Selecting & Installing Inverters & Coping with Radio Frequency Emissions

Transcription

1 Selecting & Installing Inverters & Coping with Radio Frequency Emissions The recent large increase in the use of inverters (drives) in the Heating Ventilating & Air Conditioning ( HVAC) industry has brought a number of trades (technicians with different skills sets) together, working with these products. The common use of inverters has brought many technicians into the area where they are responsible for the installation & commissioning of these devices very often without any kind of training or guidance in what can be termed Good Practice, to ensure that the installation, when complete, has chance of being successful & trouble free for the owner/operator. This paper is aimed at qualified installers who are directly involved in the installation and commissioning of drives, with the objective of providing additional guidance, above that normally found in a typical user manual supplied with a drive. This paper is based on many years of practical experience in trouble shooting problematic installations and contains information probably not found in any other document. Subjects covered :- 1. Drive selection. Drive and motor capacity i.e. drive larger or smaller than motor Magnetising current. 2. Selection of installation area. Efficiency of drive. IP ratings RFI filters. Local environment 3. Cabling Useful tips. 4. When it goes wrong! 1. Drive Selection. The recommended method of selection of a drive, is by using the data available on the motor nameplate i.e. kw or full load current drawn (preferred). However, drives can be considered as intelligent Current Sources. Therefore the kw rating of the motor is in fact immaterial as the main requirement is that the drive should be able to provide sufficient current to satisfy the demand from the motor. Drive Capacity Larger than Motor capacity If the drive is much larger in capacity than the motor, this will be an uneconomic installation, but does sometimes occur, however the drive will cope with this situation without problems. If the drive is very much larger in capacity than the motor, there will be a loss of accuracy in the measurement circuits, in particular the 3 motor phase current transducers may have difficulty in producing accurate values. 09/11/04 1 Installing Inverter & emissions.doc

2 Drive Capacity Smaller than Motor capacity. In this scenario, more care is required as there is much more chance of an unsuccessful installation. If the load can be considered as HIGH STARTING TORQUE i.e. a crane or conveyor, the drive should be selected using its HIGH OVERLOAD rating. If the load is for example a centrifugal fan or pump, then it is normal for these machines to have a low starting torque. In this case the drive can be selected using the LOW OVERLOAD rating. This usually results in a more economic selection, however generally this also means that the MAXIMUM ambient temperature surrounding the drive should not exceed 40 C. Often, this factor is not taken into account by the installer and the result is an unreliable installation, which on occasion will result in a drive trip due to high heat sink temperature. This is most likely to occur when the drive is on a heavy load, which is usually the most inconvenient time for the owner/operator. Since the drive can be considered a current source, there are occasions when a smaller drive can be selected than that described above. This could be for example where the drive is installed in a fully air conditioned space and also where for whatever reason, the motor load is very low, resulting in relatively low current demand. In this situation it could be possible to select a drive with 1 frame capacity less than normal. However the motor normal accelerating & running current MUST be less than the drive full load current. There is potential for an unexpected problem, which is often not anticipated by application engineers embarked on this course. This problem is caused by the Motor Magnetising Current. The Motor Magnetising Current is a relatively fixed value, irrespective of the load on the machine i.e. if the motor is run completely disconnected from any load it will still demand the same level of current. Once the motor is connected to a load and begins to produce torque, the current drawn by the motor will rise above this minimum value. The magnetising current of a motor varies inversely according to its capacity, i.e small capacity motors have a high percentage magnetising current, relative to their full load current. Large capacity motors have a relatively small level of magnetising current in relation to their full load current. A typical value to use would be 33% - but this will vary. The drive has a motor model within it, which is a pre programmed Matrix of Data that reflects the expected current against a profile of operation. If the drive capacity is smaller than the motor capacity and is operating with default configuration, it is highly likely the drive will trip, due to a perceived Motor Overheat. This usually occurs down in the region of 60% or less of the rated output frequency. There are parameters available, which can be adjusted to prevent these trips occurring. 09/11/04 2 Installing Inverter & emissions.doc

3 2. Selection of the Installation Area. 2.1 General. Drives are relatively expensive electronic devices that deserve an appropriate environment in which to be installed. They also operate very efficiently, nevertheless, they do emit some heat and if they are more than just a few KW in capacity, this can be considerable. A typical modern drive will have an efficiency of around 96-98%. This means that a drive of 110 KW capacity will emit possibly 2-4 KW when on full load. This quantity of waste heat can be used with a benefit, if the area chosen for the location of the drive is an area that would normally require heating. Chose the location wisely & the waste heat from the drive can be reused, thus reducing overall energy consumption. 2.2 IP/NEMA Rating Drives normally come with a variety of Protection Ratings ( IP ratings). IP00, IP20, IP21 & IP54 are quite common in the industry. (See Fig 9 below for brief description on IP ratings) IP00 rated drives have exposed high voltage electrical components, they are also unprotected from dust or water from any direction. IP00 drives must be installed within additional enclosures to ensure that no person is exposed to the risk of the electrical hazard & the drive is not exposed to damage from dust or water. IP20/21 is the minimum protection rating for the installation of any equipment within a cubicle. This is to ensure during inspection, the unwary finger or hand cannot accidentally touch live equipment. If the area considered for the installation can be secured and there is a very remote risk of water entering the area, then IP20/21 rated equipment could be installed here. IP54 rated equipment should be selected where the installation is to be undertaken in an area where access is unrestricted and there is the possibility of water being sprayed or falling from other equipment. IP54 equipment is NOT suitable for installation in areas exposed to the weather i.e. additional protection would be required. 2.3 RFI Filters. The drive will generally be fitted with an appropriate RFI filter and choke (to reduce High & Low frequency disturbances on the power network). The current EU Directive for Power Drive Systems, EN61800 Part 3 Amendment 11 does allow for drives to be installed without filters under certain circumstances. However it much easier to use a standard drive already equipped with an RFI filter and some kind of choke in the first instance, than it is to correct any ensuing problems 09/11/04 3 Installing Inverter & emissions.doc

