Power Measurement of Open Transition Motor
Power Measurement of Open Transition Motor
(OP)
I have a 1500 ton water chiller at 480 VAC. It is an open transition wye start, delta run motor. Each of the three phases is replicated to produce six phases which are connected across the motor coils in such a way that each coil (in the run state) will have a set of conductors that brings the current in and out of the coil. Different than a standard delta connection where each coil current is mixed in the delta-corner connected conductors (i.e. phase current does not equal line current in a normal delta connection, but on the open transition delta they are equal). Each of the six phase connections has three conductors (3 x 500 MCM). Two of the conductors on the day of measurement read about 60-75 amps, where the third conductor measured 100-115 amps. Here is the difficult part, one of the conductor's currents is of opposite polarity of the other conductors on the same phase. I need to measure power, but am unable to really understand how one conductor in a phase could have a negative power while the other two would be positive. Any thoughts?





RE: Power Measurement of Open Transition Motor
The six (groups of) leads are the only way to achieve wye-delta starting. Can you reply with the nameplate current and conductor size for [each?] motor? Is this and NEC-jurisdiction site?
Assuming you are familiar with the assorted Code requirements for parallel conductors—it’s possible that the conductor-current imbalance stems from departure from Code constraints. (You are probably aware that current needs to divide evenly between multiple per-phase/coil conductors.) I do not see open-transition starting having an appreciable effect on the installation or measured currents. The maximum current listed seems like it corresponds to a fraction of full load—which would normally be sized based on 0.577x nameplate.
RE: Power Measurement of Open Transition Motor
RE: Power Measurement of Open Transition Motor
Counts are:
Motors - 1
Phases - 3 (configured to bring two conductor bundles to the motor per phase)
Conductors - 6 bundles x 3 conductors per bundle = 18
Motor Terminals - 6 (2 phase A, 2 phase B, 2 phase C)
Conductors - Copper
If you have a 2002 Code Handbook, look under article 430.22, part D, and the accompanying handbook explanation.
I agree that the open transition likely does not affect this measurement, but actually makes the power measurement easier, as the phases do not interact in the line current. In this configuration, coil current per phase equals phase current equals line current (similar to a wye connection).
RE: Power Measurement of Open Transition Motor
Sorry—no 2002 NECH.
RE: Power Measurement of Open Transition Motor
my first thougt when reading tdunruh's last post was the same as yours, but he has not talked about measuring the current in the three phases without changing the voltage connection but only in the three parallel connections per phase.
RE: Power Measurement of Open Transition Motor
RE: Power Measurement of Open Transition Motor
RE: Power Measurement of Open Transition Motor
RE: Power Measurement of Open Transition Motor