# Step down transformers & efficiency?

1. Dec 1, 2007

### mxtiger67

My question is why the power efficiency % goes down with Voltage in step down transformers.

The #of coils is less, therefore internal resistance is less too. Why the loss of efficiency in power?
I hope someone can help

2. Dec 1, 2007

### Staff: Mentor

Generally it is because the current becomes larger as the voltage gets smaller. Larger current leads to more power loss via ohm's law.

3. Dec 1, 2007

### TVP45

Can you explain your question a little more. I don't recognize the phenomenum you describe.

4. Dec 1, 2007

### mxtiger67

I guess what I'm asking is, where is the power loss, where does it go?

5. Dec 1, 2007

### Staff: Mentor

It is dissipated as heat.

6. Dec 2, 2007

### stewartcs

Transformer losses include copper losses (I^2*R losses) in the primary and secondary windings, and the core losses (hysteresis and eddy-current losses) in the iron. These losses are the same whether operating in the step-up or step-down mode. Hence, the efficiency is the same in either mode as well.

7. Jan 17, 2008

### CurriousShawn

I have to put a stepdown transformer in a new store.

The landlord put the wrong heating unit on the roof so i have to convert 480 Volt to 120-208 Volt power. Does anyone here know if I will use more electric because of this? Are todays transformers more efficient? Should I expect a higher electric bill? If so by what percentage?

Shawn

8. Jan 18, 2008

### TVP45

Yes, you will. It depends on the specific heating unit and the specific transformer, but should be in the neighborhood of 2-10% more.

9. Jan 18, 2008

### stewartcs

Why don't you tell the landlord to change the heating unit to the correct type?

If you step up the voltage (or down) there is some power lost as described above. So yes, it will cause you to use more energy by whatever the efficiency of the transformer is.

CS

10. Jan 18, 2008

### Staff: Mentor

I take it the existing 480-120/208V transformer was checked to make sure it was too small before a second one was added....?

11. Jan 19, 2008

### CurriousShawn

In a way the landlord is saving me  by putting in a 480 Volt Rooftop unit (RTU) in. This is a 10 Ton unit needing a crane to lift and put in and a 3 month wait in manufacture, no one that we know of stocks these things. That will run direct without the Step-down Transformer. The problem arose when the contract had called for 120/208 service and the transformer before the meter was only 120/208 with no means to supply the RTU. The landlord had to dig a new trench and install 600 ft of large gauge copper wire x 4 to supply the RTU. The step-down will power all of the lighting and 110 outlets. So by your comments I will be paying a premium in electricity of 2-10% on our store feed for all the other stuff. Now you all have the complete story. PS in case your interested www.floor-n-more.com tells a little about what we do.

Shawn

12. Jan 19, 2008

### stewartcs

Transformers are very efficient when operated at rated load, usually around 98% or so. That means you'll lose 2%, i.e. use 2% more energy than you would without the transformer.

That doesn't mean your power bill will go up by 2%, unless of course you have nothing else that uses electricity in your store.

CS