Chapter 3 - AutomationDirect
Contents
1. 5 1 From Table 1 use the estimated savings figure for the next higher rating ie 55 kW The savings would be approximately 3 5 x 37 5 kW 1 3125 kW 2 37 5 kW 2 pole motor From Table 1 use the estimated savings figure for the next higher rating ie 55 kW From Table 2 apply the pole number factor of 0 5 96 The savings would be approximately 3 5 96 0 5 96 x 37 5 kW 1 125 kW 3 37 5 kW 2 pole low slip motor From Table 1 use the estimated savings figure for the next higher rating ie 55 kW From Table 2 apply the pole number factor of 0 5 96 and the slip factor of 0 5 The savings would be approximately 3 5 96 0 5 1st Ed RevA 09 2011 0 5 96 x 37 5 kW 0 938 kW SR44 Series Soft Starter User Manual 3 19 Chapter 3 Applications lll oo M e nR 3 5 Chapter 3 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 SR44 current limit is only active during start up where it contributes to the motor control function This feature is particularly useful when starting high inertia loads that require an extended start up period See also O verload Level Direct O n Line DO L The direct connection and disconnection of a motor from the AC main supply by means of a contactor or switch Acceleration and operation is2. Complete motor data Current Voltage Power Speed Rotor Inertia Speed Torque Current curves 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 and 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 3 2 3 Reserved 3 2 4 Resistive Loads AutomationD irect does not support the control of resistive loads with the SR44 Soft Starter 3 2 5 Frequent Starting High starting frequencies require careful consideration of the soft starter thermal capabilities In many cases a standard sized SR44 may be suitable as start times are generally shorter for this type of application If this is not the case then a larger soft starter may be required 3 2 6 Optimizing Drives which operate for long periods of time at less than their rated capacity can benefit from the energy saving function optimizing of the SR44 which adjusts the thyristor triggering to reduce the excitation losses of the motor This feature will lower the run
3. 3 4 Principles of the Optimizing Mode 3 4 1 Principles Every wound field electric motor must consume some minimum amount of energy to provide a magnetic field which enables it to work at all With DC motors the field is under separate control so that the amount of magnetizing energy can be adjusted to be sufficient to overcome losses and provide an armature reaction appropriate to the load The squirrel cage AC induction motor has no such provision with the result that energy is wasted at any load less than its rated full load at full speed When a squirrel cage motor is supplied at a constant terminal voltage as when it is connected directly to the supply without a controller of any kind the strength of the field flux is fixed by the supply voltage At normal running speed the field will take a fixed quantity of energy regardless of the torque demanded by the mechanical load The energy required to support the load torque is determined by the torque demand As load torque increases the rotor slows down a little ie slip increases causing induced rotor currents to also increase in order to increase the torque These additional currents in the rotor are balanced by additional current in the stator coils Conversely if load torque demand falls the slip decreases the rotor currents decrease and the current in the stator decreases accordingly But at constant terminal voltage the current providing the stator field flux remains unchanged
4. Full Load Torque FLT My Synchronous speed Pull up Torque Figure 3 3 2 3 E SPEED 2 Torque Speed Curve Accelerating Torque lstEd RevA 09 2011 SR44 Series Soft Starter User Manual 3 9 Chapter 3 Applications lli N RN N E H H Tja 3 3 2 The Induction Motor continued O bviously the larger the difference between 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 Atthe 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 Itis possible to combine the twin requirements
5. 0 0 cece eee ees 3 6 3 2 9 Replacement of Fluid Couplings 0 00 cece eee 3 6 3 2 10 Two speed Motor Applications 0 0 ces 3 6 3 2 11 Multiple Motor Starting 0c ee 3 6 3 2 12 Overhauling Loads 22 03 622i dewedwedigcwddicedbuedw anid 3 6 3 2 13 Application Table exer ue We d E E ExwESQd ADR wed ees 3 6 Table of Contents continued next page 3 la APPLICATIONS Table of Contents continued from previous page 3 3 Concepts and Principles of Fixed Speed Induction Motor Starting and Control 3 8 3 3 1 IMWOGUCUON 126542 dus a eme wewkswa csdx Ra xRexqens He ee es 3 8 3 3 2 The Induction Motor e 2222 asi es wp ERRGeOS Komeda euh s 3 8 3 3 3 Starting Induction Motors 2 3 acanx oc xa X ERR ERR eae ROC ORE RA 3 10 3 3 4 Electro M echanical Methods Of Starting 000 eau 3 11 3 3 5 The Semiconductor Motor Controller aanas cee ee 3 12 3 3 6 Running Induction Motors eese 3 13 3 3 7 Reliability Considerations nee 3 14 3 4 Principles of the Optimizing Mode 3 15 3 4 1 Principles seeeeer RR hh 3 15 3 4 2 Advantages of Energy Optimizing 0 0 0 3 16 3 4 3 Additional Benefits in Practice llle 3 16 3 4 4 Optimizing with the SR44 System 1 es 3 17 3 4 5 How Much Energy 22 6265 4sbadte te teense eee eiase ee oes 3 18 3 4 6 Estimating Energy Savings suuxcedces e xS ES
