Stellar SR55 Soft Starter User Manual SR55-M
Contents
1. B APPENDIX B GLOSSARY OF TERMS Breakaway Torque The minimum torque required to achieve rotor movement for the motor with its load Current Limit The current at which the ramp is held For the SR55 current limit is only active during start up where 1t contributes to the motor control function This feature 1s particularly useful when starting high inertia loads that require an extended start up period See also Overload Level Direct On Line DOL The direct connection and disconnection of a motor from the AC main supply by means of a contactor or switch Acceleration and operation is at full mains voltage only IERS Intelligent Energy Recovery System An advanced motor control technology proven to reduce the energy consumed in fixed speed motor applications It matches the power consumption to the load required by intelligently monitoring and regulating energy consumption voltage current and power factor during the motor starting and running stages IERS automatically bypasses itself when it is not needed and continues monitoring to re engage itself as needed Inrush Current or Locked Rotor Current The current that flows at the instant of connection of a motor to the power source It is limited by the impedance presented by a de energized motor and the applied voltage Usually expressed as a multiple of motor full load current Kick start Voltage The percentage of supply voltage applied before commencing ra2. the Wauchope starter which eliminate or reduce the reconnection transients However such starters are expensive and have reliability implications and they are not widely applied for these reasons The wye delta starter also has disadvantages due to the restricted starting torque available 1f you need 40 LRT to break away you can only increase the motor size or revert to direct on line Combined with the severe effects of the re switching surges and the additional costs of bringing six conductors from the motor to the starter instead of only three wye delta only offers an Imperfect solution to the problem of starting the induction motor METHOD C PRIMARY RESISTANCE STARTER It has long been recognized that the transition step in the wye delta system was a source of problems such as welded contactors sheared drive shafts etc For many years a method of stepless control has been available in the form of the primary resistance starter This type of controller inserts a resistance in one or more often in each of the phase connections to the stator at start up after which it is progressively reduced and shorted out at the end of the acceleration process Frequently the resistances are movable blades that are gradually inserted into an electrolyte liquid The mechanism is usually large and expensive both to purchase and to maintain and considerable heat is created by the passage of current through the electrolyte resistor This limits t
3. 3 6 RUNNING INDUCTION MOTORS Once a start has been completed the motor operating efficiency becomes of interest When working at or near full load the typical three phase induction motor 1s relatively efficient and readily achieves efficiencies of 8596 to 95 However as shown below motor efficiency falls dramatically when the load falls to less than 50 of rated output FIGURE B 3 6 1 MOTOR EFFICIENCY LOAD CHARACTERISTIC 100 EFFICIENCY 0 1 2 1 1 LOAD Motor Efficiency Load Characteristic In fact very few motors actually experience consistent fully rated operation and the vast majority operate at much lower loads due to either over sizing a very frequent situation or natural load variations For Fan and Pumping applications the affinity laws will allow the inverter drive to show very considerable energy savings over virtually all other methods of control through varying the speed of the motor in response to changes in load Where motor speeds cannot be varied an optimizing version of semiconductor motor controller such as the SR55 will also produce energy savings in lightly loaded motors Less sophisticated systems of soft starters remain at full conduction and the motor then behaves as if it were connected directly to the main supply However at light loads and mains voltages induction motors always have excess magnetic flux and efficiency loss and power factor degradation result By detecting the load at any ins
4. ON until the current through the device falls to zero which conveniently happens at the end of each half cycle in alternating current supplies By controlling the switch on point of a thyristor relative to the voltage zero crossing in each half wave of an alternating current it is possible to regulate the energy passing through the device The closer the turn on point is to the voltage zero crossing point the longer the energy is allowed to flow during the half cycle Conversely delaying the turn on point reduces the time for the energy to flow Putting two thyristors back to back or anti parallel in each of the phase connections to a motor and by precisely controlling their turn on points an electronic soft starter continuously adjusts the passage of energy from the supply so that it 1s just sufficient for the motor to perform satisfactorily So for instance by starting with a large delay to the turn on point in each half cycle and progressively reducing it over a selected time period the voltage applied to the motor starts from a relatively low value and increases to full voltage Due to the motor torque being proportional to the square of the applied voltage the starting torque follows the same pattern giving the characteristic smooth stepless start of the soft starter Stellar SR55 Series Soft Starter User Manual Ist Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations B
