AC Theory Question: Transformers

In summary, a short circuit test was performed on a 40-kVA 4800/240V 60HZ transformer, yielding a ze of 7.92<64 ohms. The short circuit test involves shorting out the load impedance to isolate ze and calculate it using V/I. The source used is 4800VAC with no internal resistance. The current and magnitude of the load impedance are still unknown. To find the current, the voltage drop across the load impedance can be solved and the voltage regulation can be determined. The angle of the current can be found by breaking ze down into resistance and inductance and using the power triangle.
  • #1
Ryan231
14
0
Question: A short circuit test is performed on the low voltage side of a 40-kVA 4800/240V 60HZ transformer and the following data are obtained: W = 240W, V= 66V, and I = rated current. Determine the percent voltage regulation with a 0.88 lagging PowerFactor load connected.

On the Short cct test I found
Impendence (Ze) to be = 7.92<64 ohms

Basically now we've got a simple series circuit with
Source = 4800V
Ze = 7.92<64 ohms
ZL = x< -28.4 ohms

I'm totally stuck on how to find either the current (I) or the magnitude of ZL (x). If I could find either of these I could solve the voltage drop acrossed ZL and find the VR.
 
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  • #2
Sorry for my dumb question, but how do you perform this short circuit test? Is the 66V the input voltage when the output is shorted? What is the assumed source impedance for the 240VAC mains feeding the transformer during this short circuit test?
 
  • #3
berkeman said:
Sorry for my dumb question, but how do you perform this short circuit test? Is the 66V the input voltage when the output is shorted? What is the assumed source impedance for the 240VAC mains feeding the transformer during this short circuit test?

The short cct test is just shorting out the Load Impedence (ZL) this allows us to isolate Ze and calculate it by doing:

Ze=V/I = 66/8.33 = 7.92
To find the angle we break Ze down into Re (resistance Equivilant) and Xe (inductive). cosX = re/xe = 64 degrees.
We found these values using the power triangle.

The 66v tells us the amount of voltage the circuit uses I guess.

I'm not sure what you mean by the last part, but we assume the sources are perfect for these questions (ie no internal resistance).

The final circuit looks like this

Source 4800VAC (no internal resistance)
Load Ze = 7.92<64 ohms
Load ZL = x< -28.4 ohms

This is where I am stuck how can I find the current when all I have is a source voltage and 1 of the 2 loads?

Also, the teacher mentioned that he may give us the current of the circuit WITHOUT the angle, how would I go about finding the angle of the current?
 
Last edited:

1. What is the purpose of a transformer?

A transformer is used to change the voltage level of an alternating current (AC) electrical circuit. It can either step up or step down the voltage, depending on the number of windings in the primary and secondary coils.

2. How does a transformer work?

A transformer works on the principle of electromagnetic induction. When an alternating current flows through the primary coil, it creates a changing magnetic field which then induces a voltage in the secondary coil. This voltage can be either higher or lower than the input voltage depending on the number of windings in each coil.

3. What are the two types of transformers?

The two types of transformers are step-up transformers and step-down transformers. Step-up transformers have more windings in the secondary coil, resulting in a higher output voltage. Step-down transformers have more windings in the primary coil, resulting in a lower output voltage.

4. What are some common applications of transformers?

Transformers are used in a variety of applications, including power transmission, voltage regulation, and electrical isolation. They are also used in electronic devices, such as cell phone chargers and laptop power supplies.

5. How do you calculate the voltage and current in a transformer?

The voltage and current in a transformer can be calculated using the transformer equation: V1/V2 = N1/N2 = I2/I1, where V1 and V2 are the input and output voltages, N1 and N2 are the number of windings in the primary and secondary coils, and I1 and I2 are the input and output currents. The transformer must be designed with a specific turns ratio to achieve the desired output voltage.

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