[MRCA] Dyno Starts Trip Breaker on PP-7333

[Radio Station KW1I]

Robert,

Is this the article you were thinking of?

"Running Dynamotors With Computer Grade Supplies"   ER #79   Nov. 1995 Page 
18

Dale



----- Original Message ----- 
From: <WA5CAB at cs.com>
To: <mrca at mailman.qth.net>
Sent: Monday, October 29, 2007 10:29 PM
Subject: Re: [MRCA] Dyno Starts Trip Breaker on PP-7333


> Chris,
>
> The PP-7333 is too new for me to have anything on it, even to the correct
> nomenclature.  But if it was built to run 28 VDC vehicular or aircraft 
> radio
> equipment and it is actually putting out 30 VDC, (a) it's seriously out of
> adjustment and (b) that's your problem.  The open circut voltage of a 
> fully charged
> lead-acid cell is 2.1 volts.  So ignoring nominal 6 and 12 volt systems, 
> the
> fully charged voltage of a 24 volt lead acid battery (12 cells) is 25.2 
> volts.
> The system buss voltage in 24 volt buss vehicles and aircraft is a 
> trade-off
> between how fast you recharge the batteries between engine starts and 
> battery
> life.  Depending upon contactor, the design figure varies between 26.5 and 
> 28.0
> volts.  It should never exceed 28.0 volts.
>
> This subject has come up several times recently and I wish someone would 
> come
> back with the issue numbers of ER containing an article I co-wrote years 
> ago
> so I could just refer to it.  But I can't remember.  The first thing you
> should do before hanging booster batteries (or capacitors) onto the output 
> of an AC
> operated supply is to determine whether it can survive having full output
> voltage on its output terminals when it is turned off.  Some (70's & 80's 
> vintage
> HP bench supplies, for example) can't.  It eats the pass elements and
> sometimes other components.  Since your PP-7333 has apparently already 
> survived I'll
> assume it can.  What you need to do is to drop the output voltage to the 
> point
> where the battery takes over the surge starting current before the output
> breaker trips but ideally isn't being discharged after the dynamotor 
> starts.  The
> simplest way to do that is to try one PN drop.  If that doesn't work, 
> reset
> the breaker and try two.  Etc.  Once you have it working, check and make 
> sure
> that the battery is still being charged at least during key-up periods.
>
> In a message dated 10/29/2007 7:40:26 PM Central Standard Time,
> aj1g at sbcglobal.net writes:
>> I finally got a chance to put the 60A 28VDC PP-7333 power supply I picked
>> up at Gilbert on service with my ART-13 and GRC-19.  I am floating my 
>> pair of
>> series connected 12V deep cycle batteries across the output of the power
>> supply to take up the starting surge of the dynos.  The setup works fine 
>> steady
>> state, with the PP-7333 carrying the load with no strain, but the unit's 
>> DC
>> output breaker keeps tripping on dyno startups, such as keying the mic on 
>> the
>> ART-13 in voice, even though the batteries have a full charge.  For now, 
>> I am
>> using a work around of starting the dynos directly off the batteries with 
>> the
>> DC output breaker on the power supply open then shutting it to take the
>> steady state load.
>>
>> I am thinking that the trips are being caused by the output voltage on 
>> the
>> PP being slightly on the high side relative to the batteries, which I 
>> suppose
>> could also cause them to overcharge at some point.  Since the PP voltage 
>> is
>> on the high side, there is a lot of current draw from it, and pop goes 
>> the
>> breaker!
>>
>> The previous supply I was using was designed to float a pair of batteries
>> and the paralleled load.  I would set the output voltage so that it was 
>> very
>> close to the battery voltage, hence little current draw at "idle" and 
>> most of
>> the starting surge came out of the batteries.  I never had a DC supply 
>> breaker
>> trip problem on dyno startup with that set up.
>>
>> I do not yet have a manual for the PP-7333, just a schematic from the 
>> inside
>> of the unit.  Is there a way to slightly lower the output voltage to 
>> about
>> 27.6 V (2X13.8) which is about what the fully charged batteries voltage 
>> would
>> be?  Right now, I think I'm getting, accordng to the PP's voltmeter, 
>> about
>> 30V out. There is no ammeter on the PP7333.
>>
>> If the PP output is not adjustable, maybe a high current dropping diode 
>> or
>> two in series with the output?
>>
>
> Robert Downs - Houston
> <http://www.wa5cab.com> (Web Store)
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