400V Motors and Wye secondaries grounded?

[dinkelja]

We are working at a plant with 480/277V, and is distributed using 3 wires + ground. The incomming feeders are 13.2kv to a delta - wye transformer. I guess the wye secondary is grounded somewhere at the substation? Although I only see a delta-star diagram on the nameplate. Should / does the sec. neutral of a delta-wye xfmr have to be grounded (per NEC) or can it float (star)?

Okay, so each MCC / PDP has 3 incomming wires. They derive a neutral from the structure somewhere nearby - for single phase 277 lighting. Is this good practice, the phases are by no means balanced so there is current flowing there. Why wouldn't a 4th wire be fed from the substation ground bus? Again, I am assuming that the Wye of the transformer is grounded.

My main issue is the MCC's have mostly 3 phase 460V motors. The engineers' plan to feed a 460/230V delta/delta transformer from the MCC, to run some small 1/3hp 230V motors. (We are in the US by the way.) It would be nice to have all 460V motors, chances are those motors are 230/460V rated anyways, but we can't be sure. There were also 3hp 400V motors included in the project. The engineer was going to demand the 400V motors be swapped for 460's. Why delta/delta anyways?

Lead me in the correct direction if you would. My thought was to get a 460/230V delta-wye transformer. The 460V and 230V motors should be connected wye (star), and the 400V motor in Delta. I thought that if the engineers 460/230 delta/delta was used it would yield 230V delta (of course) and 132V wye. I pointed this out what, what I thought would be the case, the response I received was all motors are connected as wye's.

Last thing, though these are small motors, is the real purpose of a 400V motor (in the US) so it can be wye/star started? BTW, I've been searching through all the old posts, lots of good info, just haven't had time to digest it all.

Jared

 

[waross]

#1 If you haveloads connected to the neutral of a wye transformer (or bank) the code requires the wye point to be grounded.
#2 Where a neutral is used there must be a neutral conductor seperate from the grounding conductor, and the neutral conductor must be adequately sized to carry the maximum single or two phase load.
#3 400 volt motors are often 200/400 volt motors. They are intended for use on 208 volt and 416 volt system respectively.
#4 For voltage ajustment for small motors, I have used an open delta autotransformer connection. I have usaed it for single motors and groups of motors. I have seen it used for a large portion of an entire plant.
When the transformers are connected as autotransformers the losses are less and smaller transformers may be used.
#5 Most motors in the US and Canada are internally connected star. Externally only the three line wires are connected and it doesn't matter if the supply is star or delta, the motor can't tell the difference.
#6 You mentioned 460 volt transformers. $80 volt transformers are commonly use for 460 volt motors. (And 240 volt transformers for 230 volt motors.) This gives a margn of safety for voltage drop in the feeders, transformers and motor conductors.
7# Rather than use a wye delta transformer, consider two transformers in open delta autotransformer configuration, as I have mentioned.
Have I left any issues unaddressed?
respectfully