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What is the alternator phase current?

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    wayneups is offline Junior Member Pro Subscriber
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    What is the alternator phase current?

    A delta-connected three-phase alternator is driving a wye-connected resistive load. The alternator output voltage is 480 V. The load resistors are 12 ohms each. What is the phase current in the alternator?
    Your Answer: 13.33 A
    Correct Answer: 40 A

    The correct answer is not correct.

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    Quote Originally Posted by wayneups View Post
    A delta-connected three-phase alternator is driving a wye-connected resistive load. The alternator output voltage is 480 V. The load resistors are 12 ohms each. What is the phase current in the alternator?
    Your Answer: 13.33 A
    Correct Answer: 40 A

    The correct answer is not correct.
    Agree with your assessment. My math:

    Find phase voltage of Wye connected load => 480V/ sqrt(3) = 277.128 V

    Find phase current of load => I = E/R = 277.128V / 12 ohm = 23.094 A

    Find phase current of source => current of phase = current of line in wye connection => line current = 23.094A => current of phase in Delta = current of line / sqrt(3) = 23.094A / sqrt(3) = 13.33A

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  5. pallen is offline Junior Member Pro Subscriber
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    Quote Originally Posted by kingRADi88 View Post
    Agree with your assessment. My math:

    Find phase voltage of Wye connected load => 480V/ sqrt(3) = 277.128 V

    Find phase current of load => I = E/R = 277.128V / 12 ohm = 23.094 A

    Find phase current of source => current of phase = current of line in wye connection => line current = 23.094A => current of phase in Delta = current of line / sqrt(3) = 23.094A / sqrt(3) = 13.33A
    I calculated the same Amp values as above at first. Upon review i realized that the current calculation need to be done for the current circulating in the delta connected alternator and not the wye connected 3 phase load.
    So no sqrt 3 conversion is necessary
    add the 3 phase ohm values because they are all connected too the delta alternator.
    12Ohms x 3= 36
    480v line voltage divided by 36ohms = 13.33A is the phase current in the delta connected alternator.

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