REMOTE CONTROL AND MONITORING OF AN INDUCTION MOTOR

Transcription

1 Proceedings of COMADEM 2007 The 20 th International Congress on Condition Monitoring and Diagnostic Engineering Management Faro, Portugal, June 13-15, 2007 REMOTE CONTROL AND MONITORING OF AN INDUCTION MOTOR A. Antonelli, C. Boccaletti, G. Duni, G. Fabbri Department of Electrical Engineering, University of Rome Sapienza, Via Eudossiana, Rome, Italy, tel: (+39) , fax: (+39) ABSTRACT The paper deals with the design and implementation of a tool for the control and the monitoring of a remote power system. Commands can be sent to an equipment, and measurement parameters, such as current or temperature, can be monitored with feedbacks that can be sent back and stored. Operations are performed along the Internet, between one or more clients and a PC-server that is connected to the equipment to be monitored. The client sends a command by means of an to the PC-server through the Internet; the PC-server receives and decodes it, and let the terminal execute it if the client has the necessary authorization; the PC-server also detects the feedback quantities and sends them back to the client. The tool can be very effective when it is not possible to reach distributed systems by means of wire connections and only GSM/GPRS connections are available. It also aims at optimising the costs of installation and of the connections with the monitoring times fixed by the user. The experimental setup was built by using conventional components that are commonly available on the market. The whole architecture was developed through the following steps: design and assembly of the electronic circuits, selection of electrical and electronic devices, coding of the software for managing the complex Internet connection - client - electronic and electrical devices. The experimental prototype has been extensively tested with satisfactory results. KEYWORDS Remote control, monitoring, GRPS, GSM, Internet INTRODUCTION Nowadays the Internet is well accessible by many users, and can be used not only for information, communication or file transfer, but also for remote control. Remote control can be used for diagnosis and monitoring of devices - such as motors or drives even when the user is not present. If a problem arises in the remote system, it is possible to analyse the drive performance and make appropriate parameter changes on a remote basis to solve the problem. This can be done without an engineer being on site, by utilizing the communication port of the drive and the available 505

2 COMADEM 2007 network (Errath and Dominguez, 1999; Lee and Schneeman, 1999). The aim of this work is the design and implementation of a tool for remote control and monitoring of a power system via the Internet. The Internet wireless telemetry can be obtained by means of a GPRS/GSM connection; the cabling costs can be further reduced and power systems that are distributed in a territory where no other kinds of communication are available can be easily reached. The connection costs can be minimised since the monitoring times can be fixed by the user. Such a system and the relevant software are described in the following. Tests have been performed and the results show the effectiveness of the system. SYSTEM DESCRIPTION One or more clients, the Internet, a PC-server and a remote equipment (the motor) interact one with the other in the system. A client with the necessary authorization can send a command to the PC-server via the Internet (TCP/IP); the PC-server checks and decodes it and lets the motor execute it by means of an electronic interface. The PC-server also checks the measured parameters of the motor and sends it back to the client together with the report of the commands that have been executed. All the exchanged messages and the performed operations are stored in a database. Figure 1 shows the connections between the main elements of the project. The communications are performed via and a particular syntax is required. Figure 1. Scheme of the architecture 506

3 Faro, Portugal, June 13 15, 2007 The electronic interface An electronic interface allows the communication between the PC-server and the motor. It consists of two main sections (A and B): an actuator and an analog/digital converter, as shown in Figure 2. Figure 2. Scheme of the electronic interface The main functions are as follows: the actuator receives the signals from the PC-server and allows the opening and closing operations of four relays (5V DC) that select different power drives of the motor (speed, direction). It has been designed to fit the voltage and current levels of the PC-server. An electronic circuit between the PC-server and the relay has been also included. Thus, the safety is ensured and the reliability improved. Section B is devoted to the measurement of a motor quantity such as temperature or current. In the experimental setup it consists of a temperature sensor fixed on the motor (a 1 kω NTC thermistor) and an AD converter (ADC0803) that allows the discretisation of the measured temperature and the generation of a signal to be sent back to the client (Anton et al.,2004). A LED circuit is added to indicate the binary conversion from the analog temperature signal. The interface is connected to a parallel port of the PC-server (DB-25 connector). The motor An asynchronous monophase motor has been chosen for the remote control tests (see Fig 3 and Table 1). Two speeds are possible by means of different alimentation. Two of the four windings generate a twelve pole magnetic rotary field for the lowest speed; the other two generate a two pole field corresponding to the highest speed. The motor is supplied through four relays that are connected with logical ports. Signals coming from the PC-server are combined with the ones coming from the NTC sensor to determine the voltage level for the relays. 507