5. User Manual 1st Ed Rev A 08 17 2015 Page B 7 Appendix B Soft Starter Application Considerations WAUTOMATIONDIRECTE P Pm s vv TT B 3 CONCEPTS AND PRINCIPLES OF FIXED SPEED INDUCTION MOTOR STARTING AND CONTROL Since its invention one hundred years ago the standard three phase induction motor has become one of the most familiar items of industrial equipment ever known Due to its simplicity of construction low cost reliability and relatively high efficiency it is likely to remain the prime source of mechanical energy for the foreseeable future B 3 1 INTRODUCTION Energy conversion from the electrical supply to rotating mechanical energy is a characteristic of all motors To regulate energy flow most motor circuits require a mechanism to connect and disconnect them from their electrical power source Electro mechanical switches known as contactors are the standard means of achieving this control Even today more than one hundred years after their introduction contactor based systems remain the most widely used method of motor control Nevertheless there is a definite trend towards more sophisticated electronic systems of control being applied to fixed speed motor drives This section will discuss these forms of control namely electronic microprocessor controlled optimizing soft starters such as the SR55 B 3 2 THE INDUCTION MOTOR In order to appreciate the benefits of usin
6. motors Page B 8 Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations B 3 2 THE INDUCTION MOTOR CONTINUEDO FIGURE B 3 2 1 TORQUE SPEED CURVE INDUCTION MOTOR Pull Out Torque M TORQUE Se Locked Rotor Torque LRT Ma Pull up Torque Full Load Torque FLT M Synchronous speed 0 SPEED S Torque Speed Curve Induction Motor FIGURE B 3 2 2 TORQUE SPEED CURVE COUPLED LOAD Each load coupled to an induction motor has its own speed torque curve Pull Out Torque M TORQUE Locked Rotor Torque LRT MA Full Load Torque FLT My Synchronous speed 0 SPEED S Torque Speed Curve Coupled Load FIGURE B 3 2 3 TORQUE SPEED CURVE ACCELERATING TORQUE The acceleration of a motor load system is caused by the difference between the developed torque motor and the absorbed torque load and is shown by the shaded area in the next figure Pull Out Torque M TORQUE Locked Rotor Torque LRT Mj Full Load Torque FLT My Pull up Torque Synchronous speed 0 SPEED S Torque Speed Curve Accelerating Torque Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 Page B 9 Appendix B Soft Starter Application Considerations VAUTOMATIONDIRECT R B 3 2 THE INDUCTION MOTOR CONTINUED Obviously the larger the difference bet
7. unloaded 50 85 Conveyor vertical lifting loaded 100 175 Conveyor vertical lowering unloaded 10 40 Conveyor vertical lowering loaded 10 25 Crusher not rock unloaded Drilling machine unloaded 25 75 10 Can be high inertia Fan axial flow propeller 20 40 Feeder screw 100 175 Needs high starting torque motor Feeder vibrating motor driven 100 150 Needs high starting torque motor Grinder unloaded 10 25 Usually high inertia Hammer mill 20 125 Eccentric load needs high starting torque motor Mills flour etc 30 50 Mixer dry contents Mixer fluid contents 35 75 10 40 Mixer plastic contents 75 125 High torque motor offers advantage Mixer powder contents 75 125 High torque motor offers advantage Pelletizers 50 100 Press flywheel 50 150 Needs high starting torque motor Pump centrifugal 10 25 Soft stopping useful Pump positive displacement piston type 100 175 Needs high starting torque motor Pump vane type positive displacement Rolling mill 100 150 Needs high starting torque motor Saw band Saw circular May be high inertia Plug brake may be useful Screen vibrating Transformers voltage regulators Change firing mode Tumblers Can be eccentric load may need high torque motor Stellar Y SR55 Series Soft Starter
8. with this directive For the SR55 this guidance is given in the EMC guide which is Appendix C of this user manual It is essential that users and installers understand and comply with the requirements described in these sections B 1 14 FUSES Page B 4 Circuit protection fuses should be rated to allow for the extended start times associated with the use of a Soft Starter Traditional HRC motor fuses may need to be rated higher than the motor rated current for normal low inertia applications but modern extended start fuses will generally give full motor protection See also section B 2 2 relating to high inertia loads Semiconductor fuses are available for the short circuit protection of the thyristors in the SR55 See the Fuse and Current Ratings section in chapter 2 Electrical Installation of this manual for semiconductor fuse recommendations and details of the Overload incorporated into the SR55 Stellar SR55 Series Soft Starter User Manual Ist Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations I l AAaaaa d B 2 RULES FOR SPECIFIC APPLICATIONS B 2 1 IN DELTA OPERATION The SR55 control system allows the soft starter to be installed in the delta connections of the motor which can permit the use of a lower current rated unit However in this mode of operation it 1s important that the soft starter is connected in accordan
