[ARC5] 28 volt high-current supplies?

Sat Feb 26 20:01:01 EST 2022

Thanks for the suggestions. I have a couple of those transformers but 
not big enough... I am currently using a 1 kva transformer with taps 210 
through 250 vac and a nominal 18 volt secondary at 60 amps. Although our 
line voltage (unloaded) is closer to 250, with the 210 tap connected I 
have 28.5 dc with no load. I figured that 50-60 amps DC would be enough 
at least for a half-load test, but as you see, I am having problems.

I hate to be negative, but I see a few issues with your ideas:

Even a pair of 1 kva transformers is only about 60% the size it needs to 
be if I want to load this inverter to its capacity, which requires 115 
amps at 28 dc. The math is left as an exercise :)

Another problem with your doubler suggestion (besides the price of BFC's 
in that size!) is that 30-40% of the power would be dissipated as heat  
in addition to the 55% full load efficiency of the inverter itself (ok 
in the winter, not so much in summer), and would have to be drawn from 
the line. To get 3 KW dc I'd need a 30 amp 240 volt outlet, and more 
iron than two 1 kva cores too.

As I'm discovering, making dc supplies in the kw range, even linear 
ones, is not a trivial exercise.

...

Meanwhile, the plot thickens!
I shortened the 8-ga wires (from the transformer to the bridge) from 2 
feet to one foot each, which helped a little.

I also discovered why there wasn't any 400 Hz output. The transfer relay 
(24-28v coil) was pulling in at first, but with the inrush (I measured 
220 amps  ACat the transformer secondary, with an Amprobe clamp meter) 
pulling down the dc, it would drop out, and with the voltage not coming 
back up enough to pull it in, the contacts were open. Sigh.

So, actuating the relays via the Switch On terminal from an external 
power supply, then I had 400 Hz on both A and C phases. My Triplett 
630-A showed 185 volts ac when it stopped rising. At first I thought it 
may actually be 200 (as in 115/200 three phase) by the time I get it 
spinning at full speed with 24 volts. Or maybe the regulators aren't 
regulating when unloaded, since A is allegedly 115 volt single phase, 
not 200... but that's for another day once I have enough power supply.

But even unloaded, the input current never dropped below 110 amps AC 
(again, measured at the transformer secondary) although I only had 16 
volts on a panel meter at the inverter terminals, and with lots of ripple.

Will the motor current drop when the voltage comes up and it spins 
faster? I don't remember all the details of compound-wound DC motors...

