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AMAZON multi-meters discounts AMAZON oscilloscope discounts General Installation and environmental requirements Modern power electronic AC VVVF converters, which are used for the speed control of electric motors, are usually supplied as stand-alone units with one of the following configurations. The first two are the most common configurations. • IP00 rating Designed for chassis mounting into the user's own enclosure, usually as part of a motor control center (MCC). • IP20/IP30 rating -- Designed for mounting within a 'clean environment', such as a weatherproof, air-conditioned equipment room. The environment should be free of dust, moisture and contaminants and the temperature should be kept within the specified limits. • IP54 rating -- Designed for mounting outside in a partially sheltered environment, which may be dusty and/or wet. General safety recommendations The manufacturer's recommendations for installation should be carefully followed and implemented. The voltages present in power supply cables, motor cables and other power terminations are capable of causing severe electrical shock. In particular, the local requirements for safety, which is outlined in the wiring rules and other codes of practice should always take priority over manufacturer's recommendations. The recommended safety grounding connections should always be carefully installed before any power is connected to the variable speed drive equipment. AC variable speed drives have large capacitors connected across the DC link. After a VSD is switched off, a period of several minutes must be allowed to elapse before any work commences on the equipment. This is necessary to allow these internal capacitors to fully discharge. Most modern converters include some form of visual indication when the capacitors are charged. Hazardous areas In general, power electronic converters should not be mounted in areas which are classified as hazardous areas, even when connected to an 'x'-rated motor, as this may invalidate the certification. When necessary, converters may be mounted in an approved enclosure and certification should be obtained for the entire VSD system, including both the converter and the motor. Environmental conditions for installation The main advantage of an AC variable speed drive (VSD) is that the TEFC squirrel cage motor is inherently well protected from poor environmental conditions and is usually rated at IP54 or better. It can be reliably used in dusty and wet environments. On the other hand, the AC converter is far more sensitive to its environment and should be located in an environment that is protected from:
When installing an AC converter, the following environmental limits should be considered:
++++ Typical temperature de-rating chart for PWM converter In regions or environments where there is a high ambient temperature above the accepted 40 deg. C specified in the standards, both the motor and the converter need to be de rated, which means that they can only be run at loads that are less than their 40 deg. C rating to avoid thermal damage to the insulation materials. The manufacturers of AC converters usually provide de-rating tables for high temperature environments that are above 40 deg. C. A typical table is given below for a modern PWM converter. This table should be used as a guide only and should NOT be taken to apply to AC converters in general or any converter in particular. The design of AC converters is different from various manufacturers, so the cooling requirements are never the same. The cooling requirements of different models from the same manufacturer may also be different. De-rating for high altitude At high altitudes, the cooling of electrical equipment is degraded by the reduced ability of the air to remove the heat from the motor or the heat-sink of the converter. The reason is that the air pressure falls with increased altitude, air density falls and, consequently, its thermal capacity is reduced. In accordance with the standards, AC converters are rated for altitudes up to 1000 meters above sea level. Rated output should be de-rated for altitudes above that. The manufacturers of AC converters usually provide de-rating tables for altitudes higher than a 1000 m. A typical table is given below for a modern IGBT-type AC converter. Note that this table is NOT applicable to all AC converters. The de-rating of converters with high losses, such as those using BJTs or GTOs, will be much higher than the de-rating required for low loss IGBT or MOSFET converters. The higher efficiency of the latter requires less cooling and would therefore be less affected by altitude changes. ++++ Altitude de-rating chart for IGBT-type converter Power supply connections and Grounding In accordance with accepted practice and the local, power is normally provided to a VSD from a distribution board (DB) or a motor control center (MCC). Adequate arrangements should be made to provide safety isolation switches and short-circuit protection in the connection point to the power supply. The short-circuit protection is required to protect the power cable to the AC converter and the input rectifier bridge at the converter. The converter provides down-stream protection for the motor cable and the motor itself. Adequate safety grounding should also be provided in accordance with the local wiring rules and codes of practice. The metal frames of the AC Converter and the AC motor should be grounded to keep touch potentials within safe