9. AUTOMATIONDIRECT XO B 2 7 SOFT STOPPING Soft stopping can reduce positive surge pressures in pipelines on shutdown It is necessary to make sure that the ramp down time is long enough to remove the energy from the fluid before the firing of the thyristors Is stopped Otherwise the surge pressure may still be present Soft stopping can also be successfully applied to loads such as conveyor belt systems where sensitive items such as bottles are being transported B 2 9 REPLACEMENT OF FLUID COUPLINGS Soft starters can replace fluid couplings yielding benefits of higher efficiency running and lower costs to the user If the coupling is used to magnify the available breakaway torque it may be necessary to replace the installed motor with another of a larger size or one with a high starting torque characteristic before a soft starter can be employed B 2 12 OVERHAULING LOADS Certain applications can overspeed the motor as part of normal operation Power then flows from the motor to the power supply It is important to disable the ERS optimizing mode during over speed conditions and reinstate the optimizing mode during normal conditions External control or communication is required to disable and reinstate iERS B 2 13 APPLICATION TABLE The table on the following page shows many common motor applications that suit the SR55 soft starter It lists typical breakaway torque requirements as a percentage of motor full load
10. R55 Series Soft Starter User Manual Ist Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations gt m ss ssnN gt gt 1STHT q avr sx B 1 4 MAXIMUM MOTOR CABLE LENGTH The length of the cable between the output terminals of the starter and the motor should not normally be greater than 100 meters 328 ft B 1 5 POWER FACTOR CORRECTION CAPACITORS Power factor correction capacitors applied to a single motor must ALWAYS be connected by a separate contactor placed on the SUPPLY side of the SR55 Soft Starter Capacitors should be switched into the circuit after top of ramp full line voltage 1s reached and switched out of the circuit before a stop 1s initiated It is important that any total system PFC scheme that automatically corrects for a range of inductive loads 1s not operated in such a way as to leave it heavily over compensated since this might introduce oscillations leading to damaging over voltages B 1 6 LIGHTLY LOADED SMALL MOTORS Lightly loaded small sized less than 2kW 2 7 hp star connected motors can produce high voltages at the motor terminals when shut down by simply opening the line contactor As these voltages can damage the soft starter it is safer to control the opening of the line contactor with the soft starter run relay contacts B 1 7 MOTORS INSTALLED WITH INTEGRAL BRAKES Motors that inclu
11. Replacement of Fluid CouplingS lr rrr rrr s B 6 B 2 12 Overhauling LoadS s d som mom wow ox Eos xo ow om cR OR wow Re OE A B 6 Biz Lo Application Iable uoo ox demo REOR xm RO GC e S s AAA B 6 B 3 Concepts and Principles of Fixed Speed Induction Motor Starting and Control B 8 Bo Lo nodes xau aw Ww aoe Ye oe RUE ROW 2 Q X9 4 wow hee d d Ww ee g B 8 B 3 2 The Induction Motor x uos oe deor RR EUR sus s AAA sos os XO B 8 B 3 3 Starting Induction Motors saoao B 10 B 3 4 Electro Mechanical Methods Of Starting o aa rrr B 11 B 3 5 The Semiconductor Motor Controller o o e B 12 B 3 6 Running Induction Motors 1 ce a B 13 B 3 7 Reliability Considerations 4 hrs B 14 Appendix B Glossary of Terms 4 4 a s a r r a sz a B 15 Stellar Y SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 Page B 1 Appendix B Soft Starter Application Considerations VAUTOMATIONDIRECT B 1 MOTOR SUITABILITY AND ASSOCIATED CONSIDERATIONS The SR55 Soft Starter is based on a series of microprocessor based optimizing soft starters which have been used world wide in more than 100 000 critical and non critical systems Since 1983 these soft starters have successfully operated with almost every type of load and environment from the Antarctic to the jungle The design has proven to be both reliable and adaptable and provides
12. SoFT STARTER APPLICATION CONSIDERATIONS APPENDIX B SOFT STARTER APPLICATION CONSIDERATIONS APPENDIX Ve B 1 Motor Suitability and Associated Considerations B 2 Bl Letal errada A ARA AER RADA eee ee A B 2 B 1 2 Induction Motor Characteristics oo B 2 Bio Rda noo se Mug 2 2 K 2 eee 2 z S eae ee eee eee eee B 2 B 1 4 Maximum Motor Cable Length B 3 B 1 5 Power Factor Correction Capacitors a c leen B 3 B 1 6 Lightly Loaded Small Motors B 3 B 1 7 Motors Installed with Integral Brakes 22s B 3 B 1 8 Older Motors A aw aie we RARA Se eR ee X X Aro s B 3 B 1 9 Wound rotor or Slip ring Motors B 3 B IgO BHloSH eS xe arie X EORR XUL AROS a SUR w e Se S P RARA we B 3 BALLEN amp z x 2 2 Ide B S EE Q k EORR SEL ROUES EROR REO REESE EG we B 4 B 1 12 High Efficiency Motors es B 4 B 1 13 EU Compliance with the EMC Directive B 4 NBSP E PPP B 4 B 2 Rules for Specific Applications a rz rz a sr s s e B 5 B 2 1 In Delta Operation a a oso woo kw bow OE XD KO EO aras EE E B 5 B 2 2 High Inertia Loads lee es B 5 B 2 4 Resistive LoadS 30 ms e AAA ARA E e ERED ESO HESS B 5 B29 FICQUCH SIGNING uuu uw a deu bode eae N Q Qw Suca de wem A A B 5 E D OOM escitas EE E S SUR P Q Q SRU E REOS Q E EE eee eae Qh Q ee B 5 Biz 90 EE v ee ARA ERE SUS Q P w A OE Qh Qo B 6 B 2 9