4 Once the general site is has been chosen for the installation, it is now time to choose the final location. Try to avoid areas where heat, dust or water may at some time be generated. In particular avoid areas that are close to outside walls which during periods of heavy rain, may be subjected to the overflow from roof guttering etc. Also avoid any inside wall that may be directly in front of a South facing window, as during the summer months, direct sunlight may fall upon the drive & easily raise the surface temperature to more than 50 C. 3. Cabling. Once the general area has been chosen, the specific fixing area should be identified. With both Panel Mounted & Free Standing drives, try to choose a metallic surface or frame that is galvanically connected to the buildings metallic structure i.e. beams or columns. Avoid non-conducting masonry work if at all possible, as this type of material provides no additional earth path for noise currents. If the drive is to be panel mounted, the following steps should be taken to ensure maximum potential or a successful installation. Ensure the motor cable is correctly glanded at the enclosure take particular care with the tightness and cleanliness of the connection. See fig 5 & fig 6. Bond the entry gland plate to the back plate of the enclosure with a short braided cable. This should be in addition to the Protective Earth cable (PE cable). See fig 7 Bring the cable tray carrying the motor cable as close as possible to the drive. Bond the enclosure to the cable tray with a short multi-strand earth braid. Bond each successive section of cable tray. Where any other cable tray, trunking, pipework electrical cubicle passes within a few cm of the motor cable tray, bond this also with similar multistrand braid. Continue this process up to the motor. Where the motor cable run is broken by a local isolator, ensure that the cable armouring in and out of the isolator are cross-bonded with similar braiding. See figs 2 & 3 below. Avoid using flexible conduits like that shown in the picture. Terminate the motor cable armouring at the motor, remove any paint or rubber seals, between the gland and the motor terminal box. See fig 4 below. If possible, within the terminal box, add additional bonding between the terminal box and the motor body. If the motor is installed on an anti-vibration mounting, then bond across the mountings to any metallic chassis below the mounting. This is particularly important within air handling units (AHU), as the metallic structure of the AHU provides an excellent additional earth path. (See Fig 1 below) 09/11/04 4 Installing Inverter & emissions.doc

5 Do not allow ANY control cables or other power cables to be installed close to the motor cable from a drive. In particular, control cables & network cables should be segregated from motor cables from drives, by at least 0.5 Metres. If possible use screened motor cable, within the enclosure & segregate from any other cable. See fig 8 power input on left screened motor cable on right If the drive has to be fixed on masonry the following steps should be taken to ensure maximum potential for a successful installation. Ensure the motor cable is correctly glanded at the drive take particular care with the tightness of the connection Bring the cable tray carrying the motor cable as close as possible to the drive. Bond the drive to the cable tray with a short multi-strand earth braid. Bond each successive section of cable tray. Where any other cable tray, trunking, pipework electrical cubicle passes within a few cm of the motor cable tray, bond this also with similar multistrand braid. Continue this process up to the motor. Where the motor cable run is broken by a local isolator, ensure that the cable armouring in and out of the isolator are cross-bonded with similar braiding. See fig 2 & 3 below. Avoid using flexible conduits like that shown in the picture. Terminate the motor cable armouring at the motor, remove any paint or rubber seals, between the gland and the motor terminal box. See fig 4 below. If possible, within the terminal box, add additional bonding between the terminal box and the motor body. If the motor is installed on an anti-vibration mounting, then bond across the mountings to any metallic chassis below the mounting. This is particularly important within air handling units (AHU), as the metallic structure of the AHU provides an excellent additional earth path. (See Fig 1 below) Do not allow ANY control cables or other power cables to be installed close to the motor cable from a drive. In particular, control cables & network cables should be segregated from motor cables from drives, by at least 0.5 Metres. If these measures are applied, there will be a significant chance of a reliable trouble free installation. 4. When it Goes Wrong. The first signs of a problem usually occur long after the drive and its supporting cables have been installed. This means that any rectification work usually entails 09/11/04 5 Installing Inverter & emissions.doc

