# Inverse expected transformer efficincies with increasing frequency at 1v AC!!!

Discussion in 'Homework Help' started by tfxuk7, Dec 12, 2009.

1. ### tfxuk7 Thread Starter New Member

Dec 10, 2009
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I am currently experimenting with pwer tranformers and the parameters of the experiment are 1V AC into a 120V - 6V 12VA step down transformer. Measurements are taken in input and output voltage and amps and recorded against frequency change. the secondary side has a 10kohm reistor for load. frequencies are varied from 30Hz - 30kHz. the input voltage output voltge and output amps remained similar but the input amps decreased with increading frequency --significantly ....this results in an increasing efficiency why? any ideas greatfully received

2. ### beenthere Retired Moderator

Apr 20, 2004
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Transformer efficiency is measured by comparing power in to power out. That is not measured properly with a fixed load that draws below the secondary current rating.

What factor do you think might have influenced the drop in current through the input winding? How might this be expected to influence power in the transformer?

3. ### tfxuk7 Thread Starter New Member

Dec 10, 2009
3
0
I presume that the increasing frequency response within the transformer is increasing impedance in the primary coil and this reduces input I. this will increase impedance in the secondary coil too but the output I remains relitively constant (when using multi sim it does not change at all).

Quote
'Transformer efficiency is measured by comparing power in to power out. That is not measured properly with a fixed load that draws below the secondary current rating'.

are these statements true?
the I out remains the same, is this indicative of the draw I of the 10kohm fixed load?
the transformer will not operate efficiently until it just below saturation levels?
is the decreasing Input I with increasing F as a result of increing Z within the primary coil?

I'm not sure as to why this will effect the power in the transformer.

4. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Of course power will be affected by the increase in impedance decreasing the current in the primary. P = I * E. Your driving voltage remains constant, so power decreases as current. Power out of the secondary cannot equal, much less exceed, that in the primary.

5. ### tfxuk7 Thread Starter New Member

Dec 10, 2009
3
0
thanks for your continued replies, I am quite new to transformers and electrical principles in general. However i am still failing to grasp the principles at work here. i was under the impression that in a transformer experiement with increasing frequency this would have an increasing hysteresis effect in the core, plus other factors like magnetostriction and heating (although i would think negligible at such a low test I and V) reducing the efficiency of the transformer with increasing frequency. but by using the below saturation fixed load are you suggesting that the effects at work here are due to increased impedance within the primary coils. thus preventing amp entry into the primary and hence 'artificially' or temp improving effficiency rather than decreasing it.
Quote
'Power out of the secondary cannot equal, much less exceed, that in the primary.'

so as the frequency is increasing thus decreasing the I in, this will reach a point where it will reduce the secondary load draw if the frequency is raised high enough?

got a presentation tomorrow so could do with some definitive answers lol thanks again

6. ### beenthere Retired Moderator

Apr 20, 2004
15,808
295
Look at what affects the size of Xl, and what that might have to do with the effect you noted. Power is voltage * current. You have decreased input power, so you will have also decreased the output power.