Re: MOT testing

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Tim,
       I hate to spoil the party but things get worse:

On 27 May 2002, at 14:54, Tesla list wrote:

> Original poster: "Alexander Rice by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <alex-at-rices.myip-dot-org>
> 
> >Original poster: "Tim J by way of Terry 
> Fritz <twftesla-at-qwest-dot-net>" 
> <digital_raven45-at-hotmail-dot-com>
> >
> >The best way to measure a MOT is by 
> feeding 120V or 240V (depending on MOT) 
> >into the secondary. The primary should be 
> around 5 to 6 Volts output. Then 
> >you take 120V or measured mains voltage 
> and divide by primary Volts.
> >
> >120 V
> >_____  =  20:1 transformer or 2400 VAC
> >
> >6 V
> 
> that is fine to measure turn ratio
> 
> >
> >
> >......then once your done that step use 
> the mA current setting to measure 
> >output current on the primary. This will 
> tell you what sort of drawing rate 
> >the transformer has.
> 
> Tim,
> 
> um, no actually it will just blow the 
> fuse, if a transformer transforms voltage 
> in the ratio 1:20 then it transforms 
> current in the ratio 20:1 . since you are 
> operating the device in reverse you have 
> 120/20 = 6v but at (guessing 13A 
> 'secondary' current) 13*20 = 260A which 
> will do _bad things_ to your meter! 
> 
> regards
> 
> alex

While those measurements might give you some idea of the reflected 
load current you might see in the primary of a normal transformer 
they take no account of the very high magnetizing (idling) current 
drawn by a MOT while it is doing nothing other than being connected 
to the mains.

Regards,
malcolm