ABSTRACT. The SANS series of standards have selected sections in various locations addressing power drive systems.

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1 MIE TALK June 2016 Compiled by: Schalk J. P. Kruger ABSTRACT Concerns have been raised in the literature regarding the use of power drive systems (PDS) feeding motors (low voltage and medium voltage) in explosive atmospheres. Power drive systems are extensively covered in the SANS series of standards and motors are addressed in the SANS series of standards. These documents however do not provide specific requirements for assessing, testing and certification of power drive system motor combinations for use in explosive atmospheres. The SANS series of standards have selected sections in various locations addressing power drive systems. EXECUTIVE SUMMARY As the utilization of the combination of motors and Variable Frequency Drives (VFD) in hazardous areas is critical certification of the combination is required. To enable certification a set of parameters must be utilised by the certification body that are based on sound engineering practices and does not replicate approved manufacturing standards of the individual components. There are standards defining the specifications for manufacturing of electrical motors in the SANS range. The same stands for the manufacturing of the Complete Drive Module (CDM) in the SANS range. The requirements of hazardous environments are defined in the SANS for flammable gasses and vapours; and the SANS range for flammable dusts and fibres. As the standards referred to above define the specifications of the equipment it is not in the scope of this document to address manufacturing only the testing and certification of the application of Power Drive Systems (PDS). The challenge become in the application of the different standards and inference is required in some cases. Motors manufactured for PDS is defined. The standards do not define the requirements of the CDM used to drive the motor. CDM is defined and the standards do not define the motors attached to the CDM.

2 TABLE OF CONTENTS Abstract... 1 Executive Summary... 1 Table of Contents... 2 List of Figures/Charts... 3 Nomenclature Applicable Standards Power Drive System Components Power Drive Systems in Hazardous Locations Sources of Arcs, Sparks or Hot Surfaces Voltage Common Mode Voltage Electrostatic Voltages Current Common Mode Current Paths Circulating current Capacitance Capacitive Discharge Current Flux Ring Flux Linking Shafts Homopolar Fluxes Cabling Inter-Machine Arcing Electromagnetic Interference (EMI) Installation PDS Installations and safety Page 2 of 38

3 5.2 Motor Selection Risk management of sparks Risk Management of Excess Temperature Bearings Certification Certification for Arc and Sparks Creation Certification for Hot Surfaces Protective Device Marking Additional Marking Inspections Documentation References and Bibliography LIST OF FIGURES/CHARTS Figure 1: PWM PDS... 8 Figure 2: Symmetrical sine waves, the sum of the three = zero Figure 3: The sum of the input phases is not equal to zero Figure 4: How the noise currents flow in a BDM Figure 5: Earth paths for high-frequency current Figure 6: Overview of potential earth current paths Figure 7: Shaft grounding current equivalent circuit Figure 8: Addition of direct earth connection Figure 9: Induction of rotor circulating current Figure 10: Equivalent circuit for circulating current Figure 11: Capacitances of motor winding Page 3 of 38

4 Figure 12: Typical waveform of motor frame earth current Figure 13: Capacitive discharge current equivalent circuit Figure 14: End view depicting asymmetric field Figure 15: Shaft current and voltage due to asymmetric magnetic field Figure 16: Homopolar fluxes around stator, rotor and shaft Figure 17: Currents adding in a bearing Figure 18: Underground inter-machine arcing Figure 19: Potential difference between machines Figure 20: Illustration of RF bonding and its effect Page 4 of 38

5 NOMENCLATURE ASD BDM CDM CM DM EDM EMC EMI EPE Homopolar IGBT PDS Permeance PVC PWM RF RTD SS VFD VSD XPLE Adjustable Speed Drive Basic drive module consisting of power input, control and power output sections (SANS ). Complete drive module consisting of BDM and auxiliary sections, but excluding the motor and motor-coupled sensors ) Common Mode Differential Mode Electro-Discharge Machining Electro-Magnetic Compatibility Electro-Magnetic Interference Explosion Protected Equipment having equal or constant electrical polarity Insulated Gate Bipolar Transistor Power Drive System, comprising CDM, motor and sensors, but excluding the driven equipment and sensors (SANS ) The property of allowing the passage of lines of magnetic flux Polyvinyl chloride Pulse width modulation Radio Frequency Resistance Temperature Detectors Soft Start Variable Frequency Drive Variable Speed Drive Cross linked poly ethylene Page 5 of 38

