Paralleled Motor Wiring

[Kevingeldard]

In a paralleled motor feeder application would there be any adverse issues to connect 1 of the Paralleled L1 feeder wires to T6 and the other Paralleled L1 wire to T1? Typical for the remaining connections for L2 & L3.

This is an 300 HP inverter duty rated 480 Volt, 3 Phase Motor connected for typical Across-the-line Delta Run, L1 to T1-T6, L2 to T2-T4, L3 to T3-T5. The motor is started using a VFD with a load side reactor in which the motor is overrun to 65Hz. The VFD is released at the overrun speed and the motor is then connected across the line with an magnetic motor starter contactor.

The new motor has parallel wring for each motor winding connections. Example: 2 wires for T6, 2 wires for T1, etc., 12 Total wires for motor. Not a 12 lead motor. If connected normally the L1 motor connection would have 2-250 Parallel wires, 2-Parallel Motor wires for T6, and 2-Parallel Motor wires for T1 for a total of 6 wires in each motor connection. This becomes a bulky connection. I have an electric hydraulic crimp tool and my preference is to use crimp lugs with grade 8 bolts for a positive and secure electrical connection. The connection would be insulated with 600 volt rated cold or heat activated heat shrink tube insulation.

My thoughts are that this should not be an issue because:

• Each winding connection of T6 & T1 are receiving the same phase.
  • The motor feeder wires of L1 are joined in the reactor thereby connecting T6 & T1 together bridging the motor windings. Additionally, the motor feeder wires of L1 are joined at the motor contactor.
• If this was a Wye Start Delta Run application, the motor winding connections would be installed individually and joined together at the motor control center starter.

Is there anything that I have overlooked?

 

[LarryFine]

Welcome to the forum.

It sounds like you're asking whether it would make any difference if, instead of actually connecting each phase's conductors in parallel (both ends of each phase's wires tied together), you connect only the supply ends of each phase together, and connect the load ends separately to each of the motor's winding sets.

I would think the genuine parallel connection would perform better, with a lower overall impedance, and the motor would behave more like a single-voltage motor.

 

[synchro]

If you close the delta connection at the supply end instead of at the motor, then each wire will carry the same current as that of its corresponding motor winding. But if you make the delta connection at the motor, then for equal winding currents each of the two paralleled wires will carry cos(30°) = 0.866 times that of the winding current. In the latter case the total line current will be the familiar 2 x 0.866 = 1.73 times the winding current. And so if you close the delta at the supply end, the currents in each wire will be 1/0.866 = 1.155 or 15.5% higher than if you had connected the pair of wires at the motor.

The fraction (i.e., "component") of the current in each of two connected windings which is in-phase and contributes to the line current is cos(60°) = 0.866 . The remaining fraction of the winding current is at 90° from the line current and is sin(30°) = 0.5 times the winding current, with the current component in one winding at +90° and the other at -90°. And so this 90° component of the winding current flows between the connected windings and not through the line conductor. This is similar to how balanced L1-N and L2-N load currents on a 120/240V single phase system will flow between the loads and not through the neutral feeder.
And as expected, because the current flowing between windings at their connection is at 90° to the line current, 0.866² + 0.5² =1, and all of the winding current is accounted for.

A similar discussion is at https://forums.mikeholt.com/threads...with-single-phase-loads.2565677/#post-2728173, and in Julius Rights preceding post.

 

[Kevingeldard]

Larry & Synchro,

I am glad to be included in the forum! Thank you both for your kind and generous contributions to my question. Your comments have provided me and hopefully others with a clear explanation to my question. I also reviewed the additional attached discussion.

It is refreshing to have exceptionally knowledgeable and helpful professionals in a collective support group that are willing to provide informative and detailed explanations. Continual growth in our knowledge, no matter where we are in our career, is an essential requirement to becoming an better professional.

I did fail to include in my original post that the parallel motor feeder is in a single 3" underground PVC conduit located about 35 foot from the motor to the load side of the reactor and disconnect. The MCC, VFD and across-the-line motor starter is located another 40' from the reactor and motor disconnect for a total distance of 75'. If I were to choose the Line side Delta connection in this particular application, the motor delta connection would occur in the middle of the motor feeder circuit at the reactor. I understand that the appropriate line current calculations would apply as described above.

Your assistance is very much appreciated!

Thanks again for your support and have a great day!