Transformer question

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ammklq143

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Location
Iowa
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Electrician
I received a call from a customer about installing some machines in their factory. They purchased them from a company that went out of business. They have a 240 V three-phase delta service and the equipment they purchased is 480 V three-phase. I'm looking at putting a step up transformer in for them. There are two motors that would need to run on this transformer. The nameplates say FLA is 30 A on one and 7 A on the other at 460 volts. Would this call for a 30 kVA transformer?
Thank you.
 
If the motors are dual voltage (often they are on small motors) I may just change the connections for low voltage and adjust/replace overloads starters as needed, may need to change supply taps on any control transformers as well.
 
But if not, then you also have to think about how they are started. If started across the line (most likely at that size) it's best to size the transformer kVA at 2 to 2-1/2 times the motor HP in order to avoid significant voltage drop. You didn't say the HP, but at those FLA values at 460V I'd hazard a guess at 20HP and 5HP. If they never start simultaneously you might get away with a 37.5 kVA, but if they do, 45-50 kVA is better. Alternatively you could just let the voltage drop on the 480V side as long as it doesn't cause a VD further up stream. If the motors have VFDs, 30kVA should be fine, but make sure the 380 secondary is solidly grounded wye.
 
Jraef said:
But if not, then you also have to think about how they are started. If started across the line (most likely at that size) it's best to size the transformer kVA at 2 to 2-1/2 times the motor HP in order to avoid significant voltage drop. You didn't say the HP, but at those FLA values at 460V I'd hazard a guess at 20HP and 5HP. If they never start simultaneously you might get away with a 37.5 kVA, but if they do, 45-50 kVA is better. Alternatively you could just let the voltage drop on the 480V side as long as it doesn't cause a VD further up stream. If the motors have VFDs, 30kVA should be fine, but make sure the 380 secondary is solidly grounded wye.
Click to expand...
Hint: 37.5 is standard single phase transformer size but three phase usually jumps from 30 to 45.
 
Another benefit of oversizing any transformer would be frequent motor starting. If this machine switches on and off a lot, heat from the inrush current doesn't have time to dissipate and the transformer will run hot.
 
The motor controllers have VFD's in them. I did initially look at replacing those with 240 volt ones but the cost was higher and there wasn't room in the control cabinet for one of the larger VFD's. So, I am looking at using the 45Kva transformer to power these and just wondering if what I have planned is the best way to go or if anyone has any other ideas. I do also need to feed a 240 volt 3 phase motor and a 120 volt motor in the same set up so I was thinking of running 240 volt 3ph to the room and put a 100 amp 3ph panel so I can feed these two additional lower voltage motors. From that panel I am planning on feeding to the transformer and out of the transformer to another 100 amp 3ph panel.

Do I need a disconnect between the first (240 volt) breaker panel and the step up transformer or is the breaker that feeds the transformer suitable if it's right next to it?

The service entrance is in another room approx. 100 feet away. It has a 400 amp main disconnect. If I run #2's to the new 240 volt panel........I need to add a 100A disconnect near the 400A service disconnect to feed my new panel.....right? If so, does it have to be right next to the main disconnect or how far away can it be?

Lastly.....if I feed each of these motor controllers and the other motor that are in the same room as the transformer and new breaker panels do I have to put a disconnect at each controller and motor?

I haven't done much industrial electric work for many years so I appreciate the help.
Thank you.
 
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