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Coil supply voltage questions

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Topic starter


Did a quick search of existing posts but didn't find much on my issue. A company I do contract development projects with picked up an old eddy current dyno and would like to get it set up (on the cheap) to do sustained power loading of smaller industrial engines with modern controls and capabilities. I'm looking into the requirements and plans for the project right now, and YourDyno is perhaps among our feasible options.

The absorber is an Eaton Dynamatic, coil model A-53397-0220, real vintage stuff. Rated spec is 0.87A at 220 VDC. Seems like the small dyno industry has settled on 190VDC max since this thing was cranked out, and the 0.87 is in the basement. Coil windings only have 2 accessible connections, so there aren't much for options for splitting into parallel coils at lower voltages or anything either. Haven't found absorber limits at other-than-spec-conditions yet, although my company just found some more documentation that I'll hopefully dig into soon. The absorber capacity is already a bit lower than ideal, so taking a hit on max load capacity right away due to inadequate supply voltage isn't a good start to the project. Any thoughts on that issue?

Similar story, running on 120 AC has been judged "highly desirable" for the project. There are several parts of the building where we run testing, and also outside on occasion (either for noise or cold weather reasons). The current power supply system (a huge metal enclosure that we'd really like to replace) has 2 large transformers and judging by the part numbers another engineer theorized that it steps up 120AC voltage twice in order to eventually obtain the 220 VDC. Some other budget commercial systems I've looked at offer 190VDC supply as an added-cost option, but still require a 220AC hookup. Perhaps the worst of both worlds for us there. 

And lastly, I had initially envisioned a fixed supply voltage and using PWM to control load. Supply voltage manipulation looks like the more common strategy though. I was probably envisioning the coil as a very resistive load rather than inductive. Is that a major reason for avoiding PWM?



Posted : 09/03/2021 6:28 pm
Posts: 1268
Member Admin


The most straight forward solution is solution is a step up transformer, then an SCR or PWM based motor speed controller or general power supply that can drive an inductive load.

The SCR (Thyristor) is my favorite due to its simple and sturdy design, but both are ok. The PWM is simple in principle, but you do need a passive rectifier bridge first and it is more tricky to design them so they can tolerate all failure modes (open circuit, short circuit, etc), and they also need more electrical filtering because they are inherently more noisy. Of course if you buy one off the shelf then the designers have built all this in hopefully. 

There are modules like the United Automation AFM-11 which can be suitable here. You need the SCR bridge in addition. 

Or you can buy the YourDyno power supply, but it is a bit overkill for such low power. 

Posted : 09/03/2021 10:38 pm
Posts: 2
Topic starter

Thanks for the quick reply. Looked at the data sheet for the AFM-11, and it mainly further exposes my ignorance. There are a handful of things in your post that I'll have to research before getting excited. Shortcuts on a DIY project would be great, especially in the power supply side of things, which is much more in the air than RPM/TC and analog inputs/user interface parts of the project.

I'll keep poking around the YourDyno site for more information, been pretty useful already.



Posted : 10/03/2021 6:35 pm
Jostein reacted
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