No 240V3PH-480V transformer allowed to power motor...per utility????

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craff

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We have a 240V 3P3W service in a older building. There is a transformer (ACME, 480V3PH prim, 240V3PH sec.). It's backwired to provide 480V3W to a 100HP compressor for HVAC. The compressor went bad and the local utility says we need remove the transformer and replace compressor with a 240V3W unit Are they worried about power spikes or the proximity to the substation???????????????
 
We have a 240V 3P3W service in a older building. There is a transformer (ACME, 480V3PH prim, 240V3PH sec.). It's backwired to provide 480V3W to a 100HP compressor for HVAC. The compressor went bad and the local utility says we need remove the transformer and replace compressor with a 240V3W unit Are they worried about power spikes or the proximity to the substation???????????????
They are worried about power sags due to the energizing inrush of the transformer and coordination with their protective devices.
 
They are worried about power sags due to the energizing inrush of the transformer and coordination with their protective devices.

I figured as much. the main gear for the building is dual fed, with two meters. One, for 240V3PH3W loads and the other 120/208V3PH4W for lighting and convienience loads. The utility says that keeping the transformer is against their req's and installing a 240V3PH3Wcompressor will extend the units lifespan, but it's a expensive fix for our client. Not that I'm complaning about the work.
 
Craff, welcome to the forum!

Two points to remember:

1. For a given load size, the current at 240v is twice what it is at 480v.

2. Reverse-connected transformers usually have excess inrush current.

Since the compressor need replacing anyway, why not go with the 240v?
 
Was it just the motor that went bad? Or is the whole compressor that is shot? I can see the added expense if it was just the motor because the starter would have to be changed to a larger size, along with the feeder wiring. The service size should already be sufficent, since the actual load will not change. Would the POCO entertain changing the starter to a soft start type to reduce the spiking?
 
Thanks it good to know there is a place for all of life's pressing questions.

Yes, the whole unit is bad. So the entire control setup needs to be replaced to compensate for the voltage and load changes. Additionaly, the transformer is fed in parallel, so we will have to upize the wire and changeout conduit. Our HVAC division is looking into changing the higher efficiency motor in hopes of lowering the load. We have concluded that the entire run, conduit, wire, disconnect need to be redone. The existing breaker is 400A, which with brain based calcs seems like it'll barely make it. I'm apt to upsize the breaker to a 600A for good measure. We are going to have the project pushed through a electrical engineer for good measure and CYA.
 
There is another option, albeit zany: Switch the entire service to 480v, drive the compressor at 480, and flip the transformer around to serve the 240v loads (if it's big enough.)

Hey, I said it was zany. :roll:
 
Thanks it good to know there is a place for all of life's pressing questions.

Yes, the whole unit is bad. So the entire control setup needs to be replaced to compensate for the voltage and load changes. Additionaly, the transformer is fed in parallel, so we will have to upize the wire and changeout conduit. Our HVAC division is looking into changing the higher efficiency motor in hopes of lowering the load. We have concluded that the entire run, conduit, wire, disconnect need to be redone. The existing breaker is 400A, which with brain based calcs seems like it'll barely make it. I'm apt to upsize the breaker to a 600A for good measure. We are going to have the project pushed through a electrical engineer for good measure and CYA.

If the exsisting breaker on the 240 volt side was working before, it should still work with the new 240 volt compressor, your load on that side will actually be a little less than before, because now you don't have any extra power lost in the transformer. I got to thinking about this, and you will still have the same amount of startup amperage on the 240 volt side as you had before. The POCO will not gain anything from you changing the compressor to 240, as the total startup load will be nearly the same as before. I am assuming that you are not turning the transformer off and on with the compressor, so inrush would not be an issue. Consider a air screw type compressor, the compressor motor continues to run even though the demand is satisfied, thus you start it in the morning, shut it off at the end of day. A smaller compressor may be able to do the same job as the big one that you have now.
 
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