCHUAN VS-616G5 digital inverter was used in this design. The PG card and rotary encoders must be equipped with VS-616G5 inverter in the speed control system of elevator in order to supply motor speed and feedback [7]. Rotary encoder was connected with the coaxial of motor to detect the motor speed. A, B were represented as two phase pulse outputs from rotary encoder. When A phase pulse was ahead of B phase pulse 90' , the electric motor was operated in the state of forward rotation. Similar, when the A phase pulse was lag of the B phase pulse 90' , the electric motor was operated in the state of reverse rotation.
According to phase sequence of A, B pulse, the direction of motor rotation could be determined by rotary encoder. Motor speed could also be measured by frequency or period of A, B pulse.
Feedback signals were sent into VS-61605 through PO card to operation regulation [8]. The pulse waveform of A, B phases were shown in figure I.
B. Calculation of Inverter Capacity and Braking Resistor
We had supposed that elevator motor power of traction machine was defined as P1,operate speed of the elevator was V1, lift weight was W1 , lift truck was W2,additional weight W3, acceleration of gravity was g , Power of inverter was P. With the maximum load, the traction power P2 in the process of elevator rise [9].
Which,
was friction; δ could be negligible;
inverter power P should be close to electrical power P1.the safety surplus capacity must be considered relative to traction power P2. Therefore, P=1.5P2, we could known from experience value,
Therefore, the inverter capacity was selected about 15KW.
As elevator was potential energy load, variable frequency speed regulation system should have a brake function since elevator could generate renewable energy during the process of operation. When VS-61605 was used in the system of speed control, the braking resistors must also be configured. When the elevator was worked in the deceleration, the electric motor was run in the power generation state and feedback power to the inverter. DC part voltage from AC- DC-AC inverter could increase when sync speed dropped [10].The role of braking resistor was feedback power and suppresses DC voltage. Renewable energy were consumed in the braking resistor through the brake unit by use of the energy consumption braking mode, which has lower cost and good results.
The value of energy consumption braking resistor R2 should be made the value of the braking current I z does not exceed the half of the inverter current.
Which, U0 was defined as DC bus voltage of inverter in the rated state, R2＞30Ω .Because the work of braking resistor was not continuous long-term work, the power could be significantly less than the power consumed.
III. HARDWARE DESIGN
The hardware of elevator control system was composed by Car Lateral Control Panel, Elevator Landing Door Signal, Inverter, and Adjustable Speed Drives. The structure chart of control system was shown in figure 2.
The function of adjustable speed was completed by inverter and logic control section was
completed by the PLC controller. The logic relatives of various signals were responsible for PLC to send on/off control signals to inverter. At the same time, the working status from inverter was transported to PLC .the bilateral contact relationships were established. The speed loop and position loop were established by established to complete speed test and feedback by equip with PO card and Rotary Encoder, which could be connected with Motor coaxial connector. In addition, the system also must be configured with the brake resistor [11]. When the elevator deceleration, DC voltage hypertension must be suppressed due to motor working in the state of renewable generation power, which will back power to inverter. A. Motor Drive Control System
Similar as other elevator control systems, control system based on PLC was also mainly composed of two parts, that is, signal control system and motor drive control system. The basic structure of PLC elevator control system was shown in figure 3. The main hardware had been included by PLC controller, CPU memory, machinery, car lateral control panel, call elevator plate outside of hall, layer device, gate machine, adjustable speed drives, main motor drive system and so on.
According to the position detection method of Car Lateral from rotary encoder, a high-speed counter was required in the programmable logic controller. FX2N PLC from Mitsubishi Company made in Japan was chosen. The control instructions from elevator were realized by PLC software. The management and control functions of command signals were completed by PLC controller, such as start, acceleration/deceleration and stop of elevator traction motor and open/close door motor, operate direction, floor display, landing call, car lateral internal operation command, security and so on.
B. Signal Control System
Input control signal to the PLC have included that run mode selection, operation control signal, Car Lateral internal instructions, elevator plate calls, signal security information, rotary encoders, optical pulse, on/off door controller and limit level signals, door area and flat layer signals. The control system of six level elevators was used as an example in this paper. According to the need of control switch, about 48 input points and 30 output points need to be control. FX2N a 128MR model of PLC was chosen when we have considered about 10% to 15% surplus capacity of control points [12]. The signal control system of elevator was shown in figure 4.
