[Amps] Re: Aluminum plate trick?

[Jeffrey Madore]

----- Original Message -----
From: "BEAR" <bearlabs at netzero.net>
To: <amps at contesting.com>
Sent: Saturday, June 28, 2003 3:59 PM
Subject: [Amps] Re: Aluminum plate trick?


>
>     IF it's the aluminum doing something that steel wouldn't do it is
> causing
> eddy currents to flow through it, shunting the magnetic field slightly...
> perhaps this is causing a change in the lams slapping themselves, or the
> lams against the chassis behaving like an electromagnet (buzzer).
>
>     Usually, the lift from the chassis and isolate works well enough.
>
>     I'm thinking that the eddy current thing might have merit, since
> there seems to be some effect upon audio output trannies when they
> are lifted off the chassis (no coupling).
>

I believe it does, Bear.

I'm thinking out loud, theorizing, pulling this out of the ether, so I
welcome correction, as always:

We can assume that the additional transformer / chassis magnetic interaction
is due to the higher primary current at 50Hz, due to lower XL.

Aluminum has much higher magnetic reluctance than steel and, therefore,
shields the chassis (somewhat...it's thin) from the effect of the primary
magnetic field.

However, as you mention, the eddy current in the aluminum is going to be
much higher than the eddy current in the steel, as the electrical resistance
of aluminum is much lower than that of steel. The voltages caused by this
current are so low, that the oxide insulates it from the steel, just as the
varnish insulates the transformer laminates from each other.

This current flowing in the aluminum now produces a small counter EMF, as
the magnetic field produced by this current is 180 degrees out of phase with
the primary's magnetic field.

This CEMF is tightly coupled to the steel chassis and, therefore, bucks or
cancels the effect of the primary's magnetic field on the steel chassis.

That's my theory...shoot...

Bear, what effect do you notice with audio transformers? For the better, or
worse?  I wonder if the magnitude of this effect is different on an aluminum
chassis vs. steel?  Do you do any work with pro audio?

Thanks es 73,
Jeff - K1LE -CT ><>






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