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Digital Phase Shifter


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Since it has been nearly a year since this Very Fine Discussion is Verifiably Finished, Defunct, may I throw in another acronym? DPS -in this context, a digital phase shifter (although I shudder to think what the IFI crew will make of THIS one.) Myung Youn sells these through Amazon. I don't know if this is just a (oxymoron alert) NON variable VFD, or a different way of getting where a rotary will get you. 

I bought one for my surface grinder. The surface grinder may benefit from a VFD once I convert to belts, but until I understand what else is in there (it has a 120 v outlet on the back) I figure a simple phase converter would be best. Unlike  VFD's, you can just leave this hooked up and use the normal controls of the machine rather than leaving the machine on and controlling from the VFD. At least that is my understanding. 

What isn't obvious from the advertising, but is VERY obvious once you read the manual that comes with the unit, is that getting a DPS that is rated for much higher power than needed can harm the motor. My motor is 2hp. I originally ordered a 7.5 hp DPS figuring that I might need something beefier in the future. Once I read the manual (I guess something good can come from being laid up from ankle surgery. I normally would have just hooked it up and tried it.) I packed it up for return and ordered a 3 hp DPS. That is due in today. I'm still laid up, but once I try it I'll post the results.

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I've not heard about Digital Phase Shifters (or Digital Phase Converters, as some of them appear to be called), but a little quick research did turn up this interesting tidbit:

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A digital phase converter uses a rectifier and inverter to create a single voltage with power electronics, which is added to the two legs of the single-phase source to create three-phase power. Unlike a VFD, it cannot vary the frequency and motor speed. Since phase converters generate one leg that must match the voltage and frequency of the single-phase supply, it does have the advantage of a sine-wave output voltage and excellent voltage balance between the phases.

The same site also clarifies that a DPS/DPC is not the same thing as a static converter:

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in which the motor is run at less than full efficiency mainly on two of the legs of the three-phase motor. Current is sometimes injected into the third leg with a capacitor or transformer arrangements that provide imperfect phase shift. In these systems the motor must be de-rated.

This information comes from the website of a company called "Marshall Wolf Automation"; I will not link to it directly, as it is a commercial site. I haven't yet found a technically informative website that isn't actually trying to sell something, but I'm sure there's more out there.
 

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John - Thanks for the info. It makes sense, and yet I'm still not clear on why having an over-rated DPS/DPC would cause a problem.

Just to further my discombobulation, although I'm forbidden to work in my shop until I get my stitches out, I did sneak in and finally figured out how to open the electrical box. I found another plate in there that says max load 7.5 hp! That is just the size DPS/DPC I sent back! Wiring on this sucker looks like #2  but I can't find anything on the jackets. I opened the back panel and although I haven't cleaned up the motor yet, it doesn't look even as big as the one on my Reed belt grinder. I have a suspicion that the interior rating plate is for a relay and is probably standard for heavy three phase equipment and the wiring is selected for the relay rating, not the grinder motor. In other words, this wiring is serious overkill.

However, there is another system in this grinder called a "one shot oiler". It has an oil reservoir and I see tubes going to gear mesh locations. My DPS/DPC says that I should use on one motor at a time although it is OK to use with two motors simultaneously.

Once my stitches are out I'm going to just hook it up and see what happens. If I burn up something I'll just replace it. Eventually I plan to run the grinder motor with a VFD anyway when I convert it to belt. The DPS/DPC is really better suited to drive the wood splitter press I'm going to build anyway.InsideGrinderrate.jpg.0a4225a744a47a14391205a7d41b0808.jpg

insideGrinderElecBox.jpg

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Update: The DPS did not work in this application. The surface grinder has a transformer that converts the input three phase 220 V into 120 V for using a magnetic chuck and/or coolant pumps or whatever. There is also an automated one shot oiler (at least I haven't found a button or lever to activate it manually) and I don't know what powers that. My guess is that the DPS isn't compatible with those.

The DPS was labeled U, V, and W. My surface grinder is labeled L1, L2, and L3. I initially hooked them up respectively. I switched power onto the machine and hit "start". The wheel moved slightly and stopped. I hit "Stop" and switched power off. The manual said that if the machine didn't work well, to try switching the order on the three phase output. I swapped the U and V so that U was going to L2 and V to L1. This time when I powered the unit I heard a loud hum. Before I could hit the "Start" I heard a pop and saw bluish smoke shooting out of the DPS. I immediately de-energized and switched off the grinder. With the grinder off I re-energized the circuit and saw further smoke.

I contacted Myung Youn and they requested pictures of the spec plates. I provided those and they had some further questions that kind of echoed my own. The main plate that stated 5.5 A also had "Control voltage 120 V". I'm not sure what that means in a 3-p 220V system. To their credit, they offered to send another unit if we could figure out why the first one blew. 

I declined the offer, figuring the cost as part of my continuing education. I've ordered a VFD that I will wire directly to the motor, separating it from the rest of the grinder's electrical system.

When I get around to converting my wood splitter to a press I may give the DPS another look as it would probably be ideal for direct drive of the 3-P motor that came off my Little Giant when I converted it to 1-P. It will replace a 5 hp gas engine to drive the hydraulics on the press.

Now I need to figure out how to get that oiler system working.

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