Electro Magnetic Compatibility for Boiler Controller - Research Paper Example

Electro Magnetic Compatibility for Boiler Controller . Introduction Electromagnetic waves are available in nature due to radiation from atoms and molecules when they transit from one energy level to another. These waves are also inherent in lightning discharges. Electromagnetic radiation involves the propagation of Electric and Magnetic fields . Any change in the electric field induces a magnetic field as stated by Maxwell and any change in the magnetic flux density produces an electric field as stated by Faraday. These are the basic postulates for the time varying Electro – Magnetics . The electric and magnetic fields present in an electromagnetic wave interchange their energies as they propagate. Electromagnetic Interferences between electronic / electrical products are caused by the electric and magnetic fields produced from a source component. These electric and magnetic fields may be absorbed or received by another electronic / electrical component that is susceptible to electro magnetic radiations. The coupling between the electromagnetic transmitting device and the electromagnetic receiving device can be one of – capacitive coupling , inductive coupling , conduction or radiation. The radiated electromagnetic field ( EMF ) coupling , can be of near field coupling and far field coupling. In the near field coupling the Electric ( E ) field and the Magnetic ( H ) field are considered separately , but in the far field they are considered as plane wave coupling. These interfering fields affect the normal operation of any device or component and hence their effects have to be reduced. The common approaches to reduce the electromagnetic radiation effects involve shielding , grounding , isolation , separation , proper orientation , noise cancellation methods and of course proper design for Electro Magnetic Compatibility ( EMC ) . This report discusses such EMC design rules pertaining to a specific product – Boiler Controller. If the EMC is not considered in the design sequence , it would be too hard to meet the EMC requirements later . The EMC design approach depends on estimated system cost and production volume says Williams .T. ( 1996 ) . Role of EMI / EMC in Electronic Design : The common electromagnetic effects are described in terms of Electro Magnetic Interferences ( EMI ) , Electro Magnetic Compatibility ( EMC ) , Electro Magnetic Pulse ( EMP ) . The types of Electro Magnetic Interferences may be classified as continuous interference and pulse interference . Continuous interferences include radiations in the audio frequency ranges and is mainly due to power supply units. Other continuous interferences include radio frequency interferences and broad band noise. The radio frequency interference may be due to wireless transmissions, industrial equipments , high frequency signal in electronic circuits . Broad band noise occurs both in audio and radio frequency ranges. The pulse interferences are otherwise called as transient interferences . Sources of these pulse radiations include switching circuits , electrostatic discharge , nuclear discharges , surges in power lines , electric transients , etc. Electro Magnetic Compatibility ( EMC ) , refers the ability of a device or component to show disturbances or deterioration when exposed to Electro Magnetic Interference . The assurance to Electro Magnetic Compatibility ( EMC ) of a device can be achieved by maintain certain standards for emissive and susceptive levels , design standards , testing the device or component for Electro Magnetic Compatibility ( EMC ) . EMC design rules : The common design methodology for Electro Magnetic Compatibility ( EMC ) could include the following stages . Identifying the source of noise - the noise can be internal that is generated by the device itself or it can be external to the system . Control the ground path – any noise is an unwanted current signal . If this unwanted signal is properly grounded , then the disturbance due to that noise signal can be eliminated easily . This method of grounding is highly helpful in controlling the internal and the external electromagnetic noise interferences . Segmenting or partitioning of the system - this method of partitioning the system separates the sensitive circuitry from the noisy circuits . It also enables the usage of different levels of filters and shielding circuits to eliminate the electro magnetic noise from reaching the inner most part of the system . Checking RF immunity - external RF signals are easily picked up by long cables or I/O wires . To avoid this special EMC filters can be used or low equivalent series resistance capacitors have to be used. Handling ESD and other transients -Electro Static Discharges ( ESD ) and other high frequency transients can be controlled by using RF filters and large series resistors and components like over voltage protectors . Provision of decoupling – good decoupling of the power lines is a prime requirement for stability of any system . For digital circuits the supply current is of short spikes , and may switch to high spikes under abnormal conditions . Layout grounding - zonal grounding is essential for circuits involving analog and digital devices placed together . they can be separated into layers and grounded separately to avoid Electron Magnetic Interference. Shielding – shielding the circuit with a pressurized container like structure reduces the Electro Magnetic noise . A shield which is semi closed and connected to the ground can be more effective and acts like a ground plane. Filtering - to avoid interference currents through cables , they need to be decoupled at the point where they enter the enclosure. If the circuit involves surges of high energies , surge limiters have to be used on the mains . These are the common design procedures for Electro Magnetic Compatibility ( EMC ) of a product . The product – Boiler Controller : The