13. a powerful mechanism with which to control fixed speed induction motors However due to the intrinsic differences between electronic and electro mechanical starting systems there are a number of simple rules and observations to follow when using the SR55 Soft Starter This section introduces guidelines for the user and those incorporating the unit as part of their system design B 1 1 SUITABILITY In principle any three phase induction motor can be started by a soft starter Normally the breakaway torque of the load should be less than the full load torque of the motor unless a motor with a high locked rotor torque characteristic is employed As a quick assessment any load which has a low or no load start with a moderate starting time or which can be started with a star delta wye delta starter auto transformer or other forms of reduced voltage starting can be considered a potential application for a soft starter B 1 2 INDUCTION MOTOR CHARACTERISTICS Three phase induction motors are required to provide sufficient torque to accelerate the motor and its load from standstill to full speed and to maintain full speed efficiently at all torque levels up to the design full load torque Most modern three phase induction motors have characteristics that are wholly suitable for use with soft starters However the characteristics vary considerably between different manufacturers and design types It is important that the motor is c
14. apable of providing sufficient torque to drive the load at all speeds from standstill to rated speed to enable the SR55 to function properly It is particularly important that the motor to be soft started does not have a low pull up or saddle torque or the load may not be accelerated correctly The primary function of the soft starter is to act as a torque regulating device It cannot apply a torque greater than that which the motor generates For this reason problematic applications for which many different starting methods have been tried but failed may need analysis of the motor or load performance before a soft starter can be successfully applied B 1 3 RATING Page B2 For most applications except high inertia loads the starting demands and the inertia of the rotating masses are small enough to be insignificant This means that no special consideration needs to be given to the rating of the soft starter other than to ensure that it is equal or marginally greater than the rated voltage and current of the controlled motor Alternatively if the number of poles of the motor and the moments of inertia of the load Jjgaq and motor rotor Jmotor are known a soft starter will be suitable if the figures comply with the criteria given in the bottom row of Table B 1 3 Table B 1 3 Number of Poles Synchronous Speed rpm 60 Hz Section B 2 13 contains a table showing the more common applications Stellar S
15. ce with the relevant wiring diagram The power circuit wiring diagram in Chapter 2 Electrical Installation gives detailed instructions for this configuration If motor rotation is incorrect the connections should be changed as described in the diagram It should be noted that six connections are required between the motor and soft starter The Firing Mode parameter must be set for delta mode which also disables IERS Chapter 3 Configuration and Parameters For SR55 soft starters an in line isolation contactor controlled by the soft starter MUST be used with the In Delta Firing Mode and motor connections B 2 2 HIGH INERTIA LOADS High inertia loads such as centrifugal and axial fans grinders flywheel presses etc may require a larger size soft starter than the motor For example a 75kW 100 hp starter may be needed for a 55kW 75 hp motor This is necessary to allow for the extra heating effects of the prolonged over current on the soft starter thyristors during the extended starting time If very high inertia loads are involved then an analysis of the starting characteristics should be made This requires accurate data about the motor and the load e Complete motor data Current Voltage Power Speed Rotor Inertia Speed Torque Current curves e Complete load data Type of load Speed Inertia Speed Torque curve Power absorbed or Full load Torque Consideration must also be given to thermal overload an
