125kV power Ideal Transformers WILL be on exam 1 1. The city of Murray, Utah. di
ID: 2248260 • Letter: 1
Question
125kV power Ideal Transformers WILL be on exam 1 1. The city of Murray, Utah. distributes power to neighborhoods with a 12.47-kV three-phase system. (12.47 kV is the line voltage.) Each group of houses is served from one phase and ground, and transformed to 240/120 V by a pole transformer, as shown. 240 V center-lapped To eanh ground a) What is the turns ratio (primary/secondary turns) of the pole transformer to give 240 V,center-tapped? b) When a 1500-W toaster-oven is turned on, how much does the current increase in the high-voltage wire? Assume the power factor is unity and the transformer is 100% efficient. c) Repeat b) for a clothes drier that draws 15 A 2. A single-phase transformer is designed to operate at 60 Hz, Its voltage ratings are: primary, 500 v secondary, 200 V. The maximum permissible load is 30kVA a) What will be the magnitudes of primary and secondary currents when the device is full-loaded b) Loading is accomplished by an impedance connected across the 200-V terminals. How many ohms will correspond to full-load of the transformer? (Use IT model.)Explanation / Answer
Answer:-1)a) We have line voltage of 3-phase system is 12.47 kV and phase voltage is 12.47/(3)0.5 kV = 7199.55 V.
From transformer the output phase voltage is 240 V. Hence transformer turn ratio = (7199.55 / 240) = 29.998 or 30.
b) When no load connected, no current flows. When load drwaing power of 1500 W is connected then current output from line transformer is = power/voltage = 1500/240 A = 6.25 A. Since transformer ratio is 30, and output current is 6.25 A hence transformer input or primary current is 6.25/30 = 0.208 A = 208 mA.
Note:- (Number of turns in primary / Number of turns in secondary) = (Input transfromer voltage in primary / output transformer voltage from secondary) = (output transformer current from secondary / Inpput transformer current in primary)
c) Similarly for 15 A output from secondary, primary must have current value of 15/30 A = 0.5 A.
Answer:-2)a) Full loaded means 30 kVA power device is connected. So for 30 kVA power at primary with voltage 500 V, the full-load current is = 30000/500 A = 60 A. For secondary having voltage of 200 V, the full load current is 30000/200 A = 150 A.
b) Value of impedance connected across 200 V terminal is 200/150 Ohm = 1.33 Ohm.
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