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  1. arbil

    Transformers for grounded and ungrounded system

    Thanks for the replies .
  2. arbil

    Transformers for grounded and ungrounded system

    We need one generator transformer (11/7.2 KV ,Dyn11 ) .The star side will be connected to ungrounded system and hence neutral will be isolated.However the delivery of the transformer is bit delayed .We are having a spare transformer of same rating , the only difference being that the spare...
  3. arbil

    Half cycle peak current in short circuit study

    What is the significance of calculating half cycle peak current during short circuit study ? Whether it will effect the circuit breaker ratings only or the bus bar ratings also?
  4. arbil

    NiCd battery or lead acid battery

    Well exactly ,I have to replace an existing NiCd battery bank . However I don't know whether I should go for NiCd and incur extra cost or simply go for lead acid which I suppose will require more maintenance but save me lots of initial spending.This is considering that charger will be suitable.
  5. arbil

    NiCd battery or lead acid battery

    Which battery is better , Nickel Cadmium or Lead acid .If we want to replace an existing NiCd battery bank with lead acid , what are the factors we would have to consider ?
  6. arbil

    Star delta winding selection for motors

    Thanks Marke , that was exactly what I wanted to know ,i.e. if the motor is designed to operate at delta , and we change the connection to star (and vice versa), how will the motor behave . As per your post , we can rule out the possibility of changing star designed motor to delta.But we can...
  7. arbil

    Auto transformer start motor tripping on prolonged starting time

    But for carrying out the motor current signature analysis , the motor should run .As per our ommunications with agencies carrying out the tests , the requirement is for the motor to run with at least 50% load .
  8. arbil

    Auto transformer start motor tripping on prolonged starting time

    7anoter:But how to confirm/rule out the possibility of a broken rotor bar without actually opening up the motor .We have run the motor in decoupled condition .The current seems to be steady.
  9. arbil

    Auto transformer start motor tripping on prolonged starting time

    7anoter : Infact , its standby motor was once detected with a broken rotor bar .But in that case the symptoms were different .The motor used to start but during running there was a continuous hunting observed in the load current .Moreover in the vibration signature , sidebands were observed at...
  10. arbil

    Auto transformer start motor tripping on prolonged starting time

    One more thing I may not have added : This is an existing installation which is in operation for the last 25 years. Also the fluid which is being pumped is water .
  11. arbil

    Auto transformer start motor tripping on prolonged starting time

    LionelHutz : Yes, we have monitored the current during starting .As per the datasheet it should be around 280 Amps .The actual current is more than that (approx. 320 -330 Amps) .We have also tried to monitor the speed during starting .Although the measurement has not been very accurate , the...
  12. arbil

    Auto transformer start motor tripping on prolonged starting time

    This motor is normally started once every day and runs for approximately 3 hrs . We have the thermal withstand curve for the motor which has a plot between stator current and time .For 60% tap , the current is 1.83 times FLC and the time against that current comes out to be approx. 17 seconds...
  13. arbil

    Auto transformer start motor tripping on prolonged starting time

    raghun : we don't have any way of measuring the actual voltage at motor terminal .The voltage at the bus is OK . What we are thinking next is to modify the control circuit a bit to take trial run on DOL .
  14. arbil

    Auto transformer start motor tripping on prolonged starting time

    Thanks all for your replies edison123 It is an existing installation .Previously also similar problem has occured ,but we have always been able to trace it back to either pump or discharge valve passing .Only this time everything seems to be OK .Pump was also resent to our workshop for checking...
  15. arbil

    Auto transformer start motor tripping on prolonged starting time

    We are having a 6.6 KV,1500 KW motor with auto transformer starting .The tap setting of auto transformer is at 60% .As per the motor data sheet/curves,at 60% tap, the motor starting time is 11 seconds and thermal withstand time is 17 seconds. As per the control scheme , the changeover from auto...
  16. arbil

    Cable sizing calculation : short circuit withstand time selection

    For determining the cable size we are using the following formula S= I*SQRT(t)/K , where S= nominal cross sectional area of the conductor in mm2 I= Fault current in Amps k= constant based on type of cable t= short circuit withstand time in sec My question is , whether there is any standard for...
  17. arbil

    Star delta winding selection for motors

    What is the basis for selection of star/delta winding for motors ? If we chage the connection of an existing star motor to delta or vice versa , what will be the impact ?

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