limits. The chassis of the AC converter is equipped with one or more protective ground (PE) terminals, which should be connected back to common safety ground bar. ++++ Power supply, motor and grounding connections Power supply cables The variable speed drive should be connected to the power supply by means of a cable that is adequate for the current rating of the VSD. Reference can be made to U.S. standard when selecting cables. The AC converter requires a 3-phase supply cable (red/white/blue) and a protective ground conductor (green/yellow), which means a 4 core cable with copper or aluminum conductors. A neutral conductor is not necessary and is usually not brought to the frequency converter. The AC converter is a source of harmonic currents that flow back into the low impedance of the power supply system. This conducted harmonic current is carried into other electrical equipment, where it causes additional heat losses and interference. Sensitive electronic instrumentation, such as magnetic flow-meters, thermocouples and other microprocessor based equipment, ideally should not be connected to the same power source, unless via a filtered power supply. Also, interference can be radiated from the power supply cable and coupled into other circuits, so these cables should be routed well away from sensitive control circuits. The power supply cable should preferably be laid in a metal duct or cable ladder and shielded in some way to reduce the radiation of EM fields due to the harmonic currents. Steel wire armored (SWA) cables, are particularly suitable for this purpose. If the power cable is unshielded, control and communications cables should not be located within about 300 mm of the power cable. The conductor sizes should be selected in accordance with normal economic cable selection criteria, which take into account the maximum continuous current rating of the VSD, the short-circuit rating, the length of the cable and the voltage of the power supply system. The relevant local safety regulations should be strictly observed. However, when selecting the cable cross-sectional area for the power supply cables and upstream transformers, a de-rating factor of at least 10% should be included to accommodate the additional heating due to the conducted harmonic currents. If a supply side harmonic filter is fitted at the converter, this may not be necessary. Three phase systems composed of three single-conductor cables should be avoided if possible. Power cables with a trefoil configuration produce a lower radiated EM field. Cables between converter and motor The cable from the AC converter to the motor carries a switched PWM voltage, which is modulated at high frequency by the inverter. This results in a higher level of harmonics than the power supply cable. Harmonic frequencies are in the frequency spectrum of 100 kHz to 1 MHz. The motor cable should preferably be screened or located inside a metal duct. Control and communications cables should not be located close to this cable. The level of radiated EM fields is higher for cables with 3 separate single cores, laid horizontally on a cable ladder, than a trefoil cable with a concentric shield. The recommended size for the cable between the AC converter and the motor should preferably be the same as the power supply cable. The reasons are: • It will be easier to add a bypass device in parallel with the frequency converter at a later date, using the same cable, cable lugs and connections. • The load-carrying capacity of the motor cable is also reduced by harmonic currents and additionally by the capacitive leakage currents. It should be borne in mind that the AC converter VSD provides short-circuit and overload protection for the cable and motor. A separate ground conductor between the converter and motor is recommended for both safety and noise attenuation. The ground conductor from the motor must be connected back to the PE terminal of the converter and should not be connected back to the distribution board. This will avoid any circulating high frequency currents in the ground system. When armored or shielded cables are used between the converter and motor, it may be necessary to fit a barrier termination gland at the motor end when the cable is longer than about 50 m. The reason is that the high frequency leakage currents flow from the cable through the shunt capacitance and into the shield. If these currents return via the motor and other parts of the grounding system, the interference is spread over a larger area. It’s preferable for the leakage currents to return to the source via the shortest route, which is via the shield itself. The shield or steel wire armor (SWA) should be grounded at both the converter end and to the frame of the motor. Control cables The control cables should be provided in accordance with normal local practice. These should have a cross-sectional area of at least 0.5 mm^2 for reasonable volt drop performance. The control and communications cables connected to the converter should be shielded to provide protection from EMI. The shields should be grounded at one end only, at a point remote from the converter. Grounding the shield to the PE terminal of the drive should be avoided because the converter is a large source of interference. The shield should preferably be grounded at the equipment end. Cables which have an individual screen for every pair provides the best protection from coupled interference. The control cables should preferably be installed on separate cable ladders or ducts, as