Inside the delta vs DOL with soft start. 4

[wrsharper]

Not too long ago we had a star-delta motor controller blow apart but did not hurt the motor, at least from megging and ohms measurements. I read this forum with enthusiasm and followed your collective advise and asked the boss to order a soft start. He ordered a soft start controller to handle the motor FLA. He also wanted to connect the soft start in "inside the delta" configuration. I said that this was not neccessary since we could DOL with the soft start since it was rated for motor HP. I also told him that engineers at this site generally frowned on that configuration. Now for my question, the motor is a 12 lead motor that was being used in star-delta. I have been installing the soft start and intend to wire the motor to the soft start controller just like it would be if it was connected to M1 and M2 in the old star-delta configuration during run. In that:
Output soft start controller,
Phase 1 tied to T1, T7, T6 and T12.
Phase 2 tied to T2, T8, T4 and T10.
Phase 3 tied to T3, T9, T5 and T11.
That is how the motor was connected to the line when both M1 and M2 are energized and the motor is up to speed and in run. This is a little late in asking but is there anything wrong with my design?

 

[davidbeach]

But what you have now is not per code; the NEC prohibits installations with an impedance mismatch between parallel conductors.

 

[jraef]

He said that was a temporary connection for testing. I trust he has proven the point and will connect all 4 cables now. Kudos back to you wrsharper for sticking with it and making it right.

If your line is more balanced that your load, it is sometimes reflective of connection problems or some mechanical problems in the machinery. A thorough thermal scan of all connection points can be enlightening of any hidden connection issues that may not be enough to see visually. Mechanical harmonics get reflected back to the motor as torque requirement fluctuations, and the motor may not react as fast as that harmonic so current harmonics are created, which can be additive to the point of creating current imbalance. That, combined with minor deviations in winding resistance and flux penetration in the motor can add up. "Rolling" your conductors would be a good idea to start with, followed by a vibration analysis IMHO.


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[wrsharper]

All of the problems have been worked out and the boss has ordered connection blocks per jraef that will accept all of the cables for a correct NEC installation. The extra flex motor lugs will keep their high compression crimp lugs. Thanks again to all.