16. d fuse protection systems when extended start times are involved This is the case for heavy duty starting as a standard thermal overload will trip under these conditions A heavy duty start thermal overload or an electronic overload with dual settings for start and run is recommended Modern HRC motor fuses will allow for some overload during the start but the fuse curve giving time current data will give an indication of suitability for the particular application B 2 4 RESISTIVE LOADS AutomationDirect does not support the control of resistive loads with the SR55 Soft Starter B 2 5 FREQUENT STARTING High starting frequencies require careful consideration of the soft starter thermal capabilities In many cases a standard sized SR55 may be suitable as start times are generally shorter for this type of application If this 1s not the case then a larger soft starter may be required B 2 6 OPTIMIZING Drives which operate for long periods of time at less than their rated capacity can benefit from the energy saving function 1ERS optimizing of the SR55 which adjusts the thyristor triggering to reduce the excitation losses of the motor This feature will lower the running temperature of the machine and help to extend its life See Chapter 4 Principles of the Energy Saving Mode 1ERS Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 Page B 5 Appendix B Soft Starter Application Considerations V
17. de an integral electrically operated brake internally connected to the motor input terminals can only be soft started when the brake is re connected to the supply through its own contactor Do NOT soft start the brake B 1 8 OLDER MOTORS The action of the fully controlled soft starter introduces harmonic currents and voltages to the motor Therefore it is important to ensure that the motor employs techniques such as rotor skewing in its construction to suppress the effects of harmonic fluxes and avoid rough starting This 1s rarely a problem with modern motors because nearly all motors designed in the last 20 years employ these techniques B 1 9 WOUND ROTOR OR SLIP RING MOTORS Slip ring induction motors ALWAYS need some resistance in the rotor circuit to ensure that sufficient rotational torque is generated to overcome any alignment torque which is present at start up The resistance can be safely shorted out in the normal fashion with a contactor controlled by the programmable relay set as top of ramp contacts B 1 10 ENCLOSURES Thyristors are not perfect conductors and the passage of current through them causes heat dissipation in the body of the soft starter which in turn causes the heatsink temperature to increase As a guide the heat generated 1s 1 watt amp phase which equates to a dissipation of 30 watts from the heatsink for a line current of 10 amps Therefore all cabinets or enclosures that house soft starte
18. er Application Considerations VAUTOMATIONDIRECT g VC N C T P lt lt r rrr Page B 16 Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015
19. form of control METHOD B WYE DELTA AND OTHER REDUCED VOLTAGE STARTING SYSTEMS Reduced voltage starting makes use of the fact that motor torque is proportional to the square of the terminal voltage The most familiar type of reduced voltage starter is the wye delta or star delta starter Consisting of three contactors and a time switch which can be mechanical pneumatic electrical or electronic the wye delta starter changes the motor winding configuration from an initial wye connection to a delta connection as the motor accelerates The change over or transition point is controlled by the time switch and is usually arranged to be approximately at 80 of full speed The effect of starting in the wye connection is to alter the voltage across each stator winding to 58 of normal This reduces the starting torque to a third of locked rotor torque LRT with a consequent reduction in starting currents and acceleration forces Although an apparent improvement over the direct system significant disadvantages still remain The transfer from wye to delta momentarily removes the motor from the supply During this time the motor 1s under the mechanical influence of the rotating load and at the instant of disconnection current will still flow in the rotor bars due to the time delay necessary for the magnetic flux to die away Therefore there is a residual flux frozen on the surface of the rotating rotor which cuts the stator windings genera
20. g an electronic controller it is important to have some understanding of the characteristics and limitations of the induction motor and the electro mechanical systems currently used to control them The standard fixed speed induction motor fulfills two basic requirements e To accelerate itself and its load to full speed or speeds with multi speed motors e To maintain the load at full speed efficiently and effectively over the full range of loadings Due to the constraints of materials and design it can be difficult to achieve both objectives effectively and economically in one machine So how does a motor start 1n the first place As mentioned previously motors convert electrical energy drawn from the power supply into a mechanical form usually as a shaft rotating at a speed fixed by the frequency of the supply The power available from the shaft Is equal to the torque moment multiplied by the shaft speed rpm From an initial value at standstill the torque varies up or down as the machine accelerates until reaching a peak at about two thirds of full speed and then dropping to zero at synchronous speed This characteristic means that induction motors always run at slightly less than synchronous speed in order to develop power the slip speed and hence the term asynchronous Figure B 3 2 1 shows a graph is of an induction motor torque speed curve and illustrates this important characteristic of asynchronous three phase induction
