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The purpose of this assignment is to highlight and investigate different standards and testing procedures which are required to ensure electromagnetic compliance of the speed controller system for an AC motor which is being used in the manufacturing plant.
EMC issues concerning the environment in which the speed controller circuit is being operated are discussed; analyzed and correct testing procedures are identified in order to get the product compliance with the EMC Directives and standards.
Table of Contents
Aim: To discuss and analyze the techniques and good practices and make recommendations in order to ensure that the speed controller system becomes compliant to the EMC directives and standards.
Objectives: Following objectives are to be met within the current proposed report:
1: Application of good design rules for the system.
2: Research regarding EMC directives.
3: Identification and recommendation of the correct testing procedures for EMC compliance
Structure of the Test Plan
Following structure of the test plan is suggested and various aspects of the plan will be covered in the present report wherever possible in context of means by which the equipment will be tested for compliance with Emission and immunity EMC standards and directives. In addition to this report the test plan should cover aspects such as: the purpose and operation of the Speed controller circuit, limitation and absolute maximum rating of the device and also a report clearly showing the test results. But this report is only limited to identifying the EMC compliance standards and making recommendations for testing procedures considering the environment in which the device is placed.
Description and Operation of the Speed controller System
Electrical and mechanical characteristics with absolute maximum ratings
Failure criteria and range of performance degradation
Explanation of the Test facility to be used
Requirement of the Test Equipment
Formal Test report showing results of various testing procedures
Speed Controller System for AC Motor
The speed controller system for the AC motor can introduce substantial amount of electromagnetic interference to the surrounding equipments in the plant. In addition to this speed controller can be subjected to electromagnetic interference due to the operation of the high powered devices in manufacturing plant. In order to comply with the EMC standards it is of vital importance that the speed controller productâ€™s operation does not affect the operation of other surrounding devices by inducing EMI. Moreover, its own operation should not be degraded due to the surrounding EMI, the system should have certain level of immunity.
Product Environment: The environment of the product is a large scale manufacturing plant. In addition to the AC Motor, several other control equipments such as: high powered equipments, chemical mixing furnace, heat monitoring system, conveyor belt and other safety critical products are also present.
Major concerns regarding the current product: Due to the fast switching action of the semiconductor switches in the AC speed control systems electromagnetic emission can occur. The intensity of emission is dependent on the frequency at which the semiconductor switches are being turned on and off. In addition to this due to the presence of capacitive and inductive components within the circuit, lower resonance frequencies can be the cause of EMI.
Emission: Emission includes all types of electrical and magnetic disturbance which is omitted from an interference source in our case, AC speed control system. Coupling paths are heavily dependent on the intensity of frequency emission.
EMC Standards for low frequency emission: Due to the LC circuits within the AC speed controller, for frequency which is less than 9 kHz compliance with the following test standards is required. The reason for selecting each standard in context of the current system is also given.
EN61000-3-3/ -11: Voltage fluctuations due to the fast load changes at the AC motor
EN60146-1-1: Due to the commutation circuit in the input circuit, periodic voltage dips can occur.
EN61000-32/-12: Due to the superimposed harmonics voltage distortion can occur.
EN 61000-2-2/4: Voltage imbalance and frequency changes.
The effect of the above disturbances will be massive on the surrounding equipments in the manufacturing plant. There can be voltage dips and reduction in the power factor value which can make certain hazard monitoring systems, such as a watchdog which is monitoring the belt alignment, non responsive. Cables connectors and circuit breakers can get overloaded. Certain test and measuring equipments will stop working which could have devastated effect on the production line and can put safety of the equipment and operators at risk. Parallel Resonance excitation can also occur among surrounding equipments.
Safe design: In order to lower the harmonics, improve the power factor and also lower voltage distortion, the load connected to the AC motor should be only 15 to 25% of the total supply power. Moreover, if possible a separate transformer should be used to provide power to the motor speed drive circuit. Special LC filters that are harmonized to the frequency of the disturbing harmonics can be used to lower harmonics.
EMC Standards for High Frequency Emission: High Frequency emission of more than 9 kHz can also affect the surrounding environment of the AC speed drive circuit.
The output circuit of an AC drive converter typically consists of fast-switching IGBT modules. These modules operate with switching frequencies of between 2 and 16 kHz. Identified EMC Standard concerning high frequencies are given below:
EN55011: Due to the cable born disturbances, in the range of 10 kHz to 30 MHz and radiate disturbances, in the range of 30 MHz to 1GHz
Due to the cable disturbances imposed by the high frequency, interference current and voltages can have negative effect on the unshielded cables and can cause the damage of the equipment within the plant. Switching frequency of the semiconductors should be critically monitored and not allowed to be in a range that it will interfere with the operation of the other control products in the plant.