6. hundred years after their introduction contactor based systems remain the most widely used method of motor control N evertheless 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 SR44 3 3 2 The Induction Motor In order to appreciate the benefits of using 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 fulfils two basic requirements To accelerate itself and its load to full speed or speeds with multi speed motors 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 in 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 dow
7. setting feature which is particularly useful in plug braking where the braking settings need to be different from the normal starting ramp time and current limit level See section 2 4 4 and Figure 2 4 6 of the Electrical Installation chapter for details 3 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 3 2 10 Two speed Motor Applications Two speed motors whether Dahlander connected or with dual windings can be soft started at each speed provided that the start is initiated when the actual motor speed is less than the synchronous speed for the winding selected This is particularly important when changing from high to low speeds The SR44 has a dual setting feature which allows different operating settings for the high and low speeds and the ability to switch rapidly between them 3 2 11 Multiple Motor Starting See section 2 4 2 of the Electrical Installation chapter for details 3 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 Itis important to disable Optimizing Mode during over speed con
8. 44 Series Soft Starter User Manual lstEd RevA 09 2011 Chapter 3 Applications 3 3 6 Running Induction Motors 1st Ed Rev A O nce 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 is relatively efficient and readily achieves efficiencies of 85 to 95 However as shown below motor efficiency falls dramatically when the load falls to less than 5096 of rated output Figure 3 3 6 1 Motor Efficiency Load Characteristic 100 EFFICIENCY 0 1 2 1 1 Figure 3 3 6 1 pone 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 SR44 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
9. APPLICATIONS Contents of this Chapter 3 1 Motor Suitability and Associated Considerations 3 2 Deel E Uo MC PCT ere se pcr 3 2 3 1 2 Induction Motor Characteristics assas hod a Ree im aed 3 2 3 1 3 Rating nnana naana aaa 3 2 3 1 4 Maximum Motor Cable Length asaan aaa 3 3 3 1 5 Power Factor Correction Capacitors s asana aaea aaa 3 3 3 1 6 Lightly Loaded Small Motors ror Ba d RR Rr en 3 3 3 1 7 Motors Installed with Integral Brakes sess 3 3 3 1 8 Older Motors 2 xk or rU dee iod e xe i RUP RC ERAS RCR RR 3 3 3 1 9 Wound rotor or Slip ring Motors assas es 3 3 3 1 10 EBDEIOSN BS io btbe these ERES beth bed ete deee nea dd Pd ds x 3 3 3 1 11 EMCNCY T 3 4 3 1 12 High Efficiency Motors ccc eens 3 4 3 1 13 EU Compliance with the EMC Directive 0 ccc eee 3 4 CMM FUSS PC e Asay ene eee e ea e a ares 3 4 3 2 Rules for Specific Applications 3 5 3 2 1 In Delta Operation 633994593 3109013 1 or drap eant ER ER D 3 5 3 2 2 Higlidertalboeatls ie acc uude pig ELEESSCREEE See Pd ds 3 5 3 2 3 Reserved oct nnns 3 5 3 2 4 Resistive Loads eee eee nnn 3 5 3 2 5 Frequent Starting usado seeds du corvos writ de edt 3 5 3 2 6 ODUMIAING ionetestheecice ad dee heteen er US eaea CENE 3 5 Bag Soft HOPPING 5 2 2 2 3 2 209 5 9 RREGUR Cw X 1X eain en BORD EE X4 o CHORI Bo 3 6 3 2 8 Reversing Drives and Plug braking
10. Applications _e_ __ _ _ _ _ _ _ _ _ _ _ _ z amp _ _ _ E_ __ _ _ _ _ _ _ RR RR RR RR qt There are methods of control for example 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 if you need 4096 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 l
11. REREe dein y CE esd 3 19 3 5 Chapter 3 Glossary of Terms 3 20 3 6 Power Factor Conversion Chart 3 21 3 1b APPLICATIONS Chapter 3 Applications 3 1 Motor Suitability and Associated Considerations The SR44 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 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 SR44 Soft Starter This section introduces guidelines for the user and those incorporating the unit as part of their system design 3 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 w ye delta starter auto transform
12. al to select a standard motor with a rating somewhat higher than the maximum demand of the driven load The motor selected for any given application will almost certainly be over rated for this reason alone and therefore energy could be saved even at full load when supplied at rated voltage Furthermore there are those applications where the size of motor has to be chosen to provide for high loadings which occur only intermittently or for an arduous start even though the load demand at most times is much lower 3 16 SR44 Series Soft Starter User M anual 1st Ed RevA 09 2011 Chapter 3 Applications MM 3 4 4 Optimizing with the SR44 System During start up the SR44 software uses a patented method to compute and store a reference value for the power factor When the motor has reached full speed and is driving the load at the demanded torque the SR44 enters the motor running stage At this stage if required the motor may also operate in O ptimizing Mode Entering this mode can be pre set from the SR44 keypad and stored for automatic operation which will suit the majority of applications where it is required This is the default operating mode for the SR44 It can also be toggled on and off while running by using either the O PTIMIZE button on the keypad or through external circuitry connected to one of the programmable inputs and controlled by the driven process In this mode the reference power factor is continuously compared w
