Transformer efficiency conversion

[zebra1707]

Greetings physics gurus

Im getting bogged down on conversion of the VA to A

Homework Statement

A small transformer used to power a laptop computer has the following characteristics printed on it:

• input AC 220 V, 50 Hz, 15 VA.

• output DC 5.0 V, 1.0 A.

Is this transformer efficient? Support your answer with calculations. Determine value for efficiency

Homework Equations

Efficiency = energy output/energy input x 100%

The Attempt at a Solution

Im not sure if the Hz impact on the solution - but I am getting a very low efficiency. 33%

Just need a little guidence to get me going with this question.

Cheers P

 

[bejoynp]

Transformers are one of the most efficient electrical devices as there are no moving parts. The only way you can actually check a transformer's efficiency is via a load test as there are different loses involved like: Iron loss (Core loss), Copper Loss (I square R), Hysterisis loss, Leakage inductance loss, Magnetostriction etc.. Generally for electrical machines, the efficiency curve increases with the power rating.

Since you have mentioned a laptop adapter, there is a possibility that it might be an SMPS. In that scenario, the efficiency calculation will differ.

 

[Mindscrape]

Actually, switching supplies are the most efficient electrical power supplies, which is actually what most laptop computers are using these days to provide DC voltage. Yep, 1/3 efficiency is right, and that's about what your average transformer will do.

 

[zebra1707]

Hi there

Many thanks, can you advise on how to set the calculation out.

Im confused about the 15VA part of the question can I use 15 VA in the equation or do I need to convert it to another unit type?? and also determining a value for the efficiency, how can that be done?

Cheers P

 

[bejoynp]

Hi

Watts is applicable in DC circuits as well as in Resistive AC Circuits. When Inductive and Capacitive loads come into picture, you have to consider Power factor:

(VA) X Power Factor (PF) = Watts

VA=Apparent Power
Watts=True Power drawn by the equipment

Only when PF is near/equal to 1, VA is equal to Watts. That is why we have Power factor correction circuits in sensitive equipments like computers. It is not easy to find out the power factor for a load and it is a complicated process. In your case:

If your laptop adapter is transformer type and if it is a good one (i.e. does not get heated up when not charging the laptop but plugged in) you can assume the PF to be 0.92. This is just an assumption.

 

[zebra1707]

I have a further question about the efficiency calculation - to ensure that I get Watts for both input and output - can I convert VA to the following.

A small transformer used to power a laptop computer has the following characteristics printed on it:

• input AC 220 V, 50 Hz, 15 VA (Does this mean that 15 VA = 220 V x 0.0682 A)

• output DC 5.0 V, 1.0 A.

Is this transformer efficient? Support your answer with calculations. Determine value for efficiency

 

[wwshr87]

Like bejoynp said, you will need to know the power factor to convert VA to Watts.

 

[bejoynp]

Hi

You need to know the power factor in order to convert VA to Watts.

1. First Please find out whether your laptop adapter is an SMPS type or Transformer type. An SMPS type adapter is one fourth the weight of a transformer type.

2. SMPS power conversion is the most efficient technique. Their efficiencies can go upto 95% or more. Smaller transformers are not very efficient. Generally transformers of rating ~kVA can have efficiencies upto 88% or more. (I conducted a load test on 2 kVA transformer in my undergraduate college lab).

In your case, you have to assume a value for power factor to calculate the efficiency.

 

[zebra1707]

Hi all

I am still a standstill on this question as I don't have a power factor included in the question.

Assistance greatly appreciated.

Cheers P