6 1. APPLICABLE STANDARDS The following standards have references to PDS. Standard Name SANS The installation, inspection and maintenance of equipment used in explosive atmospheres Part 1: Installations including surface installations on mines SANS The installation, inspection and maintenance of equipment used in explosive atmospheres Part 2: Electrical apparatus installed underground in mines SANS Rotating Electrical Machines Part 11: Thermal Protection SANS Rotating electrical machines Part 17: Cage induction motors when fed from converters - Application guide SANS Rotating electrical machines Part 18-41: Qualification and type tests for Type I electrical insulation systems used in rotating electrical machines fed from voltage converters SANS Rotating electrical machines Part 18-42: Qualification and acceptance tests for partial discharge resistant electrical insulation systems (Type II) used in rotating electrical machines fed from voltage converters SANS Rotating electrical machines Part 19: Specific test methods for d.c. machines on conventional and rectifier-fed supplies SANS Rotating electrical machines Part 25: Guide for the design and performance of a.c motors specifically designed for converter supply SANS Rotating electrical machines Part 31: Selection of energyefficient motors including variable speed applications - Application guide SANS Explosive atmospheres Part 0: Equipment General requirements SANS Explosive atmospheres Part 7: Equipment protection by increased safety "e" SANS Explosive atmospheres Part 14: Electrical installations design, selection and erection SANS Explosive atmospheres Part 15: Equipment protection by type of protection "n" SANS Explosive atmospheres Part 17: Electrical installations inspection and maintenance SANS Safety of machinery - Electrical equipment of machines Part 1: General requirements SANS Safety of machinery - Electrical equipment of machines Part 32: Requirements for hoisting machines SANS Semiconductor convertors - General requirements and line commutated convertors Part 1-1: Specification of basic requirements SANS Semiconductor convertors - General requirements and line commutated convertors Part 1-2: Application guide Page 6 of 38

7 SANS Semiconductor convertors - General requirements and line commutated convertors Part 1-3: Transformers and reactors SANS Semiconductor converters Part 2: Self-commutated semiconductor converters including direct d.c. converters SANS Semiconductor convertors Part 6: Application guide for the protection of semiconductor convertors against overcurrent by fuses SANS Adjustable speed electrical power drive systems Part 1: General requirements - Rating specifications for low voltage adjustable speed d.c. power drive systems SANS Adjustable speed electrical power drive systems Part 2: General requirements - Rating specifications for low voltage SANS adjustable frequency a.c. power drive systems Adjustable speed electrical power drive systems Part 3: EMC requirements and specific test methods SANS Adjustable speed electrical power drive systems Part 4: General requirements - Rating specifications for a.c. power drive systems above V a.c. and not exceeding 35 kv SANS Adjustable speed electrical power drive systems Part 5-1: Safety requirements - Electrical, thermal and energy SANS Adjustable speed electrical power drive systems Part 5-2: Safety requirements Functional SANS Adjustable speed electrical power drive systems Part 6: Guide for determination of types of load duty and corresponding current ratings Page 7 of 38

8 2. POWER DRIVE SYSTEM COMPONENTS The Power Drive System (PDS) consists of a couple of parts as depicted below. The figure below shows a standard PWM system without a number of components (refer to the list below). Figure 1: PWM PDS The following is a more detailed list of the items that forms part of the PDS. Input Cable Input Transformer Input Filter Rectifier DC Link DC Link Discharge Inverter Output Inductor Filter Output Common Mode Filter Output Cabling Motor Earthing and Bonding Controls Page 8 of 38

9 Active Protection 3. POWER DRIVE SYSTEMS IN HAZARDOUS LOCATIONS Only the motor and driven load shall be installed in the potentially explosive atmosphere, with the Complete Drive Module (CDM) in a safe area. In the case of mines the energy source and the CDM is located in the hazardous location and requires the use of explosion prevention techniques for the total system. The PDS changes the motor operating conditions mostly due to: reduced cooling for self-ventilated motors due to speed reduction resulting in cooling airflow reduction; additional heat due to speed variations i.e. speed control by acceleration and deceleration of the load as required by process; frequency of starting; regeneration i.e. stopping of high inertia loads using the motor as a break; DC braking non-sinusoidal supply leading to increase losses (compared to sinusoidal supply) resulting in increased temperature rise; additional heat generation as a result of harmonic currents; dielectric heating due to high frequency/voltages; induced voltages leading to currents through the bearings; For these reasons attention must be paid to certification of PDS. 4. SOURCES OF ARCS, SPARKS OR HOT SURFACES 4.1 VOLTAGE COMMON MODE VOLTAGE The definition of common mode voltage is as follows: Page 9 of 38

10 Vcm = (Vag+Vbg+Vcg)/3 Where Vag, Vbg and Vcg are the voltages measured at the respective terminal of the variable frequency drive with respect to ground. Figure 2: Symmetrical sine waves, the sum of the three = zero Figure 3: The sum of the input phases is not equal to zero ELECTROSTATIC VOLTAGES Electrostatic voltages are not due to the basic design of the machine but rather to do with special circumstances, for instance, low humidity environments, or the nature of application, e.g. belt and pulley driven loads. The shaft voltage continues to build up until a discharge occurs through the bearings. Sometimes all that is needed is a little friction of a belt or pulley to set up electrostatic charges. Sources for electrostatic voltages in the PDS are as follows: Page 10 of 38