far away from the power cables as possible. If control cables are installed on the same cable ladder as the power cables, the separation should be as far as possible, with the minimum distance being about 300 mm. Long parallel runs on the same cable ladder should be avoided. Grounding requirements As mentioned earlier, both the AC converter and the motor must be provided with a safety ground according to the requirements of local standards. The main purpose of this grounding is to avoid dangerous voltages on exposed metal parts under fault conditions. When designing and installing these ground connections, the requirements for the reduction of EMI should also be achieved with these same ground connections. The main grounding connections of an AC converter are usually arranged. The PE terminal on the converter should be connected back to the system ground bar, usually located in the distribution board. This connection should provide a low impedance path back to ground. Common cabling errors The following are some of the common cabling errors made when installing VSDs: • The ground conductor from the AC converter is run in the same duct or cable ladder as other cables, such as control cables and power cables for other equipment. Harmonic currents can be coupled into sensitive circuits. Ideally, instrument cables should be run in separate metal ducts or steel conduit. • Running unshielded motor cable next to the supply cable to the AC converter or the power cables for other equipment. High frequency harmonic currents can be coupled into the power cable, which can then be conducted to other sensitive electronic equipment. Other cables should be separated from the motor cable or converter power cable by a minimum of 300 mm. • Running excessively long cables between the AC converter and the motor, these should be no longer than 100 m. If longer cables are necessary, motor filters are necessary to reduce the leakage current. Alternatively, the switching frequency may be reduced. Start / Stop control for AC drives The protection requirements for AC variable speed drives is covered in considerable detail: Protection of AC converters and motors. The protection of the mains supply side of the converter requires short circuit protection either in the form of a set of adequately rated fuses, usually as part of a switch-fuse unit, or a main circuit breaker. The stop/start control of the AC drive can be achieved in a number of ways, mainly:
The first method is the recommended method of controlling the stopping and starting of an AC converter. This may be achieved by stop and start pushbuttons wired directly to the control terminals of the converter. Alternatively, if the control is from a remote device such as a PLC, the control can be wired from the PLC directly to the terminals of the AC converter. The second method is the one most commonly used for the direct on line (DOL) starting of normal fixed speed AC motors. Following from previous DOL 'standard' practice, this method is also quite commonly used in industry for the control of variable speed drives, particularly for conveyors. It’s usually a safety requirement to interrupt the power circuit when an emergency stop or pull-wire switch has been operated. While this method satisfies the safety requirements by breaking the power supply to the motor, there are a number of potential hazards with this method of control. The main problems are: • Contactor on supply side of the AC converter • Opening/closing the supply side of the AC converter for stop/start control should be avoided because most modern converters take their power from the DC bus. Every time the power is removed... - Power to the control circuits is lost - Control display goes off - Diagnostic information disappears - DC capacitors become discharged - Serial communications are lost When the AC variable speed drive needs to be restarted, there is a time delay (typically 2 secs) while the DC bus charging system completes its sequence to recharge the DC capacitor. This stresses the charging resistors, the DC capacitor and other components. The charging resistors of many AC converters are short-time rated and, although sometimes not highlighted in the user manual, there is a limit to the number of starts that can be done. Many users have the concept of 'run on power up' is acceptable and unrestricted. The following is an extract from the manual of one of the leading manufacturers of AC converters: ATTENTION: The drive is intended to be controlled by control input signals that will start and stop the motor. A device that routinely disconnects and then reapplies line power to the drive for the purpose of starting and stopping the motor is not recommended. If this type of circuit is used, a maximum of 3 stop/start cycles in any 5 minute period (with a minimum period of 1 minute rest between each cycle) is required. These 5 minute periods must be separated by 10 minute rest cycles to allow the drive pre-charge resistors to cool. Refer to codes and standards applicable to your particular system for specific requirements and additional information. • Contactor on motor side of the AC converter Opening/closing the 3-phase power circuit on the motor side of the AC converter for stop/start control should also be avoided, particularly while the AC drive is running. Breaking the inductive circuit to the motor produces transient over-voltages which can damage the IGBTs and other components. Many modern AC converters have RC suppression circuits (snubbers) to protect the