product of concern , Boiler Controller , has five separate sub systems . the first system is a power supply feeding the other four sub-systems, this is a switched mode type (running at 40kHz), that produces 5Volts at 0.5 Amperes and 12 Volts at 5 Amperes from a 240Vac 50 Hz supply. The other systems are connected in series . the second sub system is a true centigrade temperature sensor - LM35 . It is connected via a two-meter lead to the third sub system which is a signal conditioning unit . The LM35 needs between +9 and 20 volts to power it. It’s response is 10mV/ C over the range -55 C to +150 C. The signal conditioning circuit takes the voltage from second sub system and changes it between 1 volt and 5 volts. The fourth sub system is a microprocessor board with an on-board ADC and an oscillator running at 100 MHz. The final sub system is a display and interfacing board, with outputs to a five-digit LCD, an alarm circuit and disabling system for a boiler. The last output is connected to an RS232 socket connecting to the control room 20 meters from the boiler . A keypad is mounted on to this board to program, set the temperature and assist in calibrating the system. EMC design for the Boiler Controller : In this product , the first unit is a Switched Mode Power Supply ( SMPS ) . The noise due to SMPS could be transient noise , power supply spikes and ground bounce . In any SMPS a component of RF noise occurs between live and neutral , line to line at the input , and between the positive and negative in the output .These are differential - mode interferences . these can be reduced by using differential mode filters as shown in fig.1. Here XCAP is a harmonic filter designed to absorb all the harmonics which are multiples of the switching frequency of the SMPS . Fig .1. Differential Mode capacitor filter for SMPS. Additional capacitors may be added after the rectifier in SMPS , to suppress the harmonics further . According to Zainal .S. the British Standards ( BS 2135 ) states that the ground leakage current limit for SMPS is 0.25-5mA; 250V/50Hz and the maximum value of XCAP is defined to be 0.0032-0.064uF. A common mode interference is also possible in all power supplies with respect to the common ground . To avoid these interferences a common mode filter as shown in fig. 2 could be used. Fig.2 Common Mode Inductive Filter for SMPS. The second sub system is the LM35 Precision Grade Temperature Sensor . according to datasheets the National Semiconductors ( 2000 ) adds that like ‘any linear circuit connected to wires in a hostile environment, its performance can be affected adversely by intense electromagnetic sources such as relays, radio transmitters, motors with arcing brushes, SCR transients, etc, as its wiring can act as a receiving antenna and its internal junctions can act as rectifiers’. The remedy for this LM 35 sensor is to use a bypass capacitor from VIN to ground along with a series RC damping circuit with R value being 75Ω in series with a capacitor of value 0.2 or 1 µF between output and ground . The third sub system is a signal conditioning circuit . For such a circuit the required EMC precaution would be shielding and grounding techniques . The shielding should include mechanisms to shield the two meter lead and the successive digital LCD display circuitry . There has to be magnetic shielding also which should account for the reflective effect due to Eddy currents which is according to Len’s law . Bad grounding leads to conducted interference . there must be proper isolated ground for the analog and digital circuits separately . also suppression capacitors which act like dampers can be used to reduce the conducted interferences. The EMC precautions for the Microprocessor sub system would include , harmonic filters that are multiples of the oscillator frequency of the microprocessor , the oscillator section has to be of special type which has ultra low EMI emissions. The next sub system is the 5 digit LCD display system. For such a system the EMC guidelines according to red- lion control ( 2001 ) would be - using shielded cables , connect shields to the panel when frequency is over 1 MHz. The signal cables should not run in the same raceway along with the AC power lines or solenoids or heaters . EMI suppression core like Ferrite Suppression Cores have to be used . The final sub system is the RS232 cable that it has to be according to the EMC directives Recommendation of System Hardening : In spite of proper design and implementation any system may be susceptible to EMI . to reduce this effect an EMC hardening may be required. This hardening may involve controlling the signal amplitude , proper frequency assignment , correct placement and orientation of each and every component , prioritizing the operating schedule of the system. The other EMC system hardening techniques could be usage of additional fuses , tripping switches , transient absorbing devices, circuit breakers , etc . Conclusion : This report discusses the various concepts related to electro magnetic interferences . initially the role of EMI in electronic circuits and their adverse effects have been dealt in detail. The design rules to reduce Electro Magnetic Interference or the rules for Electro Magnetic Compatibility have been outlined. Later the specific EMC design guidelines for the product mentioned - Boiler Controller has been discussed in depth. Finally the different system hardening recommendations for Electro Magnetic Compatibility ( EMC ) has been explained. References : National Semiconductor , November 2000 , LM35 Precision Centigrade Temperature Sensors . www.datasheetpro.com Red- Lion . September , 2001 . ADJUSTABLE TIMEBASE 5-DIGIT RATE INDICATOR , Bulletin No. DT9 - X.< www.redlion-controls.com > Tim Williams . 1996 , EMC for Product Designers. 2nd edition , Newnes, Oxford, 1996 . ISBN 0 7506 2466 3 Zainal Salam . Power Electronics and Drives. Page 1 of 7. NOTES ON EMC STANDARDS, .... BS 2135. Read More