11 Air friction due to the rotation of the rotor Connection of the PDS to a liquid pump system that creates static voltages due to friction in the pipes. 4.2 CURRENT During VFD frequency conversion process, 2% to 3% of power is lost in a form of heat. The process also yields overvoltage spikes and harmonic current distortions. During its operation there are two kinds of noise currents that flow between the VFD unit and the power system. These noise currents are called common mode current and differential mode current, the converter inverter circuit on the diagram below, illustrates the flow behaviour of these currents. Figure 4: How the noise currents flow in a BDM During the VFD operation the following has to be taken into consideration: Common Mode noise Current CM current flows out along all of the conductors in a cable or cable bundle at the same time, and returns via a different path to complete its loop, in its return any path will do, however one that always exists, is via the earth structure. Common Mode creates a voltage between each phase and earth Note: Where there is no conductive path and since these noise currents are of high frequency, the noise current can flow through the air or insulation through stray capacitance. Page 11 of 38

12 Differential Mode noise Current In a case of a delta connected VFD as the on Figure 4: How the noise currents flow in a BDM above, DM current flows out and back along the three mains phase conductors. The DM current also flow out and back along the three motor phase conductors, implies that current flows along one phase and returns via the other phase. Differential Mode creates a voltage between phases independent of earth COMMON MODE CURRENT PATHS Bearing currents are discussed in SANS (2006). Rotating electrical machines Part 17: Cage induction motors when fed from converters Application Guide, section 10. The current in the capacitances returns to its source at the inverter drive through the earth paths. Where there are several possible routes, it divides between them in a proportion depending on their high-frequency impedance, which is mainly a function of their inductance. This is where the possibility of earthing-related bearing current arises. If there are earth paths present which pass through the bearings, and if the inductances are such that a significant part of the earth current flows through this route, then bearing damage may arise. Figure 5: Earth paths for high-frequency current, illustrates the essential features of this effect. Figure 5: Earth paths for high-frequency current I1 is the current flowing into the designated earth return path to the inverter. Page 12 of 38

13 I2 returns to the inverter through an alternative path, such as the motor mountings. It does not pass through the bearings, and therefore is harmless to them - it may cause EMC (Electro-Magnetic Compatibility) problems. I3 passes through the motor bearings to the shaft and hence to earth via the driven machine. I3 may be harmful, as well as being a possible cause of EMC problems. Figure 6: Overview of potential earth current paths, shows an overview of the complete system and earth paths. Figure 6: Overview of potential earth current paths The current paths shown dashed in Figure 13 are the return routes for highfrequency earth current to return to the supply. The flow of common-mode current in the supply may be responsible for electrical interference problems, and measures such as filters are used to ensure that the return route is restricted to within the drive system. Figure 7: Shaft grounding current equivalent circuit, shows an equivalent circuit. The inductance Lfe represents the inductance of the two parallel earthing paths which carry I1 and I2. The circuit shows how the earth currents I1 + I2 result in a voltage on the motor frame relative to earth, which can cause a current I3 to return to earth through the bearing and shaft. Part of I3 flows in Crf and is harmless, but if the voltage across the bearing is sufficient then it may suffer dielectric breakdown and carry current, with resulting damage. Note that in this case the source of the current is coupling through Csf, which has a much higher value than Crf and therefore passes a higher current for a given rate of change of stator winding voltage. This is why shaft grounding current, if it occurs, is more serious than capacitive discharge current. Page 13 of 38

14 Figure 7: Shaft grounding current equivalent circuit The relative values of I1, I2 and I3 depend on the relative impedances of their paths. To minimise I3, it is necessary to ensure that the paths for I1, and to some extent I2, have much lower impedance than the paths for I3. At the high frequencies associated with modern inverters, the impedances are predominantly inductive. The problem of earthing for high frequencies is well understood from EMC considerations, and to a considerable degree the solutions are the same ensuring a low-inductance earth path returning from the motor frame to the inverter drive, and using screened cable whose mutual inductance effect minimises the earth difference potential. In addition, provided the potential of the driven machinery is the same as that of the motor frame, there will be no tendency for earth current to flow through the bearings. Therefore the provision of low-inductance connections between the motor frame and the driven machine is the single most effective precaution required. This is illustrated in Figure 8: Addition of direct earth connection, where it can be seen that the direct earth connection between the motor frame and the driven machine prevents a potential from appearing across the bearing. The current I3 still flows into the driven machine, but does not pass through the shaft and bearings. Page 14 of 38