21. he starting frequency because the electrolyte has to condense back to liquid before a new start can proceed and these restrictions prevent this starter from being a popular option when selecting a control system However it has the distinction of being the smoothest and least stressful method of accelerating an induction motor and its load METHOD D OTHER ELECTRO MECHANICAL SYSTEMS Other control methods such as auto transformer starting popular in North America primary reactance starting etc are employed to a greater or lesser extent to compensate for some of the disadvantages of each type of starter discussed Nevertheless the fundamental problems of electro mechanical starters remain and it 1s only in the last decade or two that their dominance has been challenged by the introduction of power semiconductors controlled by electronics B 3 5 THE SEMICONDUCTOR MOTOR CONTROLLER Page B 12 During the 1950s much effort was put into the development of a four layer transistor device which had the power to switch large currents at high voltages when triggered by a very small pulse of current This device became known as the silicon controlled rectifier SCR or in Europe the Thyristor and it is the basis on which all soft starting systems are built The characteristic of most interest 1s the ability of the thyristor to switch rapidly in about 5 millionths of a second from OFF to ON when pulsed and to remain
22. izing soft starter 1s its impact on the maintenance requirements of associated electro mechanical equipment Optimizing lowers the surface temperature of the motor by reducing the losses within the motor This prolongs the motor life and reduces heating of the surrounding atmosphere in the process If the atmosphere is subject to air conditioning reducing the heat input will reduce the air conditioning costs Reduced starting and running currents reduces cable losses and contactor switching operations are carried out under the most advantageous conditions No current flows on switch on since all switching is carried out by the thyristors which virtually eliminates the need for contact replacement Indeed there are a growing number of installations where contactors are no longer employed being replaced by controllable circuit breakers or isolators instead In summary electronic controllers for most fixed speed applications are opening new ways of increasing the efficient operation of induction motors as well as offering significant benefits in control Prospective users should ensure themselves of the quality and performance of any products they expect to fit and this can be reasonably expected if compliance with the appropriate IEC standards is demanded Page B 14 Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations AAMMMM
23. ll modes of induction motor operation B 3 7 RELIABILITY CONSIDERATIONS Reliability is an aspect which 1s of increasing concern regarding electronic controllers for induction motors There is little point in installing an expensive item of electronic equipment to save potentially considerable amounts of money if the device is unreliable to the point that vital processes are constantly interrupted There are electronic products in the market place which appear to offer soft starting more cheaply However they almost always rely on less advantageous technologies such as analog control or half control where one of the two thyristors in each phase is replaced with a diode There are systems which only control the energy flow in one phase while the other two are directly connected Owing to the variable quality and performance of many so called inverters and soft starters available to the unsuspecting purchaser international standards for these products have been developed So far IEC 60947 4 2 AC Semiconductor Motor Controllers and Starters defines the soft starter in every important respect including thermal and overload performance as well as electromagnetic compatibility By ensuring that any motor controller equipment purchased conforms to IEC 60947 4 2 a user should be reasonably safeguarded from shoddy or inadequate products when specifying equipment for future installations A particular advantage of the use of the optim
