Voltage and Motors

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djn602

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I am in the middle of a project where we are bidding out the service upgrade of the winery I work at (I am in maintenance). I am trying to unsterstand and learn so I am hoping some people will weigh in here.

We currently have multiple seperate 277/480Y and 240Delta services. So far two seasoned ECs have bid. One proposes switching us to 120/208Y (fed from a new main distro of 277/480Y) which makes sense to me becasue we have many panels where that high leg is unused. The other EC proposes staying with the Delta. My questions really come from how will our motors do? Some are rated 208/230 some are just rated 230. I understand that all(?) motors are built with a voltage range of +- 10%.

I guess it "all depends" is the motor fully loaded, does the POCO dip in the summer or scale back, is the circuit wiring large enough and not too loaded to abate some voltage drop there.

I, in no way shape or form, what anything to do with killing some expensive motors prematurely.

What do we do?
 
The main issue is one of balanced loading. 240V 4W Delta services are at greater risk for unbalanced loads because by design, the single phase loads will be unable to be balanced. So they are only intended to be used for applications where there is a small percentage of the load that is single phase, typically no more than 50% of the total load on the transformer* (taken from utility service rules, which also limit the total maximum service size to 100A). So if MOST of your load is 3 phase motors, stick with the 240V delta 4 wire service, it's better for the motors. But if the majority of your load are 1 phase, use the 120/208V service and balance the loading on the phases better.

*Side note, I couldn't remember the exact loading value, I thought it was a lot lower, but this was what I found. It may not be right.
 
091008-1050 EST

Jraef:

In our area, DTE, wild leg deltas are fairly common. It seems that they put in one transformer large enough for the single phase load and its three phase contribution. A second transformer is added for the three phase wild leg power. If later more three phase power is required they enlarge the second transformer, or add a third transformer.

At the building where my son's shop is located I believe the total transformer KVA is about 170. Tomorrow is heavy rain. Later I will see if I can read the ratings on the transformers. This is considerably larger than a 100 A capability.

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