13. at full mains voltage only Inrush Current or Locked Rotor Current The current that flows at the instant of connection of a motor to the power source Itis 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 ramp 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 O verload Level The level of current at which the controller overload begins to integrate For the SR44 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 pr
14. at any level of load torque demand As a consequence the efficiency of an induction motor decreases as the load decreases Figure 3 4 1 1 Typical duty cycle for a machine load where the Torque D emand varies Load Torque Demand at working speed 40 60 80 100 120 140 160 seconds Figure 3 4 1 2 Torque Demand converted to an equivalent current with the motor magnetizing current added Current Demand due to torque and flux 7 Magnetizing current L Torque current 0 40 60 80 100 120 140 160 seconds 1st Ed RevA 09 2011 SR44 Series Soft Starter User M anual 3 15 Chapter 3 Applications lic M Q OE ERR ERR Q R 3 4 2 Advantages of Energy Optimizing A soft starter with an energy optimizing feature alters the motor operation The energy optimizing function reduces the terminal voltage applied to the motor so that the energy needed to supply the field is more closely proportioned to the torque demand The effect is shown in the Figure below Figure 3 4 2 1 Full Speed end of the conventional Torque Current curves Voltage Torque balance Bex Torque at rated voltage The present considerations do not affect A nad al Full load Torque reduced voltage at rated voltage soft starting options or strategies i F r Reduced Point A on the current curve is the 100 M lt lt Torque operating point of the motor when the demand motor terminal vol
15. ditions and reinstate O ptimizing during normal conditions External control is required to disable and reinstate O ptimizing M ode 3 2 13 Application Table The table on the following page shows many common motor applications that suit the SR44 Soft Starter It lists typical breakaway torque requirements as a percentage of motor full load 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 50 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 3 6 SR44 Series Soft Starter User M anual 1st Ed RevA 09 2011 Table 3 2 13 Applications Chapter 3 Applications SS SM Application Breakaway Torque FLT Agitator 35 Air compressor rotary unloaded start 25 35 Air compressor reciprocating unloaded start 50 100 Air compressor screw type unloaded start 30 Usually two pole motor Ball mill 30 50 Eccentric load needs high starting torque motor Carding machine 100 Often high inertia Centrifuge 50 90 Usually high inertia Centrifugal fan dampers closed 10 25 Usually high inertia Centrifugal fan dampers open 10 25 Usually high inertia very long ramp ti
16. er 3 Applications M 3 2 Rules for Specific Applications 3 2 1 In Delta Operation The SR44 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 itis important that the soft starter is connected in accordance with the relevant wiring diagram The connection diagram in Figure 2 3 2 of the Electrical Installation chapter gives detailed instructions for this configuration If motor rotation is incorrect the connections should be changed as detailed in Figure 2 3 2 It should be noted that six connections are required between the motor and soft starter Parameter P6 must be set to 1 for delta mode which also disables optimizing An in line isolation contactor controlled by the soft starter MUST be used with the In Delta Firing Mode and motor connections 3 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
17. er or other forms of reduced voltage starting can be considered a potential application for a soft starter 3 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 capable of providing sufficient torque to drive the load at all speeds from standstill to rated speed to enable the SR44 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 3 1 3 Rating 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
18. eristic of most interest is the ability of the thyristor to switch rapidly in about 5 millionths of a second from OFF to ON when pulsed and to remain 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 is 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 3 12 SR