IGBTs from this type of switching. The following is an extract from the manual of one of the leading manufacturers of AC converters: ATTENTION: Any disconnecting means wired to the drive output terminals U, V and W must be capable of disabling the drive if operated during drive operation. If opened during drive operation, the drive will continue to produce output voltage between U, V and W. An auxiliary contact must be used to simultaneously disable the drive or output component damage may occur. The objective is to ensure that the AC converter is OFF before the contacts between the converter and the motor are opened. This will avoid IGBT damage due to transient over-voltages. In addition, closing the motor side contactor when converter output voltage is present can result in a motor inrush current similar to DOL starting. Apart from the stress this places on the converter, the drive will trip on over-current. Repeated attempts at closing the motor contactor after the converter has started may eventually lead to IGBT failure. If a contactor has to be installed into the power circuit of an AC variable speed drive system to meet local safety requirements, then it’s better to locate this contactor downstream of the AC converter. It’s then necessary to include an auxiliary contact on the contactor which disables the converter control circuit before the contactor is opened or, alternatively, closes the enable circuit after the contactor has been closed. This means that a late make - early break auxiliary contact should be used on the contactor and wired to the converter enable input. While the above configuration will protect the AC converter from failure, this method of routine stop/start control is not recommended. It should be used for emergency stop conditions only. Routine stop/start sequences should be done from the AC converter control terminals. An alternative method of ensuring that plant operators follow this requirement is to install a latching relay and a reset pushbutton. The latching relay needs to be reset after every Emergency Stop sequence. Install AC converters into metal enclosures If the environmental conditions are likely to exceed these accepted working ranges, then arrangements should be made to provide additional cooling and/or environmental protection for the AC converter. The temperature limits of an AC converter are far more critical than those for an electric motor. Temperature de-rating needs to be strictly applied. However, it’s unlikely that a modern PWM converter will be destroyed if the temperature limits are exceeded. Modern AC converters have built-in thermal protection, usually a silicon junction devices, mounted on the heat-sink. The main problem of over temperature tripping is associated with nuisance tripping and the associated downtime. Although the efficiency of a modern AC converters is high, typically ± 97%, they all generate a small amount of heat, mainly due to the commutation losses in the power electronic circuits. The level of losses depends on the design of the converter, the PWM switching frequency and the overall power rating. Manufacturers provide figures for the losses (watts) when the converter is running at full load. Adequate provision should be made to dissipate this heat into the external environment and to avoid the temperature inside the converter enclosure rising to unacceptably high levels. Converters are usually air-cooled, either by convection (small power ratings) or assisted by cooling fans on larger power ratings. Any obstruction to the cooling air flow volume to the intake and from the exhaust vents will reduce efficiency of the cooling. The cooling air volume flows and the power loss dissipation determine the air-conditioning requirements for the equipment room. The cooling is also dependent on there being a temperature differential between the heat-sink and the cooling air. The higher the ambient temperature, the less effective is the cooling. Both the AC converter and motor are rated for operation in an environment where temperature does not exceed 40 deg. C. When AC converters are mounted inside enclosures, care should be taken to ensure that the air temperature inside the enclosure remains within the specified temperature limits. If not, the converters should be de-rated in accordance with the manufacturer's de-rating tables. In an environment where condensation is likely to occur during the periods when the drive is not in use, anti-condensation heaters can be installed inside the enclosure. The control circuit should be designed to switch the heater on when the drive is de-energized. The heater maintains a warm dry environment inside the enclosure and avoids moisture being drawn into the enclosure when the converter is switched off and cools down. AC converters are usually designed for mounting in a vertical position, to assist convectional cooling. On larger VSDs, cooling is assisted by one or more fans mounted at the bottom or top of the heat-sink. Many modern converters allow two alternative mounting arrangements: • Surface mounting, where the back plane of the converter is mounted onto a vertical surface, such as the back of an enclosure. • Recessed mounting, where the heat-sinks on the back of the converter project through the back of the enclosure into a cooling duct. This allows the heat to be more effectively dissipated from the heat-sinks. Sufficient separation from other equipment is necessary to permit the unrestricted flow of cooling air through the heat-sinks and across