24. mp up when a load has a high breakaway torque and the standard settings of pedestal voltage may not allow sufficient torque to be developed by the motor to cause acceleration Locked Rotor Current Same as Inrush Current defined above Overload Level The level of current at which the controller overload begins to integrate For the SR55 the overload detector is always active and provides protection against prolonged over current operation Pedestal Voltage The voltage that the unit applies to the motor at start up It is expressed as a percentage of the rated supply voltage Power Factor The ratio expressed as a trigonometric cosine of the real power consumption to the apparent power consumption Top of Ramp TOR The unit achieves Top of Ramp TOR when it completes the start up stage of motor control This occurs when the voltage applied to the motor first equals the main supply voltage Soft start The regulation by electronic means of the supply voltage from an initial low value to full voltage during the starting process This over comes the inherent drawbacks of a switched supply The motor torque is modified in proportion to the square of the voltage applied Trip A trip occurs when the unit removes power to the motor because its operation equals the limit imposed by one of its self protection features Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 Page B 15 Appendix B Soft Start
25. onous speed 0 SPEED S Torque Speed Curve High Starting Torque However most induction motors are designed to have a standard characteristic that provides a compromise between starting torque and operating efficiency To summarize an induction motor will only start and accelerate when it produces more torque than the connected load absorbs This is true for all speeds including standstill and full speed B 3 3 STARTING INDUCTION MOTORS Starting a demagnetized induction motor from standstill is a demanding and complex process At the instant of switching all the energy must be present that is necessary to magnetize the motor to provide the acceleration force to supply the kinetic energy of the rotor and load and to overcome the mechanical and electrical losses To do so at full supply voltage places considerable stresses on the supply the motor windings and the iron cores of the stator and rotor Excessive acceleration of a rotor when the mechanical load is small can produce torque oscillations in the shaft causing severe wear to transmissions gears and drives Excessive acceleration when the load inertia 1s high such as in centrifugal fans causes belts to slip on the pulleys producing rapid wear and early failure Page B 10 Stellar SR55 Series Soft Starter User Manual Ist Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations gt m s
26. rs should have adequate ventilation For more detailed information refer to Chapter 1 Mechanical Installation Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 Page B 3 Appendix B Soft Starter Application Considerations VAUTOMATIONDIRECT _ _ _ B 1 11 EFFICIENCY Although the use of the soft starter introduces a power loss the system still retains an overall efficiency of approximately 99 5 If the ERS function is selected then the gain in motor efficiency at partial loads is far greater than the loss of efficiency arising from thyristor heat losses If prolonged operation at full load 1s expected the thyristor loss can be eliminated as in some matched motor pump drives by closing the internal bypass contactor around the thyristors SR55 soft starters have an internal bypass contactor that is automatically activated when at full load when ERS is turned on B 1 12 HIGH EFFICIENCY MOTORS Due to an inherently steep front to the speed torque curve high efficiency motors can exhibit instability when lightly loaded and the ERS parameter group may need to be adjusted to compensate B 1 13 EU COMPLIANCE WITH THE EMC DIRECTIVE When considering the use or fitting of any Soft Starter users and installers in European countries must comply with the EMC Directive 2004 108 EC The manufacturer of the soft starter has a statutory obligation to provide a guide for compliance
27. s ssnN gt gt 1STHT q avr sx B 3 4 ELECTRO MECHANICAL METHODS OF STARTING METHOD A DIRECT ON LINE The most simple means of controlling energy flow to an induction motor is to interrupt the power supply by a single contactor Very widely applied the method is known variously as direct on line across the line direct etc and 1s the usual form of control where low cost is the first and most important consideration As a result it is most often used on small motor sizes up to approx 22 kW 30 hp or where the supply is strong enough to withstand the inrush and starting current surges without causing unacceptable voltage drops The harsh damaging effects described earlier are all imposed by direct on line starting and as a control method it is the most destructive of equipment Its simplicity and apparent low cost although attractive at first sight hide large cost penalties in the shape of increased maintenance reduced transmission equipment life and higher risk of motor failure particularly when frequent starting and stopping is needed In larger sized motors special strengthening Is necessary at higher cost before they can be safely used with direct on line starting However the shortcomings of the direct on line starter have been recognized ever since motors have been used and alternative systems have been developed over the years to reduce the damaging effects of this
