Transformer %Z impedance and voltage drop

[mull982]

I need a refresher on what a transformers %Z means in terms of voltage drop across the transformer. For example if a transformer has a 5% impedance does that mean that that the transformer will drop 5% voltage during full load conditions. So in this case a transformer with a 480V rated secondary would result in a voltage of 456V during full load conditions?

 

[ron]

You are thinking of transformer voltage regulation.
So for a simple example, a transformer delivers 100 volts at no-load and the voltage drops to 95 volts at full load, the regulation would be 5%.
This is impacted a bunch by impedance, but even more by the load power factor.

StackPath

If you know the internal characteristic of the transformer such as X and R, along with the load characteristics you can calculate the VR with these formulas

https://www.electronics-tutorials.ws/transformer/voltage-regulation.html

 

[winnie]

The tutorials go into more detail which I won't try to repeat, but a key point to remember is AC voltage drop is a vector value and that the transformer impedance is highly inductive.

If you imagine the transformer impedance in series with your load, then the voltage across the load + the voltage across the transformer impedance form two sides of a triangle, with the 'ideal' output voltage of the transformer the third side.

If the load is near resistive power factor then it won't see much of the voltage drop. If the load has a capacitive power factor than the transformer impedance might even cause a voltage _rise_ across the load.

-Jon

 

[electrofelon]

This is a fun calculator to play with:

Transformer Calculator – Voltage Disturbance

 

[Russs57]

The voltage rise is something so many have a problem grasping. Thank you for pointing that out Winnie.