On 2/26/22 18:18, millerke6f at aol.com wrote:
> Hi
>
> Might I suggest that using magnetics retrieved from medium to large 
> sized computer UPS unit might be another source of high current DC for 
> the large loads.  Check with you local E waste folks and see what's in 
> their bins.  I've found nice units in the 1KW + core range with 
> nominal 16 vac secondary ratings with tapped primary taps that will 
> allow for about a ten percent increase in the secondary voltage.  
> Couple a pair of these with a pair of 100 amp bridge rectifiers from 
> the Bay and some huge used pooter grade caps and you have all the 
> makings for a really stiff DC supply.  And if the load is within the 
> 1KW rating of the magnetic's rating, use one unit and a pair of pooter 
> grade caps in a voltage doubler using part of the 100 amp bridge 
> rectifier and you will have a nice brute force DC supply in the 45 
> volt no load region.  A string of 2n3072s and a 723 will take care of 
> that excess or string up a bunch of variable LDO  three terminal 7 amp 
> units on a chunk of heat sink as they are cheap too and being tab 
> mounted easy to mount and wire up.
>
> Fud Fer Thot
>
> Bob, KE6F
>
> -----Original Message-----
> From: J Mcvey via ARC5 <arc5 at mailman.qth.net>
> To: Christopher Bowne <aj1g at sbcglobal.net>; ARC-5 
> <arc5 at mailman.qth.net>; Charles <charlesmorris800 at centurytel.net>
> Sent: Sat, Feb 26, 2022 1:32 pm
> Subject: Re: [ARC5] 28 volt high-current supplies?
>
> Did you try measuring the start and run current that you are drawing?
>
>
> On Saturday, February 26, 2022, 04:24:14 PM EST, Charles via ARC5 
> <arc5 at mailman.qth.net> wrote:
>
>
> It's not the capacitor on phase A. Oh pooh. Now I have to look for 
> zebras rather than horses...
> Yes, I am aware of the basic motor physics ;) That "about a second" is 
> with a stiff supply. Mine is apparently not stiff enough <that's what 
> she said> :D
> My original plan was to acquire a 618T with the 400 Hz three-phase 
> supply - usually much cheaper than the 28 vdc or the 400 Hz single 
> phase versions. But this inverter is becoming a real white elephant. 
> It would be running all the time, not PTT like the ART-13.
> Anyway I am tempted to go buy a pair of car batteries and find out, 
> just because I'm stubborn that way! (My two "ARC-5" receivers have 
> their dynamotors, but I only run one at a time, and the 28v supply I 
> built with a handy Stancor RT-204 transformer has no trouble).
> Charles, WB3JOK/0 since '76
>
> On 2/26/22 15:13, Christopher Bowne wrote:
>> No, just a an Ames (Harbor Freight) 80 buck multimeter with a clamp 
>> on AC or DC ammeter.
>>
>> Peak current is always at initial 0 RPM, as the motor winds up the 
>> counter EMF brings the current down in about a second. Basic
>> DC motor physics.  Considering the instrument pedigree I take no 
>> responsibility for the accuracy of the reported result.
>>
>> As an alternative to floating batteries on the output of a DC supply, 
>> one could implement a step starter circuit, but you would need
>> some hefty dropping resistors and contactor relays.  Wouldn’t lend 
>> itself to quick PTT operation on voice.
>>
>> Chris AJ1G
>> Stonington CT
>>
>> Sent from my iPhone
>>
>>> On Feb 26, 2022, at 15:38, Charles <charlesmorris800 at centurytel.net> 
>>> <mailto:charlesmorris800 at centurytel.net> wrote:
>>>
>>> Thanks, there's a data point of 5 times the nameplate.. What was 
>>> the decay time of that 160 amp peak (did you use a scope)?
>>>
>>> If I HAD a couple of spare car batteries I'd certainly try them ;) 
>>> but have you seen the prices lately? they start at $55 plus core 
>>> plus tax at Wally World...
>>>
>>>
>>> On 2/26/22 13:37, Christopher Bowne wrote:
>>>> I’ve measured the peak inrush current on my ART-13’s DY-17A 
>>>> dynamotor,  it was a nominal 160 amps.  Just float a couple of 12 
>>>> batteries across the output of your supply, make sure that they are 
>>>> fully charged and full sized automotive or deep cycle marine type, 
>>>> small one may not deliver enough peak current to preclude tripping 
>>>> the output breaker in on your high current supply.
>>>>
>>>> Chris AJ1G
>>>> Stonington CT
>>>>
>>>> Sent from my iPhone
>>>>
>>>>> On Feb 26, 2022, at 13:57, Charles via ARC5 <arc5 at mailman.qth.net> 
>>>>> <mailto:arc5 at mailman.qth.net> wrote:
>>>>>
>>>>> I am sure I would have no trouble starting a DY-17 with a 33 amp 
>>>>> full load rating. This beast is 3.5 times that full load draw and 
>>>>> consequently any surge current would likely also be 3.5 times that...
>>>>>
>>>>> Anyway I am going to investigate shorts on the AC side, and my 
>>>>> prime suspect is the (noise reduction?) capacitor on A phase. Hope 
>>>>> it's not the armature. It does have a repair request tag from the 
>>>>> 80's or 90's (don't remember at moment) and an unreadable 
>>>>> "Serviceable" tag. The electromechanical regulator (carbon piles!) 
>>>>> will get a look after I check the cap. Something else I have no 
>>>>> experience with.
>>>>>
>>>>>
>>>>>
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