Why load current and no load current out of phase?

Discussion in 'General Electronics Chat' started by Silhorn, May 21, 2013.

  1. Silhorn

    Thread Starter New Member

    Apr 9, 2013


    Above picture is from http://www.electrical4u.com/theory-of-transformer-on-load-and-no-load-operation/

    It is part of the phasor diagram for a transformer.

    IO = No load current with core losses only
    I1 = Primary current on load
    I2' = Additional current drawn because of load
    I2 = Secondary current

    By looking at this diagram, it says that once a load is applied, it can alter the phase angle of the primary current. I always thought whatever the phase angle of the secondary current affected by the load, the primary current phase angle will always be the same.

    Could someone explain why an increase in load will alter the phase angle of the primary current?
  2. LDC3

    Active Member

    Apr 27, 2013
    The current in the primary is forcing a current in the secondary (by magnetic induction). Since there is a load on the secondary, the electrons resist moving. This current creates it's own magnetic field, which pulls at the primary magnetic field, which results in a change of phase.

    These abstract ideas are difficult to explain. :)
    Silhorn likes this.
  3. Silhorn

    Thread Starter New Member

    Apr 9, 2013
    One more thing,

    Will this change in phase be lagging or leading the voltage compared to the no load phase angle?

    The diagram I posted shows that it will lead the phase back to the voltage but is that always the case, like resistance in an inductive circuit?
  4. t_n_k

    AAC Fanatic!

    Mar 6, 2009
    Suppose for the moment the transformer is ideal. There would be no magnetizing or core losses in that case. Hence there would be no primary current with no secondary side load connected - the unloaded case.

    Keeping with the ideal situation for the moment. Once a load is connected the primary & secondary ampere-turns values are balanced as the primary and secondary MMF's would be balanced.

    So with a primary-to-secondary turns ratio of N:1, the primary current magnitude would be the secondary current magnitude divided by N. The phase relationship between the primary and secondary currents would be identical. So if the load has a lagging power factor in relation to the secondary voltage, the primary current would also have the same lagging power factor in relation to the primary voltage. Ditto for the case of a leading power factor load.

    Returning to the case of a non-ideal transformer one must add the current required for core magnetization & losses to the ideal referred secondary load current. The addition of these two current components in the primary is not algebraic - it requires one to apply phasor additions to determine the resulting primary current.

    The primary magnetization current is highly lagging in relation to the applied primary voltage. While the magnetizing current magnitude tends to be very much less than the transformer rated current it will effect the primary power factor to a varying extent depending upon the applied secondary load. The general result will be that the magnetizing current makes a lagging power factor load appear even more lagging with respect to the overall primary side power factor.
    Silhorn likes this.
  5. crutschow


    Mar 14, 2008
    To summarize, the primary phase angle is the sum of the magnetizing current phase angle and the load current phase angle. For a load current much larger than the magnetizing current, the magnetizing current will have only a small effect on the resulting primary phase angle, which then will be very close to the secondary phase angle.
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