15 Figure 8: Addition of direct earth connection The direct earth connection is inserted between motor and driven machine, to prevent shaft grounding current CIRCULATING CURRENT In the explanation given above, it has been assumed that the current flows uniformly from the three phases to the motor body through the motor stray capacitance. In fact the precise path of the current flow within the motor is complex, because the capacitance is distributed along the winding and the shape of the pulse edges changes as they propagate along the winding. The propagation is not normally geometrically symmetrical since the winding terminations are naturally at one end of the stator. Consequently there is a component of high-frequency current, which is not symmetrical with respect to the rotor, i.e. the currents in opposite arms of the coils are not the same. This causes voltage to be induced in the rotor -bearingframe-bearing loop by transformer action, which can result in a circulating current. Figure 9: Induction of rotor circulating current shows a simplified illustration of the effect in the motor, and Figure 10: Equivalent circuit for circulating current, shows an equivalent circuit. Page 15 of 38

16 Figure 9: Induction of rotor circulating current Figure 10: Equivalent circuit for circulating current Unlike the earth current, this circulating current is a function of the geometry of the motor and is not affected by earthing arrangements. The magnitude of the current depends on the dimensions of the coils relative to the velocity of propagation of the Page 16 of 38

17 pulses, and their rise-times. In practice the effect is significant only for motors of frame size 280 and above. 4.3 CAPACITANCE When a typical PWM voltage waveform is connected to a motor through a cable, the fast changing voltage waveform causes pulses of current to flow in all of the stray capacitances at every voltage transition. The calculation of capacitance is described in Error! Reference source not found.. The capacitance of the motor cable phase conductors to the earth conductor is often the highest in the circuit, but the most important capacitances in terms of bearing current are those within the motor: Csf - From stator winding to stator core and hence to motor frame Csr - From stator winding to rotor (i.e. through the stator slots, and from the end-winding) Crf - From rotor to stator core and frame The stator windings are embedded in the laminated steel core, and separated from it by thin slot liners which have a relative permittivity of (typically) 3 to 4. The slots are quite narrow, the air-gap is larger than the slot-liner thickness, and the air has a permittivity of only 1. Therefore Csf is much higher than Csr; typically times. This is illustrated in Figure 11: Capacitances of motor winding. Page 17 of 38

18 Figure 11: Capacitances of motor winding The common-mode voltage generated by the inverter causes pulses of current to flow in these capacitances, from the motor winding to the frame and rotor, which do not sum to zero as would three-phase current. The current pulses are very short, of the order of 1 μs, but the peak magnitude of the current in Csf may be up to 2 A peak, and their rise-time is also very short. This means that they can cause considerable voltage drops in the inductances of the power cable and the earth return arrangements. Figure 12: Typical waveform of motor frame earth current shows a typical current waveform. Figure 12: Typical waveform of motor frame earth current Vertical = 500 ma/div. Horizontal = 5µS/div. Page 18 of 38

19 The capacitive coupling can give rise to bearing current through two different mechanisms, which are generally referred to as capacitive discharge current and shaft grounding current. These are illustrated by the equivalent circuits in Figure 13: Capacitive discharge current equivalent circuit and Figure 7: Shaft grounding current equivalent circuit respectively. Some published papers use different terminology, which can be confusing CAPACITIVE DISCHARGE CURRENT Current transferred to the rotor by capacitance Csr tends to return to the earthed stator through the bearings. This is illustrated in the equivalent circuit in Figure 13: Capacitive discharge current equivalent circuit Figure 13: Capacitive discharge current equivalent circuit As explained above, Csr has a small value, and Crf is relatively large because of the rotor size and its proximity to the stator over its entire surface area. Therefore the voltage developed on the shaft is not normally sufficient to break down the bearing oil film, and the available charge is very limited. In some literature this particular mechanism of bearing current production is referred to as Electro Discharge Machining (EDM). Although this form of bearing current has been discussed in the literature, and remedies such as conductive slot wedges have been proposed, it is generally recognised that shaft grounding current is a far more important source of bearing current. 4.4 FLUX RING FLUX LINKING SHAFTS The linkage of the alternating flux with the shaft is one of the most important causes of bearing currents. This flux flows perpendicular to the axis of the shaft and pulsates in the stator and rotor cores. It results from asymmetries in the magnetic circuit of the Page 19 of 38

20 machine. The asymmetries arise from design and construction of the machine and from inaccurate alignment. Normally, the flux from each pole crosses the air-gap and if the magnetic path is symmetrical, it divides equally, half clockwise and half anticlockwise. However, if there is a difference in the reluctance of the core in one direction compared with the other, there will be unequal division of the flux and a net flux linking with the circuit consisting of shaft, bearings and frame will exist (see Figure 14: End view depicting asymmetric field). Figure 14: End view depicting asymmetric field Page 20 of 38