28. tant and adjusting the motor terminal voltage accordingly it is possible to save some of the excitation energy and load loss and therefore improve motor power factor when the motor ts running inefficiently at light loads FIGURE B 3 6 2 MOTOR EFFICIENCY LOSS CHARACTERISTIC Z LOSSES MECHANICAL AAA 0 1 2 1 1 Motor Efficiency Loss Characteristic Stellar Y SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 Page B 13 Appendix B Soft Starter Application Considerations VAUTOMATIONDIRECT eee B 3 6 RUNNING INDUCTION MOTORS CONTINUED All SR55 Soft Starters are microprocessor controlled and this gives them a number of advantages Firstly there are no adjustments to be made for the energy saving function all calculations necessary to find the best degree of phase back of the thyristors for any load condition is made by the microprocessor Secondly the start always synchronizes with the supply voltage and a special structure of turn on pulses virtually eliminates the inrush currents normally associated with motor start up This happens every time Lastly there is the absolutely stepless starting process otherwise found only with primary resistance or reactance electromechanical starters but without the wasted energy and with the opportunity to control the maximum current allowed to flow during the starting process Other features such as soft stopping are included to give considerable control over a
29. ting a voltage whose frequency depends on the rotor speed If the load inertia is small such as in a pump or if the friction is high there could be a significant loss of speed during the time the supply is disconnected In this case when the reconnection to delta is made a large phase differential can exist between the supply and the rotor fluxes This can give rise to very large current surges as much or more than full voltage locked rotor current together with massive transient torque oscillations which can peak at levels in the region of fifteen times full load torque Although the effects described are only present for a very short period of time about one fifth of a second they are sources of great stress and damage to the whole drive system and where frequent starting is necessary invoke high maintenance costs The current surges in the form of very high level short duration spikes are an increasing problem for computer control systems and other sensitive electronic equipment The voltage disturbance on the supply is very difficult to filter out and can cause severe problems especially when larger motors are involved Stellar SR55 Series Soft Starter User Manual Ist Ed Rev A 08 17 2015 Page B 11 Appendix B Soft Starter Application Considerations VAUTOMATIONDIRECT g V n AP METHOD B WYE DELTA AND OTHER REDUCED VOLTAGE STARTING SYSTEMS CONTINUED There are methods of control for example
30. torque FLT For the most satisfactory soft starter in a given application the motor should have a full voltage locked rotor torque LRT that is at least twice the breakaway torque e g for a reciprocating compressor the FLT is normally in the region of 5096 motor LRT As a general rule the higher the motor LRT is above the load breakaway torque the greater the control over the starting process Page B 6 Stellar SR55 Series Soft Starter User Manual 1st Ed Rev A 08 17 2015 WAUTOMATIONDIRECTE Appendix B Soft Starter Application Considerations TP C V gt O lt s s B 2 13 APPLICATION TABLE CONTINUED Table B 2 13 Applications Application Breakaway Torque FLT Remarks Agitator 35 Air compressor rotary unloaded start Air compressor reciprocating unloaded start 25 35 Air compressor screw type unloaded start Usually two pole motor Ball mill Eccentric load needs high starting torque motor Carding machine Often high inertia Centrifuge Usually high inertia Centrifugal fan dampers closed Usually high inertia Centrifugal fan dampers open Usually high inertia very long ramp times Centrifugal blower valve closed Centrifugal blower valve open 30 40 Can have long ramp time Conveyor horizontal unloaded 10 50 Conveyor horizontal loaded 100 150 Conveyor vertical lifting
31. ween the developed torque and the absorbed torque the faster the acceleration and the quicker full speed is reached and the greater the stresses experienced by the supply and drive systems during the acceleration process An ideal start would accelerate the load with just sufficient force to reach full speed smoothly In a reasonable time and with minimum stress to the supply and drive mechanisms Generally speaking the motor speed torque characteristic is controlled by the rotor resistance A motor with high rotor resistance can generate its peak torque pull out torque at standstill giving the high break away torque characteristic which reduces steadily as the speed increases and becomes zero at synchronous speed At the other end of the scale a motor with a very low rotor resistance will produce a low starting torque but will generate its peak torque closer to the synchronous speed Consequently this type of motor runs at full power with higher operating efficiency and low slip speed It Is possible to combine the twin requirements of high starting torque and efficient full speed operation within a single motor by techniques such as double cage or deep bar design and this usually is the motor characteristic chosen for lifting and hoisting applications FIGURE B 3 2 4 TORQUE SPEED CURVE HIGH STARTING TORQUE TORQUE Pull Out Torque Mx Locked Rotor Torque LRT Ma Pull up Torque Full Load Torque FLT M Synchr
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