the electronic control cards. A general rule of thumb is that a free space of 100 mm should be allowed around all sides of the VSD. When more than one VSD are located in the same enclosure, they should preferably be mounted side by side rather than one above the other. Care should also be taken to avoid locating temperature sensitive equipment, such as thermal overloads, immediately above the cooling air path of the VSD. Adequate provision must be made to dissipate the converter losses into the external environment. The temperature rise inside the enclosure must be kept below the maximum rated temperature of the converter. Calculating the dimensions of the enclosure The enclosure should be large enough to dissipate the heat generated by the converter and any other electrical equipment mounted inside the enclosure. The heat generated inside an enclosure is transferred to the external environment mainly by radiation from the surface of the enclosure. Consequently, the surface area must be large enough to dissipate the internally generated heat without allowing the internal temperature to exceed rated limits. The surface area of a suitable enclosure is calculated as follows: where: A Effective heat conducting area in m^2 (Sum of surface areas not in contact with any other surface) P Power loss of heat producing equipment in watts T-Max Maximum permissible operating temperature of converter in deg. C T-Amb Maximum temperature of the external ambient air in deg. 0 C k Heat transmission coefficient of enclosure material Example: Calculate the minimum size of an IP54 cubicle for a typical PWM type frequency converter rated at 22 kW. The following assumptions are made: • The converter losses are 600 watts at full rated load. • The converter is to be mounted within an IP54 cubicle made of 2 mm steel. • The enclosure is effectively sealed from the outside and heat can only be dissipated from the enclosure by conduction through the steel and by radiation from the external surface into the outside air. • The cubicle stands on the floor with its back against the wall in an air conditioned room with a maximum ambient temperature 25 C. • The converter can operate in a maximum temperature of 50 deg. C. • The heat transmission coefficient is 5.5 (typical for painted 2 mm steel). The first step is to calculate the minimum required surface area of the enclosure. This can be done by applying the formula for surface area. If the cubicle is standing on the floor against a wall, this area applies only to the top, front and two sides of the enclosure. A suitable cubicle can be chosen from a range of standard cubicles or could be fabricated for this installation. In either case, it’s important to take into account the dimensions of the converter and to ensure that there is at least 100 mm space on all sides of the converter. With these requirements in mind, the procedure is to choose or estimate at least two of the dimensions and the third can be derived from the above equation. This calculated dimension must then be checked to ensure that the required 100 mm clearance is maintained. For a cubicle with dimensions H × W × D standing on the floor against the wall, the effective heat conducting area is : A = HW + 2HD + WD Assuming that a standard cubicle is chosen with a height of 2.0 m and a depth of 0.5 m, the width is derived from A = 2.0 W + 2 + 0.5 W A = 2.5 W + 2 Using the required heat dissipation area from the above calculation 4.36 = 2.5 W + 2 …or… 2.5W = 2.36 W = 0.94 Based on the requirements of heat dissipation, the width of the cubicle would have to be larger than 0.94 m. In this case a standard width of 1.0 m would be selected. Clearances around the sides of the converter should be checked. With typical converter dimensions of H × W × D = 700 × 350 × 300, the cubicle chosen would provide more than 100 mm of clearance around all the converter and also leave sufficient space for cabling and other components. From this calculation, it’s clear that the overall dimensions of the cubicle can be reduced by the following changes: • Standing the cubicle away from the wall, at least 200 mm • Reducing the ambient temperature, turning down the air-conditioning • Providing ventilation to the cubicle to improve heat transfer Ventilation of enclosures The enclosure can be smaller if some additional ventilation is provided to exchange air between the inside and outside of the cubicle. There are several ventilation techniques commonly used with converters, but they mainly fall into two categories: • Natural ventilation Relies on the convectional cooling airflow through vents near the bottom of the cubicle and near the top, the 'chimney' effect. ++++ Natural ventilation of a converter in a cubicle • Forced ventilation Relies on cooling airflow assisted by a fan located either near the top or the bottom of the cubicle. It’s difficult to maintain a high IP rating with ventilated cubicles, so ventilated cubicles need to be located in a protected environment, such as a dust-free equipment room. For cooling purposes, a certain volume of airflow is required to transfer the heat generated inside the enclosure to the external environment. The required airflow can be calculated from the following formula: where: V = Required airflow in m3 per hour P = Power loss of heat producing equipment in watts T_Max = Maximum permissible operating temperature of converter in 0 C T_Amb = Maximum external ambient temperature in 0 C ++++ Forced ventilation of a converter in a cubicle Example: Calculate the