# Transformer primary

Discussion in 'Electronic Basics' started by Jamie, Jun 13, 2006.

1. ### JamieGuest

Another transformer question:

What prevents current overload on a primary side of a transformer
when there is no current flow in the secondary windings? As I understand
it, the current flow in the secondary opposes a flow of current in the
primary. If there is no load in the secondary, isn't there basically an
open circuit in the primary, or does the reactance of the primary prevent
current flow until a secondary field exists in the secondary windings?

The Moon is Waning Gibbous (98% of Full)

2. ### Pooh BearGuest

Yes. See magnetising inductance.

Graham

3. ### James ThompsonGuest

The term there is counter electromotive force. the magnetic field generated
from the voltage applied when reversed, counters the flow of current on the
ac swing. Balances out till some of the flux field is used by the secondary

How well the transformer core is made determines how much magnetic flux is
lost to ( eddy currents ) generated in the core material. Which ends up
being heat. If you are going to try and build your own, get a good core.
Thinner the slices on the core, will have fewer eddy currents, as in the
core of a microwave transformer has fairly thick slices and gets hotter from
my exp. with them.

4. ### Jasen BettsGuest

Nope, (when dealing with AC) current flow int the primary opposes current
flow in the primary, allowing a current in the secondary counteracts
some of that and allows more in the primary.

Bye.
Jasen

5. ### John PopelishGuest

When the secondary has no load, the primary winding acts like a very
large value inductor. It draws a small (relative to full load)
inductive (current delayed almost 90 degrees relative to the applied
voltage) current.
That is roughly backward. load current through the secondary
increases the current through the primary.
Not an open circuit, but a high inductive impedance.
Here are the basic relationships.

1. The flux in the core is proportional to the total, instantaneous
amperes wrapped around it (amperes times turns)

2. The voltage produced across each turn is proportional to the rate
of change of the flux through the core.

A load current through the secondary does not oppose the primary
current, it opposes the core flux generated by the primary (cancels
some of the flux created by the primary current). The reduced rate of
change of flux generates slightly less voltage across the primary
windings, which holds back the primary current less, so the primary
current rises.

But even with no secondary current, there must be a little primary
current, because something must generate the core flux, so there can
be a rate of change of that flux to generate the voltage that bucks
most of the primary voltage, to keep the primary from looking like the
resistance of a piece of wire across the applied voltage.

6. ### JamieGuest

(Yes).
you don't have a complete open circuit
but what ever the inductance is in the primary
windings.
placing a load on the secondary side will cause
load effects on the primary side..
one neat thing i did years ago was to use a
primary side of a xformer as part of a low freq
oscillator and cause load effects on the secondary
side to change the inductive value of the primary
thus change the frequency of the oscillator.