Re: [CAD_CAM_EDM_DRO] transformer question

----- Original Message -----
From: "Jon Elson" <elson@...>
To: <CAD_CAM_EDM_DRO@yahoogroups.com>
Sent: Saturday, May 06, 2006 8:55 PM
Subject: Re: [CAD_CAM_EDM_DRO] transformer question


> turbulatordude wrote:
>
>>Hi,
>>
>>I have a transformer with a lable of 220/460 primariy and 110/220
>>secondary.
>>
>>1kva.
>>
>>Would I be able to connect it 110 to get 55 volt ?
>>
> Yup, that's right, you'll get 55 VAC from each secondary winding.
> Note than 55 V AC into a rectifier will give 78 V on the capacitor.
>
>> and if so, any idea
>>what current I could pull out of it ?
>>
>>
> Best is to put the secondary in series and use a pair of big diodes (or
> half of a bridge rectifier) with the center tap of the windings
> connected to the
> - of the capacitor. OK, starting with the KVA rating at the design
> voltage of
> 110 V, the current would be I = (KVA * 1000)/V. So, a 1 KVA transformer
> would provide 1000/110 = 9 A. This may be a little optimistic for
> continuous
> duty at full load, but will certainly be fine for most CNC applications.
>
> Jon

Using the transformer in this way will work but it will have double
the heating. The rated current for the output of this 1KVA transformer is
8.33 amps when wired for 120 volts. Therefore each secondary winding
is rated for 4.17 amps.

When wired as center tap full wave rectifier you are doubling the rated
current in each winding for half the time. Thus you are drawing 8.33
amps from the 4.17 amp rated winding but for only half the time. This
will work but you are doubling the IR loss. Just for argument,
lets say that you drop 1 volt in the winding at 8.33 amps. That is 8.33
watts loss. Each winding's loss will be 4.17 watts but there are two
of them so the total is 8.33 watts.

The alternative connection is to parallel the windings and use a full wave
bridge rectifier. Your voltage drop would be .5 volts and at 8.33 amps
your loss would be 4.17 watts in this example.

That type of center tapped rectifier was used back when rectifiers were
expensive and transformers were cheap. Now a full wave bridge rectifier
capable of this power load is common and cheap.

Pete.