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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 6:24 pm 
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Posts: 528
Point taken, but here is why my WWII dynamotor application may be a possible fit. Please tell me what you think:

I built a 28V supply for my 28V dynamotor supplied transmitter. The in-rush current is close to 70A but only for a second, and then drops very rapidly to a steady 2A load. The large in-rush current is due to the inductive load of the motor part of the dynamotor as well as overcoming mechanical inertia. (I had to use supercapacitors to supply the inital crank and then a relay cuts them out of the supply line) So, I'm thinking that the voltage sag would only be for the initial startup of the dynamotor, then during actual operation of the transmitter, a 2A load would not result in a marked voltage sag on the part of the welding transformer.

(By the way, the reason I ask these questions is because I basically only use repurposed components for my projects. It's part of the challenge that I impose on my design because I like to restore, reuse and keep cost down. I also take pride in innovating around such constraints. All the other posts in response to my original question are definitely being used in solving my original project requirements - which I am making more specific in scope.)

Thanks to you all once again.


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 8:00 pm 
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Joined: Nov Wed 08, 2017 11:52 pm
Posts: 65
Location: Arizona
Steffen wrote:
Oh, that makes perfect sense - there is a lot of welding done in that complex of factories. Plus, that would explain better than anything the wire gauge. However, why would you say to forget about it? I would think that this would be a GREAT high current transformer for powering my 12V dynamotors for my WWII equipment, for example. (Most of my dynamotors are 28V based, but I have 12V versions too.)

OK, now I understand what you are trying to do. I had one of theses BC348 receivers with a dynomotor some 50 years ago. That is how during WWII they turned common AC military equipment as "mobile" :) devices, powered by 24V car batteries and a dynamotor. Car batteries didn't have any problems starting these dynomotors. Today, I wouldn't try to power these dynomotor using AC power converted to DC and then have the dynomotor convert the DC back to AC. Just power the damn thing straight from AC. If all you needs is 2A @ 24V then it is only 50W.

I am not sure, but you may be able to start these dynomotors with a 24v car battery charger, that would be interesting to find out. Anyway, these welding transformers are not designed to provide a constant voltage under heavy load, beside powering it with 115V AC according to your calculation you will get only 7.5V open circuit.


Last edited by upsss on May Mon 21, 2018 8:54 am, edited 1 time in total.

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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 8:24 pm 
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Joined: Feb Sat 11, 2012 3:07 pm
Posts: 528
I do power some of my military receivers with an AC power supply, but in the case where I wanted to keep the dynamotor functional, I think your idea about the battery charger is a great path to explore. I never thought of that before. It does sound like an incredible waste of power and effort to design a DC --> AC --> DC configuration. Plus, I would need 220V input with this transformer. I just wanted to use some of my equipment with the actual dynamotor. I will look into the battery charger or two car batteries in series with a charger in between uses. Just for my own curiosity, I am going to perform some experiments loading the welding transformer and plot voltage against in-rush current, short-term and sustained loads. (I bet it shares some characteristics with a swinging choke. I also bet it is designed that way in order to avoid a catastrophic failure in case the welding rod becomes welded to the other pole of the welding circuit - the voltage will quickly sag to a point where current draw will be far less and not overload the transformer windings.) Thanks again!


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 Post subject: Re: General Power Supply Regulation Question
PostPosted: May Sun 20, 2018 9:32 pm 
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Joined: Nov Wed 08, 2017 11:52 pm
Posts: 65
Location: Arizona
Most of the car battery charges have a "starting" position so even a standard 10A charger can provide very high peak current for a couple of seconds. If you cannot find a 24V charger or it will not start these dynomotors, you can try two 12V chargers in series which should work better than a single 24V charger and I bet they will start these dynomotors. If you use two 12V car chargers in series make sure that the negative terminal is NOT connected to the chassis which may be also connected to the AC ground and that will short circuit one of the chargers.


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