21 Figure 15: Shaft current and voltage due to asymmetric magnetic field This ring flux is alternating and as such it establishes a potential difference between the ends of the shaft as shown in Figure 15: Shaft current and voltage due to asymmetric magnetic field. If this potential difference is large enough to create an electrical breakdown in the bearing grease lubricating film, the thickness of which usually ranges from 1 µm to 20 µm, arcing occurs between the races and the rolling element. The resulting bearing current will cause bearing failure. In addition, experience has shown that dirt, microscopic metallic particles and irregular film thickness permit lubricant film to be bridged. Under this condition, the impedance of the bearing circuit is so low that small shaft voltages may cause substantial bearing currents HOMOPOLAR FLUXES Homopolar flux can be significant in larger machines, frame size 400 and above. It may result from an air-gap or rotor eccentricity and consequent permeance variation, leading to unbalanced ampere-turns surrounding the shaft. The path of the homopolar flux can be seen in Figure 16: Homopolar fluxes around stator, rotor and shaft. The flux crosses the air-gap in one direction and leads to local bearing currents as shown in Figure 17: Currents adding in a bearing. Page 21 of 38

22 Figure 16: Homopolar fluxes around stator, rotor and shaft Page 22 of 38

23 Figure 17: Currents adding in a bearing The homopolar flux crossing the air-gaps will generate an additional voltage as the shaft keys or keyways cut it, causing current to flow along the shaft, across the bearing and return via the bedplate and frame. At the inner region of the bearings, the circulating local bearing currents and shaft current combine. Hence, more current will flow in this region of the bearing than in the outer region. The bearing currents will have the same frequency as the homopolar flux. There is no method of insulating bearings against homopolar voltages. It is only possible to reduce the magnetic flux by compensation in the form of counter ampereturns (inverse magnetic circulation) or by increasing the magnetic reluctance. A reduction in the homopolar voltage can be achieved by careful adjustment of the airgap during construction. To prevent current from flowing towards the driven equipment, a non-conducting coupling would be necessary. However, under the pure sinusoidal supply condition the value is normally too low to warrant any remedial measure, except on large motors. 4.5 CABLING Radiated noise from cables between the VFD and the motor has been studied. Unshielded VFD cables can radiate 80 V noise to unshielded communication cables Page 23 of 38

24 and 10 V noise to shielded instrument cables. The radiated noise from foil tape shielded VFD cables is also excessive. A foil braided shield and armoured cable performs much better. Still a spacing of at least 0.3 m is recommended between shielded VFD and shielded instrumentation cables. The recommended spacing is a challenge in the case of mining and the effect should be considered during use. The cables should never cross. As a best practice, separate trays to isolate VFD and instrumentation cables should be used to avoid mistakes during plant expansions and instrumentation system upgrades. All VFD systems have reflected waves from an impedance mismatch between the VFD and the motor. The amplitude of the waves depends on the voltage magnitude and rise time from the PWM drive, the distance between the VFD and motor, and the impedance mismatch. If a reflected wave gets in-phase with the radiated wave, the voltage can double and the PVC jacketed VFD cables can be damaged. XPLE jacketed VFD cables that are capable of withstanding a high voltage impulse are recommended. 4.6 INTER-MACHINE ARCING An arc may develop between two machines when they come together due to the stray capacitance circulating on the two machines. That arcing is capable of igniting and causing and explosion in the presence of high concentration of methane gas at energy content of 250 µj and above. Figure 18: Underground inter-machine arcing shows the measurements of the spark developed between the two machines during contact: Page 24 of 38

25 Figure 18: Underground inter-machine arcing Page 25 of 38

26 Figure 19: Potential difference between machines 4.7 ELECTROMAGNETIC INTERFERENCE (EMI) The EMI can cause PDSs to suffer errors or malfunctions, e.g. un-commanded changes in speed, torque, direction, etc., where these could have consequences for safety risks, complying with IEC EMC test standards or any other EMC test standards, IEC applies the basic standard on functional safety IEC to PDS, however it does not prescribe what to do about the possibility of EMI, the following issues of concerns are listed on the IEC : Power quality issues, including: o Distorted mains supplies (including harmonic distortion and communication notches) Page 26 of 38

27 o Supply voltage deviations variations, changes, fluctuations, dips, dropouts and short interruptions o Three phase voltage unbalance o Mains frequency variations Magnetic fields Conducted continuous EM disturbances (150 khz to 800 MHz) Fast transients Radiated Continuous EM disturbances 980 MHz to 1 Ghz) Surge transients Electrostatic discharge 5. INSTALLATION SANS (2006). Rotating electrical machines Part 17: Cage induction motors when fed from converters Application Guide provides detail. I suggest some extractions from: GAMBICA/REMA (2012). Variable Speed Drives and Motors - Installation Guidelines for Power Drive Systems. Technical Guide. 4th Edition. GAMBICA. London. Additional requirements may be added for clarity. 5.1 PDS INSTALLATIONS AND SAFETY Based on the discussions above, the following has to be considered during the installation to mitigate the flow of common mode currents and differential mode currents through the stray capacitance: Inline input filter should be RF-bonded to the rectifier s metal enclosure Mains noise filter should be RF-bonded to the chopper s metal enclosure Rectifier and chopper metal enclosures must be RF-bonded Inline output filter should be RF-bonded to the rectifier s metal enclosure. Page 27 of 38

