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Calcs for supply currents to a delta connected appliance.

hertzal123
Have wired some offshore catering equipment comprising 2 heating elements and 1 lighting tx,all 3 wired in delta and designed to run at 480v,no neutral.

Have measured the current in each phase at 415v but wondered if I can find the current values on site by multiplying I x480/415.The 3 loads are dissimilar.

Otherwise,I would have to draw a vector diagram.Thanks for any help.

                                     Regards,Hz
  • 0
    Let's see if I've understood correctly...


    you've got three single phase loads, a heater connected say L1-L2, another heater connected say L2-L3 and a light (transformer) connected say L3-L1.


    It's all designed for 480V L-L but you've connected it directly to a 415V supply (no 415:480V transformer) and measured the currents in L1, L2 and L3.


    If the loads were pure and constant resistances then increasing the voltage from 415 to 480 would indeed increase the current by a factor 480/415 - so that probably is a reasonable first approximation.


    In practice though there are likely to be some complicating factors - heaters typically aren't constant resistance but have a lower resistance when cold, which then increases as they heat up - when running on 415V rather than 480V they're probably not quite as hot as they should be so their resistances might be lower than for normal 480V running - so the calculated currents for 480V might be an over-estimate. Similarly for lighting if it's of a filament type. The lighting transformer might also introduce some variation - especially so if it's of one of the electronic varieties and so attempting to present a constant load rather than a constant impedance to the supply.


        - Andy.
  • 0
    Have I understood correctly, that the equipment is designed is designed for 480 volts three phase, but has been used on 415 volts for test purposes.

    If so, then when operated on the full 480 volts it will draw more current. The heating elements will draw more current in almost exact proportion to the extra voltage.

    The AVERAGE current may decrease if the elements are thermostatic as the duty cycle will be less, but the current when "on" will increase in proportion to the extra voltage.


    The lamps will also draw increased current at the higher voltage, but not in proportion. An approximation is that increasing the voltage on a tungsten lamp by say 20% will increase the current by half that much, i.e. by about 10%.


    No great accuracy may be expected without knowing the actual voltage of the nominal 415 volt supply, and the upper and lower limits of the nominal 480 volt supply.
  • 0
    Thanks for the advice all,

                                             Regards,Hz

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