# Calculating Input Voltage of a Step Down Transformer

• ilovejava
In summary, the step down transformer has an input voltage of 88.4 volts and is capable of supplying 650 amps.
ilovejava

## Homework Statement

A pair of power transmission lines each have 0.68-Ω resistance and carry 650 Amp. If the primary voltage of the 10:1 step-up transformer is 4.6 kV, calculate the input voltage of the step-down transformer at the other end.

P=IV
V=IR
IpVp = IsVs

## The Attempt at a Solution

I recognized that when the current passes through the primary transformer it will increase, so I used V = IR and rearranged to I = V/R = (4600)/(2(0.68)) = 3382.35A I don't know how to proceed afterwards because if I try to solve for voltage I receive 4600 which is equal to the primary transformer, and cannot be the case because this is a step down transformer.

You have not used the given 10:1 step up ratio.

kuruman said:
You have not used the given 10:1 step up ratio.

So you mean to tell me that the primary transformer has 10 turns and the secondary has 1 turn? In that case would I use Vs/Vp = Ns/Np? Manipulate for Vs and then solve?

ilovejava said:
So you mean to tell me that the primary transformer has 10 turns and the secondary has 1 turn? In that case would I use Vs/Vp = Ns/Np? Manipulate for Vs and then solve?
I mean to tell you that there are two transformers. Read the problem carefully
ilovejava said:
A pair of power transmission lines each have 0.68-Ω resistance and carry 650 Amp. If the primary voltage of the 10:1 step-up transformer is 4.6 kV, calculate the input voltage of the step-down transformer at the other end.
There is a 10:1 step up transformer at one end of the transmission line and a step down transformer at the other end connected to the load.

kuruman said:
I mean to tell you that there are two transformers. Read the problem carefully

There is a 10:1 step up transformer at one end of the transmission line and a step down transformer at the other end connected to the load.

Okay, this is what I have come up with. The voltage is stepped up from the first transformer therefore, the current is stepped down. I calculate the secondary current by using Is/Ip=Np/Ns and I end up getting 65A. To find the input voltage in the second transformer I simply used V=IR noting that R = 2(.68), and I end up with an input voltage of 88.4 Volts

ilovejava said:
... and I end up getting 65A.
Is this the current in the transmission lines? The problem states that the transmission lines carry 650 A. Consider finding how much power is lost in the course of transmission. Subtract that from the power going in and that is the power that reaches the step down transformer at the other end.

## 1. How do I calculate the input voltage of a step down transformer?

To calculate the input voltage of a step down transformer, you will need to know the output voltage, turns ratio, and input voltage. The formula for calculating input voltage is: Input Voltage = Output Voltage x Turns Ratio. This means that the input voltage is equal to the output voltage multiplied by the turns ratio. The turns ratio is determined by the number of turns on the primary and secondary coils of the transformer.

## 2. What is the purpose of calculating the input voltage of a step down transformer?

Calculating the input voltage of a step down transformer is important for determining the proper voltage for the input source. This ensures that the transformer is able to properly step down the voltage to the desired level for the output. It also helps to prevent damage to the transformer or any connected devices.

## 3. Can the input voltage of a step down transformer be higher than the output voltage?

No, the input voltage of a step down transformer cannot be higher than the output voltage. This is because the transformer is designed to step down the voltage, and it cannot create more voltage than what is supplied to it. Attempting to use a higher input voltage can damage the transformer or any connected devices.

## 4. What factors can affect the input voltage of a step down transformer?

The input voltage of a step down transformer can be affected by several factors, including the turns ratio, the quality of the transformer, the type of input source, and any external factors such as temperature or fluctuations in the power supply. It is important to consider these factors when calculating the input voltage to ensure the proper functioning of the transformer.

## 5. Is there a difference in calculating input voltage for single-phase and three-phase step down transformers?

Yes, there is a difference in calculating input voltage for single-phase and three-phase step down transformers. For single-phase transformers, the formula is Input Voltage = Output Voltage x Turns Ratio, as mentioned earlier. However, for three-phase transformers, the formula is slightly different: Input Voltage = Output Voltage x Turns Ratio x √3. This is because three-phase transformers have three primary and three secondary coils, which changes the turns ratio calculation.

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