28 At the chopper end, the motor cable screen must be RF-bonded to the chopper s metal enclosure, chassis or frame. At the motor end, the motor cable screen must be RF bonded to the motor s metal terminal box, which in the motor s enclosure or frame. Prevent noise from the chopper which causes EMC problems by making the motor cable very short, less than one-tenth of the wavelength at the highest significant chopper harmonic frequency (e.g. 1 m for 30 MHz, 10 m for 3 MHz, 100 M for 300 khz). In the case of PDS the switching frequency may usually be set up to 8kHz and therefore one tenth of a cable length of m i.e m. As the highest significant chopper harmonic frequency is higher manufacturers normally requires a maximum distance in the installation requirements and this has to be complied with. o Wavelength is calculated All of the component parts of a VFD, plus its associated suppression components (chokes, filters, isolating transformers, motor cable screen bonds, etc.) must be in very close proximity, ideally all contained within one metal enclosure. All of the earth/ground conductors associated with the component parts in (h) above must connect to one point, ideally the chassis, frame, back plate or surface of the metal cabinet they are all contained within. This one point must then connect to the site s or vessel s safety earth/ground structure via one conductor. Based on the recommendations above for the VFD installations, the diagram below illustrates the application of the RF bonding and the positive effect introduced by the RF bonding. Page 28 of 38

29 Figure 20: Illustration of RF bonding and its effect By the application of the above listed requirements, the following will be achieved: Reduced harmonics Reduced DM and CM noises from flowing in the long mains cables Most CM noises now flow in a much smaller loops within the filter/drive/filter assembly Note: Dangerous Touch potentials could still be present and the technique to mitigate this risk must be tested. 5.2 MOTOR SELECTION The following safety aspects must be addressed during selection of the motor: no additional risk of sparks; no additional risk exceeding the temperature class. These aspects are considered in further detail in the following clauses. Page 29 of 38

30 5.2.1 RISK MANAGEMENT OF SPARKS The motor and converter manufacturers will ensure that bearing currents are limited and sparks are prevented using techniques including: suitable stator insulation materials and techniques; reduction of voltage transients; o electrical filters prevention of excessive bearing currents; o insulated bearings or bearing housings; reduced or optimised switching frequency; o electrical filters Recommended measures as defined in SANS and SANS must be utilised. Note: For mines, the mitigation approach is to install suitable filter systems thereby ensuring the power source, PDS and motors are certified (System Certification) as a complete system reducing touch potentials to acceptable levels RISK MANAGEMENT OF EXCESS TEMPERATURE GENERAL The temperature class of the motor shall be checked by calculation or by testing as required by the appropriate standard. There are two main methods for diminishing the risks of excess surface temperature: to have a physical feedback signal from the motor (thermal sensing element) and use this signal to initiate shut down in the case of excess temperature; to control and limit the heat (temperature) which can be generated by the motor. Page 30 of 38

31 TEMPERATURE SENSING This technique uses thermostats, thermistors or RTD devices embedded in the stator windings, with the appropriate controls to ensure that the temperatures are within the permitted limits. This does not always control any additional temperature rise within the rotating element, and for high power motors the manufacturer/notified Body may stipulate the use of additional thermal detectors at the bearings. It is also mandatory that the protection used in conjunction with the temperature detectors is suitable for the purpose (including any intrinsic safety barriers where appropriate). As the correct functioning of the protection is critical to the safety of the overall system, the functional safety of the protection should be assessed and approved in accordance with the appropriate standards. This method is applicable to all motor types. When considered specifically for an EEx d design equipped with suitable integral thermal protection, type testing can demonstrate that for a sample electrical input and motor load, the protection will trip the motor before any surface temperature reaches the limit. This must also include a period after de-energization. In this case a blanket certificate may be issued detailing only the input and load parameters CONTROL OF HEAT GENERATION Control of heating is achieved by limiting the current passing through the motor at a specific frequency. As the torque generated is directly related to current, a loadability curve may be established, which gives the maximum continuously available torque at a particular speed or frequency, when the motor is fed at the correct voltage and frequency. The curve is dependent on the motor design, and can be advised by the manufacturer. The load-ability curves must take into account the CDM technology, the surface temperature class of the motor, and the type of Ex protection. In many cases a manufacturer will publish the load-ability curves for his products to allow users to check that the load characteristics fall within the PDS capability. Figure 7 shows an example of a load-ability curve for a cage induction motor, fed by an inverter. This shows the reduction in torque capability at low speeds due mainly to the reduction in ventilation, a reduction in torque at base speed to allow a sufficient margin for safety, and a reduction above base speed due to the application of a constant voltage (field weakening). 5.3 BEARINGS For low voltage ASD application motors selected between frame 63 up to frame 280 used with an ASD switching frequency of below 10 khz, an insulated bearing is not needed. If the ASD switching frequency exceeds 10 khz, an insulated bearing on the Page 31 of 38

