...guys this is normal behavior for all transformers!
It is the result of the existence of a magnetizing inductance, usually
depicted as a parallel inductance across the primary and every transformer
has it. Don't worry about it: it's something physical and cannot be avoided!
There are higher power transformers in which this current can reach in the
amperes range, but it's reactive current, except for a few watts loss in the
ohmic component: that the reason it doesn't show up as heat. These
transformers are usually good for about 750 watts continuous, 1000 watts
ICAS.
Alex 4Z5KS
-----Original Message-----
From: amps-bounces@contesting.com [mailto:amps-bounces@contesting.com] On
Behalf Of Dr. David Kirkby
Sent: Wednesday, April 23, 2008 12:08 PM
To: Angel Vilaseca; 'AMPS'
Subject: Re: [Amps] current in primary of microwave oven transformer
Angel Vilaseca wrote:
> Hi,
>
> I have a microwave oven transformer that I plan to use for my next HV
> power supply.
>
> With the secondary unloaded and 220 V into the primary, I measured 670
> mA through it.
>
> This is a 220 V unit so that means 147 Watts, with no load on the
> secondary. Magnetic shunts were taken off.
>
> I have another two MOTs. Made the same measurement. They all behave the
> same.
>
> On the other hand, the transformers do not hum, not smoke, not even get
> hot, so I was wondering if this primary current could be "wattless"
> current, that is a current that is out of phase with voltage.
>
>
> Should I worry about this amount of "idle current"?
>
> Is this behavior particular to microwave oven transformers?
>
> Is there a way to know what is happening here without using a double
> trace oscilloscope?
>
>
> Vy 73
>
> Angel Vilaseca HB9SLV
>
>
The power dissipated is V*I*Cos(phi) where phi is the phase angle
between the voltage and current. (Assuming they are pure sine waves of
course, which is not necessary so.)
There are ICs around which measure phase, but a scope is the easiest way.
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