[ARC5] Dynamotor Regulation

[Mark K3MSB]

Just a quick reply.....with a subject in the email line...

I'm just asking if I need to worry about regulation.

Planning a PS with a cap input and pass transistor.

Back to work....not as much fun as radios but it pays better....  Will
catch up on the responses in a few hours...

Mark K3MSB
---------- Forwarded message ----------
From: "Scott Robinson" <spr at earthlink.net>
Date: Jul 28, 2017 11:31 AM
Subject: Re: [ARC5] (no subject)
To: "Mark K3MSB" <mark.k3msb at gmail.com>, "ARC5" <arc5 at mailman.qth.net>
Cc:

Hi Mark,

First off, if you use a capacitor input filter on the DC supply, you'll get
about 40V output (29 x 1.414) at no load, nd s bit less as teh load
increases. I f you have a great big choke, you  can get about 25V from a
choke input filter. The choke will need to have an inductance of at least
1.1 mH per ohm of load resistance. Fir instance, for a 14 ampere drain,
you'd need 2.2 mH.

If no choke, you'll have to reduce the voltage some other way, and a *big*
regulator is one way to do that...or turn the Variac down. Do be aware that
the current rating of a Variac is for *RMS* current, and that capacitor
input filters generate an RMS input current between 2 and three times the
DC value. For a 14A 28V load, the secondary RMS load will be between 28 and
42A, and the primary current between 6.75 and 10A. The 5V variac you may
have will not like this...but you can cheat with a fan.  The thermal weak
spot in a Variac is the interface between the brush and the winding, so aim
a small fan at that area and it will make a big difference.

How did I learn this? I toasted several 3A Variacs in a car battery charger
over a span of time; the 5A one has lasted nicely.

Other possibility: find a 120V to 24V transformer of a suitable current
rating and wire it as a bucking auto-transformer. Th will give you a no
load voltage of about 32V.

Fo course, if your dynamotor drain is other than 14A, scale appropriately.

For filter caps, you'll need a ripple current rating to match the RMS
secondary currents given  above. The big old ones with screw terminals that
everyone seems to have lying around are a good bet here. This rating is
thermal, so for intermittent use--you probably dont transmit for more than
a minute of three at a time) you can push this.

Also, use  *really* big diodes, certainly 70A or better rating. Doesn't
cost much, and they won't break.

Enjoy!

/scott robinson

On 7/28/17 7:08 AM, Mark K3MSB wrote:

> Good morning gang...
>
> My 28V power supply for my SCR-274N system isn’t quite stout enough so
> I’ve been using a standalone HV supply in place of the BC-456 dynamotor.
>  I’m planning on getting the 274 on AM and would like to use the dynamotor
> (which I’ve rebuilt and runs fine).
>
>   I have a 29-0-29 50A transformer in the pile that I’m planning on using
> for a new supply (variac controlled) to power the dynamotor.
>
>   Does a dynamotor need a regulated input?   I recall reading that if the
> supply for a dynamotor is stout enough a regulated input isn’t needed,  but
> I can no longer find the reference.
>
> Thanks!
>
> 73 Mark K3MSB
>
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20170728/87bccd50/attachment.html>