32 non-drive end will be required. For ASD application motors selected from frame 315 and above, an insulated bearing on the non-drive end shall be required. The insulated bearing used should have an impedance of at least 100 Ω at 1MHz. This will help to prevent the flow of shaft currents through the motor bearing, which causes pitting of the bearing, scoring of the shaft and eventual bearing failure. 6. CERTIFICATION 6.1 CERTIFICATION FOR ARC AND SPARKS CREATION Certification of the PDS shall include the requirements as specified for the management of the increased sources of ignition due to arcs and sparks for example bearing currents and shaft voltages etc. 6.2 CERTIFICATION FOR HOT SURFACES The temperature class of the motor must be specified based on the increased heat created by the PDS. SANS Annex A requires a test for the PDS. As per SANS motors supplied at varying frequency and voltage by a converter / soft starter supplies require that either: a) the motor has been type-tested for this duty as a unit in association with the specific converter / soft starter or with a comparable converter in reference to the voltage and current specifications and with the protective device (described below) specified in the descriptive documents according to IEC XX, or b) if the motor has not been type-tested for this duty as a unit in association with the converter / soft starter, then means (or equipment) for direct temperature control by embedded temperature sensors specified in the motor documentation or other effective measures (protective device as below) for limiting the surface temperature of the motor housing shall be provided such that the temperature is not exceeded. The effectiveness of the temperature control shall take into account power, speed range; torque, ramp up (run up) and frequency for the duty required and shall be verified and documented. The action of the protective device shall be to cause the motor to be electrically disconnected. A current-dependent time lag protective device is not regarded as an other effective measure for temperature protection. NOTE 1 In some cases, the highest surface temperature occurs on the motor shaft. NOTE 2 It is considered that soft starting is used for a short time period. NOTE 3 For motors with type of protection e or n terminal boxes, when using converters with high frequency pulses in the output, care should be taken to ensure that any overvoltage spikes and higher temperatures which may be produced in the terminal box are taken into consideration. Page 32 of 38

33 NOTE 4 The motor should be used within its electrical rating and the converter configuration should be set to match the motor rating information with respect to frequency range and any other specified parameters such as minimum carrier frequency. The converter configuration shall enable the adjustment of the parameter. NOTE 5 Permanent magnet motors operate as a generator while coasting after power is removed. For motors of level of protection eb, where the voltage can be greater than the rated voltage, the motor-converter system will be suitable for the voltages that will result. NOTE 6 Ex na motor may have its alternative temperature class determined by calculation in accordance with IEC NOTE 7 The setup of the speed control device must ensure that the motor run up is such that the motor surface temperature is not exceeded PROTECTIVE DEVICE The overload protective device shall be (SANS ): a) a current-dependent, time lag protective device monitoring all three phases, set at not more than the rated current of the machine, which will operate in 2 h or less at 1,20 times the set current and will not operate within 2 h at 1,05 times the set current, or b) a device for direct temperature control by embedded temperature sensors, or c) another equivalent device. 7. MARKING Additional marking is required on the motor and the VSD. The motor marking will entail the following at least: Specific parameter values that may not be changed The VSD must also have additional marking that will define the load being in an Ex area and that the parameter set may not deviate from the certificated values. 7.1 ADDITIONAL MARKING If the motor is installed in the hazardous area as part of the PDS the motor must also carry an additional nameplate depicting this system including certification numbering. Relevant CDM information should be supplied with the motor. The motor must be market as U (i.e. item as part of a system). This may be done using additional equipment marking or in the IA certification under special conditions of use. The additional marking should include at least: Page 33 of 38

34 relevant electrical characteristics of the converter; o this may include inverter type, switching frequency, and peak rate of voltage change maximum load torque versus speed range allowed; o in a centrifugal fan or pump application the torque at low speeds are also low but the cooling efficiency must be verified by test; o the exact value of the torque has to be considered at the minimum and maximum speed corresponding to constant torque application; o permitted duty cycle 8. INSPECTIONS SANS do not specifically address the inspections of PDS. It is therefore the proposal to add the following: Verification of the parameter set as per the certification Verification of the installation requirements regarding tagging 9. DOCUMENTATION Documentary evidence of the specific conditions of use should be obtained from the motor manufacturer, and retained, SANS Annex A. Page 34 of 38

