Allied Diesel

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Imbalanced Load and Neutral Current

One of the advantages of polyphase systems is that the neutral currents will become null if the load is balanced, therefore negating the need for a neutral current carrier and reducing wiring by 25% in a 3 phase installation. However if the load is not balanced, this will lead to two effects :

  • the Automatice Voltage Regulator will not be able to keep the voltage equal on the three phases. This is somehow mitigated by the use of our optional 3-phase sensing AVR but the effect remains and can lead to the controller shutting off the engine because one of the phase is above or below the allowed values
  • the neutral current can become quite important and in some cases even exceed the phase currents themselves.

Take the case for example where one of the phases (say phase 1) is suddenly cut or has a very low load and suppose the load on the other phases is (phase 2) 100 Amps at 0.9 pf and (Phase 3) 90 Amps at 1 pf. Enter those values (with 0 Amps for phase 1) and you will find out that the neutral current is 130 Amps !!
Even without going to such extremes, suppose you have an installation with the following load pattern:
Phase 1 : 100 Amps at 0.8 pf (all the motors are on that phase),
Phase 2: 110 Amps at 1 pf and
Phase 3: 90 Amps at 1 pf.
Apparently it looks quite balanced, doesn't it? Before proceeding try and make a guess about what the neutral current could be: 10? 20 Amps? ... or more? Enter the data in the cells below and check the resulting neutral current.
The reason why this is important is that it is customary to downsize the neutral cable to a fraction of that of the phase cables and the neutral part of the circuit breaker (if present) is usually just a switch and has no thermal-magnetic trip

To find the neutral current in your application, please enter your 3 currents and 3 cos phi (if the cos phi is not known, keep the default values of 1 but as you saw in the above example it can significantly affect the result) it is assumed that all power factors are lagging (inductive). Then press the SUBMIT button

Current 1 Cos Phi 1

Current 2 Cos Phi 2

Current 3 Cos Phi 3

The answer will appear here!