Safe design: Shielded motor feeder cables can be used to minimize the risk of damaging the equipment. Shielded motor cable ensures that the radio frequency is dampened and interference current is fed back to the source in a safe manner. The length of the shielded motor cable should be kept at minimum possible to ensure the low resistance. An EMC filter can also be used at the input of the AC motor Speed controller circuit.
EMC Compliance Testing
To achieve electromagnetic compatibility, two major aspects worth consideration are immunity to interference and emission levels of the speed controller. Both of these avenues are to be analyzed and certain test procedure are required to make sure that the speed controller system for the AC motor complies with the electromagnetic standards which are discussed in the previous section.
In order to maintain the integrity of the testing procedures it is essential to create the real working environment of the speed controller system. For this purpose, other control systems which are typical in the manufacturing plant such as conveyor belt controller, chemical furnace system and temperature monitoring equipment are placed in the test zone.
Testing procedure for Emission
Following test procedures are suggested for the Emission analyses of speed controller circuit for an AC motor.
Conducted Emission Test:
Test Equipment: Line Impedance Stabilization Network, EMI Receiver
A ground plane of specific volume that extends to the boundary of the speed controller for AC motor is used for the conducted emission measurements. In order to lower the ambient noise the testing is carried out in the screened enclosure.
By using the test equipment given above, conducted emission is measured. Power is supplied to the speed controller system via the line impedance stabilization network and emission signal is routed to the receiver which could be any other electrical system in the surrounding of the speed controller system. This approach helps in measuring the conducted emission as LISN (Line Impedance Stabilization Network) allow calibrated measurements to be made and present very low amount of impedance to the receiver circuitry.
Test Equipment: Spectrum Analyzer, Automated Computer Driven Facility
In order to measure the radiated emission it is desirable to make measurements without the presence of ambient signals, this is achieved using the shielded rooms but room resonance produces high field variations. Radio frequency absorbent material is placed inside the shielded room to make the room anechoic.
As the switching frequency of the semiconductor switches inside the speed controller is in the range of kHz, anechoic shielded room with the telescopic antenna is sufficient enough for taking the measurements. A computer is connected to the test equipment in order to measure the radiated emission
Another Radiated Emission Test is performed in the open area test site with the telescopic dipole antenna by rotating the speed controller at certain angles. Open area test has attenuation associated with it which is due to the distance between the speed controller and antenna. By measuring the site attenuation first, the final test figures are adjusted for the actual results. Spectrum analyzer can be connected to the test equipment in order to take the radiated measurements.
Testing Procedures for Immunity
In order to ensure that the Speed controller for AC motor complies with the EMC immunity standards, certain tests are carried out. Compliance is required with the EN50082-2 standard which is for the industrial environment.
In order to follow EMC immunity compliance, following performance criteria is weighed in the context of product performance:
1: The Speed controller must keep operating within the limits defined in the data sheet when exposed under continuous electromagnetic interference.
2: In case, performance of the speed controller is degraded during the test, speed controller circuit should recover as expected after the test. No data should be lost due to the degrading effect.
3: Device should recover after the temporary loss of critical function.
Following tests are to be carried out in order to comply with the EMC immunity standards.
Immunity to radiated Fields Test:
Due to the presence of high power equipments in the manufacturing plants, speed controller system will be exposed in the high radiated fields. An anechoic shielded room is suitable in order to ensure that the speed controller circuit operation is not affected by the radiated frequency range of 25 Hz to 1GHz. This test is required for compliance with IEC801-3 standard.
Resonance Test: Although this test is best suited during the development stage of the device, it is useful to know the resonance frequency of the speed controller circuit by using the magnetic probe connected to the spectrum analyzer.
Conducted Immunity Test:
Conducted immunity test is performed on the speed controller circuit by using the coupler networks on any external connection. The testing procedure by injection via couplers covers IEC801-6 standard.
Surge Immunity Test:
In order to ensure that the speed controller for the AC motor is immune against the light striking when placed in the environment which is under the influence of atmosphere, surge immunity test is to be carried out. This test is required for compliance with IEC801-5 standard.
Transient Voltage Suppression Test:
Due to the presence of electrical surges in the electrical networks which can cause damage to the device or malfunctioning, this test is carried out to ensure that the speed controller circuit can withstand electrical fast transients.
Burst of pulses in the range of 500v, which depends upon the rating of the device given in the data sheet, are applied to the external connections of speed controller circuit. This test is required for compliance with IEC801-4 standard.
Electromagnetic compatibility is of paramount importance in the design of any electrical and electronic product or system, the speed controller for an AC motor is no different. In this report, recommendations are made regarding the testing procedures which are required to comply with the EMC standards for industrial speed controller for an AC motor. Not only test equipments are mentioned but also various suggestions are made to reduce the electromagnetic interference. By following these testing procedures and ensuring correct operation, it is expected that the speed controller circuit of the AC motor will attain EMC compliance.