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Killowatt Versus Killovoltamp

  1. Aug 22, 2009 #1
    We have a synchronous, brushless (not permenant magnet) generator with the following ratings:

    3 phase brushless alternator
    Model TFW-24
    Volt 120/208
    Frequency 60 Hz.
    Phase 3
    KW (P) 24
    KVA 30
    Power Factor (Cos) 0.8
    Insulation class B
    Protection Type IP21
    Rat S1
    Standard JB/T3320-1 2000

    If a load with a power factor of 0.8 is applied and the nameplate full load current is drawn, The killowatt is 24 and the killovoltamp is 30. (corresponding exactly to nameplate ratings)
    (Since KW = I * E *1.73 * PF and KVA = I * E * 1.73)
    or in this case: 24Kw = 83 * 208 * 1.73 * 0.8 and
    30KVA = 83 * 208 * 1.73

    However if a resistive load with a power factor of 1.00 is applied and the nameplate full load current is drawn, The killowatt is 30 and the killovoltamps is also 30. (Killowatt rating is 125% of nameplate rating while current is at nameplate rating.)
    Since KW = I * E *1.73 * PF and KVA = I * E * 1.73
    or in this case: 30Kw = 83 * 208 * 1.73 * 1.00 and
    30Kva = 83 * 208 * 1.73

    Can the generator handle nameplate full load amps without overheating in both situations described above regardless of the fact that the nameplate killowatt rating is overreached when a resistive load is applied?

    From what I understand a generator's power factor rating is how low the power factor the load can go at full load amps, and although it is uncommon that it is needed, load power factors can be lower but the load amperage will need to be lowered to prevent overheating of the generator stator. This leads me to believe that if the power factor of a load is 1.00 it can run at full load amperage, which as shown in the above calculations is 125% of rated killowatt. Please let me know if this is wrong.
  2. jcsd
  3. Aug 24, 2009 #2
    First, Does this synchronous generator have a permanent magnet rotor, or does the armature have windings and slip rings?
    The generator has two ratings. One is the maximum current in the stator windings, which relates to the volt-amp rating. The second is related to the maximum power output, which is related to the maximum mechanical torque that can be applied to the input shaft at synchronous RPM (torque x 2 pi x rpm/60 = watts). I would stick to the 24 KW and the 30 KVA nameplate ratings.
  4. Aug 24, 2009 #3
    The generator is a brushless type (not permanent magnet rotor.)

    My question is that at a power factor of 1 should the KVA or KW rating be used? At full load amps the KW is overloaded with a power cfactor of 1 as described earlier.
  5. Aug 25, 2009 #4
    In practice, volt-amps is used to measure apparent power, and watts is used to measure real power.

    This is because when you buy power, you are buying real power. It is common practice to measure just voltage and current (RMS values) when simple benchmarking or testing an electrical machine as being able to measure power factor or phase difference requires more complex equipment.
  6. Aug 25, 2009 #5
    KVA is basically a limit on the current in the stator winding. The KW rating is a limit on the input mechanical torque, and associated mechanical stresses, in the alternator.
    I am not familiar with the Tawasi brand. See
    As long as the unit is under warranty, I would not exceed nameplate specifications, for either KVA or KW..
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