airflow ventilation requirements of the 22 kW converter used in the example above, using the same assumptions. The required airflow to maintain adequate cooling. An airflow of 75 m^3/h is necessary to remove the heat generated inside the enclosure by the converter and to transfer it to the outside. In this case, the dimensions of the cubicle are based purely on the minimum physical dimensions required for the converter and any other equipment mounted in the cubicle. This airflow could be achieved by the convectional flow of air provided that the size of the top/bottom openings are large enough and the resistance to airflow is not unnecessarily restricted by dust-filter pads. Alternatively, a fan assisted ventilation system would be necessary to deliver the required airflow. Alternative mounting arrangements One of the main problems associated with the ventilation of converter cubicles is that it’s very difficult to achieve a high IP rating with a ventilated cubicle. In addition, if filters are used, an additional maintenance problem is introduced, the filters need to be checked and replaced on a regular basis. A solution, which is rapidly gaining popularity, is the recessed mounting. This technique has now been adopted by many of the converter manufacturers. Most of the heat generated by a converter is associated with the power electronic components, such as the rectifier module, inverter module, capacitors, reactor and power supply. These items are usually mounted onto the heat-sink base of the converter and most of the heat will be dissipated from the surfaces of this heat-sink. The digital control circuits don’t generate very much heat, perhaps a few watts. If the heat-sink is recessed through the back mounting plane of the enclosure, most of the heat will be dissipated to the environment external to the cubicle. The portion of the converter with the control circuits remain within the enclosure. With a suitable seal around the converter, the enclosure can be relatively small and rated at >IP54 without the need for forced or convectional airflow ventilation. The heat-sink portion projecting outside the enclosure can be exposed to the environment with a lower IP rating (e.g. IP20) or it can be arranged to project into a cooling airduct system, which ducts the heat outside the building. ++++ a typical mounting arrangement of this type of converter with the heat-sinks projecting into a cooling duct. ++++ Converter mounted with heat-sink outside the cubicle Control wiring for VSDs Variable speed drives (VSD) may be controlled 'locally' by means of manual push buttons, switches and potentiometers, mounted on the front of the converter. For simple, manually controlled operations, these local controls are all that is required to operate the VSD. In most industrial applications, it’s not practical to control the VSD from the position where the VSD is located. VSDs are usually installed inside motor control centers (MCCs), which are located in switchrooms, usually close to the power supply transformer, but not necessarily close to where the operator is controlling the process. Consequently, almost all VSDs have terminals that permit remote control from a location close to the operator. VSDs have terminals for the following controls: • Digital inputs, such as remote start, stop, reverse, jog, etc, which are usually implemented by - Remote push-buttons in a manually controlled system - Digital outputs (DO) of a process controller in an automated system • Digital status outputs, such as indication of running, stopped, at speed, faulted, etc, which are usually implemented by: - Remote alarm and indication lamps in a manually controlled system - Digital inputs (DI) to a process controller in an automated system • Analog inputs, such as remote speed reference, torque reference, etc, which are usually implemented by - Remote potentiometer (10 kohm pot) in a manually controlled system - Analog outputs (AO) of a process controller in an automated system, usually using a 4-20 mA signal carried on a screened twisted pair cable • Analog outputs, such as remote speed indication, current indication, etc, which are usually implemented by: - Remote display meters (0-10 V) in a manually controlled system - Analog inputs (AI) to a process controller in an automated system, usually using a 4-20 mA signal carried on a screened twisted pair cable. Manual and automated control systems have operated very effectively for many years with this type of 'hard-wired' control system. The main disadvantage of this system is: • All the DIs and DOs require one wire per function, plus a common. • All the AIs and AOs require two wires per function, plus a shield connection. ++++ Configuration of a typical hard-wired manual control system Hard-wired connections to PLC control systems With the introduction of automated control systems using programmable logic controllers (PLCs) and distributed control systems (DCS), the 'hard-wired' control connections have been extended, with the input/output (I/O) modules replacing the manual controls. ++++ Configuration of a typical hard-wired automated control system As the control systems have grown in complexity and the amount of information required from field sensors has expanded, the number of conductors required to implement the automated control system has become a major problem, from the point of view of cost and complexity. As more and more field devices become integrated into the overall control system, this problem of more and more cables can only become