35 10. REFERENCES AND BIBLIOGRAPHY GAMBICA/REMA (2006). Variable Speed Drives and Motors - Application of the ATEX Directives to Power Drive Systems. Guide no 4. 2 nd Edition. GAMBICA. London. GAMBICA/REMA (2012). Variable Speed Drives and Motors - Installation Guidelines for Power Drive Systems. Technical Guide. 4 th Edition. GAMBICA. London. GAMBICA/REMA (2006). Variable Speed Drives and Motors Motor Shaft Voltages and Bearing Currents under PWM Inverter Operation. Technical Report No nd Edition. GAMBICA. London. JOY (2013). Presentation to SAFA Working Group Detailed Effects of VFDs onto the Power System. South Africa SANS SANS SANS SANS (2005). Rotating electrical machines Part 11: Thermal protection. SABS Standards Division. Pretoria. South Africa SANS (2006). Rotating electrical machines Part 17: Cage induction motors when fed from converters Application Guide. SABS Standards Division. Pretoria. South Africa SANS (2009). Rotating electrical machines Part 18-41: Qualification and type tests for Type I electrical insulation systems used in rotating electrical machines fed from voltage converters. SABS Standards Division. Pretoria. South Africa SANS (2009). Rotating electrical machines Part 18-42: Qualification and acceptance tests for partial discharge resistant Page 35 of 38

36 electrical insulation systems (Type II) used in rotating electrical machines fed from voltage converters. SABS Standards Division. Pretoria. South Africa SANS (1995). Rotating electrical machines Part 19: Specific test methods for d.c. machines on conventional and rectifier-fed supplies. SABS Standards Division. Pretoria. South Africa SANS (2010). Rotating electrical machines Part 24: Online detection and diagnosis of potential failures at the active parts of rotating electrical machines and of bearing currents - Application guide. SABS Standards Division. Pretoria. South Africa SANS (2007). Rotating electrical machines Part 25: Guide for the design and performance of a.c. motors specifically designed for converter supply. SABS Standards Division. Pretoria. South Africa SANS (2010). Rotating electrical machines Part 31: Selection of energy- efficient motors including variable speed applications - Application guide. SABS Standards Division. Pretoria. South Africa SANS (2005). Explosive Atmospheres Part 0: Equipment General Requirements. SABS Standards Division. Pretoria. South Africa SANS (2009). Explosive atmospheres Part 1: Equipment protection by flameproof enclosures "d". SABS Standards Division. Pretoria. South Africa SANS (2007). Explosive atmospheres Part 7: Equipment protection by increased safety "e". SABS Standards Division. Pretoria. South Africa SANS (2009). Explosive atmospheres Part 14: Electrical installations design, selection and erection. SABS Standards Division. Pretoria. South Africa Page 36 of 38

37 SANS (2010). Explosive atmospheres Part 15: Equipment protection by type of protection "n". SABS Standards Division. Pretoria. South Africa SANS (2009). Explosive atmospheres Part 17: Electrical installations inspection and maintenance. SABS Standards Division. Pretoria. South Africa SANS (2011). Explosive atmospheres - Part 19: Equipment repair, overhaul and reclamation. SABS Standards Division. Pretoria. South Africa SANS (2007). Explosive atmospheres Part 26: Equipment with equipment protection level (EPL) Ga. SABS Standards Division. Pretoria. South Africa SANS (2005). Electrical apparatus for use in the presence of combustible dust Part 0: General requirements. SABS Standards Division. Pretoria. South Africa SANS (1997). Convertor transformers Part 1: Transformers for industrial applications. SABS Standards Division. Pretoria. South Africa SANS (1997). Converter transformers Part 3: Application guide. SABS Standards Division. Pretoria. South Africa SANS (1997). Adjustable speed electrical power drive systems Part 1: General requirements - Rating specifications for low voltage adjustable speed d.c. power drive systems. SABS Standards Division. Pretoria. South Africa SANS (1998). Adjustable speed electrical power drive systems Part 2: General requirements - Rating specifications for low voltage adjustable frequency a.c. power drive systems. SABS Standards Division. Pretoria. South Africa Page 37 of 38

38 SANS (2005). Adjustable speed electrical power drive systems Part 3: EMC requirements and specific test methods. SABS Standards Division. Pretoria. South Africa SANS (2005). Adjustable speed electrical power drive systems Part 4: General requirements - Rating specifications for a.c. power drive systems above V a.c. and not exceeding 35 kv. SABS Standards Division. Pretoria. South Africa SANS (2008). Adjustable speed electrical power drive systems Part 5-1: Safety requirements - Electrical, thermal and energy. SABS Standards Division. Pretoria. South Africa SANS (2008). Adjustable speed electrical power drive systems Part 5-2: Safety Requirements - Functional. SABS Standards Division. Pretoria. South Africa Page 38 of 38