Question

I have two questions:

1) This question below is step up autotransformer but from second photo, which one is stepup autotransformer because it has 2 different stepup autotransformer? And how do you know it is one of these two stepup transformer?

2) Can you help calculate if it is step down autotransformer?

A 720-VA, 360/120-V, two-winding transformer has the following constants: R11 18.9 Ω, XH-21.6 Ω, RL-2.1 Ω, XL - nd XmH6.84 kf2. The transformer is connected as a 120/480-V, step-up autotrans- former. If the autotransformer delivers the full load at 0.707 pf leading, determine its efficiency and voltage regulation. 2.4 Ω, Rc1,-8.64 kf, a

Answer 1

It is given that two winding transformer is connected as autotransformer.

Hence the total losses that is both copper losses and iron losses remain the same as the current flowing through both the HV and LV windings remain the same, irrespective of whether the autotransformer is connected in step-ups manner or step-down manner.

If we consider the voltage rating of two winding transformer 360/120V as V_{​​​​​​1}/V_{​​​​​​2} .

Then for step-up autotransformer rating 120/480V , we can get from V_{​​​​​​2}/(V_{​​​​​​1}+V_{​​​​​​2}). Hence, from figure 2 option 'b' would be the correct connection of step up autotransformer.

And also to get step down autotransformer rating as 480/360V we get it from (V_{​​​​​​1}+V_{​​​​​​2})/V_{​​​​​​1}. Hence option 'c' would satisfy.

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