6 reworking an existing installation and this can be as expensive, or even more so, than the original installation costs. The first signs of problems usually occur when the associated plant, other than drive is being commissioned. Network problems - loss of communications when the drive/s is/are started. Fluctuations in analogue signal values high impedance signals are more susceptible, so check out humidity sensors and any thermocouple devices. Correctly operating solid state devices suddenly operating in an erratic fashion or possibly crashing. Note:- All devices used in an installation, should be marked for compliance with local standards i.e. CE,UL etc. and installed according to the manufacturers instructions. Unfortunately the application of the CE mark does not always guarantee that in fact the devices have passed the appropriate test for immunity (i.e. it is not always the fault of the drive or the installation.) If it is reported that equipment is being disturbed by one or more drives, it is best to try to inspect the installation to assess its quality, using the tips bulleted in 1. above, making corrections where appropriate. If any rework is required, a test should then be undertaken to ascertain if the corrective works have fixed the problems. If there are still problems then more detailed inspection is required, using a test instrument with a PEAK to PEAK Voltage range and with sufficient bandwidth to cope with the high frequency of the noise currents. An example of a suitable instrument would be the FLUKE ScopeMeter 123. When making tests, it is important for consistency of results, that temporary extension of the scope test probe cables should NOT be attempted. Test Procedure NOTE the drive MUST be in the RUN state, with an active output & MUST be connected to the motor To find the best performing local earth point use the above instrument (or similar) set to Peak To Peak VOLTAGE measurement. Begin by testing the Main Feed to the local distribution board from which the drive is supplied. Measure across the armoured cable gland/junction on this cable, to various points on the distribution cubicle to ascertain if there is a consistent quality of earthing across this cubicle. A value of a few 10 s of mv to 1 or 2 V Peak to Peak should be achievable. Once the general noise level has been ascertained, make the same tests between the gland on the feed to the distribution cubicle and metal work used in the structure and services of the building fig 2.(local to the cubicle.) Steel columns, 09/11/04 6 Installing Inverter & emissions.doc

7 beams, copper or steel pipework, cable tray etc. can all be used as references. Beware anything that may be isolated from earth by insulating materials. Once various metallic objects have been identified as being on a similar level of earth as the main in-comer to the distribution board (few 10 s to 1 or 2V Peak to Peak) these items can be used as references later in the test process.. It is then a simple process to work along the items identified below measuring the Peak to Peak Voltage across each junction. feed cables to each drive drive chassis. cable leaving the drive i.e motor cable. any local isolator in the motor cable. motor cable junction at the motor. motor body If the motor is mounted on an anti-vibration chassis - then the chassis to earth. Each measurement taken should be recorded. When completed the profile of the quality of the earthing can be viewed. On a well earthed cable run, a total of 10-15V Peak to Peak noise, can usually be achieved. On a poorly earthed installation as much as V Peak to Peak may be recorded ( Higher if the network voltage is higher than 400V). See below a scope image showing 549V Peak to Peak on an unearthed motor cable screen. Work should then begin by bonding the areas with the highest Peak to Peak Voltages to local earth points, using a braid that is as short as possible. This process should be repeated across all the points where high readings where recorded. When completed a second pass should be made using the same test instrument & points of measurement, again recording the Peak to Peak Voltage levels. This process 09/11/04 7 Installing Inverter & emissions.doc

8 should be repeated until the sum of recorded noise levels is less than V Peak to Peak but V Peak to Peak would be much more acceptable. Fig 1 Fig 2 This gland plate has a rubber joint the plate should be bonded to the panel backplate with a short braid This joint should be tight & cleaned of paint etc. 09/11/04 8 Installing Inverter & emissions.doc

9 Fig 3 Continue the earth path from this connection.. To this connection Change to S.W.A. or better Plastic Isolator Housing Fig 4 This joint should be cleaned of paint etc. This P.E. conductor is of little use for grounding H.F. noise currents Flexible conduit not a good screen, should be replace with S.W.A. or better 09/11/04 9 Installing Inverter & emissions.doc

10 Fig 5 Fig 6 09/11/04 10 Installing Inverter & emissions.doc

11 Fig 7 Fig 8 09/11/04 11 Installing Inverter & emissions.doc

12 Fig 9 First number (Protection against solid objects) Definition Second number (Protection against liquids) Definition 0 No protection 0 No protection 1 Protected against solids objects over 50mm (e.g. accidental touch by hands 2 Protected against solids objects over 12mm (e.g. fingers) 3 Protected against solids objects over 2.5mm (e.g. tools and wires) 4 Protected against solids objects over 1mm (e.g. tools, wires and small wires 5 Protected against dust - limited ingress (no harmful deposit) 6 Totally protected against dust 1 Protected against vertically falling drops of water 2 Protected against direct sprays up to 15 o from the vertical 3 Protected against direct sprays up to 60 o from the vertical 4 Protected against sprays from all directions - limited ingress permitted 5 Protected against low pressure jets if water from all directions - limited ingress permitted 6 Protected against strong jets of water e.g. for use on shipdecks - limited ingress permitted 7 Protected against the effects of temporary immersion between 15cm and 1m. Duration of test 30 minutes 8 Protected against long periods of immersion under pressure 09/11/04 12 Installing Inverter & emissions.doc