more difficult. A hard-wired interface between a variable speed drive (VSD) and a programmable logic controller (PLC) would typically require about 15 conductors as follows: • 5 Conductors for controls such as start, stop, enable, reverse, etc. • 4 Conductors for status/alarms, such as running, faulted, at speed, etc. • 2 or 3 Conductors for analog control, such as speed setpoint • 4 Conductors for analog status, such as speed indication, current indication If there are several VSDs in the overall system, the number of wires is multiplied by the number of VSDs in the system. Serial communications with PLC control systems Serial communications overcomes these problems and allows complex field instruments and VSD systems to be more simply linked together into an overall automated control system with the minimum of cabling. Microprocessor based digital control devices, sometimes called 'smart' devices, are increasingly being used in modern factory automation and industrial process control systems. Several 'smart' devices can be 'multi dropped' or 'daisy-chained' on one pair of wires and integrated into the overall automated control system. Control and status information can be transferred serially between the process controller and the VSDs located in the field. Parameter settings can also be adjusted remotely from a central point. ++++ Configuration of a typical serial communications system ++++ Comparison of features of EIA-232, EIA-423, EIA-422 and EIA-485 This level of the control system is usually called the 'field level' and a data communications network at this level is referred to as a 'field bus'. The physical interface standards define the electrical and mechanical details of the interconnection between two pieces of electronic equipment, which transfer serial binary data signals between them. There are a several well established physical interface standards, such as EIA-232, EIA-422 and EIA-485. The main features of the four most common EIA interface standards are compared. Interface converters While many PCs and PLCs are fitted with RS-232 interfaces as standard, these ports are not generally suitable for control of variable speed drives directly via this interface, because of differences in both the voltage levels and the configuration (unbalanced/balanced). An RS-232/RS-422 or RS-232/RS-485 interface converter can be connected between the two devices to convert the voltage levels and the connection configuration from one to the other (unbalanced to balanced). An interface converter should be physically located close to the RS-232 port (at the PC end) to take advantage of the better performance characteristics of the RS-485 interface for the longer distance to the variable speed drives in the field. It’s also preferable that these converters and the interface at the drive end optically isolate the control device from the variable speed drives to provide maximum protection against problems resulting from ground voltage differences and electrical noise. The interface between the PC and the interface converter is one-to-one, while the RS 485 side may have several devices (up to 32) connected in a multi-drop configuration. The internal connection details of an RS-232/RS-485 converter are shown below. ++++ Block diagram of an RS-232/RS-485 converter Local area networks The technique of linking digital instrumentation and control devices together to share data and pass on control commands has become an important part of automated industrial control systems. It’s fairly common for the main components of the control system to be linked together by a 'data highway'. Another name for a data highway is a local area network (LAN). A LAN is a communications path linking two or more intelligent devices. A LAN allows shared access by several users to the common communications cable network, with full connectivity between all nodes on the network. A LAN covers a relatively small area and is located within a localized plant or group of buildings. The connection of a communicating device into a LAN is made through a node. A node is any point where a device is connected and each node is allocated a unique address number. Every message sent on the LAN must be prefixed with the unique address of the destination. A device connected through a node into a network, receives all messages transmitted on the network, but only responds to messages sent to its own address. LANs operate at relatively high speed (50 kbps and upwards) with a shared transmission medium over a fairly small local area. Since many nodes can access the LAN network at the same time, the network software must deal with the problems of sharing the common resources of the network without conflict or corruption of data. In the OSI model, this level of software is called the data link layer. Some rules must be established on which devices can access the network, when and under what conditions. Most modern digital variable speed drives (VSDs) have a communications capability. The physical interface is usually based on a well known physical standard, such as RS 232 or RS-485. There are a number of well accepted industrial communications standards, For example, Devicenet, Profibus, Asi-bus, Modbus Interbus-s, etc. A suitable program embedded in the VSD can control the serial transfer of data to the VSD. There is still a considerable amount of confusion about the merits of these emerging new standards, but these will be resolved in the next few years. |
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