# Transformer copper loss calculation

1. Aug 2, 2009

### tommoturbo

Hello guys have a little problem

see if anyone can help

1. The problem statement, all variables and given/known data

Q the primary winding resistance is 125Ohms, Secondary is 0.7ohms calulate the transformers copper loss at full load.

now the transformers name plate is 240V 50Hz 28VA the earlier question explains it is a step down transformer delivering 12V

2. Relevant equations

so primary Current = Vp/Rp 240/125 = 1.92A

now im not sure what figures to use to calc the current in secondary winding at full load

i get values from 40A 38.4A

and using formula Full load current = VA rating/primary voltage i get .116A

i need Is value for copper loss formula

Copper loss W= Ip(sq)xRp+Is(sq)+Rs
Copper loss W= 1.92(sq)x125+Is(sq)+0.7 dont know the equation for secondary current Is

any help appreciated

2. Aug 2, 2009

### negitron

First of all, you're not calculating the primary current correctly. Hint: what does 28 VA mean?

Once you have that, remember that power in = power out to work out the secondary full-load current.

3. Aug 2, 2009

### tommoturbo

thanks for the reply but im lost now.

not entirely sure what to do with the 28VA but thought it was used somehow to calc secondary current

thought i did the priomary current ok though
I=V/R 240/125

am i on the wrong track?

4. Aug 2, 2009

### negitron

Very much so. Let's start at a more basic level, then:

What is VA? What does it mean and how is it calculated?

5. Aug 2, 2009

### tommoturbo

"The VA rating is the math formula used in determining the amperage at a given voltage."

So my secondary voltage of 12 volts from a 28VA transformer will produce 2.33 amps...?

e.g., 28 VA ÷ 12 Volts = 2.33 Amps. am i correct.

6. Aug 2, 2009

### negitron

Right. Now, what do you know about the relationship of primary current to secondary current?

7. Aug 2, 2009

### tommoturbo

does this apply to the primary current also

primary voltage of 240volts from a 28VA transformer will produce .116 amps...

e.g., 28 VA ÷ 240 Volts = .116 Amps.

thanks for you help in this

8. Aug 2, 2009

### negitron

You've got it! Now you can plug those values into I2R to find the primary and secondary copper watt losses.

9. Aug 2, 2009

### tommoturbo

:)

10. Feb 11, 2011

### diva_kln

hey guys
i have a doubt
will turns ratio affect the amount of copper loss or core loss
i am a little bit confused in ds matr

11. Feb 11, 2011

### Phrak

Insufficient data.

Is 12 VRMS the unloaded secondary voltage?
Is this an imaginary transformer with the unloaded primary power dissipation equal to zero?
I don't know if this one counts, but are we to pretend that 'full load' is a purely resistive load?

Last edited: Feb 11, 2011