19. have excess magnetic flux and efficiency loss and power factor degradation result By detecting the load at any instant 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 is running inefficiently at light loads Figure 3 3 6 2 Motor Efficiency Loss Characteristic zZ MECHANICAL n 0 1 2 1 1 LOSSES Figure 3 3 6 2 Motor Efficiency Loss Characteristic 09 2011 SR44 Series Soft Starter User Manual 3 13 Chapter 3 Applications ll H R BR RE ER RR ERR ERR RR ERR ERR REOR RR RR All SR44 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 duri
20. hen specifying equipment for future installations A particular advantage of the use of the optimizing soft starter is 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 3 14 SR44 Series Soft Starter User Manual lstEd RevA 09 2011 Chapter 3 Applications M
21. illations leading to damaging over voltages 3 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 3 1 7 Motors Installed with Integral Brakes M otors that include 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 3 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 is rarely a problem with modern motors because nearly all motors designed in the last 20 years employ these techniques 3 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 progra
22. iquid 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 the 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 is only in the last decade or two that their dominance has been challenged by the introduction of power semiconductors controlled by electronics 3 3 5 The Semiconductor Motor Controller 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 itis the basis on which all soft starting systems are built The charact
23. ith the running power factor The software continuously uses this comparison to compute and adjust the firing point of the thyristors in order to maintain the best power factor This method of continuous control minimizes wasted energy caused by overfluxing the motor It also maintains the power factor at the most appropriate value for every condition of load demand This can produce a significant reduction in the kVA demand This is an operating condition that may at light or partial load conditions provide the benefit of energy saving and if selected is continuous from the dwell period until a STOP command is initiated or the mode is disabled It should be noted that this function is inhibited by the software if the current being drawn by the motor exceeds 80 of the set current of the SR44 at full voltage when the motor enters its running stage with the optimizing mode selected The method of power factor management described does not affect motor performance nor does it detract from the motor s capability to respond to changes in load demand This feature of the SR44 Soft Starter is a purely electrical function which has the effect of ensuring that the motor delivers the torque demanded at all times but allows it to draw only the precise amount of magnetizing current required to support that torque output Without this feature the motor would draw the maximum magnetizing current regardless of load The optimizing function cannot improve the po
24. les in Section 3 4 6 allow a user to gain a reasonably close estimate of the savings to be achieved within the motor by using the SR44 optimizing Soft Starter The method does not include any additional savings and benefits conferred by other sources such as reduction of heating losses in cabling because of the lower voltages potential reduction of maximum demand charges further energy savings and other benefits deriving from the soft starting process itself reduced total energy demand reduced wear and tear reduced maintenance and replacement costs 3 18 SR44 Series Soft Starter User Manual lstEd RevA 09 2011 Chapter 3 Applications M 3 4 6 Estimating Energy Savings Basis for estimations 3 phase squirrel cage induction motor standard type Supply 380 to 440V 50Hz Supply voltage minimum working voltage on motor rating plate Operation 3096 rated nameplate full load Table 3 4 6 1 Estimations Energy Savings Estimations Table 3 4 6 1 Motor Size kW HP Estimated Savings 96 rated kW 5 10 6 5 3 5 2 5 More than Table 3 4 6 2 Modifying Factors 1 5 Energy Savings Modifying Factors Table 3 4 6 2 Motor Poles Motor Slip Number of Poles Add kW Slip Add kW 0 5 0 5 0 5 0 2 0 0 5 3 3 0 5 Examples of estimated savings 1 37 5 kW 4 pole motor 1
25. mes Centrifugal blower valve closed 25 35 Centrifugal blower valve open 30 40 Can have long ramp time Chillers 10 25 Usually started unloaded two pole motor Conveyor horizontal unloaded 10 50 Conveyor horizontal loaded 100 150 Conveyor vertical lifting 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 25 75 Can be high inertia Drilling machine unloaded 10 Escalator unloaded 25 50 Optimizing feature can be effective 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 35 75 Mixer fluid contents 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 tor
