[ARC5] Dynamotor Capacitor

[AKLDGUY .]

> That only applies if the dyno is supplied by a source w/ considerable
> inductance, like a generator or a long length of wire.
>
> If the dyno is w/in a foot or two of a car batter or other low impedance
> supply, you don't see big spikes.

I'm sorry I don't buy what you're saying.
If you think about the two brushes, one will always break contact before
the other, and at that instant there will be a large back emf developed
between that contact and the other brush that is still yet to break contact.

The polarity of the emf at the o/c contact is the opposite of the battery
polarity at that brush (that's why it's called a back emf), and this emf
will therefore ADD to the battery voltage through the other (still
contacting) brush. The battery will therefore play no part in suppressing
the back emf, which it would do if the polarities were the same.

There will therefore be an attempt to spark from the open circuited contact
to the brush that just left it. This is most undesirable, on account of RF
hash and brush wear.

Far from suppressing the spikes, the battery actually makes them
fractionally worse by adding 28V to the several hundred volts of back emf.

73 de Neil ZL1ANM


On Tue, Mar 25, 2014 at 3:15 PM, J. Forster <jfor at quikus.com> wrote:

> That only applies if the dyno is supplied by a source w/ considerable
> inductance, like a generator or a long length of wire.
>
> If the dyno is w/in a foot or two of a car batter or other low impedance
> supply, you don't see big spikes.
>
> -John
>
> ==================
>
>
>
> > The back EMF spike at every break of the brushes can have extremely high
> > voltage spikes. 73 - Mike
> >
> > Mike B. Feher, N4FS
> > 89 Arnold Blvd.
> > Howell, NJ, 07731
> > 732-886-5960
> >
> >
> > -----Original Message-----
> > From: arc5-bounces at mailman.qth.net [mailto:arc5-bounces at mailman.qth.net]
> > On
> > Behalf Of Mark K3MSB
> > Sent: Monday, March 24, 2014 9:35 PM
> > To: ARC5
> > Subject: [ARC5] Dynamotor Capacitor
> >
> > I just finished a complete rebuild of the dynamotor for my BC-456.
> >
> > I'm curious why the  0.006 uF shunt capacitor across the 28V side is a
> 2KV
> > cap.  That seems awfully high.
> >
> > 73 Mark K3MSB
> >
> > Sent from my Android phone
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