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Tripping coil inrush current

Jam
Hi all,


I have been asked to re-specify a UPS for an existing switchboard with 230Vac shunt tripping coils and spring motors. The installation was designed and built by others but not commissioned so I can't assume that the existing UPS is up to the job in all realistic scenarios. While we can stagger the motor operations, my concern is that, in island mode, the inrush current to the tripping coils will exceed the overload capacity of the UPS and cause the inverter to panic and shut down before the tripping operation is reliably complete.


Does anyone here know whether a 7-10x inrush assumption is reasonable on this scale (200-700 VA)? Coil operation is fast - 2 to 4 cycles, although presumably it's energised for a bit longer while everything clears - so might this already be considered in the device datasheet?


Alternatively, do small(ish = 10-20kVA) UPSs not react quickly enough to short overloads? After all, it's kind of like a downstream fault being cleared by an external device.


(I would normally use a DC battery supply and the IEEE-485 method for sizing, if it's on any relevance)


Thanks,


Jam
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  • Former Community Member
    0 Former Community Member

    Jam:


     


    OMS:

    Inrush current to shunt trips could easily be in the order of 100 x the duty VA rating if operating at 230V (for typical 25millisecond breaker operation)




    Really 100x? That seems huge, but, since I've never measured it, I'm prepared to be advised! Just checking not a typo really



     

    In an AC coil the first instant you energize it there is very limited impedance, because there is no mutual inductance from the magnetic fields of the shunt trip core. So for that instant, all you have is the resistance of the coil wire itself and that is so low that it is essentially a "short circuit", so all available fault current at the relay terminal is drawn. A cycle later, the core fields begin to interact with mutual inductance and create impedance, bringing the coil current down.


    So at switch on, the inrush is very high but very short - on a mains supply with a very low source impedance that's not usually a problem - on a UPS with very limited source impedance, that short inrush can often be a problem - it's not necessarily a case of increasing the UPS kVA - more a case of selecting a UPS that can handle that high inrush for the required time without fooling itself into thinking there is a major overload or short circuit emerging.


    Get the shunt trip data (both VA and inrush VA over the defined time  - I think you said operating time is 45mS, but you need the energising multiple of the VA (or the actual inrush peak VA)) and put that in front of a reputable industrial UPS supplier  - you may need to consider a transformer based rectifier UPS rather than a transformerless PWM machine.


    It's not impossible to achieve what you want to do, but not necessarily with a generic UPS.


    Regards


    OMS

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  • Former Community Member
    0 Former Community Member

    Jam:


     


    OMS:

    Inrush current to shunt trips could easily be in the order of 100 x the duty VA rating if operating at 230V (for typical 25millisecond breaker operation)




    Really 100x? That seems huge, but, since I've never measured it, I'm prepared to be advised! Just checking not a typo really



     

    In an AC coil the first instant you energize it there is very limited impedance, because there is no mutual inductance from the magnetic fields of the shunt trip core. So for that instant, all you have is the resistance of the coil wire itself and that is so low that it is essentially a "short circuit", so all available fault current at the relay terminal is drawn. A cycle later, the core fields begin to interact with mutual inductance and create impedance, bringing the coil current down.


    So at switch on, the inrush is very high but very short - on a mains supply with a very low source impedance that's not usually a problem - on a UPS with very limited source impedance, that short inrush can often be a problem - it's not necessarily a case of increasing the UPS kVA - more a case of selecting a UPS that can handle that high inrush for the required time without fooling itself into thinking there is a major overload or short circuit emerging.


    Get the shunt trip data (both VA and inrush VA over the defined time  - I think you said operating time is 45mS, but you need the energising multiple of the VA (or the actual inrush peak VA)) and put that in front of a reputable industrial UPS supplier  - you may need to consider a transformer based rectifier UPS rather than a transformerless PWM machine.


    It's not impossible to achieve what you want to do, but not necessarily with a generic UPS.


    Regards


    OMS

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