Transformer calculations

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As always Myself and an associate are arguing about transformer sizing.

Building a panel for 5 servos rated at 7 amps each. Controls need about 5 amps.

I say I need a 15 KVA transformer he said 30KVA can someone help me

I need the three phase 480 primary with secondary 3 phase 240 and 120 tap for controls

Thank you for you help
 
I took 2 servos off the machine. There is no coordinated motion. I would think only 2 servos would run at any given time. Probably will only have one servo at a time running. The motors are 7amps at full load I don't think they will ever see full load.
 

kingpb

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SE USA as far as you can go
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I took 2 servos off the machine. There is no coordinated motion. I would think only 2 servos would run at any given time. Probably will only have one servo at a time running. The motors are 7amps at full load I don't think they will ever see full load.

At full load, they are 7A, but what about inrush during starting.

Say each draws 4.5X FLA during starting, and you have 2 of them simultaneously, then you have 2x31.5Ax1.732x230 = approx 25kVA, then add the 5A for controls you need a XFMR around 26.8KVA because of the one phase that carrys the additional control power.

Therefore with std size xfmrs, I would have to say 30KVA given the info provided.
 
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