[ARC5] Power up Modulators

[WA5CAB at cs.com]

Frank,

The motor side of a 12/14 or 24/28 VDC dynamotor or rotary inverter is a 
series or compound wound motor.  The starting current is typically between 2X and 
3X of the rated FLA (Full Load Amps) for several tens of milliseconds.  The 
power source has to be able to (a) supply this much surge current and (b) hold 
the output voltage up at or very near the required output voltage while doing 
so.

For example, the rated FLA of the DY-17A/ART-13 is 33 amperes.  To start it 
takes typically 75-80 amps plus the relay current in the dynamotor and the tube 
heater load (which also starts high and then drops) in the T-47.  If you have 
a "modern" current limited 28 VDC supply rated at 50 amps, it almost 
certainly will not start a DY-17 by itself.  It will go into current foldback.  If 
you're lucky, it will stay there until you turn it off.  If not, it will turn the 
dynamotor start relay into a vibrator.

The three ways around this are (a) DC supply with adequate surge rating 
(PP-1104/G for example), (b) battery bank correctly wired to supply starting 
current and then drop out taking a slow charge from the power supply, and (c) 
capacitor bank with charge current limit so that it does not do the same thing as 
the dynamotor to the power supply.  Mike H. and I wrote a piece in ER maybe ten 
years ago with wiring details.

In a message dated 11/4/2005 9:26:52 AM Central Standard Time, 
fkamp at comcast.net writes: 
> >>Use enough power supply to start the TX dynamotor without
> >>grunting- 30 amps minimum, and that's if you have
> >>some big "computer" capacitors on the output to
> >>supply the starting surge, like a few hundred
> >>microFarads.
> >
> >
> >That should read "a few hundred THOUSAND microFarads."
> >
> >If you want to really do it right, WallyWorld sells,
> >in their automotive audio section, some humongous
> >1 Farad caps, good for about 20 volts.
> >Two of them (for series connection)
> >will set you back $150, but you'll never have to
> >worry about starting a dynamotor again.
> >You'll also have to figure a way to "soft start"
> >them until charged- big resistor should do.
> >
> 
> 
> "a few hundred THOUSAND microFarads." !!!????
> 
> I guess I dont appreciate the problem.  Perhaps, if someone 
> were to educate me on the need for this, I could be more 
> appreciative.
> 
> The easiest way of attaining 'a few hundred THOUSAND 
> microFarad' capability is to connect the dynamotor across a 
> hot automobile battery.  500 plus cranking amps is going 
> make that sucker spin like a top.
> 

Robert Downs - Houston
<http://www.wa5cab.com> (Web Store)
MVPA 9480
<wa5cab at cs.com> (Primary email)
<wa5cab at houston.rr.com> (Backup email)