[SEL] Generator hookup

[Listerdiesel]

On 5/11/05, Richard Strobel <Richard_Strobel7 at msn.com> wrote:
> I should have stated that differently, Curt.
> 
>  Could one take the output of this 250VDC generator which produces
> pulsating D.C. and hook it into a step-down transformer to a more suitable
> voltage?
> 
>  Or would I just let the smoke out as I believe they're frequency
> sensitive?
> 
> later,
> Rick

Nooooooooo.....

It may be pulsating DC whatever that is, but it is NOT pole-reversing
AC which is what you need to run into a transformer.

What you get from a DC genny is not AC in any sense of the word, so
don't be confused by descriptions. DC has a + and a - which are fixed
according to the output leads. AC reverses polarity at 50Hz in Europe
and 60Hz in the USA.

Going back to your question re setting the exitation down to get a
lower voltage:

The field coils are rated to provide a decent magnetic field for the
rated speed and output power of the generator. While you could drop
the excitation and reduce the voltage, there are other factors
involved.

The magnetic gap between the armature and the field coil poles is
calculated to give magnetic transference at a certain flux density of
the field. If that density is reduced then the gap should in theory be
reduced as well. In fact there is a law of diminishing returns here,
and the generator will not work properly or even may not control at
all once the field input is reduced beyond a certain point.

The armature windings are rated for the current of 250V 16A, if you
reduce to 110V you can still only pull 16A through those coils, you
can't run the field current up to get double the current as the
windings won't stand it. Ditto with the brushes.

This relates to modern stuff as well:  We took a 120kW 110V DC dynamo
to India for the Ghandi movie, but it was an 1800rpm machine. The
other end of the genny had a 50kW alternator, which the movie lighting
people wanted to run at 50Hz because of the HMI lights strobing with
the camera shutter, so at 50Hz or 1500rpm we had about 80kW instead of
120kW. The dynamo makers (Mawdesleys) said to close up the field pole
gaps and all would be OK, but we hadn't the time or the facilities to
do a major soundproof genny strip down.

Peter
-- 
Peter A Forbes
Email: listerdiesel at gmail.com
Web: www.oldengine.org/members/diesel