3-phase question

Discussion in 'General Electronics Chat' started by talwart, Jun 21, 2007.

  1. talwart

    Thread Starter New Member

    Jun 21, 2007
    I need to measure the energy consumption of a motor hooked into 3-phase, 208V AC. I know the current on each of the three wires connected to the motor (from a clamp meter). My question is what is the total amp draw of the motor.

    For example, I have:
    Blue wire: 4.2 amps
    Red wire: 3.7 amps
    Black wire: 3.9 amps.

    Is the total amp draw the simple average of these, the square root of 3 times the average, or something else?

    Thank you!
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    I believe the total draw is the sum of the three currents. I would also think that a motor with currents that far out of balance has some damage in the windings.
  3. recca02

    Senior Member

    Apr 2, 2007
    in three phase current enters from one wire to to another so three currents are not un related but same current divided in different branches at different phases.
    the current from phase to phase is vector sum of current in each phase.
    for power calculation phase current into phase voltage gives power consumed by that phase.summing wud give the total power.

    also vector sum of current for balanced 3 phases shud be zero.
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    Well, I learned something.
  5. subtech

    AAC Fanatic!

    Nov 21, 2006
    I don't know just how accurate you want to be, but here goes.
    There are a couple ways to calculate VA or watts (in your case watts, we'll ignore power factor for now)

    1. If you know the phase to phase voltage:
    Power= I*sqrt3*line volts (208V and avg. of the three currents 3.93 A)

    2. A more accurate method would have been to measure each voltage phase to neutral at the same time you were taking the amp readings and calculated power in each phase.

    Assuming a balanced phase to neutral voltage of 120 volts then:

    A phase power= 120*4.2
    B phase power= 120*3.7
    C phase power= 120*3.9
    Adding the three powers will yeild total energy consumed by the motor.
  6. talwart

    Thread Starter New Member

    Jun 21, 2007
    Thanks everyone for your help!