26. mmable relay set as top of ramp contacts 3 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 is 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 starters should have adequate ventilation Refer to the M echanical Installation procedures sections 1 6 to 1 8 for more detailed information 1st Ed RevA 09 2011 SR44 Series Soft Starter User M anual 3 3 Chapter 3 Applications ll 5 v 3 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 optimizing 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 is expected the thyristor loss can be eliminated as in some matched motor pump drives by closing a bypass contactor around the soft starter This contactor is readily controlled by the programmable relay set as top of ramp contacts as the circuit in section 2 4 1 shows 3 1 12 High Efficiency Motors Due to an inherentl
27. n 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 3 3 2 1 shows a graph is of an induction motor torque speed curve and illustrates this important characteristic of asynchronous three phase induction motors 3 8 SR44 Series Soft Starter User Manual lstEd RevA 09 2011 Chapter 3 Applications Figure 3 3 2 1 Torque Speed Curve Induction Motor Pull Out Torque M Pull up Torque Figure 3 3 2 1 0 SPEED S Torque Speed Curve Induction Motor TORQUE Locked Rotor Torque LRT MA Full Load Torque FLT Mj Synchronous speed Figure 3 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 Pull up Torque Full Load Torque FLT My Synchronous speed Figure 3 3 2 2 0 SPEED S Torque Speed Curve Coupled Load Figure 3 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 MA
28. ng the starting process Other features such as soft stopping are included to give considerable control over all modes of induction motor operation 3 3 7 Reliability Considerations Reliability is an aspect which is 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 O wing 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 w
29. ning temperature of the machine and help to extend its life See Section 3 4 Principles of the O ptimizing M ode 1st Ed RevA 09 2011 SR44 Series Soft Starter User M anual 3 5 Chapter 3 Applications ll X TP it 3 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 conveyer belt systems where sensitive items such as bottles are being transported 3 2 8 Reversing Drives and Plug braking SR44 Soft Starters used in conjunction with contactor controlled reversing and plug braked motors show considerable benefits to the user by reducing mechanical and electrical stresses particularly when utilizing the current limited start feature This type of application requires inserting a 150 350 ms delay between the opening of one contactor and the closing of the other to allow any residual flux in the rotor to die away When plug braking there should be some form of zero speed detection to stop the drive after braking has been completed otherwise the drive may either accelerate the motor in the reverse direction or switch off before zero speed has been reached The SR44 has a dual parameter
30. 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 3 3 2 4 Torque Speed Curve High Starting Torque Pull Out Torque Mx Pull up Torque Figure 3 3 2 4 0 SPEED S Torque Speed Curve High Starting Torque TORQUE Locked Rotor Torque LRT MA Full Load Torque FLT My Synchronous speed 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 3 3 3 Starting Induction Motors Starting a de magnetized 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 ca
31. onnection 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 generating 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 1st Ed RevA 09 2011 SR44 Series Soft Starter User M anual 3 11 Chapter 3
32. oportion 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 3 20 SR44 Series Soft Starter User M anual 1st Ed RevA 09 2011 Chapter 3 Applications ee 3 6 Power Factor Conversion Chart Parameter 20 is the reference power factor that the SR44 computes during the start Parameter 21 is the present power factor of the load The keypad display shows each of these parameters in angular degrees The table below can be used to convert this value 0 in degrees to the power factor cos 0 Power Factor Conversion Chart Power Factor 30 0 87 3l 0 86 32 0 85 33 0 84 34 0 83 35 0 82 36 0 81 37 0 80 38 0 79 39 0 78 40 0 77 41 0 75 42 0 74 43 0 73 44 0 72 45 0 71 46 0 69 47 0 68 48 0 67 49 0 66 50 0 64 51 0 63 52 0 62 53 0 60 54 0 59 55 0 57 56 0 56 57 0 54 58 0 53 59 0 52 Degrees WO CO NI ocu ju NY FR Oo Ist Ed RevA 09 2011 SR44 Series Soft Starter User M anual 3 21 Chapter 3 Applications OT 3 22 SR44 Series Soft Starter User M anual 1st Ed RevA 09 2011
33. que motor Pump vane type positive displacement 100 150 Needs high starting torque motor Rolling mill 30 50 Saw band 10 35 Saw circular 25 50 M ay be high inertia Plug brake may be useful Screen vibrating 30 60 Travelators walkways 25 Unloaded starting Transformers voltage regulators Nil Change firing mode Tumblers 30 100 Can be eccentric load may need high torque motor 1st Ed RevA 09 2011 SR44 Series Soft Starter User Manual 3 7 Chapter 3 Applications gi I A s C RR NR ERR NRI ERR RUE RR RR RR qt 3 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 3 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
