Application of RVSS soft starter for centrifuge applications
Contents
1. Application Notes AP03902015E Effective July 2012 Application of RVSS soft starter for centrifuge applications How to size and configure the 811 reduced voltage soft starter for centrifuge applications Application The S811 soft starter may be used to accomplish motor starting using the reduced voltage method for centrifuge applications Typically the motor horsepower or kW rating is used to determine the required size of the soft starter and the load type is used to determine operational parameter values for optimum performance With centrifuge applications the mass of the load must also be considered as current profiles and ramp times may be significantly different from other types of loads Overview Soft starter sizing is determined by the horsepower or kW rating of the motor coupled with the mains operating voltage Please note that all references to sizing the S811 soft starter are based on motor full load amps FLA rather than horsepower This is to preclude improper sizing due to variations of motor FLA among various motor manufacturers Using this information a table or graph is consulted to determine the minimum size of the soft starter for the application Centrifuge applications are con sidered to be severe duty so extended start ramp times must be considered in the selection process to ensure that SCR overheating will not occur Load currents associated with centrifuge applica tions generally approxi2. by trial and error or some selected time based on when the application is either at synchronous speed or no longer accelerating It is often assumed that if a wye delta system is adjusted for a 360 second run for example on the wye windings during the start cycle this means that the centrifuge cannot accelerate at any other rate This thinking is not necessarily correct When a centrifuge is started with the wye delta method the initial torque is approximately 33 percent of locked rotor torque due to the reduced phase voltage Prior to the end of the start timer the torque relative to phase voltage is still the same as the phase voltage This means that as the motor acceler ates additional torque is only achieved relative to the motor torque curve at the end of the start cycle therefore motor acceleration toward the end of the start cycle may not be as great as in the initial stages of the start Operational difficulties with this system may include excessive mechanical wear of belts and pulleys when the motor is switched from the start windings to the run windings and the application is not up to synchronous speed lack of operational or overload protections and failures of contactors and or control devices to name a few The reduced voltage soft starter RVSS initial torque parameter is adjusted to 30 percent This value approximates the same initial torque as would be developed with a wye delta system starting in the wye configurati
3. e motor is at least a six lead motor Cable sizing is normally adequate to meet the demands of the delta configuration start Verify that protection devices such as breakers and fuses are sized appropriately for the higher starting currents In some cases higher fuse and or breaker rating may be required Eaton Corporation Electrical Sector 1111 Superior Avenue Cleveland OH 44114 USA Eaton com 2012 Eaton Corporation All Rights Reserved Printed in USA Publication No AP03902015E Z12539 July 2012 E T N Powering Business Worldwide Application Notes AP03902015E Effective July 2012 Supporting documentation Reference Number MN03900001E 8114 User Manual Additional help In the event that additional help is needed please contact the Technical Resource Center at 1 877 ETN CARE Option 2 Sub Option 2 Eaton is a registered trademark of Eaton Corporation All other trademarks are property of their respective owners
4. ed to achieve synchronous speed plus 30 seconds This is to allow for minor variations in centrifuge operation such as remaining process in the unit during a start attempt All other operational characteristics and any troubleshooting information if needed may be found in the S811 User Manual Circuit protection devices such as breakers and fuses are to be sized according to accepted standards based on motor capacity and conductor size 2 EATON CORPORATION www eaton com Application of RVSS soft starter for centrifuge applications Wye delta conversions Many centrifuge applications utilize wye delta starting methods to reduce current levels during the start cycle The information provided here is to provide additional information to address assumptions and expectations of system operation and performance when converting from a wye delta start method to reduced voltage soft start methods Start time The typical start winding of a wye delta system provides reduced current by reducing the phase voltage in the winding of the motor Typically this value is 33 percent of line voltage which results in 33 percent of locked rotor current maximum during the initial stages of the start process This current yields approximately 33 percent of locked rotor torque to accelerate the application to synchronous speed The timer setting required for the application to transition from the start winding to the run winding is usually determined
