Re: Efficiency of Transformer, increasing output current
- From: "Bob Eldred" <nsmontassoc@xxxxxxxxx>
- Date: Mon, 4 Apr 2005 10:08:37 -0700
<charles.macleod@xxxxxxxxx> wrote in message
news:1112615498.839444.320040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> In a standard (low voltage eg 40V)transformer - same number of input
> turns as output turns, should the effiency go up with increasing the
> output current or down?
All transformers consume magnetizing current to energize the core. Some of
this is reactive while some is resistive. The resistive portion involve
copper and core losses, mostly core when the current is low. These losses
occur whether there is a load on the transformer or not. If there is no load
on the transformer, no output, there is still magnetizing loses so the
efficiency is zero. Pout/Pin = 0/(small number) = 0. As you increase the
load, the output becomes higher and the copper losses also become higher but
the efficiency increases because there is now an output. The efficiency
continues to increase with load until it reaches some maximum where the
transformer is most efficient. Once that point is reached, the efficiency
will decrease with increasing load because heating will exacerbate copper
losses (higher resistance with temperature). Any transformer can give
several times it rated output current for short durations. Heat is what
limits it. The flux density in the core and the related magnetic losses are
NOT a function of load. They are a function of the primary voltage and not
load, by faraday's law. The secondary load current balances and is in the
opposite direction of the primary current. The flux density stays the same.
When the primary goes positive, current flows into the primary, by
convention. The same polarity winding on the secondary also goes positive
but the current flows out of that winding. The two currents times their
number of turns (amp-turns) balance, one in the other out. This is reversed
every half cycle. That balance means that there is no net flux density
caused by the load.
To sum it up, the efficiency is maximum when the load is at some rated value
usually at or near the nameplate values and is zero with no load and becomes
low again when the transformer is smoking.
Bob
.
- References:
- Efficiency of Transformer, increasing output current
- From: charles . macleod
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