Question

Question: I've got a machine with a FLA rating of 125A @ 3 phase 120/208. I must install a transformer that will be fed from a 480V source to power the machine. How do I determine the transformer size needed to provide the required power?

Answer 1

Step 1: Calculate the machine power demand Phase voltage (V) = 120V Phase current (I) = 125A Number of phases (N) = 3 Power (P) = V I N Power Factor (PF)Assuming a PF of 1 (unity power factor), the machine power demand is:

P = 120V  125A  3  1 = 45,000 VA

Step 2: Determine the primary voltage of the transformer Primary voltage = 480V (given)Step 3: Calculate the transformer turns ratio Turns ratio = Primary voltage / Secondary voltage Secondary voltage is the phase voltage of the machine, which is 120V. Therefore, the turns ratio is:

Turns ratio = 480V / 120V = 4

Step 4: Calculate the transformer primary current Primary current (I_p) = Secondary current (I_s) / Turns ratio The secondary current is the machine phase current, which is 125A. Therefore, the primary current is:
I_p = 125A / 4 = 31.25A
Step 5: Determine the transformer size Transformer size (kVA) = Power (VA) / 1000 The power demand is 45,000 VA. Therefore, the transformer size required is:

kVA = 45,000 VA / 1000 = 45 kVA

Therefore, you need a 45 kVA transformer with a primary voltage of 480V and a secondary voltage of 120/208V, 3-phase. Flag for review

Answer 2

Step 1: Calculate the machine power demand Phase voltage (V) = 120V Phase current (I) = 125A Number of phases (N) = 3 Power (P) = V I N Power Factor (PF)Assuming a PF of 1 (unity power factor), the machine power demand is:

Topic: Transformer Sizing