5. mate the 3 x FLA to 4 x FLA range and start ramp time generally ranges from 120 to 300 seconds with a reduced voltage soft starter Because centrifuge applications are highly dependent on the process materials involved and are often specifically designed for a given process exact start operation parameters are difficult to predict ETN Powering Business Worldwide 811 sizing The catalog tables for the S811 are based on an inrush value of 300 450 percent FLA of the motor with maximum start ramp times of 180 seconds On motors with quadratic loads current values are generally 250 300 percent FLA With centrifuge applications start ramp currents are often 300 350 percent Additionally the ramp times also must be considered independently as when the ramp times increase even though the inrush current does not change the maximum allowable current capacity is reduced This is due to the overall heat that the SCR can tolerate and still achieve the required performance It is recommended that the S811 with the extended start ramp time option be selected for centrifuge applications This will preclude not having the option of extending the start ramp time beyond the standard 180 seconds up to 360 seconds if needed To calculate soft starter sizing requirements assume a minimum starting current of 350 percent motor FLA Refer to Table 1 to select the appro priate size soft starter under the column indicating 360 seconds maxi
6. mum ramp time The values for 180 second ramp times are shown for comparison Table 1 Maximum Current Exposure Maximum Start Current Catalog pigimum imm a Reve Number Current 180 sec 360 sec 811 T18P3S 180 990 495 811 T24P3S 240 1320 660 811 T30P3S 300 1650 825 811 U36P3S 360 1980 990 811 U42P3S 420 2310 1155 811 U50P3S 500 2750 1375 811 V36P3S 360 1980 990 811 V42P3S 420 2310 1155 811 V50P3S 500 2750 1375 811 V65P3S 650 3575 1787 811 V72P3S 720 3960 1980 811 V85P3S 850 4675 2337 811 V10P3S 1000 5500 2750 Application Notes AP03902015E Effective July 2012 Example 1 200 hp motor with 240 FLA at 460V start ramp time 360 seconds maximum 240 x 3 5 840A S811 U36N3L or S811 V36N3L at 990A minimum 811 soft starters that are larger than shown in the example may be selected to achieve higher current capacity margins Please note that the example denotes 840A for the entire 360 second ramp time In most cases the soft starter will not be exposed to this current level for the entire start ramp time Start ramp times shorter than the 360 second maximum will also provide higher operating margins Note The mass of the centrifuge is not included in the calculation as higher mass values result in higher moments of inertia which directly result in longer start ramp times up to 360 seconds rather than simply higher starting currents This is due to the fact
7. on However due to the rate of phase voltage increase during the start ramp time of the RVSS motor torque is also increasing from increased phase voltage and torque increases relative to the motor torque curve The net result is a significantly reduced start ramp time In the event that this increased motor acceleration poses an issue with the centrifuge process the accel eration rate may be modified by the use of the current limit function of the RVSS Start current Due to the difference of phase voltages when comparing wye and delta windings it is known that to achieve the same torque devel oped with a delta configuration as would be with a wye configura tion the delta configuration current increases by a factor of the V3 Attempts to limit delta configuration currents to similar values of wye configuration currents typically will not result in a successful motor start Please refer to the values of locked rotor amps LRA and locked rotor torque LRT in Example 2 Application of RVSS soft starter for centrifuge applications Example 2 200 hp motor with 1327 LRA at 460V locked rotor torque 957 lb ft Wye configuration 33 percent of full voltage LRA 435A gt 307 lb ft torque Delta configuration 435A x V3 754A 307 lb ft torque Delta configuration 435A 102 lb ft torque insufficient to start motor When converting a wye delta system to an RVSS the cables from the RVSS to the motor are doubled as th
8. that any given motor has an impedance that does not change with the load characteristics If the centrifuge is to be started under conditions other than minimum mass leftover process remaining in the unit please allow for higher current values Higher starting currents may be observed due to higher start ramp voltages prior to the motor achieving syn chronous speed In the event that a centrifuge cannot achieve synchronous speed in 360 seconds it is very likely that motor size and or drive pulley ratio is improper for the application 811 start parameters The following soft start parameter values may be used as initial values Additional parameters such as phase sequence may need to be adjusted Motor rotation should begin within two seconds of energization Increase kick start time and then initial torque to achieve the proper characteristics e Soft start config menu e Start method Voltage ramp e Soft start time 360 seconds e Initial torque 30 percent e Kick start time 1 second e Kick start torque 45 percent e Overload config menu e Overload trip FLA Set to motor nameplate FLA e Overload trip class 30 e Overload on start Disable e Protection setup menu e Motor rated volt Line voltage Note the time required to achieve synchronous speed The internal bypass contactors will close when the motor has achieved synchro nous speed rather than at the end of the start ramp time Reduce the start ramp time to the time requir
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