[Amps] current in primary of microwave oven transformer

[Manfred Mornhinweg]

Dear all,

it seems that a little basic information about transformers could be 
good here.

> I have also seen these microwave transformers draw a lot of primary 
> current with no load connected to the secondary, a lot more, in the 
> order of 3 to 5 amps..

Yes, microwave oven transformers will indeed typically take more primary 
current at no load, than some other transformers. The reason is that 
they are designed for very short duty cycles. Below is more insight.

When designing a transformer, one of the basic decisions one has to make 
is how much flux density to put into the core. There are clear 
tradeoffs. A higher flux density is obtained with proportionally fewer 
turns. So, designing for higher flux density will make the copper loss 
squarely lower, because the wire is both shorter and thicker, assuming a 
given core size. At the same time, iron loss will be higher, also 
roughly to the square of flux density. So, by choosing flux density one 
can trade iron loss for copper loss. In addition, fewer turns result in 
much lower inductance, and thus much increased magnetizing current (the 
one that is 90 degrees off-phase and thus means no power consumption).

Now the point is that iron loss is present all the time, regardless of 
load level. Copper loss instead is linked to the square of the load 
current. So, when a transformer is idling, iron loss is important and 
copper loss is not, and while a transformer is delivering a high 
current, both are important but the rapidly increasing copper loss can 
easily dominate.

For this reason, a transformer designed to be be plugged in for long 
times but actually delivering current only for short times, such as that 
for an ICAS amplifier, should be designed with a relatively low flux 
density. Instead, a transformer that will deliver full load whenever it 
is energized, such as a microwave oven transformer, is better off using 
a high flux density. So, a designer, wishing to minimize the size of his 
microwave oven transformer, will push for a very high flux density, 
because the high iron loss and also the resulting high magnetization 
current are perfectly acceptable in view of the much lower copper loss. 
Instead, someone making a linear amplifier needs to care a lot for the 
no-load dissipation, because after all the amplifier will be idling for 
easily 90% of the total on time! So, for the amp it's acceptable to have 
high copper loss during the brief moments of full power operation, in 
exchange for very low losses during the long periods of standby, or 
lower levels in the modulation curve.

Some microwave oven transformers might overheat even with zero load, 
when plugged in for hours. Others will not overheat from this, but in 
any case will get pretty hot. A transformer designed for a linear 
amplifier instead should get barely warm while idling. On the other 
hand, the voltage regulation of a microwave oven transformer will be 
better than that of a similarly sized transformer designed for an amp, 
due to the lower wire resistance.

My advice: If you like to build amps and have a cheap source of 
microwave oven transformers, by all means try them, but whenever 
acceptable arrange the circuit to power down the transformer during RX!

Manfred.

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