4 COMADEM 2007 Figure 3. The asynchronous motor Table 1. Motor technical data V 500 RPM 50Hz A W RPM The software The PC-server can be considered the core of the whole system. It is equipped with an Internet connection, an client, and a software that allows the data exchange with the electronic interface. Such software has been developed ad hoc and installed on the PC-server to manage the client and the exchange of information between the PC-server itself and the electronic interface. Its tasks are the following: management of the Internet connection, analysis of received s, generation of reply s, management of the commands to be send to the actuator and of the signal received from the ADC, database storage of the system actions and of the detected temperatures. This software was developed with Microsoft Visual Studio 6.0 with the integration of three libraries: MAPI for dealing with , I/O for the data exchange through the parallel port, Microsoft Activex Data Object for database management. The software is divided into two main blocks that correspond to two different timers: Timer 1 and Timer 2. Timer 1 calls the following modules: Internet connection check, connection to the client (e.g. Microsoft Outlook), analysis of new messages, sender check, syntax check, communication with the electronic interface, feedback to the user. Timer 2 depends on Timer 1. It checks if data are present on the parallel port, stores them in the database and manages the temperature measurements. In the case of an Internet disconnection, the command execution is suspended and a message is sent to the user as soon as the connection is again available. Figure 4 shows the user interface of the software. The commands are available on buttons and the data windows are user-friendly. 508

5 Faro, Portugal, June 13 15, 2007 Figure 4. Software user interface SAFETY ISSUES Some safety measures have been taken before setting up the system and performing the tests. The asynchronous motor should not run at speeds below the nominal ones (300rpm and 2800rpm/min), because this implies the circulation of huge currents in the rotor and in the stator. If they last for a long time, the isolation can be damaged and the whole system run a risk. If the load is high enough to slacken or even to stop the motor, the motor slip rises and this leads to greater electromagnetic forces and a dangerous increase of the rotor current. A speed sensor and some modifications in the software provide to avert this event. As better explained below, the sensor sends digital voltage signals to the PC-server, and if the signal corresponds to a lower voltage than the nominal one, after five seconds (a possible starting delay), a command is sent to stop the motor alimentation. An alert message is sent to the user to inform him about the event. The protection of the system can be subdivided into: software protection and hardware protection. The former is managed by the software in case of net logoff (see following paragraph) in case of accidental exit escape from the software, by the instant block of all functions of the card and the motor. The hardware protection consists of the thermal sensor connected to the logic circuit: the supply of the relays is disconnected if the temperature exceeds a fixed maximum value or goes below the minimum. The relay supply is only possible when the control system sends an OK signal. Network fault If the server is disconnected from the Internet, the software can manage the event in to ways: 1) All the functions are suspended a. the relays are disconnected and the motor is stopped b. the thermal sensor recording system is disabled c. an alert is generated and left in standby until the connection is resumed 509

6 COMADEM ) All the functions are kept running a. the relays and motor state is maintained for a time that can be previously fixed; if the connection is not resumed within that time, all the functions are suspended b. an alert is generated and left in standby until the connection is resumed EXPERIMENTAL SET-UP A system prototype has been assembled to be extensively tested (see Fig. 5). The relays are sized considering that at high speed the motor requires a power of 350 W at 220 V AC (the relays are chosen so that they allow 5A/220V). The PC-server is connected to the two sections of the electronic interface. Two voltage levels are possible from the PC: 0 V (binary 0) and 5 V (binary 1). The permanent Internet connection necessary for the test is established by means of an UMTS mobile phone. The thermal sensor is glued on the stator surface. Figure 5. Prototype components. On the left is the motor with the temperature sensor. TESTS AND RESULTS Before connecting the system to the PC-server, tests were performed to verify its safety. After starting the Internet connection, a set of commands were sent by the client, and the system promptly replied with changes of the drive state, and messages confirming the execution of the operations have been delivered. Also network disconnections have been manually generated to verify the tool reliability. In Figure 6 the temperature variation is shown for two cases: High Speed (HS) and Low Speed (LS). For both cases a START command is sent at time 0 and a STOP command after 35 minutes. After the machine has stopped, the temperature rises to its maximum value. This is due to the lack of ventilation in both cases, with a greater gradient for the high speed. 510

