3 phase motor - calculating power

Discussion in 'General Electronics Chat' started by mechanicalEng, Jan 31, 2008.

  1. mechanicalEng

    Thread Starter New Member

    Jan 31, 2008

    Mechanical guy here with little electrical knowledge.

    I have the amps reading on each phase of a 50hp rated motor. The line voltage is know to be 460V. Don't know if it is a Y or Delta circuit (don't think it makes any difference).

    Is the following an acceptable way to estimate the power usage of the drive?
    Ip1 = 27
    Ip2 = 28
    Ip3 = 27

    P drive = avg(27,28,27) * 460V * sqrt3 = 21.78 kW

    Or am I way out in the left field?

    Many thanks
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    Approaching it from the rated HP - 1 HP is 768 watts. 50 HP, then, is 38,400 watts. That would be a maximum power usage. Your figure is in the ball park.
  3. John Luciani

    AAC Fanatic!

    Apr 3, 2007
    The power for a three phase circuit is

    Ptotal = sqrt3 * Vline * Iline * cos(theta) where

    Vline is the line-to-line voltage, Iline is the current through a line (assmuming
    the lines are balanced) and theta is the phase angle between the voltage and current.
    The maximum power is when the phase angle between the voltage and current
    is zero (cos(0) = 1). You should check to see if the motor specifies the
    phase angle (or power factor).

    (* jcl *)
  4. recca02

    Senior Member

    Apr 2, 2007
    or power = sum of power in each phase.
    = 3*v(phase)i(phase)
    so it does matter to know if its a y or a delta.
    or simply measure voltage across each phase(phase and neutral) or phase to phase current.
    this seems correct for Y.
    again for active power from drive u shud know abt the power factor.
    also as already stated 50 HP shud be the full load power from the motor.