Hi
Are you really running a cap input? In a lot of cases, you might
not be.
It depends very much on the size of the cap. If you are getting a
voltage boost ( due to the cap charging ), you don’t get as much
current. Power out of the secondary and power into the load
need to add up ( … no free lunch). Cap not big enough to count,
not much voltage boost, not much current penalty. No, that’s not
quite the whole story, but it’s a part of it.
Unless you have an absolutely giant cap, you likely are running
effectively a “no cap / no choke” circuit when driving it into your
motor at start up. You get a bunch of ripple in the output. The motor
pretty much does not care … it starts turning. Since the diodes in
the bridge are happier running this way (larger conduction angle),
it does have other advantages.
Further complicating things, transformers are generally rated on
some sort of continuous duty basis. It’s a good bet that they will
put out a bit more for a short while. Motor startup inrush current
is very much in the “short term” range. Will it do 20% more or 2X
more? Depends a lot on how long (seconds vs minutes vs hours)
and the voltage drop you can tolerate ( is a 20% drop ok? ). Hams
have been “abusing” transformer ratings this way pretty much
forever and ever.
Lots of variables ….
Bob
> On Sep 5, 2022, at 10:04 AM, Charles via ARC5 <
arc5@mailman.qth.net> wrote:
>
> It is also necessary to remember that the transformer rating does not give DC ampere output in a 1:1 ratio unless you have a choke input filter. From the Signal Transformer Catalog 892340 (available for download online), on page 38:
>
>> The secondary currents shown in the tables are RMS ratings. Depending upon rectifier circuit configurations, the RMS secondary current is different from the DC output current. This is indicated in the chart below:
>>
>> Full-Wave Center-Tap Choke Input = RMS Secondary Current is 0.7 x DC Amps
>>
>> Full-Wave Center-Tap Capacitor Input = 1 to 1.2 x DC Amps
>>
>> Full-Wave Bridge Choke Input = DC Amps
>>
>> Full-Wave Bridge Capacitor Input = 1.6 to 1.8 x DC Amps
>>
>> For example, in a F.W. Bridge circuit with a capacitive filter, if the load is 1 Amp DC, the RMS Secondary current is 1.6 to 1.8 Amp RMS.
>>
>
> So I needed a 250 amp (5 KVA 20 volt) transformer to deliver 150 DC amps... and at that current level, the bridge rectifier repetitive forward rating is non-trivial, as is the heat generated. I used a big heat sink and a muffin fan.
>
> -Charles,WB3JOK/0
>
>
> On 9/5/22 19:31,
arc5-request@mailman.qth.net wrote:
>> From: Bob kb8tq<
kb8tq@n1k.org>
>> To: MICHAEL ST ANGELO<
mstangelo@comcast.net>
>> Cc: ARC-5<
arc5@mailman.qth.net>
>> Subject: Re: [ARC5] 24vdc power
>> Message-ID:<
4F446E03-9F04-4E3F-A0FC-8BE25F92746B@n1k.org>
>> Content-Type: text/plain; charset="utf-8"
>>
>> Hi
>>
>> One option is to stack some number of smaller transformers:
>>
>> This would be one candidate (based on a very quick search):
>>
>>
https://www.antekinc.com/an-10425-1000va-25v-transformer/ <
https://www.antekinc.com/an-10425-1000va-25v-transformer/>
>>
>> The same folks who made that come up with a wide variety of
>> stuff up into the 1.5 KVA range. What they have today likely
>> will not be what you find in a couple of months.
>>
>> If you bought four of the ones above, you would be up
>> around 150A into your load. Voltage would be a tad high.
>> A lower voltage / higher current version likely will pop
>> up at some point. They have had them ( = 20V versions)
>> in the past.
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