Power Measurement of Open Transition Motor

[tdunruh]

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?

 

[busbar]

tdunruh-- can you elaborate on "...one of the conductor's currents is of opposite polarity of the other conductors on the same phase"? Sorry, but I can't visualize the situation. [FWIW, it doesn’t sound right.]

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.

 

[busbar]

Pardon me, but I see that the conductor size is listed—500kcmil (copper?) Still need count per motor and number of motors.

 

[tdunruh]

busbar - I agree that it doesn't sound right. That is my perplexing concern. The installation is one motor in a chiller. Since this is an open delta connection, I don't need to apply the two-wattmeter measurement method with my Fluke 39 meter, but can measure the current through each coil independently of the other. When connecting the voltage of the Fluke to two phases, and then the current to each of the three conductors for the phase, two read positive kW and one reads negative kW. Careful attention was given to the direction of the current probe, and the measurement was repeatable on each phase.

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).

 

[busbar]

OK it sounds like the problem may be in connection of the 39 meter. Are you leaving the voltage connections the same for different current measurements? For power reading to be anywhere accurate, each current ‘connection’ will have a different pair of voltage connections. Look over the meter instruction book on connections and readings for three-phase circuits. (Fluke doesn’t seem to have the operating manual on line.)

Sorry—no 2002 NECH.

 

[electricuwe]

busbar,

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.

 

[busbar]

Then the comment: "one of the conductor's currents is of opposite polarity of the other conductors on the same phase. I need to measure power..." is a bit ambiguous. tdunruh, can you clarify? May need to get upstream of the starter cabinet to get meaningful readings; either current or power.

 

[jbartos]

Suggestion: It appears that the measuring method is the culprit. Can you perform a different measuring set-up, e.g. three-wattmeter method?

 

[mrbean]

I ahve a problem. Before me is a three phase motor. Out of is comes six wires. I need to determine which is which. With an ohmmeter I can find the three pares of stator windings. Now how do I find to start and end positions of each winding without opening the motor?

 

[jbartos]

Suggestion: Try two-channel oscilloscope and apply Lentz law. Approximately simultaneous +voltage ordinates should be on beginnings or ends of coils. It appears that you will be able to determine all beginnings or all ends of coils only. The result will therefore be relative only. How would you like to define the beginning of the coil and end of the coil?