7 Faro, Portugal, June 13 15, 2007 Temperature [ C] HS LS Time [m] Figure 6. Measured temperature during two tests, at High Speed (blue diamonds) and at Low Speed (pink squares). Start command is given at t=0 and Stop command is given at t=35 min. Other tests have been performed, inserting a speed sensor in the system to measure and control the motor rotation speed. Like the temperature, the speed is converted into a digital signal by the same successive approximation A/D converter used for the temperature sensor and the detection is indicated by a LED array. For this reason a dynamo tachometer has been chosen as speed sensor instead of other types like, for instance, an encoder (see Fig. 7). A proper circuit has been inserted between the tachometer and the A/D converter to make the signal suitable to the latter. Three possible rotation speeds has been considered according to the motor application. The chosen speeds and the corresponding voltage levels and digital signals are shown in Table 2. The software provides a control on the voltage level (first four BITs of the sequence): if it is below one of the values set by the user (with a certain tolerance), after a time delay of 5 s (corresponding to the starting time) the motor is switched off and the user receives a message. Figure 7. Prototype components. In the foreground is the dynamometer. 511

8 COMADEM 2007 Table 2. Results of speed measurements. Speed Analog signal Digital signal (RPM) (V) (BIT) CONCLUSIONS AND FUTURE DEVELOPMENTS In this work a robust tool has been developed and tested for the integration of information systems, the Internet and power systems for control, monitoring and diagnostic purposes. The electronic components used for this work are available in the market at low price. Moreover, standard communication softwares have been used. Since the digital signals are very simple, no particular QoS (Quality of Service) issues are needed (Wang et al., 2001). Such modular system can have several applications. It can suit to all the cases in which the presence of an operator is not needed or not possible for a particular application. It is possible to know in real time the machine state by means of different sensors, whose signals (speed, temperature, etc.) can be decoded and sent to the PC-server and then to the final remote user. The information on the machine state can be obtained by request or delivered in case of fault. If there is more than one sensor, one converter is needed for each of them, or a switching system can be used. Since all the messages and events are stored in a database, statistics could be drawn for the relevant parameters (time, duration, sequence, etc.). The tool could be provided also with learning capability; for instance, such an expert system could take decisions in case of a network fault on the basis of the previous behaviour. A possible modification of the system is the introduction of a different mean for the remote connection, e.g. GSM. In this case an SMS protocol manager could be introduced in the software. This would be useful in all locations that are not reached by an Internet connection or a wireless network (Hossain and Fathizadeh, 2005). Control of more motors is possible by means of some modification of the software. More PC-servers are needed if the motors are located in different remote places. If they share the same location, the introduction of ad hoc codes can manage the distribution of the commands to the motor to be operated. E-learning is also an interesting application, online experimentation being available for students operating remote instruments (Anton et al., 2004; Yan et al., 2005). In conclusion, the system is reliable, can be built with cheap components, and is very easy to be modified according to different needs. REFERENCES Anton, D., Bragos, R., Riu, P.J (2004), Remotely accessible laboratory for instrumentation and sensors, Proceedings of the 21st IEEE Instrumentation and Measurement Technology Conference, Vol. 2, pgs Errath, R.A, Dominguez, J.J (1999), Remote drive condition monitoring, IEEE-IAS/PCA Cement Industry Technical Conference, Conference Record, pgs Hossain, A.; Fathizadeh, M (2005), Advanced Computer Control and Testing of Remote Rotating Machinery Using Dependable Wireless Conduit, Sensors for Industry Conference, pgs Lee, K.B., Schneeman, R.D. (1999), Internet-based distributed measurement and control applications, 512

9 Faro, Portugal, June 13 15, 2007 IEEE Instrumentation & Measurement Magazine, Vol. 2, Issue 2, pgs Wang, Q.P.; Tan, D.L.; Ning Xi; Wang, Y.C. (2001), The control oriented QoS: analysis and prediction, Proceedings of IEEE International Conference on Robotics and Automation (ICRA 2001), Vol. 2, pgs Yan, Y., Liang, Y., Du, X. (2005), Controlling remote instruments using Web services for online experiment systems, Proceedings of IEEE International Conference on Web Services (ICWS 2005) 513