34. 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 Jigaq 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 3 1 3 Table 3 1 3 Number of Poles Synchronous Speed rpm 9 60 Hz loadY motor less than Section 3 2 13 contains a table showing the more common applications 3 2 SR44 Series Soft Starter User Manual lstEd RevA 09 2011 Chapter 3 Applications MM 3 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 3 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 SR44 Soft Starter Capacitors should be switched into the circuit after top of ramp full line voltage is reached and switched out of the circuit before a stop is initiated It is important that any total system PFC scheme that automatically corrects for a range of inductive loads is not operated in such a way as to leave it heavily over compensated since this might introduce osc
35. tage is at its nominal 80 M or rated value and when the load is the maximum for which the motor is rated If the load decreases a motor supplied at a fixed voltage will speed up slightly the current demand will reduce and the operating point moves along the curve to point B Because the torque developed by a motor is proportional to the square of the applied voltage lowering the terminal voltage reduces the torque If the reduced voltage is correctly chosen the working point at the reduced torque demand becomes the point A By reducing the terminal voltage the motor has in effect been exchanged for one which has a lower rated power output A reduced terminal voltage also means a reduced field energy requirement and this simple relationship enables the optimizing function to maintain the efficiency of the motor over nearly the entire load range from no load upwards In practical terms no load means no external load There are still internal mechanical and electrical losses to overcome friction and windage of the rotor at speed and the electrical heating and hysteresis losses The ideal response to the no load condition would be to supply precisely the amount of magnetizing current needed to provide the armature reaction to balance the losses This is what the energy optimizing feature of a soft starter seeks to do automatically and continuously 3 4 3 Additional Benefits in Practice It is norm
36. 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 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 5896 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 is under the mechanical influence of the rotating load and at the instant of disc
37. using severe wear to transmissions gears and drives Excessive acceleration when the load inertia is high such as in centrifugal fans causes belts to slip on the pulleys producing rapid wear and early failure 3 10 SR44 Series Soft Starter User M anual 1st Ed RevA 09 2011 Chapter 3 Applications M 3 3 4 Electro M echanical M ethods 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 is the usual form of control where low cost is the first and most important consideration As a result itis 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
38. wer factor beyond what it would ordinarily be at full load but it does make the optimum improvement possible at any partial load 1st Ed RevA 09 2011 SR44 Series Soft Starter User M anual 3 17 Chapter 3 Applications lli HR HR ERR 3 4 5 How Much Energy The amount of energy used by a squirrel cage induction motor operating with a soft starter in energy optimizing mode is shown in the following figure Figure 3 4 5 1 for the same duty cycle as Figure 3 4 1 2 By reducing the voltage when torque demand is below maximum the magnetizing current is proportioned to the torque current Compare Figure 3 4 5 1 energy optimizing with Figure 3 4 1 2 non optimizing These graphical representations are illustrative only and not to scale Arriving at any exact figure for the energy cost saved requires each individual case to be examined in detail taking into account the motor rating type and any special characteristics such as load load characteristics duty cycle supply voltage and the cost of electricity Figure 3 4 5 1 Energy Savings Current demand with Energy Saving in operation 7 Magnetizing current L Torque current 0 40 60 seconds The calculations to cover all the likely or possible conditions would be laborious An empirical method for arriving at a usefully realistic estimate has been devised Used with a proper sense of engineering circumspection the tab
39. y steep front to the speed torque curve high efficiency motors can exhibit instability when lightly loaded and the optimizing rate parameter P19 may need to be adjusted to compensate 3 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 89 336 EEC The manufacturer of the soft starter has a statutory obligation to provide a guide for compliance with this directive For the SR44 this guidance is given in the EMC guide which is chapter 4 of this manual Itis essential that users and installers understand and comply with the requirements described in these sections 3 1 14 Fuses 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 3 2 2 relating to high inertia loads Semiconductor fuses are available for the short circuit protection of the thyristors in the SR44 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 SR44 3 4 SR44 Series Soft Starter User M anual lstEd RevA 09 2011 Chapt
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